Seveneves

DAY 700

ON DAY 700, ALSO KNOWN AS A+1.335 (ONE YEAR AND 335 DAYS SINCE the destruction of the moon), the Cloud Ark, as seen from the Earth, looked like a bright bead strung on a silver chain. For the reasons that Dr. Dubois Jerome Xavier Harris had tried to articulate during his soliloquy aboard Arklet 2, back on A+1.0, it was expensive, in terms of propellant, to maintain an actual cloud or swarm of arklets around Izzy. Much cheaper and more reliable was to have them precede or follow the space station along the same orbital path, like a queue of ducklings with Mom in the middle. Once an arklet had found its place in that train, changing its position was a maneuver whose complications were a perennial source of surprise and consternation to newly arrived members of the so-called General Population.

Arkies as such — people who had been selected in the Casting of Lots, who had spent up to two years in training for this mission, and who had been sent up here specifically to manage, and live in, arklets — understood it implicitly. As of Day 700, there were 1,276 of those, with about two dozen more coming up each day during the final surge of launches. New arrivals were assigned to empty arklets that awaited them at the head or the tail of the queue. These were launched on separate heavy-lift rockets about four times a day. Since an arklet consisted mostly of empty space, it weighed almost nothing compared to the lift capacity of a big rocket, and so they were always crammed from boiler room to front door with vitamins. These had to be extracted and stowed before the arklet could be occupied. Each arklet had its own unique manifest of stuff. Some of them were just full of compressed gas, such as nitrogen, which would later be used to fertilize crops. Others might be packed with enough diverse and seemingly random goods to open a space bazaar: medicines, cultural artifacts, micronutrients, tools, integrated circuits, spare parts for Stirling engines, Arkies’ personal effects, and, in one notable case, a stowaway who was found dead on arrival. With the exception of the stowaway — who was stashed in the morgue with the rest of the deceased — all of these items had to be extracted, cataloged, and stowed appropriately. Each arklet had some onboard storage space, so to some extent the storage was distributed — that being a fundamental tenet of the whole swarm-based Arkitecture. Bulk materials like gases could be pumped into external tanks or bladders: little ones dangling from arklets, big ones distributed around Izzy’s periphery where they could serve as extra shielding against radiation and micrometeoroids. So-called dry goods were likewise stashed in mesh bags that lived “outside” until such time as they were needed. Scarce and crowded “inside” space was reserved for organisms and goods that actually needed air and warmth. So, compared to the way it had looked a year ago, Izzy was spare and clean on the inside.

Anyone who had not been chosen in the Casting of Lots and trained as an Arkie was categorized as General Population. There were 172 of those. The number grew only slowly, since most of the people who were qualified, and who were needed, ought to have been sent up a long time ago. Adding new members was attended by much political controversy down on the Earth. The Crater Lake Accord had ratified the general scheme of a Cloud Ark populated by those chosen in the Casting of Lots. It had been obvious, and uncontroversial, that experienced specialists were also needed, and so no one had quibbled over sending up the Scouts and the Pioneers. The concept of the General Population had been written into the Crater Lake Accord precisely to allow for that. People like Rhys Aitken, Luisa Soter, Dubois Harris, Moira Crewe, and Markus Leuker had been sent up under the “GPop clause” because they knew how to do things. For every one of them who got sent up, however, a hundred with similar qualifications were stranded on the ground, where some of them called their congressmen, chancellors, presidents, or dictators. The politics had become so involved as to throttle the incoming flow to a trickle. The remaining available GPop slots were being hoarded by national governments. They were being filled grudgingly and with byzantine premeditation.

For Arkies and members of the GPop alike, it was easy to underestimate the “distance” separating Izzy from an arklet that was only a few kilometers ahead of or behind it in orbit.

The difficulties entailed in moving from one arklet to another could be mitigated by physically docking arklets to a common structure, forcing them to fly in a rigid formation. Or so it might seem to people who had not been steeped in the laws of orbital mechanics. But the fact was that an arklet docked to the end of a truss far off to the port or the starboard wing of Izzy was not in a proper orbit at all. Left to its own devices — free, that is, of the constraints imposed, and the forces exerted, by the truss — it would converge on Izzy, cross through Izzy’s orbit, diverge from it, turn around, and converge again, on the same ninety-three-minute cycle that clocked Izzy’s orbits around the Earth. An arklet mounted “above” Izzy on the zenith would want to go “slower” and lag behind; one mounted “below” on the nadir would want to race ahead. To the extent that the truss structure prevented those things from happening — to the extent, in other words, that it succeeded in its basic function of holding all the modules and arklets in a fixed configuration — it was undergoing stress, exerting forces on those arklets to prevent them from doing what they wanted to do. Humans in those arklets would notice themselves drifting and bumping into the walls as their natural trajectories, as ordained by Sir Isaac Newton, were perturbed by the structure of Izzy. The larger Izzy grew, and the more arklets and modules were connected to her, the greater those forces became, and the closer she came to breaking.

There was another, even more compelling reason for limiting Izzy’s sprawl. She was taking shelter behind Amalthea.

The space station’s original orbit had been carefully chosen. Any lower, and the thicker air would make the orbit decay too fast. Any higher, and the danger from micrometeoroids would increase. That was because the rocks zipping around in space were subject to the same slow orbital decay as Izzy herself. Which was a good thing, since it dragged them down into the atmosphere and destroyed them, leaving clear space for Izzy to sail through. Her orbital altitude of four hundred kilometers was a Goldilocks compromise between “too much orbital decay for Izzy” and “enough orbital decay to sweep the sky of dangerous rocks.”

The attachment of Amalthea to Izzy’s forward end a few years ago had changed all of that for the better. Orbital decay was less of a problem because of the rock’s high ballistic coefficient, and micrometeoroids tended to get stopped by the massive nickel-iron cowcatcher.

The White Sky, however, was going to put many more rocks into their path. Big ones could be detected from a distance and avoided, but little ones could do a lot of damage, and so the most important parts of Izzy needed to take shelter in the lee of Amalthea, crowding up against her aft surface. Some rocks might still come in from unexpected directions, but in general there would be a “prevailing wind” in the drift of lunar debris. Amalthea was aimed into it.

But Amalthea couldn’t protect any parts of Izzy that projected beyond its silhouette. Dinah and the rest of the Arjuna Expeditions crew had made some progress in “embiggening” the asteroid, carving out slabs of metal and then elevating them like flaps on an airplane wing to extend the sheltered envelope, but it could only get so big. At some point it had been necessary to draw a line under the expansion and “fix the envelope,” meaning that Izzy took on a definite shape and size. This had occurred on A+1.233. Since then they had found ways to jam more modules inside that envelope, or, where that wasn’t possible, to pack bags and bladders of stored material into gaps. And they had tacked on additional storage in the unprotected volume outside of that envelope. But nothing had been added to her before or since. She couldn’t grow in the aft direction because Amalthea’s protective shadow only extended back so far — bolides could “wrap around” in any direction, since they weren’t in perfectly parallel trajectories. Anyway, boost engines were needed back there, and being in the path of rocket exhaust made standing in hellfire seem like a pleasant summer’s day by comparison.

Amalthea was now enveloped in scaffolding, anchored directly into the nickel-iron by connection points welded on, or drilled in, by Dinah’s robots. A proboscis extended forward from that cloud of trusswork and supported a little cluster of radar and communication antennas. Forward of that, the closest arklets — seven of them, docked to a hexagonal framework — kept station about a kilometer away, far enough that the firings of their thrusters would not blast those antennas with jets of hot gas. Other heptads, as the seven-arklet clusters were called, were spaced ahead of that one at the same interval. Beyond a certain point these petered out and were replaced by triads — three arklets on a triangular frame — and beyond that were singletons.

A similar tapering could be observed aft of Izzy, though the distance to the first heptad was greater, respecting the danger posed by Izzy’s boost engines. These heptads and triads were a bit like Lego or Tinkertoy parts, making it possible to cluster arklets together without much ado; hamster tubes were laced through their trusses so that, once an arklet was docked, persons and material could be easily moved to other arklets on the same frame. Adapters were also floating around that would facilitate nose-to-nose coupling, but these had been found to be not as useful as the hep and tri frames.

Farther out toward the ends of the train, it was not uncommon to see bolos. Each of these spun with its center of gravity — the grapple joining the two cables — tracking along the shared orbital path of Izzy and the other arklets. For now, though, almost all such coupling was for training purposes. Only about three weeks remained before the White Sky. The Arkies could survive that long in zero gravity. Formation of bolos and simulation of Earth-normal gravity was a practice intended for the long haul, when people might live their entire lives in arklets and would need gravity to build and preserve their bones, their eyesight, and other body parts that went bad in its absence.

The Cloud Ark passed through a complete day/night cycle every ninety-three minutes. Time was arbitrary in space, so the ISS had long ago settled on Greenwich time, also known as UTC, as a reasonable compromise between Houston and Baikonur. The Cloud Ark had inherited that system, and Day 700 began at midnight at the Royal Observatory in Greenwich, or A+1.335.0 in ark time. About one-third of the population woke up to begin a sixteen-hour shift. Others would wake up at A+1.335.8, or “dot 8,” and at “dot 16.” The system ensured that about two-thirds of the population was awake at any given time. Awake people needed more oxygen and generated more heat than sleeping people, so it put less of a strain on life support systems, and enabled the Cloud Ark to support more people, if the waking and sleeping cycles were staggered. A reason for the popularity of triads was that each of the three component arklets could operate on a different shift, observing its own artificially imposed period of darkness and quiet. In a heptad, the same basic scheme could be used, with two of the arklets asleep at any given time and the one in the middle of the hexagonal frame being “always on.”

Doob had requested, and been granted, a position in the third shift, meaning that he was basically operating in the same time zone as Amelia, Henry, and Hadley on the West Coast of the United States. He had awakened at dot 16 of the day before, or four in the afternoon in London, which was eight in the morning in Pasadena. So, at the stroke of A+1.335.0, when the first shift of that day began, he had been awake for eight hours and was feeling like a brief nap might do him good. But he knew that this would only make it more difficult to get to sleep at dot 8 and so decided, as usual, to gut it out.

Finding that his brain was too addled to make any sense of the latest figures from Caltech on the continued exponential breakup of moon debris, he went to the “gym,” which was a module containing several treadmills. To prevent their users from bouncing off them in zero gravity, these were equipped with waist belts and bungee cords that held the occupants “down,” pressing their feet against the belt of the treadmill and forcing the legs to do some real work. Supposedly it was good for the bones and muscles. Amelia kept sending Doob emails asking him whether he’d exercised today, and he liked to make her happy by answering yes.

A few minutes after he began his exercise routine he was joined by Luisa Soter, who had just awakened, as she was on the first shift. She liked to do her “jogging” first thing in the morning, so it was not the first time they had intersected in this way. Six treadmills had been mounted to the walls of this cylindrical module; the users’ feet pointed outward and their heads projected in toward the center like spokes converging on a hub, bringing them rather close together and making conversation easy. For extroverted, social people like Doob and Luisa, this was a great setup; more solitary exercisers would don headphones and pointedly focus on a tablet or a book.

“Did you go to Venezuela when you were out collecting Arkies?” Luisa asked him.

The way she stressed the word “go” suggested that Venezuela was an obvious topic of conversation — the thing that well-informed persons would naturally begin talking about first thing in the morning. Doob didn’t know why. He had heard a few people talking recently about Kourou, which was the place in French Guiana where the Europeans, and sometimes the Russians, launched big rockets into orbit. In the last two years it had become one of the most important launch sites for arklets and supply ships. So he had the vague sense that something was afoot there, something that people were concerned about.

He had been focusing all of his attention in the other direction, on Peach Pit and its iron-rich “children.” These were still visible, through increasingly thick clouds of rocky debris. When the White Sky happened they would vanish into a cloud of dirt, and he might not see them for a while. So he had been looking at PP1, PP2, and PP3 while the looking was still good, nailing down their exact orbital parameters, taking high-res photographs. PP3 was especially interesting. It was a congealed glob of mostly iron, similar in composition to Amalthea. It was some fifty kilometers in diameter. And it had a deep cleft on one side, comparable in size to the Grand Canyon, apparently formed by a collision that had rent its outer skin while it was partly congealed. Doob had begun calling PP3 Cleft.

“Doob? You still with me?” Luisa asked. “I was going to say ‘Earth to Doob, Earth to Doob,’ but it doesn’t apply anymore.”

“Sorry,” he said. He had gone into a reverie thinking about that huge crevice on the side of Cleft, imagining what it must look like from the inside. “What was your question again?”

“Venezuela,” she said. “Did you do any of your ‘abduction runs’ there?”

“No,” he said. “Closest I came was Uruguay. Which isn’t that close. And by that time I was pretty burned out.”

“Why were you burned out?”

Typical Luisa!

“Overscheduling?” she went on. “That is, was it physical burnout? Or more emotional/spiritual?”

“I had just had it,” he said. “It’s hard. Taking young people — the best and the brightest — away from their families.”

“But it was for a good purpose, right?”

“Luisa, where are you going with this?”

“Are you aware of what is going on offshore of Kourou?” she asked in return.

“No,” he said flatly.

“You’ve checked out,” she said.

“I talk to my family every day. But other than that? Yes, Luisa, I have checked out of planet Earth. Nice place. Lovely people. But I have to focus on what comes next.”

“So say we all,” she said. “But one could argue that things happening in Old Earth’s final three weeks could have repercussions on New Earth.”

“What’s going on?” he asked.

“Apparently not a single one of the seventy-five Venezuelans who were picked in the Casting of Lots has actually been sent into space,” she said.

“You know that the overall ratio has ended up being something like one in twenty,” Doob said. Meaning that for every twenty candidates chosen in the worldwide Casting of Lots and brought to the training centers, only one had found a place up here in the Cloud Ark. Not a figure to be proud of. But it was the best they’d been able to do, and they hoped to bring the number down to more like one in fifteen, or even one in ten, with a last-minute surge of launches.

“Yes. And the Venezuelans know that too. So they’re saying that three or four of their seventy-five ought to have made it up here by now.”

“Statistically, that is not a valid—”

“These people do not look like statisticians.”

“Politics.” Doob sighed.

Luisa chuckled. “I hear you, sugar. I’m not gonna say you’re wrong. But I have to warn you that this is the word—‘politics’—that nerds use whenever they feel impatient about the human realities of an organization.”

“And I’ve been in enough faculty meetings at Caltech to know how right you are,” Doob said. “But I meant it in a different way. The way that the Venezuelans ran their selection program was overtly political. In most countries they took the Casting of Lots idea with a grain of salt. There was a random element, yes — but they also filtered for ability. The Venezuelans chose not to do that. So, they ended up sending in kids from the boondocks, truly chosen at random. Many of whom had fine personal qualities. If I had my way, we’d get some of them up here. But I’m not the one who is choosing. The people who are, are choosing on the basis of math ability and things like that. So it makes me sad that other people are in line ahead of the Venezuelans, but it doesn’t surprise me.”

“Three weeks ago, boat people started squatting on Devil’s Island,” Luisa said, “refusing to move.”

“Isn’t that a penal colony?” Doob asked. “Why would anyone—”

“It used to be a French prison, yes,” Luisa said. “Hasn’t been for a long time. Hardly anyone lives there. But it’s right under the flight path for launches out of Kourou. So, whenever there’s a launch, they evacuate it.”

“It must be evacuated all the time then, given the amount of traffic.”

“For the last two years, yes. But then a bunch of people showed up there and camped out and refused to move.”

“I’m guessing that the French and the Russians went right on launching.” In fact, Doob knew as much, since he saw arklets and supply ships coming up from Kourou all the time.

“Yes. So the occupation was more of a symbolic gesture at that point.”

“These squatters were Venezuelans, I take it.”

“Yes. It is a fairly easy cruise along the coast from Venezuela to French Guiana — a few hundred kilometers.”

Something was itching in Doob’s memory. “Does this have anything to do with the supply vessel that failed to show up yesterday?”

“And the day before. There’s been a two-day interruption, going on three, in launches out of Kourou.”

“A few squatters on Devil’s Island can’t explain that,” Doob said. Then he added, as a joke, “Unless they have surface-to-air missiles.”

Luisa said nothing.

“Are you shitting me?” Doob said.

“It’s not so much the ones on Devil’s Island as the ones in the blockade,” Luisa said. She handed her tablet to Doob. She’d pulled up what looked like an aerial photograph, probably shot out the window of a helicopter. In the foreground was the European Space Agency launch complex, which he’d seen before. It was separated from the Atlantic by a couple of kilometers of flat ground, banded with low, scrubby beach vegetation. In the distance was a trio of small islands, a few miles off the coast; he assumed that Devil’s Island was one of them.

The waters between the beach and the islands were choked with vessels: mostly small, but a few rusty freighters as well, a full-sized oil tanker that looked the worse for wear, and some ships that he could have sworn were military.

“When was this taken?” Doob asked.

“A few hours ago,” Luisa said.

“Are those naval vessels?”

“The Venezuelan navy is coming out to maintain order,” Luisa said.

“And you weren’t kidding about the surface-to-air missiles?”

“The pirates who showed up in that oil tanker claimed that they had Stingers, and that they would use them against the next rocket that lifted off from Kourou.”

“That is nuts,” Doob said.

“Politics,” Luisa said. “But we always knew it was going to happen, right?”

“Good morning, Doctors,” said a new voice: that of Ivy Xiao, entering the module to begin her own “morning” exercise routine.

“Good morning, Doctor,” said Luisa and Doob in unison, though for Doob it was afternoon.

“Did I hear the P-word?”

“Yes,” Luisa said. “We were just talking about you, honey.”

Doob was appalled. But Ivy laughed delightedly.

Ivy had been replaced, something like eight months ago, by Markus Leuker, the Swiss fighter pilot, mountain climber, and astronaut. Or, to put it more precisely, a new position had been created that made Ivy’s post redundant. Izzy was no longer just Izzy; it was the combination of the Cloud Ark fleet plus the vastly enlarged complex that Izzy had turned into. As such, a new leadership structure was required. The person at the top of that structure would shortly become the most powerful leader in human history, in the sense that 100 percent of all people alive would be under his or her authority. It was an altogether different job from being the first among the twelve equals who had been manning the International Space Station of two years ago.

Nevertheless, Ivy could have done it. Everyone who really knew her agreed on that much.

They had replaced her anyway. It was partly a matter of global politics. Placing overall command of the Cloud Ark under a representative of the United States, Russia, or China would have been seen as a provocation by the two countries that had “lost.” So it had to be someone from a small country. Preferably one that was seen as politically independent. This narrowed the list of candidates down to basically one: Markus Leuker. The dark horse being Ulrika Ek, the Swedish Arkitect and project manager, who had been launched up to Izzy at the same time as Markus — but on a different vehicle, in case one of them crashed. No one had ever really expected Ulrika to get picked, however. The choice was explained in terms of Markus’s dynamic leadership style, his charisma, and other such buzzwords that, as everyone understood, boiled down to the fact that he was a man.

Ivy had failed, in the eyes of the Russians and of many in the NASA hierarchy, by not taking a firmer hand with Sean Probst. That wasn’t the only complaint they had about her, but it was the one around which everything else had crystallized. Once everyone had begun to see her as the overly book-smart, well-meaning but week-kneed technocrat, everything she did had been viewed through that lens. Dinah’s rescue of Tekla had been examined under what doctors called “the retrospectoscope” and been deemed a failure on Ivy’s part to enforce necessary discipline. New arrivals to the Cloud Ark, prepped by Internet comment threads and television pundits to see Ivy as a weak leader, began finding ways to make that into a self-fulfilling prophecy. The success of the Bolo One test, which in other circumstances might have bolstered her, came to be viewed as an almost literal handoff of authority from Ivy to Markus. Presented with some choice opportunities to say supportive things about Ivy, Ulrika had declined to do so, and it wasn’t clear whether that was mere absentmindedness or an attempt to cement her position as number two.

In any case it was all squarely in the realm of Politics. Doob had avoided bringing it up around Ivy, not wanting to raise an awkward topic, and so he found it horrifying when Luisa went straight to it, and fascinating when Ivy laughed.

People.

“What is on your docket for today?” Luisa asked.

“Looking at spreadsheets,” Ivy said. “Trying to figure out the consequences of losing Kourou.”

“That is one monkey wrench we didn’t need,” Doob said.

“To be sure,” Ivy said, “but in a weird way, I’m almost. .” and she trailed off.

“Glad? Relieved?” Luisa guessed.

“It’s like the starting gun finally went off,” Ivy said. “We’ve been prepping for this for almost two years. Awaiting the disaster. Waiting for all hell to break loose. And now it has. Just not in the way we expected.”

“What were you expecting?” Luisa asked.

“That we’d get hit by a bolide and take a lot of casualties,” Ivy said. “Instead something unexpected happened. Which is good training, in a way.”

“How’s your sweetie?” Luisa asked.

Doob shut off his treadmill and unbuckled the padded belt that connected him to the bungee cords. He was the only man in a room where two women were talking about one of their boyfriends. He knew his cue to make himself scarce.

“He went dark on me two days ago,” Ivy said. “Which means he’s probably underwater.”

“I’m sure he’ll pop up for air soon,” Luisa said. “Can he send email when the submarine is submerged? I know nothing about it.”

“There are ways—” Ivy said, but by that time Doob was floating out of the module.

He made his way aft down the Stack to H2, then clambered down a spoke into the rotating torus T2, which Rhys Aitken had been sent up to build. Gravity in it was one-eighth of Earth-normal. Originally designed as a space hotel for tourists, it had never been quite up to the requirements placed on it by the Cloud Ark project, and so Rhys had been put in charge of building a larger one, concentric with T2, and inevitably called T3. Never one to rest on his laurels, he had invented a completely new system for building it. Unsurprisingly to Dinah, this had consisted of assembling a long loop of high-tech chain and setting it in motion around T2, then adding stuff onto it incrementally. It spun around the same hub at the same RPMs, but because it was bigger, its simulated gravity was a little stronger, about equal to that on the moon. It housed the closest thing the Cloud Ark had to a bridge: a segment of T3 about ten meters long, used by Markus as his headquarters. Attempts had been made to dignify it with names such as “command center,” but at the end of the day it was just an upgraded version of the Banana: a conference room with some television screens and power feeds for tablet computers.

Izzy didn’t have a helm. It didn’t have controls as such. No big wheel to turn for steering it through space, no throttle. Just a bewildering assortment of thrusters controlled through a web interface that could be pulled up on any tablet, provided you had the right password. So, the control room, the bridge, the command center could be anywhere. People had ended up calling the room Markus had chosen the Tank. Adjoining it on one side was a smaller office that served as Markus’s sanctum sanctorum. Next to the Tank on the other side was a larger room with a number of cubicles, bizarrely like a suburban office park, where people supporting Markus could sit and work. It had been called the Cube Farm for about ten minutes and was now simply called the Farm. Adjoining the Farm on the other side was a maze of cramped rooms where one could obtain food or use the toilet.

Doob had found that the Farm was frequently the least crowded place on Izzy, just because it didn’t occur to people to go there. The gravity was good for his bones and the availability of coffee and toilets were obvious pluses. So he tended to swing by a couple of times a day, get a beverage, see what was happening, and, if things were quiet, grab a vacant carrel and do some work.

He got there at about dot 2. The walls of the Farm, and of the adjoining Tank, were lined with projection screens, known in NASA-speak as Situational Awareness Monitors. They acted as windows onto various parts of the Cloud Ark and its environs. One was showing the Earth below them, another the cloud of debris that had been the moon, another the approach of a supply module from Cape Canaveral getting ready to dock, another the progress of a bolo coupling drill being conducted by some newly arrived Arkies several kilometers aft. Some just displayed statistics and bar charts. The biggest, at the far end of the Farm, was occupied mostly by a grainy video feed from some part of the Earth where it was dark. A label superimposed at the bottom identified it as KOUROU, FRENCH GUIANA. Once Doob had that information to work with, he could make out the general scene: a galaxy of lights from the thousands of boats that had joined the “People’s Justice Blockade,” and in the background the much more orderly precincts of the spaceport, where an Ariane stood on one pad and a Soyuz on the other, ready to launch, but still unable to do so because of the threat of those Stingers.

The silhouette of a military chopper passed between the camera and the lights of the launch complex.

This was a twenty-four-hour news network. The crawl running across the bottom of the screen was being updated every few minutes by the current BFR, or Bolide Fragmentation Rate, which had started out at zero on A+0 and been climbing ever since then; this was the number that, when it caromed through the bend in its exponential curve, would signal the onset of the White Sky. The networks had been tracking it obsessively. There was an app for it. A bar in Boston had begun offering end-of-the-world drink specials whenever the BFR broke through certain milestones, and the promotion had been copied widely.

Above the crawl was a smaller video inset showing an empty lectern in the White House Briefing Room. Apparently they were expecting some sort of announcement.

Doob sat in one of the carrels, spent a few minutes checking email, and then tried to get back to his main task, which was to write a memo about the distribution of metal-rich moon fragments, how they might be reached and exploited, and why that should be interesting to the management of the Cloud Ark. He was only a few sentences in, however, when movement on the big Situational Awareness Monitor caught his eye, and he looked up into the eyes of the president of the United States.

She was staring into the camera, or rather the teleprompter screen in front of the camera, and delivering some sort of terse announcement. She looked pissed off.

Pinned to her lapel was a loop of ribbon. All the important people had been wearing these for the last few weeks, and they had become popular among the hoi polloi as a gesture of solidarity with the mission of the Cloud Ark. Selecting the color scheme had consumed resources equivalent to the gross domestic product of a medium-sized country. They had settled on a thin red line down the center, symbolizing the bloodline of the human race, flanked by bands of white, symbolizing starlight, flanked by bands of green, symbolizing the ecosystem that would keep the Arkies alive, flanked by bands of blue, symbolizing water, and, finally, edged by bands of black, symbolizing space. The discussion had been as lively as the results were complicated. Black symbolized death to Westerners, white symbolized it to Chinese, and so on. This design offended everybody. It had gotten loose on the Internet as the “official” ribbon design even though the commission charged with designing it had become hopelessly deadlocked and was still evaluating twelve different candidate designs submitted by schoolchildren from around the globe. Factories in Bangladesh had been repurposed to hurl this stuff out by the linear kilometer, they had shown up in kiosks and souvenir shops from Times Square to Tiananmen Square, and the world’s leaders had bowed to the verdict and begun wearing them. The president had attached hers using a lapel pin consisting of a simple disk of turquoise rimmed in platinum. The blue disk on the white field was meant to echo Crater Lake amid the snows of November; it was a visual emblem of the Crater Lake Accord, and the closest thing that the Cloud Ark had to a flag.

The sound had been turned down, so Doob couldn’t hear what J.B.F. was saying, but he could guess it well enough, and a few seconds later the highlights began to show up in the crawl at the bottom of the screen. The so-called People’s Justice Blockade was no grass-roots movement but an operation planned and carried out by the Venezuelan government. It was a reprehensible political stunt that was actively interfering with the all-important building of the Cloud Ark. It was not true, as some had been whispering and the Venezuelan president was now openly saying, that the White Sky was a hoax. The blockade was not, as its sympathizers would have you believe, a peaceful civil disobedience protest; armed intruders had begun landing on the beaches of French Guiana a few hours ago, and were now being held at bay by the French Foreign Legion, bolstered by a multinational force including United States and Russian marines. Doob, doing his best to tune it out, couldn’t defeat the irrational feeling that J.B.F. was staring directly at him: a feeling he had come up here partly to get away from.

One of the PR flacks down in Houston reached him over a video chat link that Doob had not had the presence of mind to disable. He talked Doob into spending the next hour writing a little homily about how everyone down on Earth needed to unite behind the all-important mission of the Cloud Ark, and detailing how the Kourou blockade was affecting that. Doob wanted in the worst way to tell this guy to get lost, but he had a soft spot for people who had only three weeks to live.

Doob called Ivy — for Izzy had its own cell-phone system now — and got her to supply a few hard quotable numbers, which he rounded off and typed into his script. He then devoted a minute to psyching himself up to adopt the Doc Dubois persona. Formerly the ruin of his first marriage, the basis of his livelihood, and his ticket to the Cloud Ark, Doc Dubois was a person he rarely had to be anymore. That guy seemed as dated as a character from a 1970s television serial. Getting into the persona was nearly as cumbersome as donning a space suit. It required an extra cup of coffee with sugar. When he felt he was ready, he turned on his tablet’s video camera, identified himself as Doc Dubois, greeted the people of Earth, and read his little script.

When he was finished, he emailed the file down to Houston. Then he tried to go back to his memo, but he became distracted when the Situational Awareness Monitor flashed up a red BREAKING NEWS banner and began to show footage of indistinct flashes against a dark background. Some sort of hostilities had broken out on the ground in French Guiana, between the perimeter of the spaceport and the beach. The French Foreign Legion was participating in what might be the last battle ever fought. But the television news cameras couldn’t get anywhere near the action, so the coverage mostly consisted of journalists interviewing each other about how little they knew.

In the middle of all that, the flack in Houston got back to him and asked if he could please relocate to a part of Izzy where zero gravity prevailed, and rerecord the little pep talk. Conspiracy theorists were saying that the Cloud Ark didn’t really exist and that it was actually just a bunch of movie sets in the Nevada desert. Whenever they saw video from parts of the space station with simulated gravity, they cited it as evidence, and added millions of friends and followers to their social media profiles.

Doob said he’d see what he could do, and departed from the Farm. Nothing was going on here anyway; Markus wasn’t around at the moment. He ascended a spoke to H2 and thus entered zero gravity.

H2 had been the aft-most piece of the Stack — the train of modules that ran up Izzy’s central axis — until several weeks ago when the Caboose had been launched up from Kourou and mated to its aft end. The main purpose of the Caboose was to house a large rocket engine, burning hydrogen and oxygen, which would do most of the work of boosting Izzy’s orbit. It wasn’t possible to extend Izzy any farther back because, beyond that point, anything tacked onto it would no longer reside safely within Amalthea’s protective envelope. And indeed there had been long discussions of contingency plans for the case where the Caboose took a hit and its engine was destroyed.

Putting his back to the Caboose, Doob began to drift forward up the Stack. H2 led to H1, which led in turn to the old Zvezda module. This had formerly sported small photovoltaic panels to its port and starboard sides, but these — like most of Izzy’s solar panels — had been folded up and removed to make space for other construction. During an intermediate stage of the Arkitects’ labors, power had come not from photovoltaics but from little nukes, the same as those on the ark-lets. These still protruded from attachment points all over the space station, aglow with red LEDs meant as a warning to spacewalkers and pilots. And they still produced a significant amount of power and served as a valuable backup. But most of the station’s power now came from a full-fledged nuclear reactor, adapted from those used on submarines, which was mounted on a long stick that projected to nadir from the Caboose. There were a number of reasons why a big power plant might be needed, but the most important of these was to produce rocket propellant by splitting water into hydrogen and oxygen. And this explained why the reactor was where it was. The Caboose housed the big boost engine that was Izzy’s largest consumer of propellant. And it was also the central nexus of the Shipyard complex, where smaller vehicles could be assembled from a kit of parts. Once assembled, they too would need propellant.

Zvezda’s forward terminus was a docking station with ports to the zenith and nadir sides, where scientific laboratories had been connected back before Zero. This tradition had been preserved, in a way, by turning that docking site into Grand Central station for work related to the Cloud Ark’s primary function of preserving the Earth’s heritage. If Doob went “up” in the zenith direction, he entered a long module whose main purpose was to support multiple docking ports where other vessels could be, and had been, attached. In general these were crammed with priceless cultural relics, but some of them also supported server farms where digital recordings were stored. Certain relics were easier than others to send into space; the Magna Carta had made it up here, but Michelangelo’s David was still on the ground. Considerable effort had gone into sealing up heavy relics in “everything-proof vaults” and leaving them on the bottom of the oceans, or in deep mine shafts, but Doob had long since lost track of the progress of that undertaking.

If instead Doob went “down” in the nadir direction, he entered a similar three-dimensional maze of modules. Most of these were devoted to storage of genetic material: seeds, sperm samples, eggs, and embryos. All of these needed to be kept cold, which in space wasn’t too difficult; it was primarily a matter of shielding the storage containers from sunlight, which could be done with a featherweight piece of metallized Mylar film, and secondarily a matter of just preventing the warmth of surrounding objects from seeping into the samples. Doob always paused when passing by that hatch. He was not a spiritual man, but he could not humanly ignore the fact that his potential fourth child, the embryo he and Amelia had created, was in there somewhere. Along with tens of thousands of other fertilized embryos waiting to be thawed out and implanted in human wombs.

He passed into Zarya, which was the next module forward in the Stack. Having been mildly spooked by thinking about embryos, he now had a vague intention of going into the Woo-Woo Pod to rerecord his video. This was a spherical inflated structure, ten meters in diameter, with several large domed windows. It was accessed from Zarya via a hamster tube on the nadir side, so it was aimed at the Earth. Ulrika Ek had drawn the ire of every religious group on the planet by refusing to provide separate places of worship for every single one of them in the Cloud Ark. Instead of sending up a church pod, a synagogue pod, a mosque pod, etc., she had provided this one structure, which was something like the interfaith chapel at an airport in that all the different religions had to share it. Internal projectors would display crosses, Stars of David, or what have you on its interior surfaces, depending on what sort of service was happening there at the moment. It had a long, cumbersome, politically correct name, but someone had dubbed it the Woo-Woo Pod and the name had stuck.

That someone paused for a few moments at the entrance to the hamster tube that led to it, and detected the haunting tonalities of the Muslim call to prayer. Too bad. He’d thought that the Pod might actually be a good backdrop for the message he was meant to deliver. But he would have to find another place. Directly across, a hatch led into the rambling group of modules that served as Izzy’s sick bay. This had consumed much of the space formerly used by the port-side solar panels. At its farthest extremity, blocked off by an insulated hatch, was the surplus module that had been used as Izzy’s morgue and graveyard since the first Scout launch on A+0.29, when two of the cosmonauts had been found dead on arrival. The terrific mortality rate of those first few weeks had half filled this thing with freeze-dried bodies. Since then, fourteen more had died of various causes: one of a subarachnoid hemorrhage that could just as well have happened on the ground, one of a heart attack, two of suicide, two of equipment failure, four just a few days ago in the sudden depressurization of an arklet struck by a bolide. Those, plus the dead stowaway, were all stored in the morgue. The whereabouts of the other four fatalities could only be guessed at. One was a spacewalker who had simply disappeared. The remaining three had been sleeping in a Shenzhou spacecraft docked to the end of a hamster tube, which had been struck by a bolide the size of a coffee table and essentially vaporized. Performing his video surrounded by free-floating, freeze-dried corpses would shut up the “truthers” but otherwise had nothing to recommend it.

On the opposite “wing,” where the starboard solar panels had once operated, was a roughly symmetrical arrangement of modules used by the General Population for miscellaneous living and working purposes. These connected to the Stack mostly by way of the old American modules: Unity, Destiny, and Harmony. Consequently, there tended to be a lot of humans flying around in those modules, getting from one part of the space station to another or clustering for the equivalent of watercooler chats.

Beyond Harmony was Node X. NASA liked to give these things names by organizing contests for schoolchildren, which was how Harmony had ended up being called that, but the Node X naming project had been defunded before achieving a result, so Node X it was. It had never really found a purpose, so it had become the place where the life sciences gear was stored — or rather the central connector to which the life sciences modules had docked, one by one, as they had been sent up. This part of the Stack was very close to Amalthea, and accordingly well protected, and so it was a good place to store that irreplaceable equipment during the wait for it to become useful. Doob poked his head into several of those modules, hoping to encounter Moira, then remembered that, London girl that she was, she was on third shift, not due to wake up for another three hours — it was about dot 5, predawn in London.

Beyond Node X was the considerably larger SCRUM, which was literally bolted onto Amalthea at its forward end. So it was the forward-most thing in the Stack. Before Zero it had been nearly deserted. Since then it had grown and developed into the space-based headquarters of Arjuna Expeditions. People called it the Mining Colony. They had plugged in more modules until all of its ports were occupied, and then they had begun to attach scaffolding and additional modules — rigid and inflatable — directly to the aft surface of Amalthea.

It was around now that Doob forgot entirely about the task that the flack in Houston had assigned him, and decided to hang out here for a little while and see what was going on. By all rights this ought to have been his favorite part of the Cloud Ark. Yet he never visited, because coming here put him in mind of politics, which stressed him out and distracted him. His earlier conversation with Luisa had brought home to him, however, that ignoring politics might not be the wisest long-term strategy. He might not care about politics, but politics cared about him. And besides, the people who actually worked here — people like Dinah — were terrific. He had no problem with them personally. He should spend more time with them. Right now he was three hours short of the end of his waking cycle. This was the rough equivalent of mid-evening. Time to kick back and grab a beer. No better people to do that with than miners.

The Mining Colony was political for two reasons. First of all and most obviously, it had originated from a public-private partnership of which the private half was Arjuna Expeditions — Sean Probst’s company. Which had been all well and good until he had burst into H2, raised hackles, and ruffled feathers all over the place. Secondly, but much more murkily, there seemed to exist some kind of fundamental disagreement about what the Cloud Ark was supposed to be and how it was expected to develop in the years following the White Sky. Was it going to stay in place, i.e., remain in the same basic orbit? Transition to some other orbit? Would it stay together as a compact swarm or spread out? Or would it split up into two or more distinct swarms that would try different things? Arguments could be made for all of the above scenarios and many more, depending on what actually happened in the Hard Rain.

Since the Earth had never before been bombarded by a vast barrage of lunar fragments, there was no way to predict what it was going to be like. Statistical models had been occupying much of Doob’s time because they had a big influence on which scenarios might be most worth preparing for. To take a simplistic example, if the moon could be relied on to disassemble itself into pea-sized rocks, then the best strategy was to remain in place and not worry too much about maneuvering. It was hard to detect a pea-sized bolide until it was pretty close, by which time it was probably too late to take evasive action. A strike from a rock that size would perforate an arklet or a module of Izzy, but not destroy it; people might get hurt and stuff might get broken, but the worst case was that a whole module or arklet would be destroyed with the loss of a few lives. On the other hand, in the more likely scenario where the Hard Rain included rocks the size of cars, houses, and mountains, detection from a distance would be easier. Evasive action would be not only feasible, but obligatory.

Or at least it was obligatory for Izzy. For a single arklet, it didn’t matter whether it got struck by a rock the size of a baseball or one the size of a stadium. It was equally dead in either case. Izzy, on the other hand, could survive the first of these with the loss of a few modules, but the second would obliterate the whole space station and probably lead to the slow death of the entire Cloud Ark. Izzy had to be capable of maneuvering out of a large bolide’s path.

“Maneuvering” conjured images, in nontechnical minds, of football players weaving among their opponents in an open field. What the Arkitects had in mind was considerably more sedate. Izzy would never be agile. Even if she were, maneuvering in that sense would waste a lot of fuel. If an incoming rock big enough to destroy her were detected long enough in advance, she could get out of its path with a thruster burn so deft that most of her population would not even know it had happened. So, the optimistic view of how this was going to work was that Izzy would remain in something close to her current orbit, with occasional taps on the thrusters that would move her out of the way of any dangerous bolides hours or days in advance of the projected collision. The analogy was made to an ocean liner gliding through a field of icebergs, avoiding them with course changes so subtle that the passengers in the dining room wouldn’t even see the wine shifting in their crystal stemware.

There was, inevitably, a more pessimistic vision in which Izzy was more like an ox blundering across an eight-lane highway in heavy traffic. Depending on who was making the analogy, the ox might or might not be blindfolded and/or crippled.

Which of these analogies was closer to the truth boiled down to a statistical argument in which were braided together assumptions about the range and distribution of bolide sizes, the amount of variation in their trajectories, how well the long-range radars worked, and how good the algorithms were at sorting out all the different bogeys and deciding which ones were dangerous.

Somewhere in the middle, between the ocean liner and the blind ox, was the football player pushing the wheelbarrow.

It didn’t matter whether “football” for you was soccer or the American sport played by men in helmets. In either case you were meant to envision a player trying to weave a path downfield among defenders. A skilled player could succeed at this when running unencumbered but would fail if obliged to push a wheelbarrow with a boulder in it. The boulder, of course, was Amalthea, and the wheelbarrow was the asteroid mining complex that had been constructed around it. If this analogy were the one closest to the truth, then the wheelbarrow would have to be abandoned.

The image was sufficiently clear, and sufficiently alarming, that some had begun to argue for ditching Amalthea as far back as Day 30. More levelheaded analysts pointed out that if the ocean liner analogy applied, there was no need to take such drastic action, and if Izzy were a blind, crippled ox on a freeway, there was no point anyway.

Doob had his own bias, a bias frankly rooted in a certain frozen embryo, which was that the Mining Colony should be preserved at all costs. When he tried to filter out that bias and to look at the models and the data in a completely objective way, he concluded that the jury was still out. So, technical discussions of the matter tended to be unproductive, except insofar as they revealed the biases that the participants had brought into the room with them. And here was where it started to get difficult for him personally, because he couldn’t understand why anyone would harbor a bias different from his own. Why would anyone not want to keep the Mining Colony? What were they thinking? How could the Cloud Ark, and the human race, have a future without those tools and capabilities?

In any case the controversy had ramifications that extended into many seemingly mundane aspects of the Cloud Ark program. If Izzy was going to maneuver with Amalthea attached to it, then the structure holding the rock to the space station needed to be strong. To put it another way, the stronger it was, the more heroic maneuvers could be achieved without breaking it. The ability to perform such maneuvers made the survival of Izzy more likely, and so requests for additional structural work had a kind of self-justifying force. Conversely, a weaker structure limited maneuvering ability and increased the odds that they would have to jettison the Mining Colony in order to survive. And why dump scarce resources into beefing up a subassembly that was going to be abandoned anyway? A similar dynamic obtained in the case of propellant. More of it was needed to maneuver an Izzy with a big rock on it, which meant less of it for the arklets, limiting their autonomy and operating range. Thus physics drove politics to the extremes of “ditch the rock now” or “keep the rock at all costs.”

The Mining Colony now comprised eight modules, plus an inflatable dome that was attached directly to the asteroid. The robots had spent several weeks welding a three-meter-diameter ring onto a circular groove that they had prepared on Amalthea’s surface. The inflatable had been mated to it about a hundred days ago, and filled with breathable atmosphere. It was not quite a shirtsleeves environment, since the asteroid was cold and chilled the air in the dome. And many of the robots’ normal operations produced gases that were toxic, or at least irritating. But that wasn’t the point of having a dome. The point was to recapture and reuse the gases used by the robots’ plasma torches, making it possible to excavate and reshape the asteroid much faster than had been possible in the early days, when all of those gases had leaked away into space. Since then Dinah’s complement of robots had been heavily reinforced by newer and better versions of the same basic models that had been shipped up from Earth. And Dinah herself was now managing a crew of twelve, working in shifts around the clock. They’d been expanding the tunnel she had carved into the asteroid long ago to protect her circuit boards from cosmic rays, making slow progress on hollowing out the asteroid, carrying bits of metal away to a bigger and better smelter that was turning them into steel. Since there was no real place for it in Izzy’s master plan, they’d been putting that steel to work in reinforcing Amalthea’s structural connection to Izzy, feeding back into the political argument again.

Doob glided through a few of the Mining Colony’s modules, asking people where Dinah was, and got noncommittal answers. When he made a move in the direction of her shop, he sensed an uptick in nervous tension, and did not understand why until Markus Leuker emerged, greeted him personally, and engaged him in friendly, inconsequential chitchat. Stalling for time, as Doob understood, so that Dinah could have a few minutes to herself.

It had been known for several months that Dinah had been having sex with Markus, an activity referred to on Izzy as “climbing the Daubenhorn.” Two other women were known to have attained that summit, not long after Markus’s arrival, but since then Dinah had had him all to herself. By the standards of earthbound organizations, be they corporate or military, it was an eyebrow-raising violation of ethical standards for the boss to be sleeping with a subordinate. But a month from now every living human would technically be one of Markus’s subordinates, so he either had to break the rules or be celibate for the rest of his life. No one who knew him very well saw the latter as a realistic option, unless he were to have his testicles surgically removed (a procedure that certain people on Izzy were longing to see performed). That being the case, there was a certain logic in his having settled, quite early, on Dinah. It might be unethical, but at least everyone knew where matters stood. Dinah was no one’s idea of a pushover; no sane person could be worried that she was in any way feeling pressured or harassed. And on the other side of that coin, people seemed to feel more comfortable knowing that Dinah was not on the prowl. By the mundane standards of Izzy gossip, her dalliance with Rhys Aitken had been sensational, their eventual breakup a big story, detailed in London tabloids. After that she’d been unable to have coffee with any male crew member without stirring up more whispers. Being unequivocally in the bag with Markus was a lot simpler. And yet it still had to be treated as if it weren’t happening, which was why Markus and Doob had to take part in this charade.

“I don’t know if you heard,” Doob told him, “but fighting has broken out on the ground, between the spaceport and the beach.”

It was clear that Markus hadn’t heard, which was hardly surprising given that (a) it wasn’t his problem and (b) he’d been occupied. He was, understandably, quite relaxed at the moment, and it took a while for him to bring his formidable powers of concentration to bear on the matter at hand.

“I can’t believe they will let it go on like that,” he said.

“The president made a statement. She looked like she was eating bolts.”

“A government run by doomed persons is nothing to trifle with,” Markus said, “but I suppose the same could be said of the Venezuelans.” He sighed. “I wonder if we should just accommodate some Venezuelan Arkies. There must be a few bright sparks.”

“That would have worked a couple of days ago,” Doob said, “but now it’s turned into one of those ‘we don’t negotiate with terrorists’ things.”

A trace of a dry smile came over Markus’s lips. He had washed his face with the towelettes they all used; Doob could smell the industrial fragrance with which they were permeated. “Of course,” he said, “it wouldn’t do to set a precedent that might be abused during the next three weeks.”

The joke, such as it was, would have been completely unacceptable when uttered in public, or even in a meeting, and so this was a way of saying to Doob, You are in my confidence. Doob wasn’t a leader, but he was fascinated by people who were, and how they went about their work.

“Ivy’s figuring out the ramifications of not having those arklets, those supplies.”

“Thank God for Ivy,” Markus said. Since winning command of the Cloud Ark he had never lost an opportunity to praise her — another skill that Doob reckoned must be inculcated into leaders in whatever mysterious Leader Academy churned them out. More likely it was an instinct.

“Well, my day begins,” Markus continued. “Thank you for the briefing.” Markus, like a lot of the Europeans, ran on third shift, which meant that he was, in fact, beginning his day a couple of hours early.

“Mine is winding down,” Doob said. “I thought I would get drunk with some miners.”

“No better people for it,” Markus said with a wink. “I believe Dinah will be out in a minute. I think she would enjoy seeing you.”

With that Markus pulled his phone from the pocket of his coverall and turned his attention to its screen while using the other hand to pull himself out of this module and down the Stack.

Doob was left floating in the middle of the SCRUM. The only thing between him and Dinah was a privacy curtain. He was about to say “Knock knock!” when he heard a string of beeps emerge from a speaker on the other side. An incoming Morse code transmission, which he had not the skill to understand. To that point Dinah had been quiescent, but he now heard her going into movement, peeling herself out of her sleeping bag. He thought better of bothering her just now, and decided to check his own email.

SHE RAN ON FIRST SHIFT, WHICH MEANT THAT THIS WAS MIDAFTERNOON for her: traditionally a time when she began to feel a little drowsy even when Markus had not just been helping her relax. She felt that going fully to sleep would be a bad idea, partly because she had work to do and partly because it would lead to more gossip than was happening already. She could hear Markus chitchatting with Dubois Harris on the other side of the curtain. She knew that he was stalling for her, giving her some time to pull herself together; she was duly appreciative, and she made the most of it, gliding in the liminal zone between dozing and waking until her radio began to beep. She knew immediately that this was not Rufus; she could tell as much by the “fist” of the transmission. It was faint and it was clearly not the work of an experienced ham.

Her eyes opened as a thought came to her: maybe this was the source known as the Space Troll. That term had originated with Rufus, who had first mentioned it several days ago: Have you heard from the Space Troll yet? It was his name for a transmitter that he had begun picking up recently, and it matched what Dinah was hearing now.

She ejected herself from the bag, turned up the volume on the receiver, and listened while pulling on a T-shirt and some drawstring pants. The signal sounded as if it was coming in from a home-brew transmitter. The owner had a sketchy understanding of the practices and etiquette of the CW (Morse code — using) radio world. His dots and dashes were perfectly formed, and came rapidly, as much as proving that he was using a computer keyboard and an app that automatically converted keystrokes into Morse. He was sending out a lot of QRKs and QRNs, which were queries about the strength of his own signal and the degree to which it was being interfered with. So, he seemed a little insecure about the quality of his equipment.

According to Rufus, as soon as you started transmitting back to the Space Troll he would shoot back a spate of QRSes, meaning “please transmit more slowly,” further proof that he was a novice using a computer keyboard to form the groups, but not very good at deciphering what came back. He transmitted on one frequency only, which was the one that Rufus had, until a year ago, generally used to contact Dinah. This had become known to the Internet in the wake of a human interest story about the MacQuarie family, and so for a few weeks it had been damned near unusable as every CW ham on the planet had tried to use it to contact Dinah. Then word had gotten around that the MacQuaries père et fille weren’t using it anymore and it had gone pretty silent, except for a few people who apparently hadn’t gotten the memo, such as the Space Troll. Anyway, Rufus had gone back to monitoring that frequency again and Dinah was now doing likewise. She had not personally heard any transmissions from the Space Troll. This was not remarkable. Her antenna was nothing compared to the one that Rufus had installed above his mine, and her receiver was something out of a fifth-grade science project. Except when Izzy was passing over his meridian, she and Rufus would naturally “hear” different stations.

According to Rufus, having a conversation with the Space Troll required patience or a sense of humor. The fact that novice hams were screwing around on the radio, which would have driven Rufus into a spasm of righteous fury a few years ago, now just seemed like a sign of the times. Of course people were getting interested in amateur radio; the Internet was expected to go down as soon as the Hard Rain started. And of course many of them were novices.

When they finally did begin an intelligible conversation, Rufus would send QTH, which meant “where are you?” and would get back QET. This was an unofficial Q code, a sort of corny joke meaning “not on planet Earth.”

And that was why Rufus called this guy the Space Troll. Because, among other oddities, he didn’t have a call sign, or at least didn’t use one. The signal she was hearing now was QRA QET, repeated every few seconds; it meant, basically, “Hello, this is E.T., is anyone listening?”

Dinah generally kept the transmit side of her rig turned off when not in use. She turned it on now, but kept her hand well clear of the brass telegraph key. Lurking and listening were harmless, but as soon as she touched that thing, the Space Troll would hear an answering beep, and then she might never be rid of him. More likely, though, was that the Space Troll would give up after a while. Then she could transmit to Rufus, who’d be coming up over the horizon in a few minutes, and let him know that she too had heard from the mysterious “extraterrestrial.” It would be good for a laugh, and a few minutes’ distraction. Her father sounded like he could use some of both.

It had long since become obvious that he and a number of his mining industry friends had mounted a serious operation to prepare for an extended stay beneath the surface of the Earth. They were hardly the only ones to think that way; people were digging holes in the ground all over the world. Most of them would be dead within hours or days of the beginning of the Hard Rain. Constructing an underground complex that could sustain itself for thousands of years was an operation of which few, if any, organizations were capable. Most of those were governmental or military. But if any private group could do it, it was Rufus and his network. The sorts of questions he had been asking her for the last two years left nothing to the imagination. To the extent that the experts on Izzy knew anything about long-term sustainability of artificial ecosystems, Rufus now knew it too.

Distracted by thinking about Rufus and his mine, Dinah became aware that the Space Troll’s transmission had changed. Instead of the familiar QRA QET, it now began with QSO, which in this context meant “can you communicate with. .?” This was followed by an unfamiliar call sign, which she didn’t recognize as such because it was so long: a string of digits and letters that didn’t follow any of the standard conventions for radio call signs.

The third time this transmission was repeated, she wrote it down: twelve characters in all, a basically random assortment of digits and letters. But she did notice that all the letters were in the range A through F. Which was a strong hint that this was a number expressed in hexadecimal notation: a system typically used by computer programmers.

The fact that it had twelve digits was also a clue. The network chips used by almost all computer systems had unique addresses in that format: twelve hexadecimal digits.

And here was where Dinah got a weird feeling on the back of her neck, because the first few digits in that string looked familiar to her. Network interface chips were produced in large batches, with unique addresses assigned to each chip in sequence. So, just as all Fords rolling off the assembly line in a given week might have serial numbers beginning with the same few characters, all the network chips in a given batch would start with the same few hex digits. Some of Dinah’s chips were cheap off-the-shelf hardware made for terrestrial use, but she also had some rad-hard ones, which she hoarded in a shielded box in a drawer beneath her workstation.

She opened that drawer, pulled out that box, and took out a little green PC board, about the size of a stick of gum, with an assortment of chips mounted to it. Printed in white capital letters directly on the board was its MAC address. And its first half-dozen digits matched those in the transmission coming from the Space Troll.

She reached for the key and coded in QSO, meaning, in this context, “yes, I can communicate with. .” and then keyed in the full MAC address of the little board in her hand — different from the one in the original transmission. It was a way of saying, “no, I can’t communicate with the one you mentioned, but I can communicate with this other one.”

QSB, came the answer back. “Your signals are fading.” Then QTX 46, which she guessed meant something like “Will you be available on this frequency forty-six minutes from now?” As anyone on Izzy would understand, this meant “I will call you back when you have orbited around to the other side of the planet.”

QTX 46, she answered back. “Yes.”

They were passing over the terminator, currently dividing the Pacific into a day side and a night side.

WHO THE HELL ARE YOU TALKING TO?

This was a transmission from Rufus, loud and clear. She looked out the window to see the West Coast of North America creeping over the horizon toward them, identifiable as a pattern of lights delineating the conurbations of the Fraser Delta, Puget Sound, the Columbia River, San Francisco Bay. Which meant that Alaska had line of sight to Izzy.

“Knock knock!” came the voice of Dubois Harris through the curtain. He’d been waiting there a long time.

“Come in,” Dinah said, and keyed back a brief transmission to Rufus making a joke about the Space Troll and telling him she would be in touch later. She checked the world clock app on her computer screen. It was shortly before dot 7, therefore 7:00 A.M. in London, therefore ten in the evening for Rufus in Alaska.

A somewhat distracted and scattered conversation followed, Dinah trying to maintain a train of thought with Doob while fielding sporadic, peremptory interruptions from Rufus. “Something kinda weird just happened on the radio,” she said. “Do you want a drink? It’s evening for you, right?”

“I pretty much always want one,” Doob said. “Let’s not worry about it. What’s up?”

Dinah related the story. Doob looked distracted at first, perhaps because of all the ham radio jargon, but focused when she showed him the MAC addresses.

“The simplest explanation,” he pointed out, “is that it’s a troll, just messing with you.”

“But how would a troll know those MAC addresses? We don’t give those out — we don’t want our robots getting hacked from the ground.”

“The PR people have come through here, haven’t they? Taking pictures of you and your robot lab. Mightn’t it be the case that a picture got snapped when you had that box open, and some of those PC boards visible?”

“There’s no gravity in here, Doob. I can’t leave things lying around on my desk.”

“Because,” Doob said, “obviously what’s going on here is that someone wants to talk to you through a private channel—”

“And they are proving their identity by mentioning numbers that could be known only to a few people. I get it.”

“And all I’m saying is that a really sophisticated troll would look for some detail like that, in the background of a NASA publicity photo, as a way to fool you.”

“Noted,” Dinah said. “But I doubt it.”

“Who do you think it is, then?”

“Sean Probst,” Dinah said. “I think it’s the Ymir expedition.”

Doob got a distracted look. “Man, I haven’t thought about those guys in ages.”

IT WAS STRANGE THAT A STORY AS EPIC AND AS DRAMATIC AS THE voyage of Ymir could go forgotten, but those were the times they lived in.

The ship had stopped communicating and then disappeared against the backdrop of the sun about a month after its departure from low Earth orbit (LEO) around Day 126. A few sightings on optical telescopes had confirmed that it had transitioned into a heliocentric orbit, which might have happened accidentally or as the planned result of a controlled burn. Assuming it was following its original plan, Ymir should then have made almost two full loops around the sun. Since its orbit was well inside of Earth’s — the perihelion was halfway between the orbits of Venus and Mercury — it would have done this in just a little more than a year, grazing the orbit of Greg’s Skeleton — Comet Grigg-Skjellerup — a couple of hundred days ago. But this would have occurred when it was on the far side of the sun from the Earth, making it difficult to observe. The next event would have been a small matter of impregnating the comet’s core, or a piece of it, with an exposed nuclear reactor on the end of a stick, and then turning it on to generate thrust by blowing a plume of steam out the entry hole. They would have done a large “burn”—pulling out the reactor’s control blades, powering it up, and releasing a plume of steam — that would have altered the comet’s trajectory by about one kilometer per second, enough to put it on a collision course with Earth, or at least with L1, a couple of hundred days later. The timing was awkward, and many had griped about it, wondering why Sean hadn’t gone after some other comet, or plotted some other course that might have brought it home a little sooner. But people who knew their way around the solar system understood that it was near-miraculous good fortune for any comet core to be in a position to be grappled and moved in such a short span of time. The hasty shake-and-bake nature of the Ymir expedition, which had stirred up so much controversy, had been forced by the implacable timeline of celestial mechanics. Time, tide, and comets waited for no man. And even if it had been possible to bring a comet back sooner, it would have been reckless, and politically impossible. What if the calculations were wrong and the comet slammed into the Earth? So, the plan of the Ymir expedition was the only one that could have worked.

If, indeed, it were working at all. And since much of the action — the rendezvous with the comet and the “burn” of the nuke-powered, steam-fueled engine — had occurred while it was on the far side of the sun, this had been very much in doubt until a couple of months ago, when astronomical observations had proved conclusively that Comet Grigg-Skjellerup had changed its course — something that could only have happened as the result of human intervention. The comet was headed right for them. It would have triggered mass panic on Earth had Earth not already been doomed. Since then they had watched its orbit converging slowly with that of Earth, and plotted the time when it would disappear against the sun once more as it reached L1. The reactor would then have to be powered up again, as a huge “burn” would be needed to synchronize Ymir’s orbit with Earth’s and pilot it through L1 to a long ellipse that would bring it their way.

“I THINK ABOUT THEM EVERY DAY,” DINAH ANSWERED.

“When are they supposed to hit L1?”

“Any time. . but it’s going to be a long burn, they might sort of grease it in over a period of a few days rather than trying to do one sharp impulse.”

“Makes sense,” Doob said. “One high-gee maneuver might cause the ice to break apart. When was the last time they communicated?”

“On the X band? The real radio? A few weeks after they left. Almost two years ago. But clearly they’re still alive. So it must have been radio failure.”

“Well, let’s go with that theory,” Doob suggested. “Jury-rigging a new radio that would transmit over such a distance would be kind of hopeless. The best they could hope for would be to cook something up that might work when they got closer. . and to settle for lower bandwidth.”

“My dad used to talk about spark gap transmitters,” Dinah said. “It was a technology they used—”

“Back before transistors and vacuum tubes. Yes!” Doob said.

Dinah telegraphed down:

DOES QET SOUND LIKE AN OLD TIME SPARKY TO YOU?

Rufus returned:

YES COME TO THINK OF IT

“They took some of my robots with them,” Dinah said. “All they would have to do is jot down the MAC addresses on those units’ interface boards, and they’d have sort of a crude proof of identity. As a matter of fact. .” and she began to pull up some of the records she had made, almost two years ago, of the robots and part numbers issued to Sean and his crew. Within a few minutes she was able to verify that the MAC address that had come in via Morse code a few minutes ago matched one on a robot that had been taken to Ymir.

“Who has access to the file you just consulted?” Doob asked, still in devil’s advocate mode.

“Are you kidding? You know how Sean is with the encryption and everything? All of this stuff is locked down. I mean, I’m sure the NSA could get in, but not some random prankster.”

“Just checking,” Doob said. “It seems awfully roundabout, is all I’m saying. Why doesn’t he just broadcast something like ‘Hey, Dinah, it’s me, Sean, my radio’s busted’? That would seem easier.”

“You have to know Sean,” Dinah said. “Look. Anything he sends out over that channel is getting broadcast to basically the entire Earth. It’s going to go up on the Internet. . everyone’s going to know his business. He has no idea what the situation is. There’s no Internet up there and his radio’s been out for a long time. He doesn’t even know if anyone is alive up here. Or if there’s been a military coup or something. He doesn’t want to come back here if we’ve turned into the Klingon Empire.”

“I think you’re right,” Doob said. “He’s going to ease into it, test the waters.”

Forty-five minutes later Dinah was taking down a new message from QET. It started with RTFM5, then the number 00001, and went on as an apparently meaningless series of random letters.

“The only part I understand is ‘read the fucking manual,’” Dinah said, “followed by the number five.”

“Did he bring any manuals up with him?”

“He brought a bunch of stuff,” Dinah said, “from the engineers in Seattle, and left some of it here. .”

“You have a faraway look in your eye, Dinah. .”

“I remember asking him, ‘Why did you print that stuff out, why not use thumb drives like everyone else,’ and he said, ‘Owning your own space company brings some perks,’” Dinah said.

She found them after a few minutes’ rummaging in storage bins: half a dozen three-ring binders, volumes 1 through 6 of the Arjuna Expeditions Employee Manual. The entire stack was a foot thick.

Doob whistled. “Given the cost per pound of launching stuff into space, this is probably worth more than the Gutenberg Bible that showed up last week.”

They went straight to volume 5, which for the most part looked like any other corporate employee manual. But in between the sexual harassment policy and the dress code was a half-inch-thick stack of pages with no readable content at all. Random sequences of capital letters had been printed all over them, in groups of five, column after column, row after row, all the way down each page. Each of these pages had a different number at its top, beginning with 00001.

“This is the boy adventure secret code shit that Larz always used,” Dinah said. “But I’ll be damned if I know—”

“I’m embarrassed to say that I know exactly what this is,” Doob said. “These are one-time pads. It’s the simplest code there is — but the most difficult to break, if you do it right. But you have to have this.” And he rattled page 00001 in his hand.

Once Doob had explained how it worked, Dinah was able to begin decrypting the message by hand, but in a few minutes Doob had written a Python script that made it easy to finish the job. “I came here thinking I was going to have a drink and a chat about asteroid mining,” he said.

“Oh, stop grumbling — this is way more interesting!” Dinah said.

The message read:

TWO ALIVE. THRUSTING AT FULL POWER. SEND SITREP.

“There were six in the original crew, right?” Doob asked.

“Something must have happened,” Dinah said. “Maybe they hit a rock or something, damaged the antenna, lost some people. Maybe the radiation got to them.”

“Well, it sounds like they are coming back,” Doob said.

“Yeah, unless—”

“Unless what?”

“Unless he just wants to hang out at L1. That would be a hell of a lot safer. I don’t think any moon shards are going to make it out that far.”

Doob reread the message.

“You’re right,” he said. “All he says is that they’re thrusting. Nothing about transferring back to low Earth orbit. Then he asks for a situation report.” He put his hands over his face and rubbed it. “I’m fading,” he announced. “I should be Skyping my family right now.”

“Get outta here,” Dinah said. “I can work on the report. And I can encrypt it, now that you showed me how it works.”

Doob pushed off and drifted to the exit, then caught himself and turned back. “I could figure this out myself,” he said, “but it’s late. Maybe you know off the top of your head. If Sean goes into that transfer orbit from L1 to here, how long before he shows up?”

“Thirty-seven days,” Dinah said.

“About seventeen days into the Hard Rain,” Doob said. “Awkward timing.”

Dinah looked back at him. She didn’t say a word, but he knew what she was thinking: If only awkward timing were the worst of our problems.

“Okay,” Doob said. “Thanks, Dinah.”

“Next time,” she said, and made a drinky-drinky motion with her thumb and pinkie.

“Next time,” he agreed, and pushed through the curtain.

Dinah checked the time. Now that she knew roughly where Ymir was, she understood the timing of the transmissions. During a certain part of each ninety-three-minute orbit, Izzy was on the wrong side of the Earth, and couldn’t receive Sean’s signal. Following each blackout period was a window during which they could talk. They had just burned a window taking down his transmission and decrypting it, and were about to go into blackout again. During that span of time Dinah should be able to write a short message and get it encrypted using the next one-time pad.

What to write wasn’t entirely clear. She could provide some obvious data like the number of arklets currently in orbit, the number of people, how many robots she had up and running. But she suspected that Sean wanted a different kind of information. He wanted to know what would happen were he to show up, thirty-seven days from now, with a mountain of ice. The Cloud Ark could use it, that was for sure. Likewise, Sean needed the Cloud Ark; two guys on a spaceship pushing a giant ball of ice was not a sustainable civilization. But Sean was going to be cagey. He was going to want something. He would want to make a deal.

He would want to make a deal with Dinah’s boyfriend.

One step at a time. Just sending him a few basic stats would occupy the next transmission window. Rather than driving herself crazy worrying about the longer game, Dinah focused on that through the next blackout period, writing up a message as tersely as possible and then encrypting it using Doob’s Python script.

The L1 point of the Earth-sun system was located on a straight line between those two bodies. Ymir, for all practical purposes, was at L1. So, generally speaking, when Izzy swung around the dark side of the Earth and emerged into the sunlight, it meant that they could “see” L1, and communicate with Ymir. This next occurred at about 7:30 A.M. Greenwich time, which happened to be sunrise in London. Dinah, gazing down out her little window, was able to see the terminator — the dividing line between the day and night sides of Earth — creeping over the Thames estuary down below, and lighting up a few tall spires in the London financial district. Then she turned to her telegraph key, established contact with Ymir, and tapped out her message. This ended up consuming the entire transmission window. She had to send the characters very slowly, because Sean wasn’t very good at reading Morse code. And because the message was encrypted, he wasn’t able to guess missing letters from context, and so every letter had to be read clearly. By the time she was finished, Izzy had swung almost halfway around the world and was about to plunge back into night. She finished her transmission with TBC, which she hoped they would understand as “to be continued,” then went right back to work writing and encrypting a supplement.

She was getting ready to open another broadcast window, a little before 9:00 A.M. London time or “dot 9” in Izzy-speak, when Ivy floated in without knocking.

“I want to look out your window,” she announced.

“That’s fine,” Dinah said. “What’s up?” Because obviously something was up. Ivy’s face looked funny. And she had said “your window,” not “your window.”

“What’s so special about my window?” Dinah asked.

“It’s next to you,” Ivy said.

“Is everything okay?” Dinah asked. Because clearly everything wasn’t. Her first thought was that the Morse code transmissions had been intercepted and that Dinah was in trouble. But if that were the case, Ivy would not be in here asking to look out her window.

She looked at her friend. Ivy went immediately to the window and then positioned herself to look down at the Earth. By now the terminator had advanced to the point where it had lit up the easternmost bulge of South America. Izzy was about to cross the equator, which was almost directly below them.

“I heard from Cal,” Ivy said. She said it without the usual note of pleasure in her voice.

“That’s good. I thought his boat was underwater.”

“It was until a couple of hours ago.”

“They popped up?”

“They popped up.”

“Where?”

“Down there,” Ivy said.

“How do you know?” Dinah asked. “Surely he’s not beaming you his coordinates.”

“I can tell,” Ivy said. “By putting two and two together.”

“What did he say?”

“He said to prepare for some launches out of Kourou.”

“They’re going to reopen the spaceport?”

Ivy gasped.

Dinah glided over and got right behind Ivy, hugging her and hooking her chin over Ivy’s shoulder so that she could share the same viewing angle.

They knew where Kourou was; they looked at it all the time, and sometimes even saw the bright plumes of rocket engines on the launch pads.

What Ivy had reacted to was a little different. Sparks of light were appearing along the coast, spreading, and fading. A barrage of them, peppered across the interval between the beach and Devil’s Island.

“What the hell are those?” Dinah asked. “Are those nukes?”

“I don’t know,” Ivy said.

Then Dinah’s question was answered by a much brighter light that flared along the coast to the northwest, fading slightly to a luminescent ball that tumbled upward toward space.

“I think that was a nuke,” Ivy said.

“We just nuked. . Venezuela?”

It took a few moments for their eyes to readjust. That was just as well, since their minds had to do some adjusting as well. Once the light had faded, they could see that the mushroom cloud was actually offshore of the Venezuelan landmass, a few miles out to sea.

“A demonstration shot? Visible from Caracas?” Dinah asked.

“Partly that,” Ivy said. “But yesterday they were saying that the whole Venezuelan navy was headed for Kourou to restore order. I’ll bet that navy no longer exists.”

“The smaller fireballs? Near the spaceport?”

“I’m going to guess fuel-air explosives. They would do almost as much damage as tactical nukes without contaminating the launch site.”

Ivy had shrugged loose from Dinah’s embrace and turned around so that her back was to the window. They were now hovering close to each other.

Dinah finally got it. “You said that Cal’s boat had popped up. That it was on the surface. That he knew something. You think—”

“I know,” Ivy mouthed.

Cal had received the order, direct from J.B.F., and he had launched the nuke. He’d probably launched cruise missiles with fuel-air devices as well.

People assumed that Ivy and Dinah had grown apart in the last year — but then, people had assumed that they were at odds to begin with. There was no point in trying to keep track of what people imagined. Ivy’s loss of her position to Dinah’s boyfriend hadn’t made matters any simpler. But things had never been bad between them. Just complicated.

Ivy was pretty articulate, but there wasn’t a lot about the current situation that could be talked through.

After a few minutes, though, she found a way. “I guess what sucks is that all I’m going to have of him is memories,” Ivy said, “and I was trying to cultivate some good ones to carry with me.” She wasn’t exactly crying, but her voice had gone velvety.

“You know he had no choice,” Dinah said. “The chain of command is still in effect.”

“Of course I understand that,” Ivy said. “Still. It’s just not what I wanted.”

“We knew it was going to get ugly,” Dinah said.

Her radio started beeping.

“Speaking of which. .”

“Who the hell is that?” Ivy asked.

“Sean Probst,” Dinah said. “He’s back.”

Ivy hung out in Dinah’s shop for a while as Dinah laboriously keyed out the second half of her situation report. By the time South America had passed from view, long trails of black smoke were streaking northeast from the burning wreckage of the People’s Justice Blockade and casting shadows on the wrinkled skin of the Atlantic. Bright sparks had appeared over Kourou again, but now they were the incandescent plumes of solid boosters chucking heavy-lift vehicles into the sky.

“Back in business,” Ivy said. “I guess I better revise those spreadsheets.”

“You think Cal is still on the surface? Still reachable?”

“I doubt it,” Ivy said, in a tone of voice that suggested she wouldn’t know what to say to him. “I don’t think it’s standard practice to launch a nuclear missile and then just hang out.”

CERTAIN ASPECTS OF CLOUD ARK CULTURE WERE MORE OBJECTIONABLE than others to Dr. Moira Crewe. She just didn’t think it decent to live in a place where there were no coffee shops to have breakfast in when she woke up, and no public houses in which to socialize at day’s end. This was partly the result of overcrowding; partly because people lived on three different shifts, so there was no unanimity as to when morning and evening fell; and partly because the place had been hastily designed by American and Russian engineers who were blind to the importance of such things. She’d had a number of good chats about it with Luisa, who understood, and they had formed a sort of vague resolution to do something about it once the Hard Rain had begun and the Cloud Ark had settled into some kind of long-term routine. Moira’s dream was to be the proprietor of an establishment, perhaps constructed in a single arklet, that would serve that purpose. But she had not yet worked out how to get the timing just right.

Of course, she knew that she had much more important duties: the responsibility of perpetuating the human race, and most other species, was largely on her shoulders. It wouldn’t do for her to spend hours every day pulling espresso shots and wiping down counters. They didn’t even have the capability, yet, of growing coffee or barley in space, so the supply of consumables was going to run out pretty damned soon and her pub would end up serving Tang. But it was a dream. And in the meantime she could use the coffee room adjacent to the Farm as a sort of R & D laboratory. Arising each day at dot 8, she would make her way back to H2, descend a spoke to the T3 torus, make herself a cup of reprehensible freeze-dried coffee and a little bowl of equally bad freeze-dried oatmeal, and then go to a little conference table in the middle of the Farm and have a sit. As often as not she would be joined by other sleepy-eyed third shifters. Markus Leuker was one of them, and generally too busy to just sit and drink coffee with people, but he would make time for her occasionally. Konrad Barth sometimes joined her, as did Rhys Aitken, and from time to time Tekla would show up. Of these, Tekla was the most curious and interesting case, on more than one level. To put it bluntly, she was of a different social caste. Moira, Doob, Konrad, Rhys, and many of the other members of the General Population were the sorts of people who might once have encountered each other at TED or Davos, or appeared together on panels at think tanks. Not Tekla. Her curious career as one of the rare female members of the Russian military, an Olympic athlete, a test pilot, and a cosmonaut certainly made her interesting enough to be invited to a TED-like conference, but her lack of fluency in English and a certain degree of social awkwardness would have ruled her out as a presenter. The lacerations she had suffered during her escape from the crippled Luk had been sewn up by amateurs. On Earth she’d have gone straight into the care of a plastic surgeon, but on Izzy she’d just accepted the results. Moira wished she spoke better Russian so that she could talk to Tekla about her ideas regarding appearance and grooming. The facial scars put her well outside the norms of feminine beauty and she had doubled down by electing to keep the buzz cut. In spite of this, or perhaps because of it, she was, to put it bluntly, kind of hot. Moira hated to say it. But hotness was a part of the human condition and it was pointless to pretend that it did not exist. Moira herself was largely heterosexual. When younger she had slept with two different women, one in the English Cambridge and one in the Massachusetts Cambridge. This had been perfectly all right and she in no way regretted it, but it had required an awful lot of thinking. Way too much cerebration about gender and queer theory had preceded and succeeded those brief moments of passion, and those relationships had faded from her life.

It had occurred to her, in spite of efforts to keep such thoughts at bay, that Tekla would be an altogether different style of partner, and she had to admit that she found this pretty interesting. There was an entire story about Tekla’s sexuality that had begun a few weeks after her rescue with some kind of elaborate soap opera, a love triangle or quadrangle involving both men and women, loosely enshrined in the oral history of Izzy but not something Moira had ever cared to learn more about. The gist of it was that after a few months Tekla had begun openly sleeping with other women, eliciting vast amounts of analysis and commentary and drama. The analysis had tended to come from gender theorists talking about the somewhat uncomfortable fact that Tekla was an athlete who had looked kind of butch even when she had been glammed up for the Olympics and looked a hell of a lot more so now. Her coming out (though she had never formally done so) thus tended to reinforce existing stereotypes about female athletes. The commentary came from millions of idiots on the Internet. And the drama occurred in Tekla’s relationships with the other Russians, who constituted a powerful bloc on the space station. This had faded as they’d gotten used to it, as more people of various nationalities and sexualities had joined the Cloud Ark, and as everyone had focused their attentions on larger problems. But it had turned Tekla into a curious, solitary creature, socially distant from the only people with whom she could carry on a fluent conversation. The expectation among politically correct academic-leftist observers had been that she would undergo a personal transformation and become like an academic leftist, but she seemed to have retained the same basic attitude about order and discipline that had made her a Scout in the first place and that had led her to putting Sean Probst in an arm lock and offering to choke him out. Sitting across the table from her, drinking her coffee and picking at her oatmeal, Moira idly wondered whether Tekla was aware of the fact that there was a whole genre of Internet fan pornography devoted to imaginary couplings, more or less sadomasochistic, between her and Sean Probst.

In any case Tekla’s tendency to occasionally sit with Moira during breakfast seemed like, if not a direct invitation, then at least a preliminary gambit.

Preoccupied with such thoughts, Moira was oblivious at first to the fact that the Farm was suddenly crowded with people looking at the big Situational Awareness Monitor above the end of the table where she and Tekla were having their breakfast. The viewing angle was awkward, and so she had to change her position to get a good look. It was a news channel showing scraps of cell-phone video that had been spliced together to approximate a story. At the beginning of the story the People’s Justice Blockade was riding at anchor in the waters between the beach and Devil’s Island, just beginning to catch the pink light of dawn. At the end of the story the sun was shining on a churned slurry of crushed hulls and floating corpses visible through gaps between dreadlocks of smoke. In the middle were glimpses of black motes droning in from the sea and fantastic bubbles of flame spreading to envelop vast areas before they burst and disappeared, leaving behind wreckage that looked like it had been beaten with sledgehammers and doused with napalm.

From there the video would loop to sterile three-dimensional renderings of missile submarines and cruise missiles, and footage of the White House Briefing Room, where the president had made a short statement before turning matters over to the chairman of the Joint Chiefs of Staff. Other world leaders were chiming in from Downing Street, the Kremlin, Berlin.

All of which was pretty compelling in and of itself until, just at the moment when Moira was going to look away to her oatmeal, something bright caught her eye on the screen, and she looked back to see video of a mushroom cloud rising over an ocean.

“Did I miss something?” Moira asked. “That didn’t look like a meteorite strike.”

“Nuke,” Tekla said.

Moira looked at her. Tekla’s gaze, which some found chilly, was fixed on Moira’s face. Moira didn’t find it chilly at all. Tekla, for once, glanced away shyly. “Venezuela,” she added. “Navy is no longer a problem. Rockets are launched again.” She shrugged. She was wearing a tank top. Moira couldn’t stop looking at her deltoids. She had to stop doing that. “On the beach it was fuel-air bombs,” Tekla continued. “Extrimmly destructive.” Tekla leaned back in her chair and draped an arm casually over the back of the empty chair next to her. “What is your opinion, Dr. Crewe?”

“Please, call me Moira.”

“Sorry. Russian formality.”

Tekla was, maybe, cagier than she looked. She anticipated that someone like Dr. Crewe would be horrified by the fact that we were now nuking people. She wanted to get it out in the open right away, while it was still fresh.

Lost in contemplation of the structure of Tekla’s arm, Moira was startled when a large, strongly built man slammed down into the chair next to her. She looked over to see that it was Markus Leuker. He placed a cup of coffee on the table in front of him and contemplated it for a moment, almost pointedly not looking at the video screens with their infinite multiangled replays of mushroom clouds and briefing rooms. Then he turned and looked at Moira, greeting her with raised eyebrows and a nod, and then giving Tekla the same treatment.

So Moira was absolved from having to answer Tekla’s question.

Markus answered it, even though no one had asked him. “I know that I am at somewhat of a disadvantage here because I am a speaker of German, and so there is certain baggage. So. Yes. The baggage is acknowledged. I see the awkwardness of it. The delicacy. But—”

“Did you know it was going to happen?” Moira asked him.

“No, it comes as a complete surprise to me.”

Moira nodded.

“But, had they asked for my opinion, I would have said yes,” Markus said.

“They are all going to die anyway,” Tekla said, nodding.

It struck Moira, just then, that Markus and Tekla were quite comfortable with each other. It made sense. Markus would not be the least bit troubled by Tekla’s sexuality; on the contrary, it would make things much simpler for a man like him if he knew she were unavailable. He was an ex — military pilot; so was she. Naturally they would tend to view certain things in the same way. For a while, during the Cloud Ark’s first year, Tekla had been a sort of itinerant laborer. It might seem strange that a space station could support a person with no particular job. But none of the Scouts had really been expected to survive, so none of them had been sent up with long-term roles in mind. Her alienation from the Russians who took the brunt of the spacewalking work had led her to try her hand at a number of different tasks. She knew the interior of Izzy as well as anyone, but she also knew how to operate the controls of an arklet, and she could put on a space suit and go out and weld things in space. Her period of wandering in the wilderness seemed to have ended when Markus had taken over. Moira was no longer precisely certain what it was that Tekla did for a living. But she now had the clear sense that Tekla was working for Markus directly, that he was trusting her to do something.

“They’re all going to die, yes,” said another voice. “But we’re not.” It was Luisa. She came up behind Tekla and wordlessly asked permission to use the chair on which the Russian had draped her arm. Tekla not only gave it but rose to her feet and pulled the chair out as a courtesy.

“We’re not all going to die, or at least that’s what I’m hoping,” Luisa went on, “and we have all just seen this happen. It’s in our memories now. And not just that. But in a few hours we’ll be taking deliveries from Kourou, reaping the benefits of having used fuel-air explosives and nuclear weapons against people who were basically defenseless. It’s in our DNA now.” Her eyes flicked toward Moira. “If you’ll pardon the poetic imagery, Dr. Crewe.”

Moira gave her a little smile and nodded.

Markus said, “So, do you disagree with it?”

“No,” Luisa said. “Let’s be clear, Markus, I have baggage too. I’m a brown Spanish speaker from South America. I devoted years of my life to hanging out with refugees on boats. And I’m a Jew. That’s my baggage, okay?”

“Understood,” Markus said.

“I’m not down there, I don’t know what advice J.B.F. was getting, what she knew that we don’t.”

“So what is your point?” Markus asked, crisply but politely.

“We have no laws. No rights. No constitution. No legal system, no police.”

Markus and Tekla looked at each other across the table. It was not a sneaky look, or a guilty look, or a conniving look. But it was a significant look.

“It is being worked on,” Markus said. He wasn’t kidding; ever since the Crater Lake Accord had been signed, a whole think tank full of constitutional scholars had been toiling away on it in The Hague, and one of them was now resident up here.

“I know it is,” Luisa answered, “and it is very important to me that atrocities such as what we’re seeing on these screens don’t somehow infect that process. This cannot be business as usual.”

Markus and Tekla, still looking at each other, seemed to arrive at a mutual decision to say nothing.

Moira’s phone vibrated. Looking at its screen she saw that she had an appointment in fifteen minutes. She excused herself from what had become a very strange kaffeeklatsch. Oh well, perhaps it had cured her of some sentimental ideas. She had walked in aspiring to somehow re-create the experience of breakfasting in a sidewalk café in Europe and instead been treated to half an hour of nuclear warfare, mass incineration of protesters, and serious ethical discourse, mixed in with a suddenly keen sexual tension between her and Tekla. Like quite a few other people on the Cloud Ark, she hadn’t had sex since she had come up here. Many whose consciences were unencumbered by the existence of doomed spouses or fiancés on the ground had figured out a way to make it happen, but many others were not getting any. This couldn’t possibly last. A couple of docked capsules had been set aside for conjugal visits, and everyone knew of quiet places around the space station where you could do it. Moira didn’t have anyone down on the ground. She had abstained for lack of anyone up here, and just because it was the least sexy place you could possibly imagine. But it was starting to get to her.

One of the items on her long-term to-do list, actually, was to come up with a policy for how to handle pregnancy aboard the Cloud Ark. Since pregnant people weren’t fundamentally that different from those who weren’t, what that really boiled down to was how to handle babies. The assumption made by the Arkitects was that this was going to be an orderly process, and that anyone who got pregnant would do so with the intent of having the embryo frozen so that it could be implanted later, when conditions were better for raising little ones. Having now spent the better part of a year up here, Moira doubted this. The Arkitects were, she felt, underestimating the cultural difference between the General Population and the Arkies.

Until a few months ago they’d been referred to as Arkers, and, in all official communications, they still were. Then someone had coined the term “Arkies,” and, in one of those only-on-the-Internet viral phenomena, it had swept across the planet in about twenty-four hours and become universal. A few sensitive Arkansas historians had registered objections, but they had been steamrolled.

The Arkies were just kids, and they had surprisingly little exposure to the GPop. The arklets they lived in couldn’t really change their positions in the swarm. Moving from one arklet to the next was nearly impossible — it was an epic journey in a space suit, requiring some fancy tricks with orbital mechanics. Small utility spacecraft, called Flivvers, were available to squire people around, but there were only so many of them, and qualified pilots were few. Markus, following suggestions from Luisa, had tried to make up for this by “stirring the pot,” meaning that about 10 percent of the Arkies at any given time were living and working aboard Izzy. But most of the time, most of them were stranded in individual arks or on triads or heptads, their only connection to the General Population being through videoconferencing (“Scape”), social media (“Spacebook”), and other tech that had been transplanted from the earthbound world. Moira would be astonished if some girls weren’t pregnant already, but no one had approached her about getting an embryo frozen.

And any normal person who followed Moira forward through Zvezda and “down” into the cold storage facility would understand why. There was nothing about this place that tickled the nerve endings that mattered to people who wanted to start families. It was clinical/industrial to a degree that was almost laughable.

But by the same token she hoped it would seem impressive to the new arrivals, who showed up right on time for their appointment. They had arrived several hours ago on a passenger capsule launched from Cape Canaveral: long enough for their antinausea meds to kick in and for them to pull themselves together a little bit. It was a small contingent from the Philippines: a scientist who had been working on genetically modified strains of rice, a sociologist who had been working with Filipino sailors who spent their whole lives on cargo freighters — she’d be working with Luisa, presumably — and a pair of Arkies who, judging from looks, were from ethnic groups as different as Icelanders were from Sicilians. One of them was carrying the inevitable beer cooler. As Moira knew perfectly well — for she did this at least once a day — it contained sperm, ova, and embryos collected from donors scattered around the country of origin — in this case, the Philippines. She accepted it with due ceremony, like a Japanese businessman taking another’s business card, and flipped the lid open for inspection. A few chunks of dry ice were still visible on the bottom; good. The finger-sized vials were all contained within a hexagonal cage. She sampled some of them with a pistol-shaped infrared thermometer and verified that none of them had thawed out. Then, after putting on some cotton gloves to protect her skin from the cold, she pulled a few out and spot-checked them just to verify that they had been sealed, labeled, and bar-coded in accordance with the procedures specified in the Third Technical Supplement to the Crater Lake Accord, Volume III, Section 4, Paragraph 11. They had. She’d have expected nothing less from Dr. Miguel Andrada, the geneticist.

She also guessed that Dr. Andrada suspected, at some level, that none of these samples had a snowball’s chance in hell of ever developing into sentient life-forms, but this was not a subject to talk about now. For the benefit of the others, Moira gave a little canned speech, trying to make it sound spontaneous, thanking them and, by extension, the people of the Philippines for having entrusted her and the Cloud Ark with these most precious contributions, and hinting, without promising, at a future in which a cornucopia of vibrant humanity would spring forth from each little plastic vial. It was expected that these people would go forth now to their arklets and text or Facebook the news down to their friends and family at home. The promise in those words was meant to keep people on Earth from getting too rambunctious while they waited for the end; and if that failed, as it had in the case of Venezuela, well, J.B.F. could just nuke them.

“May I see how it all works?” Dr. Andrada asked, after the rest of his delegation had been sent on their way. So it was just the two of them now, hovering in a long, slim docking module that projected to the nadir side. “Below” them its far end was sealed off by a hatch with a keypad. Most of Izzy was open to anyone who wanted to wander in and poke around; they didn’t get a lot of riffraff. But the HGA, the Human Genetic Archive, had a kind of quasi-sacred status and was kept under the digital equivalent of lock and key.

Dr. Andrada was a small, wiry man with prominent cheekbones. Like some other ag geneticists Moira had known, he had a callused, tanned, leathery look, the result of spending a lot of time in experimental plots, digging in actual dirt. Except for a nice pair of eyeglasses he could have passed for a farmer anywhere in Southeast Asia. But he had a Ph.D. from UC Davis and had been on the fast track for a Nobel Prize before the Agent had intervened.

“Of course,” Moira said. “I’d fancy a chat anyway, about how we’re going to grow things other than humans up here.”

“We need to talk about that,” Dr. Andrada agreed.

She drifted down, performing a slow somersault so that she could address the keypad, and punched the button that turned on the iris scanner. After a few moments, the device agreed that she was Dr. Moira Crewe and unlocked the hatch. Bracing herself with a handle on the wall, she pulled it open, then allowed herself to drift through into the docking module beyond. There was barely room in this for both her and Dr. Andrada. White LEDs came on automatically. Clipped to the wall was a simple nylon web belt with a few small electronic gadgets holstered in it. Moira took this and buckled it around her waist.

They had entered through the hatch on the module’s zenith side. To port and starboard were openings that had been sealed off by round plastic shields. Each of these had a handle projecting from its center. The closest to Moira was the one on the port side, so she grabbed the handle, squeezed it to release a latch, and then pulled it out of the way.

Dr. Andrada flinched at the frigid air that washed into the space in its wake. They were looking down a straight tube about ten meters long, large enough for one person to work comfortably, or for two to pass each other if they didn’t mind bumping bodies. Its walls were studded with long neat rows of smaller hatches about as wide as a splayed human hand, each with its own little handle. Hundreds of them. Closer to the entrance these bore neat machine-printed labels and bar codes; farther away they were blank. Next to each one of them was a blue LED; these provided the space’s only illumination.

“Would you like to do the honors?” Moira said.

“If I don’t freeze to death first!” Dr. Andrada said.

“Space is cold,” Moira said. “We rely on that.”

She gave him a minute to put on the cotton gloves, then opened the cooler and held it out. He removed the little rack containing the samples. Moira zapped its bar code with a handheld scanner from her belt. Dr. Andrada pulled himself into the cold storage module and began to drift deeper into it, gingerly prodding the walls in a way that marked him out as a new arrival to zero gravity. “Take the first one that’s unlabeled,” Moira said. “Leave the door open, please.”

Dr. Andrada coughed as the chilly air made his throat spasm. He opened one of the small hatches and slid the sample rack into it. In the meantime Moira was using a handheld printer to generate a sticker identifying the sample in English, in Filipino, and in a machine-readable bar code language. Once Dr. Andrada had returned to the central module, she went up to the open hatch, verified that the sample rack was properly seated in the tubular cavity beyond, then closed the hatch and affixed the sticker to its front. Printed on the hatch was a unique identification number and a bar code conveying the same thing, which she zapped and then double-checked.

The LED next to this hatch had turned red, signaling that the compartment’s temperature was too high. While Moira checked her work, it turned yellow, which suggested the cold was “soaking in.” Later she’d pull it up on the screen of her tablet and verify that it had gone blue.

She flew back out to the docking module and grabbed the round shield that sealed off the cold store. “Now you know what these are for,” she said. “Thermal insulation.” She snapped the shield back into place. “I could open the other one,” she offered, “but you would see the same thing.”

“Thank you anyway,” said Dr. Andrada, “but I have never been so cold in my life!”

They went back “up” to Zvezda and then proceeded forward to the complex of modules where most of the genetic engineering gear was stored. There was nothing to see here but boxes. They could just as easily have gone aft to one of the tori, but Moira knew from experience that new arrivals didn’t benefit from switching back and forth between zero gee and simulated gravity.

Through the nice eyeglasses, Dr. Andrada was giving Moira a look that she read as polite but skeptical. Fair enough. She decided to broach what was probably on his mind. “Forgive me that bit of ceremony,” she said. “I have done it once or twice a day for a year. I’m as much priestess as scientist. You’re meant to blog it, of course. To tell the people down below that you personally hand-carried the samples all the way from Manila to a cold storage location on Izzy.”

“Yes, I understand that. I will do so.” He paused, signaling a change in topic. “It is not exactly decentralized.”

Moira nodded. “If that thing gets hit with a rock ten minutes from now, all of the samples are destroyed.”

“Yes. That is my concern.”

“Mine as well. It all boils down to statistics and mathematics. For now, there aren’t that many rocks, and we can see them and avoid them if necessary. Keeping all the eggs in one basket. .”

“And sperm,” Dr. Andrada said, in what had become the oldest joke in Moira’s personal universe.

“. . is actually a safer bet, for the next couple of weeks, than trying to distribute them among all of those arklets. But there is a plan, Dr. Andrada, for so distributing them, which will be triggered when the BFR breaks through a certain threshold.”

He nodded. “Please call me Miguel.”

“Miguel. Moira, if you would.”

“Yes. Now, you know why I was chosen to come up here.”

“You figured out a way to make photosynthesis in rice more efficient by transplanting genes from maize. Greenpeace destroyed your research facility in the Philippines but you kept the project alive anyway, in Singapore. Starting shortly after Zero you began developing strains of that rice adapted for cultivation in low-gee hydroponic environments.”

“Sprice,” Miguel said, with an ever-so-slight roll of the eyes. The term, a contraction of Space Rice, had been coined by an enthusiastic reporter for the Straits Times and become an unkillable staple of tabloid headlines and Internet comment threads. “Do you understand, Moira, that it cannot grow without some amount of simulated gravity? There has to be an up and a down or the root system cannot develop. In this it is more difficult than algae, which doesn’t care.”

“Oh, we’re all going to be eating algae for a long time,” Moira said. “Sprice will come later, after we have constructed more environments that rotate to make gravity. And then, Miguel, then!”

“Then what?” Miguel asked.

“Sprew.”

“Sprew?”

“Space brew,” Moira said. “It’s not as good as barley, but you can make beer from rice in a pinch.”

“TAP,” MARKUS SAID. HE HAD TO SAY IT BECAUSE HE COULDN’T DO IT. The traditional way for a wrestler to tell his training partner that an unbreakable submission hold had been achieved was by tapping him or her on the hand, arm, leg, or whatever could be reached. But Markus couldn’t reach anything. Tekla had both of his arms controlled.

She let go of him moments before they drifted into the padded wall of the Circus — a large, mostly empty module reserved for exercise — and they raised their hands to absorb the impact.

Watching interestedly from the far side of the Circus were Jun Ueda, an engineer named Tom Van Meter, Bolor-Erdene, and Vyacheslav Dubsky. The three men were taciturn. Bolor-Erdene, who was nothing if not enthusiastic, permitted herself three claps, then stopped when it became clear that no one else was joining in.

“Okay,” Vyacheslav said. “Seeing is believing. It is possible to perform Sambo in zero gravity.” His eyes flicked in the direction of the others. “Or jujitsu, or wrestling, or bökh, I presume.”

“Obviously there are no throws. None of that shifting of the weight that is so important on the ground,” Markus said.

Jun nodded. “It is a subset. A little bit like ground fighting. But without the ground.”

Tom Van Meter, who’d been a collegiate wrestler en route to an engineering degree at Iowa, turned himself around to face the padded wall, then tried delivering a punch. In spite of his considerable size and strength, it landed weakly and sent him drifting backward across the module.

“We experimented with that too,” Markus said. “Punches are problematic.”

Just before striking the opposite wall, Tom flung both arms outward and slapped the mat to absorb energy. “If you’re in a torus, or a bolo, all the usual stuff is going to work,” he said. “But you’re right, martial arts in zero gee is a new frontier.”

“Once you have come to grapple,” Tekla said, “not so different.”

“The Cloud Ark is equipped with a dozen Tasers,” Markus said. “I did not request these. They were here when I arrived. No one knows about them. I am not comfortable with having some persons go around with sidearms — even if they are just Tasers — while everyone else is unarmed. And yet. We have a population of two thousand or so. There is no town on Earth of such a population that does not have police. There will be crimes. Disputes.”

“What does the Constitution say about police?” Bolor-Erdene asked. “I haven’t read it.”

All of the others laughed, appreciating her. “No one has read the bloody thing, Bo; it is this thick when you print it out!” Markus said, holding his thumb and index finger two inches apart. “Written by committee, as you would expect.”

“To be clear, Markus,” said Jun. “You are not suggesting—”

“No, Jun, I am not saying we ignore it. Believe me, I am screaming at these guys every day to make it simpler, to give us the, what do you call it—”

“Cliff’s Notes,” Tom said.

“Yes. Before we fall off the cliff. A simple owner’s manual. But somewhere in there, a police force is mentioned. I grepped it. They will have to be citizen police at first — no professionals. I have studied your personnel records. I know that you are all trained in some sort of wrestling. Wrestling is the only form of organized violence that is actually usable aboard a space station, short of absolutely crazy shit.”

“How about stick fighting?” Tom asked.

“I knew you would ask because your CV mentions a little bit of escrima,” Markus said. “It is a reasonable idea. I have a question, though.”

“Yes?”

“Do you see any sticks?”

“Maybe we could grow some trees,” Bo suggested.

“That will take a while,” Markus returned. “And so I am simply asking you this, to spend a little bit of time each day getting together in this module to practice wrestling. It might come in handy.”

DOOB HAD SLEPT SO POORLY HE SUSPECTED HE HADN’T SLEPT AT ALL. But the clock said it was about dot 15. When he’d climbed into his sack it had said dot 9. He must have dozed off for a while. But he didn’t know when.

His nightly videoconference with Amelia hadn’t gone well. It hadn’t gone badly—they hadn’t raised their voices, or come to tears — but at first it had been all about what had just happened in Kourou, and after that there’d been a failure to connect. He’d noticed the same thing with Henry.

They were running out of things to say to each other. That was ghastly, but it was true. His family members were all preparing to meet their maker in two or three or four weeks. The government had been handing out free euthanasia pills to anyone who wanted them; thousands had already swallowed them and bodies overflowed the morgues. Mass graves were being dug with end loaders. Meanwhile, Doob was preparing for — to be blunt, to be honest — the greatest adventure of his life.

He wished, at some level, that they were already dead.

He had spoken those seemingly unspeakable words to Luisa several days ago and she had nodded. “Happens all the time,” she said, “with caregivers of terminal Alzheimer’s patients, or similar cases. An enormous sense of shame and guilt comes with it.”

“But Amelia doesn’t have Alzheimer’s, she’s—”

“Doesn’t matter. Seeing her, talking to her, makes you feel bad. And at some level, your brain wants the thing that makes you feel bad to go away. Simplest reaction in the world. Doesn’t make you a bad person. Doesn’t mean you have to give in to it.”

Those thoughts had led to more tossing and turning — if those were the right words for not being able to sleep in a loose sack in zero gravity — as he had wrestled with the question of “When?” Predicting it on Day 720, plus or minus a few, had been all well and good back on Day 360. But Day 700 was now approaching its end, and the “plus or minus” thing was seriously bothering him. Lately they’d narrowed it down to “plus or minus three days,” but that was in response to political pressure. It wasn’t a legit scientific move. And it meant something different to scientists. Laypersons understood it as “certainly between 717 and 723.” Scientists would instead say that if you could repeat the experiment of blowing up the moon a large number of times, and keep track of the time-to-White-Sky separately in each case, the numbers would fall into a normal distribution, a bell-shaped curve, with about two-thirds of the instances falling within that range.

Which meant that the remainder would fall outside of that range — and some would fall well outside of it. It was not out of the question that it could happen tomorrow—that it could be happening right now — while Doob floated in a goddamned sack.

So when Dinah came and woke him up just after dot 15, he wasn’t angry at her. More relieved.

Basic politeness prevented him from saying so, but she looked a wreck. Not in the sense of being over-the-top emotional. Just drained and beat up.

“You know about Guiana?” she asked him over her shoulder as they wended their way back to the Mining Colony.

“Yeah.”

“Okay.”

She said nothing further until they were in her shop. Doob could see the wreckage of old-school communication all over the place: many sheets of paper taped to the available surfaces, dull pencils drifting around, loose pages from the “employee manual” with blocks of characters crossed out. “I had to tell Sean to knock it off,” she admitted. “I’m used up. Can’t do it anymore. Need to get some sleep. This shit is difficult, you have to be precise. Keying slow enough for Sean to copy the transmission is like walking slow.”

“Walking slow?”

“You know,” Dinah said. “Anyone can walk at a normal pace. That’s easy. But when you have to walk at half speed, like because you’re accompanying someone who has trouble getting around? It’s exhausting.”

“Got it.”

“When I started to beg off, he changed his topic. To that point it was all, ‘Hey, what’s going on, how many people are on the Ark?’ but when I applied a little time pressure he started talking about sensitivity analysis.”

Doob laughed.

“Wow,” Dinah said, looking at him keenly. “Not the reaction I expected.”

“I’ve been awake for hours thinking about it,” Doob said.

“So you know what he means? Because I’m just a dumb clodhopper, I had to ask him.”

“I assume he means, how certain are we really that it’s going to happen on Day 720? And just how unstable is the system?”

“Yep, that’s what he means.”

“The closer we get, the more it’s like a nuclear reactor about to go critical, or, well—”

“Pick your metaphor, I get it,” Dinah said.

“Anything that’s that unstable can be set off by random noise in the system. Things that we inherently cannot predict. Pretty soon it’s going to be so on edge that just looking at it funny will set it off. We just don’t know which rock is going to trigger the avalanche.”

Dinah considered it for a few moments, then broke eye contact and looked at her radio. “Sean does,” she said.

“I’m not sure I heard that correctly,” Doob said, after a long, groping pause.

“The Eight Ball,” she said. “That’s what Sean calls it. It’s a rock you don’t know about. One you can’t see coming. It’s too dark, too far away.”

“Dinah, I’m confused — are we talking about a hypothetical asteroid here, or—”

“No. A specific one. A real one. Look, Doob, you know that Arjuna Expeditions has been putting up cubesats for years. We have hundreds of eyes in the sky, drifting around taking pictures of near-Earth asteroids, cataloging them, recording their orbital parameters with as much precision as we can manage. Well, apparently he’s been lying awake at night thinking about the same stuff as you. The extreme instability of the debris cloud. Its sensitivity to any kind of perturbation. And he had the bright idea: Why not search through Arjuna’s secret database of asteroids to see whether any bad actors were going to be passing through the middle of the lunar debris cloud during the next couple of weeks, when it’s on such a hair trigger?”

“He has that database with him?”

“Sure, whatever, it’s just a spreadsheet.”

“So he opened that spreadsheet and did that analysis?”

“Yeah. Doob, listen, I’m piecing this together from circumstantial evidence. You’ve seen how spotty the communication is.”

“Understood.”

“But I think he did that analysis and found an asteroid, which he is calling the Eight Ball. I assume it’s low-albedo.”

“Black. As eight balls are,” Doob said.

“I don’t know anything about its size or its orbital parameters, any of that. But Sean thinks it’s going to pass right through the middle of the cloud in about six hours.”

“Six hours?!”

“And that it has enough kinetic energy to be, well, interesting.”

Doob was thinking about Amelia. About those emotions that had kept him awake earlier. Predictably, everything had now been reversed and he was terrified that she and Henry and Hesper and Hadley were all about to die.

Dinah misinterpreted this as him making astronomical calculations in his head. “I’m going to go and get six hours of sleep,” she said. “Good night.”

“Good night, Dinah,” Doob said.

IT WAS ABOUT DOT 16, SHIFT CHANGE TIME, THE EQUIVALENT OF four in the afternoon for third shifters. So, Markus was approaching the end of what, for any normal earthbound person, would be his workday. Of course, like almost everyone else in the Cloud Ark, he worked the whole time he was awake. Even his recreational activities — such as martial arts practice in the Circus — had a larger purpose. So the “afternoon” shift change and end of his “workday” were purely formal observances. Nevertheless, he was in the habit of using this time of day for dealing with what used to be called paperwork. And as part of that he had invited to his little private office off of the Tank the Only Lawyer in Space, Salvatore Guodian. Son of a Singaporean Chinese father and an Italian countess whose parents had gone to that city-state as tax exiles, he had been educated in a school for mostly British expats, matriculated at Berkeley, dropped out after one and a half years to join a tech startup, lost his shirt, bummed around to various other startups, finally made some money, become interested in the law, essentially bought his way into law school despite not having a bachelor’s degree, worked for fifteen years at the Los Angeles, Singapore, Sydney, Beijing, London, and Dubai offices of a white-shoe law firm, been passed over for partnership, resigned, ridden his bicycle across China, moved to San Francisco, and become the general counsel of a digital currency trading firm while in his spare time volunteering for a nonprofit cyber rights organization and going out into the desert to launch very large home-brew rockets to the edge of space. Sal, as he was universally known, had been one of the first people chosen to work on the Constitution of the Cloud Ark, and so had spent a year and a half at The Hague before getting “yanked,” as the expression went, and launched up here. He was forty-seven years old but in dim light could have passed for thirty.

As a way to deal with the exigencies of zero-gee life, and a surrender to a receding hairline, he had taken to wearing a short vacubuzz. This was the easiest thing to do with hair in space. The vacubuzzer was a machine that combined the functions of an electric trimmer and an industrial shop vac. Haircuts were self-serve and consumed about thirty seconds if you were unusually fastidious. Earplugs were recommended. In his halcyon days Sal had sported a luxuriant head of long, wavy black hair and a widow’s peak that had brought out his Italian heritage, but with a vacubuzz he looked almost purely Chinese. He spoke seven languages, and he came closer than any living human to having the entire Cloud Ark Constitution — or CAC, as he called it — in his brain. If Markus had anything to say about it — which he did — then Sal would very soon combine in one person the functions of attorney general, head prosecutor, justice of the peace, and chief justice of the supreme court.

Sal laughed. He had great teeth. “You realize that those roles are completely incompatible. They are intended to be one another’s mutual adversaries in a lot of ways.”

“Then you can appoint other people to fill them. Look, Sal, we are talking about a bootstrapping process. We have to start somewhere.”

“Let’s war-game it,” Sal said. “A male Arkie from Outer Bizarristan rapes a female Arkie from Andorra. It happens in a place where we don’t have any cameras.”

“There are very few such places,” Markus pointed out.

“Okay, fine. It happens in an arklet. Or so the victim claims. She goes to sick bay, where medical evidence is gathered.”

“Do we even have rape kits?” Markus asked.

“How should I know?” Sal returned. “But we should get some. Anyway, based on that, in some countries a judge might issue a warrant enabling the police to look at the video records from that arklet. Because in some countries, Markus, people have a right to privacy and you can’t just be surveilling them all the time.”

“And what is the situation here?”

“It’s fascinating that you don’t even know, but I’ll tell you that the CAC recognizes certain rights that, however, may be abrogated or curtailed during periods of simplified administrative procedures and structures.”

“PSAPS,” Markus said. “That, I know about. It is a euphemism for martial law.”

Sal looked somewhere between pained and amused. “May I suggest you stop thinking about it that way — or, failing that, never say it out loud.”

“But nevertheless—”

“A better analogy might be the authority a captain wields over a ship at sea. The captain can do things, like preside over marriage ceremonies or order someone confined to quarters, that would not be acceptable if the same ship were tied up to a pier in Manhattan.”

“Look, I do not have time now to war-game a whole prosecution of a hypothetical rape,” Markus said, glancing at his wristwatch — Swiss, naturally, and made specifically for him by a famous Geneva company, as a sort of legacy, a way of saying we existed once, and here is what magnificent things we were capable of. “I want to talk about something very basic, very fundamental, which is: How do I have authority? Or if I am replaced by Ivy or Ulrika, how does she have authority?”

Sal didn’t quite see where he was going. “Authority meaning. .”

When this elicited no response other than impatient muttering, Sal tried: “Authority can mean many different things, Markus.”

“In this case I am not speaking of moral authority or leadership qualities or any of that stuff. I do not mean the theoretical loyalty that Arkies have to the so-called captain of the ship. I mean, what happens if we go to arrest the rapist from Outer Bizarristan, and he decides to put up a fight, and his friends decide that they are going to fight with him?”

Sal, to this point, had been viewing the conversation as an enjoyable exercise in legal theory. He now looked more serious. “You’re talking about power. What it really means. What it really is.”

“Yes.”

“It’s an old question. A pharaoh, a medieval king, the mayor of New York City, they all have to think about the same thing.”

“Yes,” Markus said again.

“When you give an order, what assurance do you have that it will be carried out? That is the essential question of power.”

“Jawohl, counselor!”

“Normally here I would speak to you about moral authority and loyalty and all of that. But you have already ruled this out.”

“When push comes to shove, as the English expression has it—”

“The traditional answer has always been that the king has his guard, the mayor his chief of police, the commander his military police, or what have you. And it is their ability to physically coerce others that is the ultimate foundation of the leader’s power.”

“Now you’re talking. And what is that for me, under the CAC?”

“You understand,” Sal said, “that the more you actually call upon such persons to coerce, the less power you have, in a way. It is an admission of failure.”

“Sal,” Markus said, “how long have you been up here?”

“Two hundred and some days.”

“How many hours have we spent talking about the CAC?”

“I have no idea, probably a hundred hours over that time.”

“And of that, how much time have we spent talking about this one thing?”

Sal checked his own watch. “Maybe fifteen minutes.”

“So, based on that allocation of time,” Markus said, “maybe you can see that this is not all that important to me in the big scheme of things. But it is important, Sal. When the moment comes when I have to arrest a criminal who is being protected by his comrades, I must have an answer. I must know what to do. I must be prepared. This is what I do. This is why I have this job.”

Someone was knocking on the door to Markus’s office, which was unusual. Markus ignored it for now.

“Under PSAPS you can deputize specific people to enforce your decisions using appropriate levels of physical coercion. Once we get out of PSAPS. .”

“How soon do you think that is going to happen?” Markus’s tone of voice suggested he had his own opinions on the matter.

“If we are lucky enough to survive? It will be years,” Sal said.

“So we must confine ourselves to PSAPS for this discussion,” Markus said. Then he hollered at the door, “Just a minute!” Then, back to Sal: “Appropriate levels of physical coercion, what does that mean? Who decides?”

“Well,” Sal said, “if you make me attorney general, head prosecutor, justice of the peace, and chief justice of the supreme court, I guess I do.”

“If someone gets Tased, and his heart stops, and he dies, is that appropriate?”

“Jesus Christ, Markus, what has gotten into you?”

“I am war-gaming,” Markus said. “Trying to be prepared. You should do it too. Not with hypothetical rape cases but with what is likely to start happening soon.” He held Sal’s gaze until Sal answered with a nod. Then he aimed his voice at the door. “All right! Come in!”

“Door” was a landlubber term for what, on a boat or a spaceship, would be called a hatch. A convention had developed where, in a part of Izzy that had simulated gravity, it was referred to as a door. In the floaty bits, it was called a hatch.

The door opened to reveal Dubois Jerome Xavier Harris. The look on his face, combined with the mere fact that he had interrupted Markus during a meeting in his private office, suggested that something serious was happening. Markus’s mind jumped straight to the most obvious explanation: “Is the president nuking people again?”

Dr. Harris looked startled by the suggestion, then shook it off. No, it wasn’t that.

“Does this meeting require privacy?” Markus asked, with a look at Sal. Sal stood up, volunteering to make himself scarce. But Dr. Harris just got a bemused look. “It concerns the least private thing that ever happened, or ever will,” he said. “So no thank you. I have reason to believe that the timetable has just been pushed up very significantly. There is a chance that the White Sky could happen as soon as six hours from now.” He checked his watch. “Call it five.”

Markus’s eye flicked to a display on the wall. “I see no uptick in the BFR.”

“It will be triggered by the passage of an asteroid through the cloud.”

“Does anyone on the ground know?”

“It depends on to what extent this office is under surveillance.”

“So, your information does not come from the ground.”

“No. It comes from deep space.”

“Via encrypted Morse code?” Markus inquired casually. He and Sal exchanged a look. Their conversation had begun, an hour ago, with reading a memo from J.B.F. complaining about such transmissions and demanding that action be taken. It was in discussing how to take such action, and whether the White House had any authority in the matter, that Markus and Sal had wandered into their more general discussion of power. Which was how Markus liked it, for now. Because if someone was sending mysterious encrypted Morse code transmissions from Izzy, it had to be his girlfriend. And he wasn’t going to arrest her. People would howl about conflict of interest: people who would be dead soon, people who had no way to enforce their authority here.

Unless they had planted, among the Arkies or the General Population, fifth columnists with orders to execute a coup d’état if necessary.

“Markus?” Dr. Harris asked. “Are you hearing me? Do you understand what I just said?”

“I beg your pardon, Dr. Harris, I just got distracted thinking about the kind of things that Sal is supposed to think about.”

“Feel free to delegate some of that,” Sal said. “I know it’s not your strong suit, but—”

“Close the door, please,” Markus said.

Dr. Harris did.

“I am reasonably sure of no surveillance in here.”

“Noted.”

“It is Dinah, isn’t it, Doob?” Markus asked.

Doob nodded. “She’s talking to Sean Probst over an encrypted channel.”

Markus shook his head admiringly. “What a girl! My god, she is trouble.”

Doob and Sal were silent. During their silence, Markus thumbed out a one-word text message to Tekla.

“Sal,” Markus said, “I declare PSAPS.”

“I don’t think we are yet authorized to—”

“Who is going to stop us?”

Doob and Sal, again, were silent on the matter.

“Is Julia — I do not call her the president anymore — going to nuke us?” He was continuing to thumb out messages as he talked.

“She, or the Russians, or the Chinese, might have other ways of removing you from your position—”

“I have thought about this,” Markus said. “About the possibility that there are plants. Military guys with Tasers or whatever. Waiting for such an order. I have talked to Fyodor, to Sheng, to Zeke, trying to sound them out, to get a feel for it.”

“Markus,” Doob said, “with respect, I don’t think that this is what you ought to be focusing on right now.”

“Which is why I am delegating the constitutional side of it to Sal and the operational side of it to her.” Markus nodded toward the door, which had swung open without a knock. Tekla glided through and closed it behind her. “We don’t have to announce to the whole world that we are going to PSAPS. We have five hours in which to begin preparations, quietly. I will contact Moira, and tell her that we must begin preparations to disperse the genetic samples to the arklets. I will tell Ulrika that we must pull the trigger on the Surge.” By this, Markus meant a long-planned burst of launches that was supposed to happen in the few days’ grace period between the White Sky and the onset of the Hard Rain. “We can be working on these things quietly. Five hours from now, it will happen or it will not. If it does not, we go back to as we were and consider this a dress rehearsal.”

The door opened again, this time after a knock, and in came a young man named Steve Lake, preceded by his laptop and followed by his dreadlocks. For Steve, in his year and a half aboard Izzy, had not succumbed to the vacubuzzer’s siren song, but he had gotten tired of messing with his long hair and had allowed it to congeal into red ropes. Formerly employed by a consulting firm in northern Virginia that hired hackers to do secret work for intelligence agencies, he had been yanked and sent up to support Spencer Grindstaff, the networks and communications specialist who’d been one of Izzy’s original crew on Zero. Spencer was an NSA man through and through, recruited straight out of MIT to work on spooky crypto stuff. Steve seemed to be an altogether different sort of character. He looked a bit mystified just now.

“Steve,” Markus said. “It is time for us to have a conversation about power.”

Steve’s brow furrowed. “You mean, electrical power or—”

“The other kind.”

“Okay, and is this going to be, like, an abstract philosophical discussion or—”

“No, it is going to conclude with me telling you, under my PSAPS authority, to change all of the passwords and keys for Izzy’s control systems.”

“Wow!” Steve said. “Shouldn’t you be talking to Spencer then? Because he’s above me in the org chart.”

“I am familiar with the org chart,” Markus said. “Under PSAPS I have the authority to change it.”

“What is this PSAPS thing you keep talking about, Markus?”

“Sal will explain it later. For now, we may set it aside. Fundamentally we speak of your loyalty, your allegiance. I think that Spencer is extremely loyal to powers that be on the ground. I do not wish to put him in an impossible bind. He will later come with us, or he will not. You I believe to be a different kind of fellow. I ask you, in effect, to now become loyal to the Cloud Ark and the Cloud Ark alone. Not to Washington. Not to Houston. And to accept the authority of whoever is the boss of the Cloud Ark. Which for now is me.”

“Okay.”

“You’re supposed to think about it first, Steve. Not just say okay.”

“I’ve been thinking about it for a while. But I have to tell you, there might be back doors. I can change all the codes I know about. The ones I don’t know about are a different matter.”

“Then we shall just have to be vigilant.”

White Sky

DOOB COULDN’T GUESS HOW MANY TIMES DURING HIS LIFE HE HAD noted a cottony tuft of cloud in a blue sky, then looked up hours later to discover that it had developed into a bank of clouds that covered the sun and told of a change in the weather. Such phenomena happened too slowly for the mind to discern them as happening at all. During the last hours of A+1.335, something like that occurred in the cloud of lunar debris that had been hanging in the sky for the last seven hundred days. Later they would watch the movies of it in time-lapse, compressing a day’s changes into a minute of video, and it would look like an explosion. Or an epidemic of explosions. If you watched the video carefully enough, frame by frame, you could see it progress from one part of the cloud to the next as the Eight Ball shot through. Like a particle lancing through a cloud chamber, it was invisible save for the trail of consequences it left in its wake. A few months earlier it might have passed through without touching anything, but today the density of rocks in the cloud was such that it could not avoid smashing into some of them on its way through. Doob, making a crude statistical calculation, put the likely number of collisions at ten, plus or minus five. Not a large number in a cloud that now contained millions of rocks, but enough to push the system, trembling on the precipice of an exponential explosion, over the edge. Around its unseen track the White Sky took form and fury. The cloud bloomed and evoluted like cream in coffee, spreading and paling, though from place to place one could see fresh bursts as rocks hurled out in earlier collisions found distant targets and touched off smaller chain reactions of their own. In places it took on a cellular structure as curved detonation fronts spread, contacted others, and merged into lacy foams of white arcs. It had an austere, monochromatic beauty about it. There was no fire and no light other than what cold sunlight the rocks bounced back to the eye. Later, when they began to enter the atmosphere, there would be fire and plenty of it. But for now the world was ending in a fractal blooming of dust and gravel, an apocalypse in a gravel quarry.

“You pretty much nailed it,” someone told Doob, “when you called it the White Sky.”

“Being right does not always bring satisfaction,” he said.

The Bolide Fragmentation Rate shot up through all meaningful thresholds within a few hours of the Eight Ball’s arrival and Doob stopped paying attention to it. The number was probably wrong now. It was just an estimate, produced by a consortium of observatories based on the amount and distribution of light coming out of the cloud. All the assumptions that went into its calculation had now become obsolete.

He tried aiming his optical telescope at where PP1 and PP2 and Cleft — the large, metal-rich children of Peach Pit — ought to be, but saw nothing except, possibly, some local highlights in the density of the cloud, perhaps caused by rocks dashing themselves to pieces on the steely surfaces of those dark bolides. He wondered if he would ever see them again.

He no longer had an accurate visual memory of the size of the moon in the sky, and so he could not estimate how many times larger the cloud was. Of course, he could look those numbers up and calculate it. But he didn’t really care what the numbers said. The full moon had always been the same size, but sometimes it looked huge and sometimes it looked small, depending on how close it was to the horizon, and on factors that were purely psychological or aesthetic. To all but a few of the people on the night side of Earth, looking up at the cloud, those factors were the only ones that mattered. He wanted to know how big it looked to them; he wanted to know how it felt. He wanted to see it over the Chino Hills from the courtyard of the Caltech Athenaeum, which was where he had last seen the moon, a few minutes before Zero, and to know how it was to stand there on terra firma and to see it and to know it was death coming.

Like most people, he had drawn up a list of everyone in the world he needed to say goodbye to, then gone through it and ruthlessly weeded out 90 percent of the names, since there wasn’t time. And then, during his last few months on Earth, he had sought out and said goodbye to the ones he needed to see in person. From orbit he had said goodbye to others on videoconferencing links or with carefully written email messages. Once he had said goodbye to a given person, he avoided communicating with them again. It was awkward to go out for a last night of drinking with a colleague, reminisce and cry and hug and say farewell, and then find yourself emailing the same person two months later with a question about their latest observations. Consequently, his scope of acquaintances had steadily narrowed as he had worked his way down the list. By this point he was down to his wife and his children. Reaching them became a lot more difficult after the Eight Ball had done its work. The volume of communications between Izzy and the ground was limited by the total bandwidth of the station’s antennas and radios. Personal communications had lower priority than operations, and operations were peaking as the final surge of launches was prepared. Or, as Dinah called it, the Splurge. Doob sent text messages to Amelia and the kids all the time; they sat in the delivery queue for minutes or hours, and half of them never got sent at all. Just when he was about to give up hope, he’d get a message back from Henry or Hadley or Hesper. Sending those messages, and seeing the responses, became more important than sleeping, so he “broke shift,” as the saying went, and dozed whenever he could, lying on the floor of the Farm or just putting his head down on a table like a kindergarten kid, his phone right next to his face so he’d feel it jump when anything came through.

It finally became clear to him, maybe twenty-four hours after he had given the news about the Eight Ball to Markus, that he was never going to communicate with his loved ones again save through sporadic and unpredictable texts. Anything he needed to say to them directly, he ought to have said before. Which should not have come as news; he had been telling himself for a long time that you had to act as if each conversation might be the last. This did not stop him from reviewing his final video chats with each one of them, on the evening of Day 700, and wishing he’d said certain things.

How does it look from up there? Henry texted him.

Doob checked the time. It was night in Moses Lake. He imagined Henry sitting out on that crappy old couch that they’d moved out of the house in Seattle, drinking a beer between work shifts, watching the White Sky reach out for him like a spectral hand.

Doob didn’t know what to say.

I think I am seeing some spread along the orbital axis — the beginnings of rings, he texted back.

I meant Earth, Henry returned.

Doob went looking for a place where he could look down at Earth through a real window — not one of those damned Situational Awareness Monitors. This ended up being the Woo-Woo Pod. It was pretty crowded. Izzy was about to swing over the terminator from day into night. Even over the brightly lit Pacific they could see what looked like hairline scratches in the pellucid shell of the atmosphere: the white trails left by incoming bolides. Above the dark side of the Earth these became arcs of blue fire that sometimes forked, and sometimes ended in red bursts when they made it all the way to the ground. In other words, it looked the way it had looked the day before, and the day before that. This level of meteorite activity would have been the most amazing astronomical event in human history had it happened suddenly, two years ago. But beginning with the first big rock that had plowed into Peru just a few days after Zero, the ambient level of bolide strikes had steadily crept upward. People had adjusted to it. Some had posted red-faced self-portraits after suffering “bolide burn,” meaning an acute case of sunburn caused by exposure to the ultraviolet light emitted by meteor trails in the nearby sky.

Looking down at you now, Doob texted. He wanted to add Wish I was there but it would have been stupid. Looks like a big one coming in over southern BC.

I see it, Henry returned. Feeling its heat.

Busy there?

You know it. Racking and stacking the big boys, getting ready for the Surge.

Doob wondered how it worked. What was to prevent desperate people from rushing the launch pads, trying to cram themselves aboard the last of those big boys? Like the last chopper out of Saigon, people dangling from the skids as soldiers punched them in the face. Or was he underestimating human nature? Maybe it was all perfectly orderly down there.

I need you here. That one was from Markus.

Reluctantly, Doob pushed himself away from the window and got turned toward the tube that would conduct him back to the Stack. From there he would make his way back to T3, where Markus was presumably hanging out in the Tank—

Markus Leuker was hovering directly in front of him, face illuminated by the blue light of a phone. He turned it off and slid it into his pocket.

“I don’t mean that I need you in the same room as I,” he said. “I mean that I need your brain here, in space, on the Cloud Ark, not down there. Your family is dead, Dr. Harris.”

“Dead. But still talking,” Doob said, feeling the start of a slow burn that might lead to him punching Markus in the nose, if only he could get to a place with gravity.

“What is it you think they would most like to hear back from you?” Markus asked. “Lovey-dovey stuff? They know you love them. Were I in their position, you know what I would like to hear? I would like to hear ‘Sorry, my darling, but I am very busy just now ensuring the survival of our species.’ May I suggest you text something in that vein and then join me in the Tank; we have matters to discuss.”

And Markus Leuker, using one of the ropes that were strung across the Woo-Woo Pod as handholds, propelled himself toward the exit. As he passed through into the tube, Doob saw his silhouette against the circle of light, a Da Vinci Man, just for a moment. Then two others swung in behind him and spoiled the effect. That detail caught his attention. Markus now had an entourage. Or perhaps a bodyguard.

Hard Rain

LIKE ANY GOOD STORM, THE HARD RAIN BEGAN WITH A SUDDEN thunderclap: a kilometer-wide rock that lit up eastern Europe with eerie, silent flashes as it skidded in across the upper atmosphere before digging into thick air somewhere around Odessa. Its trail set fire to dry leaves and combustible litter in the Crimea, then painted a long brushstroke of burning buildings and forests across the northeast rim of the Black Sea, ending with a long elliptical crater in the steppe between Krasnodar and Stavropol. The former city was first set on fire by radiant heat from the sky and then flattened by a blast wave. The latter got only the blast, followed by a rain of ejecta. Both disappeared from human ken.

After a few hours’ respite, smaller bolides began to come down. They landed all over the world, but most often in the lower latitudes, close to the equator. Having been told, long in advance, that this would be the case, many people had moved toward the poles in recent months, prompting Rufus MacQuarie and his friends, family, and associates to establish a defensive perimeter around their works in the Brooks Range. That was a terrible place in November. The only refugees likely to make it up that far would be well equipped and well prepared, but those were exactly the kinds of uninvited visitors that Rufus didn’t want creeping around. Unencumbered by the limits on bandwidth that applied to all the other radios in the Cloud Ark, Rufus and Dinah had kept up their Morse code correspondence during the three-day “grace period” between the White Sky and the Hard Rain. Rufus was still transmitting from his truck, which he had parked before the entrance to the mine. He had considered erecting a larger antenna on the top of the mountain and hooking it up to an underground transmitter via armored cables, but Dinah, after surveying the predicted effects of the Hard Rain, had told him not to waste his time.

Ivy had said goodbye to the Maternal Organism several days earlier, immediately before the Morg had swallowed her government-issue euthanasia pill. The one person on Earth she was still in touch with was Cal, aboard his submarine, keeping station on the surface offshore of the Norfolk Naval Base, out where the water got blue enough to facilitate a deep dive when the time came. In those days Ivy’s main link to her family came through music. For the Morg had given five-year-old Ivy a choice between becoming the best pianist in Southern California or the best violinist in Southern California, and Ivy had opted for the violin. She had never become the best in Southern California, or even close to it, but she had played in various youth orchestras and developed some familiarity with the classical orchestral repertoire. She had a violin aboard Izzy, which she would tune up and play from time to time.

When the Bolide Fragmentation Rate shot up through a certain level on Day 701, marking the formal beginning of the White Sky, a number of cultural organizations launched programs that they had been planning since around the time of the Crater Lake announcement. Many of these were broadcast on shortwave radio, and so Ivy had her pick of programs from Notre Dame, Westminster Abbey, St. Patrick’s Cathedral, the Imperial Palace in Tokyo, Tiananmen Square, the Potala Palace, the Great Pyramids, the Wailing Wall. After sampling all of them she locked her radio dial on Notre Dame, where they were holding the Vigil for the End of the World and would continue doing so until the cathedral fell down in ruins upon the performers’ heads and extinguished all life in the remains of the building. She couldn’t watch it, since video bandwidth was scarce, but she could imagine it well: the Orchestre Philharmonique de Radio France, its ranks swollen by the most prestigious musicians of the Francophone world, all dressed in white tie and tails, ball gowns and tiaras, performing in shifts around the clock, playing a few secular classics but emphasizing the sacred repertoire: masses and requiems. The music was marred by the occasional thud, which she took to be the sonic booms of incoming bolides. In most cases the musicians played right through. Sometimes a singer would skip a beat. An especially big boom produced screams and howls of dismay from the audience, blended with the clank and clatter of shattered stained glass raining to the cathedral’s stone floor. But for the most part the music played sweetly, until it didn’t. Then there was nothing.

Paris is gone, she texted. Through the military systems, which were patched in with NASA’s, she could still communicate with Cal.

Dive bbs, he answered. Which by itself was pretty enigmatic, but she knew its meaning: the submarine had to dive below the surface for a little while, to avoid some danger, but he expected he’d be back soon.

But he might be wrong about that. She might never hear from him again. She decided it was long past time. She texted him a message that he would find waiting when and if his boat returned to the surface: I release you from your vow.

Then she felt a strange wave pass through her body, almost as if she were in a submarine in the Atlantic when a pressure wave rolled through from some distant meteor strike. She assumed it was an emotional reaction to what she had just done. But then she noticed that every loose floating object in her workspace was drifting in the same direction, toward the wall against which she had braced her back. Pops and creaks and groans propagated through Izzy. The space station was accelerating gently, at just a fraction of a gee. The thrusters must be firing.

The lights had turned red. The PA speaker in her module emitted a slight pop as it came on. “Alert,” said a synthesized voice. “All personnel should now be awake and at stations for urgent swarm maneuver. This is not a drill.”

So it had happened. They had been practicing this for months. But this was the first real Streaker Alert. It meant that a bolide had been detected by SI — the Sensor Integration team — on an unusual trajectory that might pose a danger to Izzy unless the course was corrected slightly.

Her first, nervous impulse was to look out the window toward Amalthea. The big rock was still there. The maneuver hadn’t caused it to snap off.

But this was Ship thinking: placing top priority on Izzy. She, and everyone else, needed to get in the mental groove of Cloud thinking. The majority of the population lived on arklets. Izzy’s purpose was to help the arklets survive.

So she wrenched her gaze away from the window — an antiquated thing, that — and brought up a display on her tablet showing the disposition of every vessel in the Cloud Ark. It was an app called Parambulator. It was not a literal rendering of what the cloud looked like, though you could make it show you that if you clicked the right menus. Parambulator was a tour de force of data visualization that would only make sense to people like Ivy, Doob, and most of the Arkies, who had spent a lot of time learning about orbital mechanics. Starting with empirical observations from Lina Ferreira and other mathematically sophisticated biologists, mathematicians like Zhong Hu had extrapolated swarm algorithms from three to six dimensions and physicists like Ivy had figured out how to make these algorithms work under the special constraints of orbital mechanics. In general, every vessel in the cloud was shown as a dot on a three-dimensional scatter plot showing information about its orbit. Six numbers — the orbital parameters, or, as everyone up here had begun to call them, the params — were required to convey everything about an orbit. Only three could be visualized in any given plot. So that was where the user-interface legerdemain came into play, and where someone like Ivy had to pay attention and engage all available brain cells. But the gist of it was that each arklet was a projectile that could strike Izzy, or another arklet, if its params were wrong. In a hypothetical, extremely simple Cloud Ark consisting of only two arklets, only one calculation needed to be performed: namely, the calculation that answered the question “Will Arklet 1 bang into Arklet 2 if both stay on their current courses?” In a three-arklet cloud, it was also necessary to figure out whether Arklet 1 would collide with Arklet 3, and whether 2 and 3 were going to collide. So, that was a total of three calculations. If the cloud expanded to four arklets, six calculations were needed, and so on. In mathematical terms these were known as triangular numbers, a kind of binomial coefficient, but the bottom line was that the number of calculations went up rapidly with the number of arklets in the cloud. For a hundred-arklet cloud it was 4,950 calculations, for a thousand-arklet cloud, about half a million. It would have flummoxed the simple computers of Apollo days but was nothing by modern standards — provided that accurate information could be had about each arklet’s orbit. An old-school, centralized approach would have been for all the arklets to report their params to a computer on Izzy, which would then do all the calculations and report the results. The reliability of that process could be improved if Izzy’s radars, observing the arklets and plotting their movements, filled in gaps in the data. And indeed something like that was happening all the time, not just on one computer on Izzy but on several. But this, again, was Ship thinking. Cloud thinking dictated that each arklet make those observations and do those calculations separately. The computer on a single arklet — call it Arklet X— might not have all the information needed to track every single one of the other arklets in the cloud, but it could identify the ones most likely to be a danger and focus on those. Others, as well as the central processors on Izzy, could assist it by sending messages to the effect of “You might not be aware of it, but you are possibly in danger from Arklet Y and might want to move it to the top of your list of things to keep an eye on.” To which it might reply “Thank you, but I’m not getting good params for Arklet Y because Izzy is blocking my view on the radar.” The cloud would then respond by in some sense becoming aware that Arklets X and Y needed to know more about each other’s params and giving a higher priority to making that happen.

The cloud, in other words, became not just a physical cloud of flying objects in space but a computational cloud as well, a free-floating, self-regulating Internet. The function of Parambulator was to give its users an Olympian perspective on all that was happening in that network, and at some level all you really needed to know about it was that scary things were shown in red. Ivy looked at it now, more in curiosity than in alarm, since they had been practicing maneuvers for weeks and she thought she knew what to expect. Whenever Izzy fired her thrusters and changed her params, red propagated through the scatter plots like a drop of blood in a glass of water. All the free arklets, and all the ones connected to bolos or to heptads or triads, now needed to evaluate their params and see whether they were in danger of colliding with Izzy. Or — almost as bad — of drifting away so far that they could never get back to the swarm, a condition shown by a yellow dot in the display. It was a simple matter for any given arklet to plot a new course that would avoid both of those fates. Much more complicated was for three hundred arklets to do it at the same time without banging into each other. So a kind of negotiation had to take place, based not on awaiting commands from Izzy but on observing what “nearby” arklets were doing and coordinating the firing of thrusters with them to minimize the amount of red showing up on the plot.

It was necessary to place the word “nearby” in scare quotes because it had a different meaning in this swarm than it did to a bird in a flock. To a bird, nearby meant just that. To things maneuvering in the six-dimensional parameter space of orbital mechanics, “nearby” meant “any set of params that is potentially interesting to me in the next few minutes,” and it could apply to objects that were currently too far away to be noticed. Once that was accounted for, however, the arklets could do as birds did when flying in flocks. In the simulations that they had seen shortly after the concept had been proposed, it had looked astonishingly like the behavior of schooling fish. And the reality of it, which had only been implemented in the last few months of round-the-clock launches from Kourou, Baikonur, Canaveral, et al., answered well to those simulations. It just happened more slowly in real time.

It was happening now, in response to Izzy’s course change. The red only spread so far, then began to recede, first fraying around the edges, then dying off in patches. A few dots went yellow, then corrected themselves as they caught up. Ivy’s expectation, based on the last few months’ tests and exercises, was that the last few red dots would turn white very soon and cease to be a concern. But this didn’t happen. Some remained stubbornly red. Spinning the plot around, looking at it in various modes, she zeroed in on those dots and queried them. Almost all of them were cargo modules or passenger capsules that had been launched during the Splurge: the last-minute effort made by all the spacefaring nations of the world to launch every last rocket they had capable of reaching orbit.

Her phone buzzed. A message had come back from Cal; his boat must have resurfaced.

What’s that supposed to mean?

He had only just now seen her last text.

It means we are no longer engaged.

That seemed a little blunt, so she added, You need to find some nice mermaid.

After a minute he answered {crying} I was going to do the same. Your odds considerably better.

She answered Bullcrap, which was an old joke between them. When she had first met him at Annapolis, he had been such a straight arrow that he was unable to speak the word “bullshit.”

SAB = Straight Arrow Babe came back.

SAB is sad:(Why did you dive?

Big surface wave came through. Bad news for East Coast.

Who tells you? Do you have a chain? Meaning chain of command.

One rung left above me. Then, after a pause, POTUS has gone dark.

She typed in Thank God for that and hesitated before sending it. But the world was coming to an end; she didn’t have to worry about repercussions. She hit Send.

She’d never talked to Cal about what had happened on Day 700: the fuel-air devices, the nuclear warhead. But she was certain it had been his finger on the button.

May God have mercy on her soul, Cal answered, and she knew the subtext: and may He have mercy on mine.

This exchange of messages was interrupted by one from Markus: need u.

She pocketed the phone to free her hands for movement through Izzy, maneuvered through the maze of habitation modules to the Stack, and headed aft, bound for the Tank. The trip down the Stack took no time at all. A week ago she would have had to maneuver around people clumped in twos and threes for conversation. Since Markus had declared PSAPS, this had changed; one of his edicts had been that the Stack must be kept clear for rapid movement of essential personnel. Right now it was as empty as she’d ever seen it. Down in the Zvezda module she saw some comings and goings, and recognized, for a moment, the spiky profile of Moira’s hair. She would be busy making preparations to disperse the Human Genetic Archive to the cloud, a project that in and of itself was at least as complicated as anything happening with swarms and params. Essential personnel indeed.

Luisa popped into view down in H1 and propelled herself up the Stack like she meant business. After nearly colliding with one of Moira’s helpers, she let her momentum carry her up into Zarya, then stopped hard at the entrance to the tube that led to the Woo-Woo Pod. She looked into it for a few moments, evaluating, then made a decision and pulled herself into it.

Ivy passed by the same location a few moments later, slowed for a moment, and glanced down the length of the tube. It was possible to see straight down its length, across the spherical Pod, and through its windows to the Earth. Normally this meant the blue light of the oceans and the white light of clouds and ice caps. Sometimes, a lot of green when they were passing over well-watered parts of the world, or some yellow when over the Sahara.

Right now the light was orange because the Earth was on fire.

People were screaming down there in the Pod. Luisa must have been sent there to calm people down. Ivy was almost drawn in by a sort of magnetic power of fascination. Earth looked as if some god had attacked it with a welder’s torch, slashing away at it and leaving thin trails of incandescence. Some of these were red and steady: things burning on the ground. Others were blinding bluish-white and evanescent: trails drawn through the atmosphere by meteorites.

She fancied she could almost feel the warmth radiating from the planet.

Markus needed her. She couldn’t help the screaming people down in the Pod. She turned her head aft and pushed on.

Hovering in the entrance to the genetic storage modules, Moira was ticking off items on her tablet, listening, dead faced, to something on a large pair of headphones. She noticed Ivy. She peeled a headphone away from one ear and aimed it at her. Ivy recognized a cappella music, medieval polyphony. “King’s College is holding up rather well,” she said. “Do you know the piece?”

“I’m certain I’ve heard it before, but I can’t place it,” Ivy said.

“Allegri’s ‘Miserere mei, Deus,’” Moira said. Thanks to the Morg’s insistence that she take Latin, Ivy knew what it meant: Have mercy on me, O God.

“It’s beautiful.”

“They would sing it at Tenebrae, in the wee hours, as they extinguished the candles one by one.”

“Thank you, Moira.”

“Thank you, Ivy.”

A minute later she was in T3. As always, she stood flat-footed for a moment to get the feel of simulated gravity, then headed toward the Farm and the Tank. Passing through the utility section she considered getting herself a cup of coffee. Then she felt shock and shame over the fact that she was thinking about coffee while her planet was being set on fire.

Then she poured herself a cup of coffee anyway and stepped into the Farm. This was crowded. Most of the Situational Awareness Monitors were showing status displays relating to the functions of the Cloud Ark. The big one at the head of the room was just showing a view of Earth through a camera aimed in that direction. But the video image had nothing like the impact of seeing it directly through the windows of the Woo-Woo Pod. The arc-light intensity of the streaking bolides was reduced to a blurry flare of maxed-out pixels. Out of habit she wondered why they didn’t change the channel to CNN, or Al Jazeera, or one of the other full-time news networks. Then she remembered what was happening.

She proceeded to the door that led into the Tank.

Flanking it was a pair of people who were doing nothing — just standing there. Odd.

She noticed that both of them had unfamiliar devices slung from their belts.

She realized that they were Tasers.

Before she could fully adjust to that, one of them — she recognized him now as Tom Van Meter, an engineer and sort of a jock — nodded politely and opened the door for her.

The Tank was a quarter the size of the Farm, just a medium-sized conference room with, at the moment, six people seated around the table working on tablets or laptops. At its far end was the door leading to Markus’s office. This was ajar. Ivy went through it, and for the first time since coming to Izzy three years earlier, she felt ill at ease doing so, as if someone might jump out and Tase her. But Markus was sitting there talking to Doob.

“Have you been watching Parambulator?” Markus asked her.

“Yes. After we made that course change, a few minutes ago.”

“The performance of the cloud was not everything we could have hoped for.”

“There were some stragglers.”

“Still are,” Doob said, and drew her attention to a projection screen on the wall.

“It looked like they were all new arrivals,” Ivy said. “Cargo modules, passenger carriers from the Splurge. I’m assuming they haven’t logged on to the cloud yet, are not with the program.”

“That is all true but it is dangerous nonetheless,” Markus said.

“Of course it is.”

“It is distracting me.”

“I’ll take care of it.”

“As far as bolides are concerned, the systems are working okay and Doob is keeping an eye out for anomalies. But I need to delegate to you, Ivy, this problem of the stragglers.”

“Consider it done.”

“We will destroy them if we have to.”

“How would you even do that, Markus? We don’t have photon torpedoes.”

“We have a module full of freeze-dried dead people,” Markus reminded her, “that we need to jettison anyway. And I would be happy to jettison it in the direction of any straggler that is threatening the Cloud Ark.”

“I will keep that in mind,” Ivy said, “as a bargaining chip.”

Luisa entered, looking a little wild, her face wet with tears.

“Luisa?” Markus said politely. “Did you find out what was going on in the Vu-Vu Pod?”

“A few people getting very emotional,” Luisa said, “as you would expect. Nothing dangerous. Whoever called that in as a disturbance was being a little paranoid.”

“Thank you for investigating it.”

“Speaking of which — you have armed guards posted outside the door to the Tank!”

“I will speak briefly to that, because I am busy,” Markus said. “My feelings about it are basically the same as yours. But I am not here to express my personal feelings but to carry out certain operations to the best of my ability. I didn’t want to be the king of the universe. Nevertheless, now I am. Everything I have ever seen in the history of human civilization, disagreeable as it might seem, says that someone in my position needs to have security.”

Luisa’s face suggested that she could make all kinds of objections to that. But she got the better of it, and just let out a sigh. “We will talk about it later,” she said.

“Good.”

“Do you know what is happening down there?”

“I can guess what is happening. It is none of my concern.”

“Understood. But I think that the king of the universe needs to make an announcement pretty soon.”

“I have one prepared,” Markus said.

“Oh, yes, of course you would have one prepared. When were you thinking of delivering it? Because there are a lot of people who need to be calmed down.”

“Is one of those people you, Luisa?” Markus asked the question clinically, but not unkindly.

Luisa drew herself up. Ivy braced herself for a sharp reaction, but then a change came over Luisa’s face as she saw that Markus was merely asking for information. Not being snide.

“Yes,” she answered. “A few minutes ago, Manhattan was struck by a hundred-foot wall of water. I presume that the same is true of most of the East Coast. I was listening to the service from St. Patrick’s Cathedral when it went off the air.”

Markus nodded and changed the display on the projection screen to a live view of Earth.

Ivy was shocked by how far the fire had spread during the few minutes she’d been in here.

She pulled her phone out of her pocket and discovered a series of messages from Cal, sent during the last several minutes.

Hey

You busy?

OK I guess you got pulled away

In case we get cut off I love you

Will look for a mermaid like you said but no substitute 4 u

Lost contact with Norfolk. No chain above me

Holy crap it is getting hot

Diving

Bye

And the last message in the series was a photograph snapped on his cell phone’s camera. It took Ivy a minute of panning and zooming to figure out what she was seeing. Cal had taken the photo while standing in the conning tower of his boat, looking straight up the ladder at the open hatch above him. This provided a tunnel-vision view of a disk of sky.

The sky was on fire.

In his other hand he was holding up his engagement ring — a simple band of polished titanium. He was holding it between his thumb and index finger, shooting the picture through the ring, making it concentric with the disk of the burning sky.

She looked up. Someone had spoken her name.

“Mine just faded away,” Doob told her.

“I beg your pardon, Dr. Harris?” Ivy said, the Morg’s manners triumphing over all circumstances.

“I had been gearing up for these final goodbyes with Amelia, with my kids,” Doob said. He spoke quietly, without marked emotion, as if relating a mildly surprising anecdote. “But, you know, the communications just broke down slowly over a couple of days, and there was never really a goodbye.”

“Very well,” Markus said, “I will make the announcement.”

HOT ENOUGH TO BAKE TATERS ON HOOD OF THIS TRUCK

GO INSIDE DAD

NOT KIDDING ABOUT THERMAL EFFECTS. PAINT BUBBLING

I AM NOT KIDDING EITHER YOU HAVE TO GET INSIDE

GOT A SPACE BLANKET TO PROTECT ME WHEN I MAKE A RUN FOR IT

THEN FOR GODS SAKE USE IT DAD

AH BUT THEN I CAN’T CHEW THE RAG WITH YOU ANY LONGER DINAH

WHAT IF YOUR GAS TANK EXPLODES

HA HA WE DRAINED IT FOR GENERATOR FUEL. WAY AHEAD OF YOU KID

GOD U R A SMARTASS

Dinah was keying this in, thankful that Morse code still worked when your vision was blurred by tears and your voice choked by sobs, when a voice came out of a speaker. It was Markus’s voice: “This is Markus Leuker.”

“I know who you are,” she answered. But then she understood that Markus was speaking on the all-Ark PA system, which supposedly reached into every corner of Izzy as well as to all of the arklets. They had tested it a few times with prerecorded messages, but never actually used it. Markus considered the thing a relic of the twentieth century, and detested it; communications ought to be targeted, busy people ought not to be interrupted by disembodied voices barking from speakers.

“The Cloud Ark Constitution is now in effect.”

Dinah drew breath, knowing what this meant. Markus spelled it out anyway. “This means that all nation-states of Earth, and their governments and constitutions, no longer exist. Their military and civilian chains of command are no more. Oaths you may have taken to them, allegiances you may have held, loyalties you may have felt, citizenships you may have had are now and forever dissolved. The rights granted you by the Cloud Ark Constitution, no more and no less, are your rights. The laws and responsibilities of the Cloud Ark Constitution now bind you. You are citizens of a new nation now, the only nation. Long may it endure.”

She keyed:

MARKUS IS CALLING IT

WHO SAID HE WAS BOSS?

Rufus’s transmission was getting scratchy. Dinah wiped her eyes and looked out her window to see Earth encircled by a belt of fire. The trails of the incoming meteorites, once a pattern of bright scratches in the air, had merged into a blinding continuum of superheated air that had set fire to anything on the surface capable of burning. Since more of the rocks were coming in around the equator, the belt of radiance and fire was brightest there; but north and south of it, long swaths of the surface were aflame, and the belt was widening to envelop the high latitudes of Canada and South America.

She transmitted:

ABOUT TO LOSE YOU, TELL BOB AND ED AND GT AND REX I LOVE THEM. AND BEV.

ALREADY DID BUT WILL AGAIN. CHRIST IT IS HOT

GET INSIDE DAD

DONT WORRY I AM RIGHT BY THE DOOR. CAN HEAR THEM ALL SINGING BREAD OF HEAVEN.

THEN GO JOIN THE CHORUS DAD

OKAY BOB AND ED ARE COMING OUT TO GRAB ME. BYE HONEY DO US PROUD QRT

QRT QRT QRT QRT

She wasn’t sure how many times she keyed that in.

She pulled herself out of her sobs, later, by imagining what had happened: her brothers, Bob and Ed, dressed in silver fireman suits, rushing out of the mine’s entrance to haul Dad out of the old pickup truck, wrapping him in the space blanket to keep him from being broiled by the sky, and dragging him inside. An inch-thick steel plate being slammed across the doorway, the welders going to work laying down fat fillets made to last five thousand years. Once that was done, the heavy machinery fired up, shoving tons of rock and gravel up against the steel plate to bolster it against any shock waves powerful enough to punch it out of its frame.

Then silence, save maybe for the distant thuds of meteorite strikes, and sitting around the table to say grace and tuck into the first of fifteen thousand or so meals that the MacQuaries and their descendants would have to prepare and eat if they were ever to escape from that tomb. They had five hundred people down there, and, at least on paper, enough food-growing capacity to keep that many alive. Exactly how you made that a sustainable proposition wasn’t clear to Dinah; she hadn’t bothered Rufus for every last little detail of his plan.

Markus’s announcement was continuing. He was telling everyone what they already knew, which was that Earth was over, and that the great dying that they had been expecting for the last two years was now in the past. Everyone knew it, but someone had to say it.

He asked for 704 seconds of silence: one second for each of the days that had passed since Zero. About twelve minutes. All nonessential duties would be suspended during that time, and it would be the sole responsibility of the survivors to think, and remember, and mourn. After that, they must put Earth in the past, as a thing that had once been, and apply their minds to what was now.

Drawn up into a fetal position, Dinah hovered alone in the middle of her shop, listening to weird squeals and hisses coming out of her radio’s speaker. Alone of all the people in the Cloud Ark, she knew that her family was still alive, and might go on being alive for a long time. It was not clear to her whether this was better or worse than simply knowing that they were dead. All she had to go on was DO US PROUD, her father’s final transmission. Morse code didn’t leave a paper trail, or an email thread on the screen of your tablet. She would never be able to scroll back and reread the exchange she’d just had with Rufus. She hoped she’d said the right things and that he’d remember it well, and that he would tell the others about it at dinner this evening.

She tried then to mourn for all the others who had died, but it was too big. Emotionally, it was little different from reading about a great war that had happened a hundred years ago. Which maybe was Markus’s whole point. Even though the dying was still going on, they had to force themselves to think about it like the Irish potato famine, or like what had happened to the peoples of the New World when Columbus had arrived and infected them with a slew of deadly diseases. Regret, even horror were appropriate. But detachment was necessary. They all had 704 seconds in which to effect that detachment.

So Dinah thought about what exactly would be entailed in doing Rufus MacQuarie proud. There was a simple answer, which had to do with doing the right thing, being honorable, upholding a few rough-and-ready ethical standards. A sort of frontier code of conduct. All of which was easy to understand if not always quite so easy to live up to. But Rufus was not a cowboy, and he certainly wasn’t a preacher. He was a miner: a delver, a demolisher, a builder, a businessman. If he lived by a simple code of ethics, it was not an end in itself, but a way to get something done without selling his soul or destroying his reputation. It was a tool to be wielded like a shovel or a stick of dynamite. Tools were for building things; and pride was something you could feel after the fact, when you stood back, looked at what you had built, and passed it on to your children. Dinah could spend the rest of her life living by her word, giving everyone a fair shake, and all of that. Rufus would no doubt approve of all those things. But it was not the charge he had given her. He had told her, though not in so many words, to get busy building a future.

“Are you about finished?”

She turned her head to see Ivy hanging in the SCRUM, looking at Dinah through the hatch.

“We’re only, like, two hundred seconds into the—”

“Markus said I could skip it. He sent me on a mission. I need your help,” Ivy said.

“Bitch.”

“Slut.”

“Shall we?”

“REMEMBER WHEN THE INTERNET WAS NEW, AND SOME PEOPLE IN your life just didn’t get it?” Ivy asked. She was preceding Dinah through the seemingly endless maze of docked modules and hamster tubes, headed toward the periphery of Izzy.

“People in my world got it pretty fast. You don’t know many miners, do you?”

“Not in my world. We had these throwbacks who would do stuff like printing their emails out on paper to read them, or asking you for your goddamn fax number two decades after you had thrown away your fax machine.”

They were hurtling through an otherwise perfectly silent space station, still only about five minutes into the twelve minutes of silence. Faces in open hatches would turn to look at them in shock, then recognize them and go back to mourning, praying, meditating, or whatever it was that they were doing.

Dinah understood that this was terribly important but was secretly pleased that Ivy had given her dispensation to get to work.

“How does that apply to—”

“The system works — Parambulator and all of that — as long as every ship in the Cloud Ark is playing by those rules. Logged on to the system, communicating with the agreed-on protocols, obeying the dictates of the swarm. If even one is just hanging out and doing its own thing, well, it might as well be a meteoroid, in terms of its destructive potential.”

“We’ve got one of those?”

“A few of them. But one in particular that is causing havoc.”

“Any collisions yet, or—”

“No, but every time it draws near it triggers an explosion of red in Parambulator and a hundred arklets have to burn fuel to alter their courses. It’s like the whole Cloud Ark is turning somersaults around the movements of this one ship.”

“What is it?”

“Optically it’s an X-37.”

“Fits,” Dinah said.

“Yeah,” Ivy said.

Translation: someone had looked at the craft through a telescope and thought it looked like a Boeing X-37 Orbital Test Vehicle, which resembled a miniature Space Shuttle. It was so miniature, in fact, that it couldn’t carry any crew; it had a cargo bay that accounted for most of its fuselage. It had been developed by DARPA in the late 1990s and early 2000s when it had become obvious that the Space Shuttle was going to be phased out and they needed a small, easily launched vehicle that could go up and, by remote control, perform maintenance tasks on the United States’ fleet of military satellites. Since then it had come in for very little actual use, but when it was used, it was for black-budget spook stuff that Dinah and Ivy wouldn’t know about. It was a footnote in history, obsolescent, not designed for the requirements of the Cloud Ark. It had probably been launched into orbit by some trigger-happy launch crew that just wanted to send up everything they could. With a sufficient amount of sifting through old emails they might be able to find some record of who had launched it, and what, if any, cargo was aboard; but for now it was easier to just go and look at the damned thing. Nearly all the engineering that had gone into it had been devoted to the problem of reentry. Most of its proudest features were therefore useless to them.

Approaching the end of a side-stack, they were able to see through the round orifice of a port into the vehicle docked to its far side: a Flivver, or Flexible Light Intracloud Vehicle. These had begun showing up a few months ago; they were the jeeps of the Cloud Ark, the small utility vehicles used to move people and valuable stuff from one arklet to another, or between an arklet and Izzy. Because they didn’t have to operate in the atmosphere, they had the same general utilitarian look as the arklets. But the pressure hull was smaller in diameter, and instead of an inflatable outer hull the Flivver had more practical stuff: two different styles of docking ports, an airlock big enough to accommodate a human in an Orlan, a robot arm, lights, thrusters. At Dinah’s suggestion they had studded the pressure hull with attachment points that a Grabb could latch on to; this made it possible for each Flivver to carry its own complement of Grabbs, Siwis, Buckies, and Nats, which swarmed all over it like crabs, remoras, and sea lice. Instead of being limited by the hard-engineered capabilities of the robot arm, the Flivver was constrained only by the imagination and ingenuity of the programmer inside, telling the robots what to do.

The silvery burr of Tekla’s head poked out in front of them; apparently she’d been dispatched to assist with closing the hatch and undocking the Flivver. She’d been waiting in the adjacent DC, or docking compartment, which was just a small side module tacked on to serve as an airlock and provide a little extra space for personnel in cases like this. She drew her head back in to make space as Ivy and then Dinah cruised by her. As soon as those two were inside the Flivver, Tekla emerged and exchanged a nod with Ivy.

“Lamprey is in airlock and is functioning,” Tekla said, and closed the hatch. Dinah had some ambivalent feelings about Tekla, but there was no one she’d rather work with in a case like this. She was all business; she got the job done without useless conversation or touch-feely stuff. Dinah closed the Flivver’s hatch and began going through the undocking sequence while Ivy, strapped into the vehicle’s pilot seat, ran down the preexcursion checklist. Befitting a craft that had been designed in a hurry to be Flexible and Light, this wasn’t that lengthy, and so Flivver 3—one of a fleet of eight — was under way before Markus’s 704 seconds of silence had quite expired. Dinah strapped into a jump seat beside Ivy’s. The Flivver’s front end dome consisted largely of windows, bolstered by a sturdy web of curved aluminum struts, so from behind Ivy looked like a bombardier seated in the glass nose of a World War II bomber. She touched the controls and made the craft rotate in a way that caused Earth to pass beneath them, and then the resemblance became stronger. Dinah was reminded of a painting Rufus had shown her, depicting a bomber flying over a burning city, red light flooding into the plane from below. The same effect held now, save that the firestorm covered most of the surface of the Earth.

“I can feel the warmth on my face,” Ivy said.

Dinah couldn’t think of anything useful to say to that. During their passage from her shop to the Flivver she had forgotten about the fact that the Earth was burning, and she didn’t enjoy being reminded of it. Instead she tried to focus on the red light emanating coolly from the screen of her tablet, which was running Parambulator. Flivver 3 had been picked up on the swarm’s collective sensorium and identified as a bogey that might potentially collide with as many as a hundred different arklets if it stayed on its current course. Rather than controlling its thrusters directly, which would lead at best to confusion and at worse to a chain-reaction disaster, Ivy was negotiating a solution with the rest of the Cloud Ark, telling it where she wanted to go and finding a way of getting there that would minimize the amount of maneuvering demanded of all the others.

It was not a speedy way of getting around, and indeed ran at right angles to the fighter-jockey ethos of many of the ex-military types who had come up here in the astronaut and cosmonaut corps. But as they got farther away from Izzy they were able to move into orbits that caused minimal consternation to the rest of the cloud, and move in a more direct way to rendezvous with the wayward X-37.

This had been placed, by whoever the hell had launched it, in an orbit with the same period and plane as the Cloud Ark, but with somewhat greater eccentricity. The orbit of Izzy, and hence of the Cloud Ark, was almost perfectly circular. The X-37’s was more oval, meaning that about half of the time it was “beneath” the Cloud Ark and the rest of the time it was “above,” but twice during each ninety-three-minute orbit it crossed through, each time touching off the havoc that was wasting so much propellant and causing so much annoyance to Markus. Right now it was “above” and due to cross over in another twenty minutes.

“Any bolides we need to worry about before I focus on this?” Ivy asked her.

“Nothing in particular,” Dinah said, meaning that there was nothing so big as to force the entire Cloud Ark to make a course change.

“Let’s make this fast then,” Ivy said, and went over to manual control. For they were now far enough away from the Cloud Ark that she could execute solo maneuvers without making Parambulator screens turn solid red. “Can you scope it?”

Dinah spent a minute refamiliarizing herself with the user interface for the optical telescope mounted to the Flivver’s nose; this was an electronic eyeball about the size of an orange. The controls were intuitive, but getting it to aim at a particular bogey took a bit of doing. Soon enough, though, she was able to see something white and bright. She locked on to it and zoomed in.

From longer zoom it was clearly a winged craft with a black nose, like the Shuttle of old, but it seemed to have taken on added parts. Zooming in further she was able to see that the cargo bay doors that constituted most of the X-37’s “back” had been opened at some point after it had reached orbit. Its payload had then been lifted out of the bay using the built-in robotic arm, which was still holding it, frozen in position. The payload was almost as big as the X-37 itself; it was yet another dome-ended cylinder. But unlike a Flivver or an arklet, it lacked thrusters or any sort of visible power source. It was just a burnished aluminum capsule, gleaming white on one side from sunshine, red below where it reflected the planetary firestorm.

Ivy was looking at it too, dividing her attention between the Flivver’s status displays and the window running this optical feed. “Can you get more detail on the forward end? There’s a fitting there that might be a—”

“Yeah,” Dinah said, zooming in and panning to center it. “That’s a docking port all right.”

“Well, I guess we’re being invited to dock with it,” Ivy said.

“It’s weird. I don’t like it.”

“I agree,” Ivy said, “but we can’t come back later. That thing is tiny. Less than four feet in diameter. If there’s humans in there, they are running out of stuff to breathe.”

“Why would they send a human up in something like that?”

“It’s some plan that went awry. An email didn’t get answered, a transmission got garbled, now these people are marooned and probably waiting to die.” Ivy spoke brusquely, a little irked by Dinah’s questions.

Dinah heard thrusters pop and felt them nudging her around as Ivy maneuvered. She knew better than to distract her friend when her brain had gone into orbital mechanics mode. She unbuckled herself from the jump seat and moved to the docking port on the Flivver’s “top” surface, steadying herself by reaching out to grab the adjacent handles whenever Ivy effected a little course adjustment.

Within a few minutes Ivy had matched orbits, maneuvered the Flivver into the right attitude, and driven it straight onto the capsule’s docking port.

“Got a positive mate,” Dinah remarked. She activated a valve that flooded the little space between the Flivver’s hatch and the capsule’s with air. “Here goes nothing.”

She opened the Flivver’s hatch. She was now looking at the outside of the capsule’s hatch, which, until a few seconds earlier, had been exposed to space.

A strange detail: taped to the aluminum hatch was an ordinary sheet of 8½ x 11 inch North American printer paper. On this had been printed a color image: a yellow ring encircling a blue disk lined with stars. Spread-eagled on its center, an eagle with a red-and-white-striped shield. The printer that had spat this thing out had been low on cyan ink and so the image was strangely banded and discolored. Exposure to space hadn’t done it any favors either.

Even though the United States had only ceased to exist a few minutes earlier — declared extinct by Markus under the authority granted him by the Cloud Ark Constitution — this image already seemed as old and quaint to Dinah as a pilgrim or a musketeer.

She heard a mechanism activating on the other side of it.

“It’s aliiiive!” she called. Then, in spite of this effort at jocularity, she held her breath.

The hatch swung open to reveal a haggard, space-bloated, sickly green face, hair floating around it in disarray. But the eyes in that face were as cold and hard as ever, and they were fixed on Dinah.

“Dinah,” the woman said. It was her voice, more than her face, that Dinah recognized. “Even in these tragic circumstances, what a relief to see a familiar face.”

“Madam Pres—” Dinah began. Then she caught herself. “Julia.”

Julia Bliss Flaherty looked as if she didn’t appreciate one bit being addressed that way.

Ivy was using the thrusters quite a bit. Now that the Flivver, the capsule, and the X-37 were all joined together mechanically into a single object, it was possible — though awkward — to maneuver them into sync with the Cloud Ark and clean up all of that Parambulator red. There was some lurching. Julia was getting knocked around a little, learning she had to keep a grip on those handles. Random stuff, including some filled barf bags and a large number of what looked like red marbles, were careering around inside her tiny capsule. Looking through it during a moment when Julia had been flung to one side, Dinah saw a man floating in the far end of the capsule. He was bloody, and he was kind of floppy too. He was dressed in the remains of a navy-blue suit. He was not the ex — First Gentleman.

“I’m sorry for your loss,” Dinah said.

“Who the hell is that?” Ivy was shouting. “Markus wants to know if we have survivors.”

“My loss?” Julia asked.

“Your husband,” Dinah said.

“He took the pill,” Julia announced, “in the limo.”

“Oh my god.”

“I’ll need your help getting Mr. Starling squared away. He’s too big for me to move.”

“No, he isn’t,” Dinah said.

“I beg your pardon?” Julia said sharply.

“You’re in zero gee,” Dinah pointed out. “So he’s not too big for you to move. But I can still help you if you want.”

“If you would be so kind,” Julia said. She got a hand over the rim of the hatch while reaching out with the other for a shoulder bag, and looked expectantly at Dinah, who was still blocking her path.

Dinah looked at the back of Ivy’s head. “Julia Bliss Flaherty requests permission to come aboard.”

Julia let out a hiss of exasperation.

“Granted,” Ivy said.

“One casualty on the way too,” Dinah said, and cleared out of Julia’s way.

Julia launched herself through the hatch too hard, flew across the Flivver, and slammed into the far side of it elbow and shoulder first. “Augh!” she cried. But Dinah didn’t think she was hurt, and so she pushed through into the capsule. One of those red marbles was drifting toward her face and she reached out with a hand to brush it away before realizing that it was blood.

Pete Starling was suffering from a number of lacerations, as if he’d been in a stick fight or a car crash. He was groggy, and gagging on blood — probably from a broken nose — which he would cough out explosively when it got in the way of his breathing. Dinah grasped the lapel of his jacket, trying to find a usable handhold. When she pulled on it, the front of the coat came away from Starling’s chest for a moment, revealing an empty shoulder holster.

No matter now. She planted her feet, put her back into it, and got him stretched out in the middle of the capsule, head aimed toward the docking port, drifting slowly in that direction. She was looking to Julia to reach through and pull her companion through the hole. But Julia, banged up from her first attempt to move, was still flailing around, learning the basics of zero gee locomotion the hard way.

Dinah was at the back of the capsule, staring at Pete’s feet, which were kicking weakly. One of his feet was stocking clad; the other still wore an expensive-looking leather shoe. She grabbed a foot with each hand and tried to push him toward the docking port, but he reacted against it. He had no idea what was going on, didn’t understand that he was in space, didn’t like having his feet grabbed. She moved forward, got her waist between his knees, hugged him around the thighs, squeezing his legs together to either side of her body, and tried to get him re-aimed toward the port.

She heard a sharp pop and felt warm wet stuff all over her arms. More of it had splashed up her throat, all the way to the point of her chin. She smelled shit and heard a loud hissing noise. Pete Starling jerked once and then went limp.

She looked up toward the source of the hiss and saw starlight through a jagged hole in the skin of the capsule. The hole was about the size of a man’s thumb. Triangles of metal were bent back away from it.

On second thought, the hissing was coming from two places at once. Another hole had been punched in the other side of the capsule. Pete Starling’s body was between the two holes. The middle of his torso was just a rib-lined crater. Blood was hurtling out of it and accelerating through both holes.

Her ears had popped several times already.

She looked down the length of the capsule at Julia, who had finally gotten herself properly oriented and was looking into the hatch, wild eyed, utterly confused.

“Julia,” Dinah said, “we’ve been struck by a small bolide. We’re losing air, but not that fast. Pete’s dead. He’s in my way. If you could reach through and grab him by the collar and pull him toward you—”

The conversation, and her view of Julia’s face, was cut off by the Flivver’s hatch swinging shut.

ANY CURVE YOU COULD MAKE BY SLICING A CONE WITH A PLANE — A circle, an ellipse, a parabola, or a hyperbola — could be the shape of an orbit. For practical purposes, though, all orbits were ellipses. And most of the naturally occurring orbits in the solar system — those of the planets around the sun, or of moons around planets — were ellipses so round as to be indistinguishable, by the naked eye, from circles. This was not because nature especially favored circles. It was because highly elongated elliptical orbits tended not to last for very long. As a body in a highly eccentric orbit went rocketing in toward the central body and executed a hairpin turn at the periapsis — the point of closest approach — it was subject to tidal forces that could break it up. It might strike the central body’s atmosphere or, in the case of heliocentric orbits, come too close to the sun’s heat and suffer thermal damage. If it survived the plunge through periapsis, it would fly out on a long trajectory that would take it across the orbits of other bodies. After rounding the turn at apoapsis — the point of maximum distance — it would cycle back across the same set of orbits on its way back in toward the center. The solar system was sparse, and so the odds that it would strike, or come close to, any given planet or asteroid on any given circuit were small. But over astronomical spans of time, the likelihood of a close encounter or a collision was high. Collision would, of course, result in a meteorite strike on the planet and the destruction of the formerly orbiting body. A mere close encounter would perturb the body’s orbit into a new and different ellipse, or possibly into a hyperbola, which would eject it from the solar system altogether. The sun still maintained a stable of comets and asteroids in highly eccentric orbits, but their number dwindled over time, and they were rare events to astronomers. In its early aeons the solar system had been a much more chaotic place, with a wider range of orbits, but the processes mentioned had gradually swept it clean and, by a kind of natural selection, produced a system in which nearly everything was moving in an almost circular orbit.

What was true of the solar system as a whole had also been true of the Earth-moon system. The moon had circled the Earth in a nearly circular orbit. From time to time, a wandering stone from deep space would blunder in through a libration point and get captured into a geocentric orbit, but sooner or later it would hit the moon, hit the Earth, or be ejected by a close encounter with one of those bodies. Thus had the moon swept Earth’s skies for billions of years and protected it from most big meteor strikes, making it a suitable place for the development of complex ecosystems and civilizations.

All the rocks that made up the White Sky had once shared the moon’s orbit, and most of them, for the time being, remained at a safe distance of about four hundred thousand kilometers. Their orbits, for now, were of low eccentricity, meaning that they were nearly circular. However, the vast number of chaotic interactions within the White Sky had spawned a diversity of orbits. Some of those orbits were highly eccentric, meaning that their apogees might be far away, but their perigees were close to the Earth: close enough to get caught up in its atmosphere or to strike it directly. Any rock whose orbit was eccentric enough to come near the Earth could also come near Izzy. In general, rocks in such orbits were moving at about eleven thousand meters per second when they were that close to Earth. A bolide the size of a peppercorn, moving at that velocity, would have the same kinetic energy as a high-powered rifle bullet.

Of course, high-powered bullets were designed to strike things with great force and do damage in a predictable way, while moon rocks weren’t designed at all. So the results of collisions could be unpredictable.

What had probably happened in this case was that a rock closer in size to a chickpea, and packing the energy of several rifle bullets, had punched through the wall of the capsule but, in so doing, fractured into several pieces that had sprayed outward across the capsule’s volume in a narrow cone, striking Pete Starling’s body something like a shotgun blast but with much more total kinetic energy. Most of that energy had gone into his flesh and caused him to basically explode. The largest single piece of the original rock had kept going through his body, or perhaps missed him entirely, and punched its way out through the opposite side of the capsule.

If the rock had passed a couple of meters to either side, it would have missed them entirely and they wouldn’t even have known it was there. In the Earth’s atmosphere, of course, it would have been a different story. The rock would have dissolved in a bright streak, turning most of its kinetic energy into heat. The air in its immediate vicinity would have gotten warmer for a bit. Had it happened at night, keen observers might have seen a streak of light. When the same thing happened on a large scale, all over the Earth, the air became so hot that it glowed, as it was doing now.

In any case Dinah now found herself locked into a capsule, lit only by a few strips of white LEDs that were darkened by blood spatter, as the air leaked out of it. She had, of course, been drilling for events such as this one for a significant part of her life. One of the first things they taught you was that the air wasn’t really leaking out as quickly as you thought. Only so much air could get through a small hole. Nevertheless, plugging those holes was life-or-death. So Dinah’s first move, once she had recovered from surprise, was to shove Pete Starling’s remains up toward the larger of the two holes: the one through which the bolide had entered. With a wet sucking sound his bloody flesh sealed that hole. Her ears now enabled her to find the smaller exit hole, which was about the size of her pinkie. She slapped her bloody hand over it. The hiss stopped and she immediately felt a space hickey beginning to form where the Big Hoover was trying to pull her flesh out into the void. It hurt, but not that badly. She listened for a few moments until she was satisfied that there were no other hissing noises — no other leaks.

A bloody bandage floated past. She snatched it out of the air, peeled her hand away from the hole, and stuffed it in there. Some of it got sucked out into space, but then it formed into a wad that moved no further. The hole was still hissing, though, so she grabbed an empty plastic bag and shaped that over the irregular mound of wet gauze. The vacuum sucked it inward and created a nearly airtight seal.

A softer hiss, more of a whooshing noise, emanated from the “back” of the capsule. Dinah’s ears felt a change in pressure, but they didn’t pop — suggesting that the pressure had just increased. She knew nothing about this capsule, but she did know how simple life support systems worked, and she knew that they would likely contain a store of compressed oxygen that would be bled in to compensate for what was being turned into CO₂ in the occupants’ bodies and absorbed by the scrubbers. The mechanism was probably trying to compensate for the air that had just been voided into space, bringing the pressure back up to normal.

If that were the case, then it should now be possible to open the hatch on the Flivver. Dinah floated toward it, reached through the capsule’s open hatch, and rapped on the metal, leaving bloody knuckle prints.

Nothing happened for a moment, and so she rapped out SOS: three dots, three dashes, three dots.

The hatch opened to reveal Ivy’s face. “My. Goodness. Gracious,” she said.

“Thanks, sister,” Dinah said, and vaulted through as Ivy got out of the way — partly just to be accommodating but largely, Dinah assumed, to avoid getting smeared with the bodily fluids of Julia’s late science advisor. Julia herself was strapped into one of the jump seats, buckled over into a fetal position suffering from the dry heaves, and keeping an eye on Dinah out of the corner of her eye.

Welcome to space! was on the tip of Dinah’s tongue, but she managed to stifle it.

“While you were, uh, busy, we flew through the Cloud Ark again. We have about forty-five minutes now on its nadir side,” Ivy said.

“Should be enough,” Dinah said. She strapped herself into the other jump seat, wiped her hands on her thighs, and pulled her laptop close. Holding it down with the heels of her hands so it wouldn’t float away, she brought up the set of interface windows that she used to communicate with robots. Over the course of a few seconds, the laptop established communication with all the robots that were within range — which is to say, that were riding along on the outside of this Flivver.

Meanwhile she pulled down a folding arm with a mitten-like contraption on its end. This was the interface for the Flivver’s external robot arm.

“Pop the airlock for me, sweetie?” she said.

“Already done, hon,” Ivy returned.

In her peripheral vision she could see Julia’s eyes swiveling back and forth, reacting to this exchange. She tried to ignore Julia in spite of — perhaps because of — her weird talent for demanding attention, and focused on the video feed from the camera on the end of the robot arm.

The airlock’s round orifice grew larger as she reached toward it, revealing the device Tekla had stashed inside.

The Lamprey was a box with a blinking light on it. On the side facing the airlock door it sported a lug, or handle. With the hand on the robot arm, Dinah was able to grapple this easily and pull the device out into the light.

“Any reason not to just ’biner it onto the X-37’s arm?” she asked.

“Can’t think of any.”

“What is it you’re doing?” Julia asked.

“Deorbiting that piece of space junk before it kills someone.”

“That piece of space junk happens to be carrying the earthly remains of a brave man who gave his life in the name of—”

Dinah said, “Ivy, you want to take this or should I?”

“I’ll do it. You’re busy,” Ivy said. Dinah could hear her twisting around in the pilot’s seat to look at Julia. She spoke as follows: “Julia. Shut up. If you say another fucking word I’ll stave your fucking head in and put your corpse out the airlock. Nothing about this is acceptable. Starting with the fact that you are flapping your gums, posing a distraction to Dinah while she is carrying out a difficult mission-critical operation to protect the Cloud Ark. You just attempted to countermand a direct order from Markus, who is in charge of everything here under the PSAPS clause of the Cloud Ark Constitution. You are up here illegally. The Crater Lake Accord specifically barred the sending of national leaders to the Cloud Ark. You have violated that commitment and found a way to be launched up here anyhow, and judging from the looks of it there was no end of dirty dealing along the way. Your vehicle approached the Cloud Ark in a manner incompatible with our safety and security procedures, endangering the lives of everyone up here, and forcing arklets and Izzy itself to expend priceless and irreplaceable fuel to perform evasive maneuvers. We were sent here on an emergency basis, placing ourselves in harm’s way and expending more scarce resources to clean up the mess that you created by your cowardly and dishonorable act. For all of these reasons I am commanding you, by my authority as the commander of this vessel, to remain silent until we have docked safely at Izzy.”

“Very well,” Julia said.

Dinah looked up from her work to see Ivy and Julia glaring at each other.

“I’m sorry,” Julia said.

“You really are asking for it,” Dinah told her. And then she went back to work.

She had already accomplished much during Ivy’s soliloquy. The task at hand was to somehow attach the Lamprey to the X-37. The connection didn’t have to look good but it did have to be solid. Back in the days when every maneuver had been planned years in advance by NASA, this would have been a several-hours-long operation making use of custom-designed hardware. But lately the people of the Cloud Ark had been obliged to get good at lassoing random pieces of floating space junk, and so she ended up using a more highly evolved version of the trick that Rhys had come up with for reining in Tekla’s Luk. On that occasion, Dinah had fashioned a whip by chaining Siwis together. It had worked, but it was much heavier and more complicated than it needed to be. After the completion of T3 had left Rhys with some free time on his hands, he had begun tinkering with surplus Nats. Being old and obsolete, these were big, clunky, slow, and stupid compared to the new models — which was fine for Rhys’s purposes. He had turned them into a new kind of robot that he dubbed the Flynk, for flying link, and taught them to be really good at forming themselves up into chains and then doing the sorts of maneuvers in space that his great-great-great-great-uncle John, and Herr Professor Kucharski of Berlin, could only have dreamed about. There was much room for creativity here, but he had focused most of his efforts on problems that needed to be solved all the time.

Such as precisely the one Dinah needed to solve right now. The robot arm of the X-37 was sticking awkwardly out into space, an obvious target for grappling. A chain with a free end would whip around it easily, just as Rhys had once ensnared Dinah’s index finger with his necklace. All Dinah needed was a suitable chain. She happened to have one: a necklace of third-generation Flynks spiraled around the Flivver’s hull, ready for use. One end of it was already connected to the Lamprey. By invoking some computer code she was able to set the rest of it into motion, unwinding itself from around the Flivver and snaking out into free space, forming a U-shaped bend, or Knickstelle, that was aimed at the X-37’s robot arm.

“Ready to undock now,” she said.

Ivy had moved back to the port through which their guest had entered. “Undocking,” she said, and began running through the checklist that undocked the Flivver from the X-37.

Dinah meanwhile moved up to the pilot’s console and punched in a programmed series of thruster burns. As soon as Ivy confirmed separation, Dinah executed the program, effecting a small delta vee that made them back away from the X-37. The Knickstelle went into motion, as if the chain were passing around an invisible pulley, and began to propagate away from the Flivver and toward the X-37. Presently the chain’s end whipped around the robot arm and spiraled about it several times before grapplers on the Flynks found each other and engaged, lashing the chain into place for good.

Dinah released the Lamprey from the grip of the Flivver’s robot arm. The Flynk chain, still following a canned program, pulled the Lamprey in and made it fast to the X-37. The Flynk chain, the X-37, and the Lamprey were now a single object, and would remain thus until they were destroyed.

Dinah brought up the interface that controlled the Lamprey. This was a fire-and-forget device, but someone did have to fire it. She spun a control wheel that adjusted the box’s attitude, aiming its business end in a safe direction.

Getting things out of orbit was almost as complicated as launching them. Once a thing was in a legitimate, stable orbit, you couldn’t just drop it toward the Earth. It would stay in orbit indefinitely unless you slowed it down. Slowing it down generally meant using thrusters, which meant spending fuel. The Lamprey was a simple alternative.

“We’re undocked,” Ivy announced, moving back forward to the pilot’s chair. “Gonna nudge us free.”

A couple of pops from the thrusters signaled that they were gaining some distance from the X-37. Ivy spun the Flivver around so that they could see the X-37 perhaps a hundred meters away, floating upside down above the burning Earth, the elbow of its arm projecting toward the nadir, the Lamprey strapped to it and blinking.

“Okay, the Lamprey is giving me all green thingies. I see no red thingies. So I am activating it in three. . two. . one. . now.” Dinah tapped the Deorbit button.

Most of the Lamprey — the entire box — jumped away, headed toward Earth, propelled by white plumes of solid rocket exhaust. After a couple of seconds the motors burned themselves out and the box continued to coast away, unreeling a wire behind it. This came to a stop a minute later, dangling half a kilometer below the X-37, and pulled taut by tidal force.

“We have positive current flow in the tether,” Dinah reported. “So it’s working.” The wire, sweeping through Earth’s magnetic field on its orbit, was picking up a weak electrical current, creating a force that would slow the X-37 down. The effect was slight, but within a few hours the X-37’s orbit would decay to the point where it no longer posed a danger to the Cloud Ark, and in days or weeks it would descend into the atmosphere and be annihilated.

Twenty minutes remained before the Flivver’s orbit would next cross Izzy’s. But the physical separation was only a few tens of kilometers and they were still “on swarm,” meaning that the Flivver’s computer was talking to the Cloud Ark network and searching parameter space for the safest and most efficient way to reintegrate with it and to dock. That, plus the Lamprey’s success in moving the X-37 out of the way, ought to have cleared up most of the red that had been maculating Parambulator displays at the time of their departure. But when Dinah and Ivy turned their attention back to those screens, they looked worse than before. It was not immediately clear why. Parambulator was a beautiful thing from the standpoint of mathematics and data visualization, but there were times when you just wanted to know what the hell was happening. You wanted a narrative.

A text came through on Ivy’s phone. It was from Markus. She read it out loud. “Approach using visual observation and manual control,” it said. “Warning: collision debris.”

“Already?!” Dinah exclaimed. It wasn’t a good start if they’d already suffered a bolide strike a couple of hours into the Hard Rain.

“It was fratricide,” Ivy said, still reading. “Looks like an arklet got cornered.”

Getting cornered was a problem that had arisen in simulations. The swarm as a whole would look for solutions that would prevent arklets from banging into each other with minimum expenditure of propellant. In a pinch, of course, it was okay to burn a lot of propellant to avoid a collision. But there were situations where a collision was going to happen no matter what, and there was nothing to do for it but choose the least damaging outcome. Getting cornered wasn’t supposed to happen; everything about Parambulator was supposed to prevent it. But the number of possible scenarios was infinite and nothing was ever certain.

“A controlled collision,” Ivy said, “no fatalities. But then some follow-on. Still being evaluated. There might be loose debris drifting around. That’s why he wants me to fly it in manually.”

“What kind of debris?” Dinah asked. “Hard stuff or—”

“Thermal protection, looks like,” Ivy said. “So that’s good.”

Apparently one of the modules, or an arklet, had lost some of the layers of reflective foil and insulation that were used to shield it from the heat of the sun. The stuff was feather light and so probably didn’t pose much of a threat to the Flivver. But it would look huge on radar and make Parambulator go crazy.

Ivy, in the pilot’s chair, was monopolizing the only window. Dinah didn’t like flying blind, so she pulled up the interface for the Flivver’s eyeball camera.

Julia began to make a weird repetitive noise, a sort of wet, gurgling drone.

She was snoring.

“Long day for her, I guess,” Ivy remarked.

“Yeah.” Dinah had no precedents to tell her how she should feel toward the ex-president at a time like this. On the one hand, her behavior had been reprehensible. On the other, she had, within the last few hours, lost her husband, her daughter, her country, and her job.

With a few moments’ panning around, Dinah was able to center Izzy in the camera’s frame, then zoom in. Izzy was on the night side of the Earth just now. In normal times — or what used to be normal — it would have been dark, but now she was lit up from below by the red glow of the atmosphere, punctuated from time to time by bluish flashes, like lightning strikes, as large bolides plowed into the air three hundred kilometers below. Of course, Dinah had never seen Izzy so illuminated, and it took a bit of getting used to.

From a distance Izzy looked fine, but at higher magnification Dinah began to see visual noise that gradually resolved into drifting bits of debris — the shredded thermal protection that Ivy had mentioned.

Izzy had become unfathomably complicated in the last two years. Dinah rarely saw it from a distance, so she didn’t have a strong sense of what was normal. But the more she zoomed in, the more certain she became that something weird had happened on the nadir side, near the junction of Zvezda and Zarya.

Complicated though she might be, Izzy was complicated in a way that was orderly, stiff, and stable. The one exception to that rule was Amalthea, but even that had become more regular as the Mining Colony’s robots had reshaped it. What Dinah was zooming in on now was messy, and it was unstable: big expanses of thermal shielding material that had been torn loose and were now stirring randomly in the nearly imperceptible wind. At a glance, it did not look like a serious matter. “Serious” would have meant a hull breach, air erupting from a hole, perhaps dragging debris, or even human bodies, along with it.

“I’m thinking maybe a grazing impact at most,” Dinah reported. “A near miss between an arklet, or something, and the nadir side of Zvezda. Destroyed some thermal shielding but caused little if any structural damage.”

“They are reporting zero serious casualties,” Ivy said. “Some bumps and sprains aboard an arklet. So maybe you’re right.”

“Maybe,” Dinah said. For they had now drawn close enough that the camera could provide more detail. What had been exposed by the damage to the thermal shielding looked unfamiliar to her at first glance: a big T-shaped construct that jutted out to the nadir side of the Stack like a pair of handlebars. It was studded with many long neat rows of small, identical objects, gleaming in the occasional flashes from below.

Finally it all snapped into place in her head: she was looking at Moira’s thing. The HGA, the Human Genetic Archive. Moira had given her a tour once, but that had been from the inside, or enclosed and pressurized part of it. Now Dinah was seeing the same thing from the outside. Until now, this had always been concealed from view by the thermal shielding. Once that was torn away, its internal structure could be seen: the rows and rows of hexagonal sample racks, each carrying its load of deep-frozen sperm, ova, or embryos, waiting in the near-absolute-zero cold and dark of space.

“How has Moira been doing with the dispersal project?” Dinah asked, forcing her voice to sound relaxed.

“Well. . obviously, the schedule got compressed when we learned about the Eight Ball. Just like all of our other preparations did. But I guess my real answer is that I don’t know,” Ivy said.

Ymir

“. . AND THEN THE FORCE OF THE VACUUM CAUGHT HOLD OF THE hatch, and to my horror I saw it slam shut right in front of me! I tried to pull it back open, but the suction was too strong. I cannot tell you, Markus, how helpless and guilty I felt when I realized that Dinah was trapped on the other side.”

Markus’s eyes went to Ivy. He had been listening to Julia for a long time, and needed a break.

Ivy threw her hands up. “I was trying to fly this ungainly contraption. I didn’t really understand what was happening even when Julia tried to explain it to me.”

“Yes,” Markus said, “I can’t believe you were able to fly that thing at all. People will be talking about it a hundred years from now.”

Assuming people still exist, Dinah thought.

Ivy was just regarding Markus, blinking slowly, looking for signs that he was being sarcastic. He wasn’t. The Markus bluntness worked both ways: he could blurt out astonishingly generous compliments as easily as he could cut and burn you with his words.

“It sure used up all of my brain,” Ivy said.

They were sitting around the conference table in the Tank. Markus had not used the term “inquest” to describe this meeting, but that was clearly what it was. Or as close as they would ever get, in any case, to a formal determination of what had happened yesterday. It had gotten off to a reasonably brisk start with a summary from Markus, then gone off the rails as Julia had insisted on telling her story “from the beginning”—which turned out to mean from the moment she had woken up in the White House next to her late husband and gone down to breakfast with her late daughter, straight through to the end of the world, and her hastily arranged launch into orbit, some thirty-six hours later. Along the way had been a sequence of mishaps and coincidences just shaggy enough to be somewhat plausible. No liar could fabricate such a story. The narration had lasted for the better part of an hour despite Markus’s increasingly frequent and obvious glances at his Swiss watch, and left all the others in a strange combination of spellbound, bored, horrified, and bemused.

She seemed to believe that they would actually care about all of her interactions with those dead people on that dead planet. It was a common enough mistake among new arrivals. In her case it was magnified considerably by the fact that she was used to being the president. Everyone was always happy to sit and listen to the most powerful person in the world.

“Thank God,” Julia said, “that we were able to—”

“Yes,” Markus said, cutting her off. Plainly he did not wish to hear any more from Julia. But just as plainly he was a little reluctant to move on to the next part of the story.

Everyone seemed to be pointedly not looking at Moira.

“Thank you, Julia,” Markus said, in a tone that made it clear she was free to leave now.

Julia looked a bit startled. “But we haven’t heard from Dr. Crewe yet.”

“But we have heard from you,” Markus pointed out.

The point sank in. Julia didn’t like it. “Very well,” she said, standing up carefully. “As I mentioned before, Markus, I am eager to make myself useful in any way that I can.”

“It is so noted,” Markus said. He looked across the table, deadpan, at Ivy. Dinah knew what they were both thinking: You are worse than useless here — which is why you were never invited. “Thank you, Julia.”

The ex-president turned away from the table. She stopped before the door leading into the Farm and turned back toward Markus one last time with a sad-puppy look on her face, perhaps expecting him to just slap his thigh and laugh at the joke and warmly invite her back to her seat. When this failed to happen, a transformation came over her face that Dinah found mildly frightening to watch.

What would it be like, she wondered, to be nuking people one day, and, less than a week later, to be asked to leave a meeting? Evidently it did not put Julia in the best of moods. J.B.F. turned her back on them, as much to hide her face as to find the way out, and opened the door. During the few moments it was open, Dinah caught sight of a young woman in an Islamic-style face veil standing just outside of it, waiting. The bottom half of her face was covered, but her eyes brightened and her body language was warm as she saw Julia emerging. Julia reached out to her affectionately and laid a hand on the small of her back as she turned. The two of them walked away shoulder to shoulder as the door closed.

Remaining in the Tank were Markus, Dinah, Ivy, Moira, Salvatore Guodian, and Zhong Hu, an applied mathematician who was their head theorist when it came to swarm dynamics. Others knew more about orbital mechanics and rocket engines — the old-school techniques for managing individual space vehicles’ trajectories — but Hu, a specialist in complex systems, was the main architect of Parambulator, and the only person who could quite understand and explain what went wrong, or right, in a swarm. He’d spent most of his life in Beijing, but with enough time in Western universities to get along fine in English. In response to a nod from Markus, he said, “I have evaluated what happened. As we already know, there was a cornering event leading to a bump.” This being the polite term for a mild collision between arklets. “But still, Arklet 214 had enough control authority that it could have avoided the second event.”

“Then why didn’t it?” Markus asked.

“The algorithm predicted a near miss and so it took no action beyond routine attitude corrections. The human operator was distracted and disoriented and so was reluctant to correct course manually.”

“I can’t fault the human,” Markus said, “since we have warned them so many times about the consequences of flying by hand. But what went wrong with the algorithm?”

“Nothing went wrong with it,” Hu said. “It had bad data. I will show you.” With a few taps on his tablet he brought up a three-dimensional model of Izzy on the big screen above the conference table. To a first approximation, this seemed reasonably up to date; it depicted modules and space vehicles that had been added to the complex only within the last couple of days. “This is the model that the system was using yesterday for collision avoidance.”

Dragging his finger around on his tablet, he rotated the model on the screen so that they were looking at the nadir side. He zoomed in on the distinctive “handlebar” shape of the Human Genetic Archive: the pair of cold storage units projecting to port and starboard below the Zvezda module. The view was much the same as what Dinah had seen from the Flivver the day before.

“Hang on, is this the exact model? Is this everything?” Ivy asked.

“Yes,” Hu said.

“This doesn’t include the thermal protection,” she pointed out. “That adds at least a meter to the collision envelope.”

“That is correct,” Hu said. “In that sense, this model is obsolete. We have replaced it now with an upgraded version.”

As all understood, this was no one’s fault. The Arkitects had been struggling for almost two years to keep their three-dimensional model of Izzy up to date and accurate: a nearly impossible task when it changed every day. Soft goods like thermal protection blankets tended to get a lower priority. Humans, looking at the model, would mentally add those. Computers weren’t that smart.

“Still,” Markus said, “we take the model with a grain of salt. No arklet should ever pass that close.”

“Let me show you what happened,” Hu said, and brought up a video, shot from an external camera apparently mounted on one of the trusses.

The Human Genetic Archive and its surrounding blanket of thermal protection were not centered in the frame — they were down in the lower right corner. So the camera angle wasn’t ideal. But they could see what happened. The arklet approached, creeping in gradually from the port side with closing velocity no greater than a slow walk.

“Is this real time?” Sal asked.

“Yes. Because it was an extremely low-speed approach it was not viewed as terribly dangerous.”

“It looks like it’s going to be a near miss,” Sal said.

“It was — until this,” Hu said, and freeze-framed the video. It wasn’t easy to make out, but they could see a tiny flash on the forward halo of Arklet 214. “The thruster fires — a small course correction under automatic control.” He stepped it forward. The flash faded but expanded into a dim gray cloud. “Exhaust gases. Expanding rapidly but moving quite fast.” He stepped it forward several more frames until they could see the thermal protection blanket recoiling from the impact of the gas. A seam parted between two adjoining blankets and one of them flailed out like a rag caught in a wind gust.

Hu let the video run now, and they saw the arklet’s rear halo snag on the loose blanket and rip it away, exposing the Human Genetic Archive to the orange radiance of Earth’s atmosphere.

Ivy said, “If that thruster hadn’t fired at the wrong moment—”

Hu nodded. “Arklet 214 would have passed underneath with two meters to spare. Not a margin to be proud of. But it would have been enough.”

After a pause, Hu added, “The HGA’s thermal protection system could have been designed better.”

Another pause in which everyone else waited to see who would be the first to laugh. If it weren’t for dark humor, they’d have no humor at all.

Hu seemed to sense it. “What I mean is that it was engineered for normal thermal loads.”

“Meaning sunshine,” Dinah said.

“Yes. Not for radiant heat shining up from the atmosphere below it.”

“The same thing is true of many parts of Izzy, of course,” Markus said. “We are having thermal overloads all over the place now. Moira, what’s the damage?”

Dinah had to give Markus credit for a kind of finesse for the offhanded way he dropped the question in. Moira, who had been quiet through the whole meeting, took a moment to snap out of her reverie.

“Well,” she finally said, “as Hu said, the thermal protection system—”

“Was bad,” Markus said. “We know.”

“There was no backup system.”

Markus said, “Of course not. The cooling system for the HGA was the rest of the universe. We do not expect to have a backup system for the rest of the universe. We can rely upon it to be cold most of the time.”

“Because of the accelerated schedule, caused by the Eight Ball—”

“Stop,” Dinah said.

Everyone looked at her.

“Let’s get this over with,” she said. “Look. When I was fourteen, one of my dad’s mines collapsed and killed eleven employees. It was terrible. He never really got better. Of course, he wanted to know what had happened. It turned out to be a long story. One thing led to another, which led to another. . the individual steps all made sense, but no one could have seen the whole thing coming. Of course, he still felt responsible, but he wasn’t, in any normal sense of that word.

“So here’s what happened,” Dinah went on. “Sean Probst started an asteroid mining company that sent up a bunch of cubesats and gathered a lot of data about near-Earth asteroids, which he kept secret. He took the database with him on his mission to Greg’s Skeleton. His radio got hit by a rock and destroyed, so he couldn’t communicate. At the last minute, when it was basically too late, he had the idea to look at the database. He learned about the Eight Ball. He alerted me, I alerted Doob, Doob alerted everyone else, and we pushed up the schedule for everything. Moira pulled the trigger on the project, which had been planned for over a year, to disperse the HGA samples to the arklets. Like every other project in the history of the universe, it went slowly at first because all kinds of snags came up. And not only that, but all of the Flivvers were spoken for and all of the space suits were busy, because of the Splurge. So not much got moved. It was obviously safer to keep the samples in cold storage in the HGA while all of these logistical problems got sorted out. The Splurge happened and a lot of random shit got launched in our general direction and made Parambulator light up like crazy. Arklets were getting cornered on a pretty regular basis. We almost lost a couple. Ivy and I took off in the Flivver to fetch Julia and probably added a lot of other noise and chaos to that problem. Then, the thing we just watched happened. Arklet 214 tore most of the badly designed thermal protection system off of the HGA and exposed it to the direct radiance of the Earth’s atmosphere. The samples all warmed up before replacement thermal protection could be jury-rigged. All of those samples have been destroyed. Right, Moira?”

Moira, apparently not trusting her ability to speak, nodded.

“Okay,” Dinah said. “So I think that what Markus is really asking is how many of the samples in the HGA actually got moved to safe cold storage in other locations before this happened. In other words, how many survived?”

Moira cleared her throat and said, in a faint voice, “About three percent of them.”

“Okay. I only have one other question,” Markus said. “Have you talked to Doob?”

“I’m sure he suspects,” Moira said, “but I have not officially broken the news to him. I wanted to be absolutely sure first.”

“Are you sure now?”

“Yes.”

Markus nodded and spent a few moments thumbing something into his phone. “I am inviting him to join me and Moira here immediately,” he said.

Everyone who was not Markus or Moira stood up to leave. Markus held up his hand to stay them. “Before you go, let me say something about the Human Genetic Archive that was lost.”

He then paused for effect, until everyone was looking at him.

“It was always bullshit,” he said.

Everyone took a moment to consider it.

“Are you going to tell Doob that?” Ivy asked.

“Of course not,” Markus said, “but the real purpose of the HGA was politics on Old Earth.”

“Is that what we’re calling it now? Old Earth?” Sal asked, fascinated.

“That’s what I am calling it,” Markus said, “in the increasingly rare moments when I actually think about it.”

“Thank you, Markus,” Moira said.

HE HAD KNOWN, OF COURSE. IZZY’S COMPLEXITY WAS SO GREAT AS to belie its tiny size: a few hundred people sorted into a volume the size of a few jetliners. News traveled fast. Everyone had known within a few hours that the Human Genetic Archive had been almost completely destroyed.

He was in the Tank with Markus and Moira. They were gazing across the table at him, patiently awaiting some kind of reaction.

“Look,” he finally said, “Doc Dubois is no more. That was a persona, you understand? Just an act. I’m a private person. I do not spontaneously emote. Especially when people are watching me and expecting it. A year from now, when I’m alone, when I least expect it, I’ll break down in sobs over this. But not now. It’s not that I don’t feel. But my feelings are my own.”

“I am very sorry that it happened,” Moira said.

“Thanks,” Doob said, “but let me say what all of us are thinking. Seven billion people died yesterday. Compared to that, the loss of some genetic samples is nothing. The embryo that Amelia and I created together, and that I brought up here with me. . well, that was a special favor that J.B.F. granted me as an incentive to come up here. No one else got that kind of special treatment. It was unfair. I knew it. I accepted it anyway. So here we are.”

“Yes,” Markus said. “Here we are. Going forward—”

“But I’m not sure I agree with you,” Doob said, “that the HGA was so insignificant.”

Markus bridled his impatience and raised his eyebrows. Doob looked at Moira. “What was the term you used? Heterozygosity?”

“Yes,” Moira said. “The stated purpose of the HGA was to ensure a sufficiently diverse genetic basis for the human race.”

“Sounds important to me,” Doob said. “What am I missing?”

“We have tens of thousands of human genomes recorded in digital form. From all different parts of the world.”

“So there’s your heterozygosity. That’s what you’re saying,” Doob prompted her. “That’s why”—he glanced at Markus—“the HGA wasn’t really needed.”

“Yes, but there’s a but,” Moira said.

“Okay, what’s the but?”

“The digitized sequences, as I’m sure you’ll understand, are only useful so long as we have the equipment needed to transcribe them into functional chromosomes in viable human cells. By contrast, to make use of a sperm sample, all we need is a turkey baster and some lube. But to make use of a DNA sequence stored on a thumb drive, we need—”

“All of the equipment in your lab,” Doob said.

Moira looked a bit impatient. “What you are referring to as my lab bears the same relationship to a proper lab as some ones and zeroes on a thumb drive does to a living human. It is a collection of crated equipment that cannot even be unpacked and used in zero gravity. And even if we set it all up and turned it all on, it would be useless without a staff of Ph.D.-level molecular biologists.”

“Really? Useless?” Markus asked.

Moira sighed. “For small-scale work, one sample at a time, well, that is easier. But to reconstruct a genetically diverse human population—”

“But, Moira,” Markus said, “we cannot do that anyway until so many other things are in place. A large population cannot live in arklets eating algae. We need to establish a viable and safe colony first. Then, we build your lab. Then, we create a more diverse ecosystem: better food, greater stability. Only then do we even begin to worry about the heterozygosity of the human population. Until that time, we have more than enough people to create healthy non-inbred children just by the usual process of fucking each other.”

“That is all true,” Moira said.

“And that is the basis of my statement that the HGA was bullshit,” Markus concluded.

“You’re saying,” Doob said, “that if we had all of the prerequisites in place — the colony, the ecosystem, the talent — needed to actually exploit the HGA—”

“—we would no longer need it, yes, this is my point!” Markus said. “Can we please stop wasting time on it now?”

“How would you prefer to be spending time, Markus?” Moira asked, giving Markus an amused, owlish look through her glasses.

“Talking about how to get there. How to realize that situation we were just talking of.”

“And how might I contribute to that, given that the HGA is ninety-seven percent destroyed and none of my equipment will be usable for a long time?”

“I want to talk of preserving that equipment,” Markus said, “preserving it against all hazards, and then getting it to a safe situation where we can one day construct this laboratory you speak of.”

“It’s about as safe as we can make it, isn’t that so?” Moira asked. “It was given a sort of privileged position off of Node X — quite close to Amalthea. It’s not living dangerously, the way we are at the moment.”

She was referring to the notion, frequently discussed by Arkitects, of the Cone of Protection that supposedly existed in the lee of Amalthea. To the extent that the paths of incoming bolides were predictable, Amalthea could be pointed into them and used as a sort of battering ram. The forward surface of the asteroid would take a beating — but a solid slug of ancient nickel and iron could survive quite a lot. Anything situated up against its aft surface would be sheltered against virtually all hazards. But the protected zone did not, of course, stretch back infinitely far. The farther you lagged behind Amalthea, the more likely you were to get hit by a bolide coming in from an off angle. The Mining Colony was in the safest position, since, by its nature, it had to be right up against the asteroid. Almost as safe was the cluster of modules connected to Node X, immediately aft of the SCRUM, which was where all of Moira’s gear had been stashed. Behind that, the protected zone narrowed, a long acute cone, finally disappearing altogether somewhere aft of the Caboose. When Moira joked about “living dangerously” she referred to the fact that T3, the third torus, in which they were sitting now, was rather wide and rather far aft, placing it close to the limits of that cone. Efforts had been made to beef up its shielding, but it was still at higher risk than many other parts of Izzy.

Markus nodded. “Your stuff is pretty safe. But it would be safer if we moved it inside of Amalthea. I have talked to Dinah about it. She says that they could mine out cavities and store things of great importance there.”

A silence while Doob and Moira pondered it.

On one level, Markus’s proposal was perfectly obvious. Of course anything would be safer inside of a huge metal asteroid.

On another level, it had ramifications.

As of a few days ago — pre — White Sky, the last time anyone had been able to think straight — the fate of Amalthea and the Mining Colony had still been subject to debate. Was the asteroid the boulder in the wheelbarrow that had to be dumped? Or was it the aegis that would shelter the entire human race? The argument had come down to statistics. They just didn’t have enough data to make a decision.

By suggesting that Moira’s equipment be moved into the interior of Amalthea, Markus seemed to be committing to a specific course of action.

It was a course that Doob instinctively agreed with. But it was a bit strange for a man like Markus to just decide on a course of action before the numbers were in.

Or did he know something Doob didn’t?

Moira, in any case, went first. “What if we Dump and Run?”

She was referring to a gambit, frequently discussed and war-gamed, in which Amalthea would be cut loose and abandoned, and Izzy, lightweight but unprotected, would boost herself to a higher orbit with fewer bolides flying around in it.

“Then we would simply have to move all of that stuff back to Node X first,” Markus said. “Or wherever we felt was safest.”

This elicited a searching look from Moira. Markus held up his hands. “But I take your point. I am increasingly biased against Dump and Run.”

“You know how I feel about the Swarmamentalists,” Moira said.

She was referring to another of the basic gambits, Pure Swarm, in which everything — presumably including Moira’s lab — would be distributed among arklets, which would then collectively move to higher orbit. People and goods would move among them through a decentralized market-based economy.

“Listen,” Markus said, “now that everyone below is dead, and we don’t have to put up so much with bullshit, you will find that Hu and the others have a more nuanced view than they were letting on before.” He referred to the fact that Zhong Hu, as the foremost swarm theorist and the brains behind Parambulator, was assumed to be a Swarmamentalist.

Doob nodded. It still took some effort to remind himself that the millions of Internet commentators arguing for this or that strategy were all ghosts now.

“You know something,” Doob blurted out. Then, as the thought was coming into his head, he added, “From Dinah. The radio.”

“Yes,” Markus said. “Ymir is coming in hot, high, and heavy.” He surrounded those three words with air quotes.

“What does that mean?” Moira asked. “She’s made of ice, how can she be hot?”

“She is approaching with a high closing velocity. Not unmanageable. But. . somewhat exciting.”

“And ‘high’?” Doob prompted him.

“Sean also transmitted his params,” Markus said. “It would seem that he did us a large favor. He executed the plane change while it was still easy to do so, way out around L1.”

“So when he says he’s coming in high,” Doob said, “he means that Ymir has a high orbital inclination — close to ours?”

“Very close to ours,” Markus confirmed. “He is dropping this big chunk of ice into our lap.”

“So,” Moira said, “on top of everything else, Sean Probst is now preparing to dive-bomb us with a comet?”

“A piece of one.”

“A big piece,” Doob guessed, “if he specified ‘heavy.’”

“The number was impressive.” As Markus said this, he shifted toward Doob and looked him in the eye.

“Oh wow,” Doob said. “Is it enough for the Big Ride?”

“If we can get Ymir to rendezvous with Izzy, then yes,” Markus said. “It is more than enough.”

The Big Ride was the third of the basic options. It meant to boost Izzy in its entirety — Amalthea and all — to a much higher orbit. It had been considered implausible because of the amount of propellant that would be needed. Not just implausible but — absent the timely return of Ymir—physically impossible. Despairing of Sean’s chances, its supporters had lately tended to suggest scaled-down variants, such as reshaping a small percentage of Amalthea into bolide deflectors and ditching most of its mass.

“Including the plane change?” Doob asked.

A trace of a smile came onto Markus’s face. He knew exactly what Doob was thinking. For, unable to get Cleft out of his head, Doob had shown pictures of his favorite piece of the moon to Markus, to Konrad, to Ulrika and Ivy and some of the others who seemed to make up the informal power structure of the Cloud Ark.

“Let me be clear,” Markus said. “When I speak of the Big Ride, I mean it for real. We take all of Amalthea with us. We raise the orbit to the moon’s. We change the plane. We circularize. And we end up safe and sound in Cleft.”

“And Ymir carries enough water for that mission?”

“Yes,” Markus said, “if we can control her and bring her in.”

“Isn’t that Sean Probst’s job?” Moira asked.

“Not anymore,” Markus said. “The information I just imparted to you was in Sean’s final transmission.”

Moira and Doob looked at him sharply.

“The health situation has been not so good, for a long time,” Markus explained. “Sean was the last member of the expedition to die.”

“Are you saying that Ymir is a ghost ship?!” Doob asked.

“Yes.”

“And there’s no way to remote-control her,” Moira guessed.

“Unfortunately Dinah’s Morse code cannot help us in that regard,” Markus agreed.

“So someone has to go and—”

“Someone has to go and land on that fucking big piece of ice,” Markus said, “and get inside of Ymir and restart the nuclear reactor and commit the final burns that will bring her into sync with Izzy.”

“Who the hell—” Doob began, but Markus cut him off by pointing to himself. He did this in a somewhat awkward fashion that, deliberately or not, looked like a pantomime of suicide by handgun. He said, “I am placing Ivy in command of Izzy and the Cloud Ark tomorrow. I am assembling a crew that will depart in a MIV and make a rendezvous with Ymir. We will board her and manually execute the procedures needed to bring her under control and get her payload to Izzy. We will then use what is left of the ice to raise Izzy’s orbit — and we will bring Amalthea with us on the Big Ride.”

“That’s. . major,” Moira said. “Who knows? When were you going to announce it?”

“I just decided it now.” Markus sighed. “Listen, it is the only way. In my heart I always considered Dump and Run and Pure Swarm both to be too risky. What happened with the HGA just makes this more obvious. The only wise course is the Big Ride. It will take a long time — two years or something. But during all that time the most important resources can be sheltered within Amalthea. And by that I mean you and your equipment, Moira. You can have whatever resources you need from the Mining Colony to create a safe location for the genetics lab.”

“Okay,” Moira said, “I’ll talk to Dinah.”

“Talk to whomever she delegates,” Markus said. “Dinah is going to have to come with me on the expedition. I need her to deal with all of those verdammt robots.”

“How can I help?” Doob asked. He wondered if Markus might dragoon him as well, and was torn between being afraid of that and tremendously excited.

“Figure out how we are going to do it,” Markus said, after considering it for a few moments. “Lay in a course for Cleft.”

“Yes,” Doob said. “I’ll do that.” The little boy in him was crestfallen that he wasn’t going on the adventure. Then he reminded himself that he was already part of the biggest adventure ever, and that, so far, it had been altogether miserable.

ALL CONVERSATIONS WORTH HAVING ABOUT SPACE VOYAGES WERE couched in terms of “delta vee,” meaning the increase or decrease in velocity that had to be imparted to a vehicle en route. For, in a common bit of mathematical shorthand, the Greek letter delta (Δ) was used to mean “the amount of change in. .” and V was the obvious abbreviation for velocity. The words “delta vee,” then, were what you heard when engineers read those symbols aloud.

Since velocity was measured in meters per second, so was delta vee. The delta vees bandied about in spaceflight discussions tended to be large by the standards of what Markus was now calling Old Earth. The speed of sound, for example — a.k.a. Mach 1—was three hundred and some meters per second, and most earthbound people would consider it awfully damned fast. But it hardly rose to the notice of most people who talked about space missions.

A common delta vee benchmark had been the amount needed to get something from an Old Earth launch pad to an orbit like Izzy’s. This was some 7,660 meters per second, or more than twenty-two times the speed of sound: an impossible figure for any object that had to fight its way through an atmosphere. Once a vehicle had reached the vacuum of space, though, things became simpler: rocket engines worked more efficiently, drag and aerodynamic buffeting were absent, and the consequences of failure weren’t invariably catastrophic. Getting it from point A to point B was a matter of hitting it with the right delta vee at the right time.

Sean Probst’s delta vee history, from his departure from Earth until his departure from life, had gone something like this. The launch from terra firma to Izzy on Day 68 had required a delta vee of 7,660 m/s according to a naive calculation; but as any old space hand would know, losses due to atmospheric friction and the need to push back against gravity would have elevated the practical number to more like 8,500 or 9,000.

Once he had collected Larz and most of Dinah’s robots, Sean had needed to execute a plane-change maneuver to get from the Izzy orbit — which was angled at about fifty-six degrees to the equator — to the equatorial orbit in which Ymir was being assembled. This was one of those circumstances in which human intuition got it all wrong. The Izzy orbit and the Ymir orbit did not seem all that different in most respects. Both of them were a few hundred kilometers above the atmosphere. Both were essentially circular (as opposed to elliptical). And both went in the same direction around the Earth. The only real difference between them was that they were at different angles. And yet the delta vee required to get from one to the other was large enough that it had been necessary to launch a separate rocket, carrying nothing but extra propellant, just to refuel Sean’s vehicle in preparation for the plane-change burn.

Once Ymir had been assembled, a delta vee of some 3,200 m/s had been needed to place her in a very elongated elliptical orbit that had taken her out to L1. En route, the plane-change problem had once again reared its head. Essentially everything in the solar system, including Comet Grigg-Skjellerup, was confined to a flat disk centered on the sun. The imaginary plane through that disk was called the ecliptic. Conveniently for people who liked seasons, but not so good for interplanetary travelers, Earth’s axis and equator were angled with respect to the ecliptic by 23.5 degrees, and so Ymir’s initial orbit had been off-kilter by that amount. Fortunately, plane-change maneuvers were much less “expensive” (meaning they required a lot less delta vee) when they were performed far away; and Ymir was, of course, going very far away. So, they had done the plane change out at L1 range, as part of the same burn, totaling some 2,000 m/s, that took her out through the L1 gate into heliocentric orbit.

That orbit, more than a year later, had intersected that of Comet Grigg-Skjellerup. As Ymir had drawn near to the comet core, she had used another 2,000 m/s of delta vee to sync her orbit with its.

All of these maneuvers, up to the arrival at Grigg-Skjellerup, had been achieved by using Ymir’s rocket engines, which were altogether conventional: they burned propellants (fuel and oxidizer) in a chamber, making hot gas, which was vented out of a nozzle to produce thrust. The final burn had emptied her propellant tanks, so this was a one-way journey unless the nuclear propulsion system could then be turned on.

No engine had ever been made that was capable of pushing a comet core around the solar system at any appreciable speed. For that, they had needed to embed the nuke-on-a-stick into the heart of the ice payload, construct an ice nozzle behind it, and then pull out the control blades, causing the reactor’s sixteen hundred fuel rods to become very hot. Ice turned to water, then steam, which shot out the nozzle and produced an amount of thrust actually capable of making a difference. So a few months had then been consumed disassembling Ymir and integrating its parts into a chunk of ice carved off the three-kilometer ball.

The question might have been asked: Why just a piece of it? Why not bring the whole comet core back, if water was so desirable? What was the point of sending a large nuclear reactor into space if you weren’t going to use it? And the answer lay in the fact that even a large nuclear reactor did not even come close to having enough power to move such a big piece of ice. The mission would have lasted more than a century, assuming the existence of some kind of a miracle reactor that could operate at full power for that long. In order to get this done in any reasonable amount of time, they could only bring back the bare minimum of ice needed to rendezvous with Izzy and accomplish the Big Ride.

In any case, Sean and his surviving band had used the nuclear engine to impart a delta vee of about 1,000 m/s to the shard they had carved off Greg’s Skeleton, thereby placing it into a somewhat different orbit that had, a few months later, glided into L1. Sean had remained alive just long enough to yank out the control blades one last time and execute a delta vee that had basically reversed the maneuver they’d used to leave the L1 gate almost two years earlier. This had simultaneously brought Ymir into geocentric orbit while executing, as cheaply as possible, the plane change needed to enable a later rendezvous with Izzy. A couple of days later Sean had tapped out the “coming in hot, high, and heavy” message and dropped dead. Of what, they could only conjecture.

The retrieval team that was now being organized by Markus was going to use a MIV, or Modular Improvised Vehicle, assembled from a kit of parts: a sort of Lego set for the construction of spaceships, neatly sorted on a stack of modules, collectively known as the Shipyard, connected to the Caboose.

The Shipyard was a generally T-shaped contraption. One arm of the T’s crossbar, projecting from the port side of the Caboose, was studded with MIV parts. The opposite arm was a cluster of spherical tanks surrounding a collection of splitters. These used electrical power to split water molecules into hydrogen and oxygen, and piped them to chillers, which refrigerated the gases until they became cryogenic liquids that could be stored in the bulging tanks.

So much for the T’s crossbar. Its long vertical stroke was a truss terminated by a nuclear reactor: not a small RTG like the ones on the arklets, but a true reactor, originally designed to power a submarine, considerably souped up for this task.

Markus dubbed the Shipyard’s first product New Caird, after a small boat that had been used in Shackleton’s expedition to Antarctica. She was assembled and made ready for use in ten days: about one-third of the time they estimated it would take for Ymir to arc in from L1 and make her closest pass to Earth.

To design, assemble, and test such a vehicle so quickly would have been unthinkable two years ago. During the interval between Zero and the White Sky, however, the engineering staffs of several earthbound space agencies and private space companies had foreseen the future need to jury-rig space vehicles from standard parts such as arklet hulls and existing rocket engines, and had provided a kit of parts, lists of procedures, and some basic designs that could be adapted to serve particular needs. In effect, New Caird had been designed a year ago by a large team of engineers on the ground, all but three of whom were now dead. Those three had been sent up to join the General Population. Building on their predecessors’ work, they were able to produce a general design — enough to begin pulling the bits together, anyway — within a few hours of Markus’s decision. Details emerged from their CAD systems as they were needed over the following week and a half, and the necessary parts and modules were shuttled about the Shipyard until the new vehicle was ready.

New Caird would have to execute one burn to reach an orbit that would intersect Ymir’s and another to match her velocity, so that the crew could board the ghost ship and take the helm. The total “mission delta vee” for that journey, from its departure from the docking port on Izzy to its arrival at a similar docking port on Ymir, was some 8,000 meters per second.

The conversation turned now to mass ratio: a figure second only to delta vee in its importance to space mission planning. It simply meant how much propellant the vehicle needed at the start of the journey in order to effect all the required delta vees.

Laypersons tended to substitute “fuel” or “gas” for “propellant,” making the obvious analogy to the stuff that had been burned by the engines of cars and airplanes. It wasn’t a bad analogy, but it was incomplete. In addition to fuel, most rocket engines needed some kind of oxygen-rich chemical (ideally, just pure oxygen) with which to burn it. Cars and planes had simply used air. Rockets stored the oxidizer in a separate tank from the fuel until the moment of use. The two chemicals were collectively referred to as “propellant,” and their combined weight and volume tended to dominate space vehicle design in a way that hadn’t been true of, say, automobiles, whose gas tanks had been small compared to their overall size.

A convenient figure for characterizing that was the mass ratio, which was how much the vehicle weighed at the beginning (including the propellant) divided by how much it weighed at the end, when all the tanks had been emptied. If you knew how good the engine was, and how much delta vee you needed, then the mass ratio could be calculated using a simple formula named after the Russian scientist Tsiolkovskii, who was credited with having worked it out. It was an exponential: a fact that explained almost everything about the economics and technology of spaceflight. For if you found yourself on the wrong side of that exponential equation, you were completely screwed.

When the relevant numbers for the Ymir retrieval mission were jacked into the Tsiolkovskii equation, the result was a mass ratio of about seven, meaning that for every kilogram of stuff — Markus, Dinah, other personnel, miscellaneous robots, etc. — that they wanted to arrive safely at the docking port of Ymir, they needed to allow for six kilograms of propellant at the moment of departure from Izzy. This wasn’t all that difficult to achieve, especially for a vehicle that would never be exposed to the rigors of passage through the atmosphere.

The payload in this case was a single arklet hull that had been augmented with a “side” door: an airlock that could accommodate one person in a space suit. Other than that, it had been stripped to the minimum complement of equipment needed to keep a crew of four alive for a few days. To its mass, of course, needed to be added that of the actual humans and their food and other essentials. The lightness of a bare arklet hull was startling; the newer hulls, made of overwrapped composites, weighed in at eighty kilograms. Stripped of everything that made it comfortable and inhabitable over the long term, and including the “side door,” the maneuvering thrusters, and a reasonable supply of thruster propellant, the mass of New Caird was about ten times that. The humans weighed three hundred kilograms. The rocket motor that would be doing all the important burns weighed another two thousand. So, in round numbers, the payload mass — the stuff that actually had to get delivered to the docking port of Ymir—was some thirty-five hundred kilograms. The mass ratio of seven meant that its propellant load, at the beginning, was going to be some twenty-one thousand kilograms of liquid hydrogen and liquid oxygen.

The Shipyard had been stocked with several cryogenic propellant tanks of various sizes, some designed to hold LH2 (liquid hydrogen) and others built to the somewhat different specifications needed in the case of LOX (liquid oxygen). The chosen tanks were bolted together in a stack with the rocket engine mounted “below” and thermal protection wrapped all about. New Caird proper — the arklet with the humans in it — projected forward on a scrap of scaffolding just long enough that her maneuvering thrusters wouldn’t damage any of the other parts when they came on.

While the MIV was being constructed, twenty-one thousand kilograms of water had to be split into hydrogen and oxygen, chilled to cryogenic temperatures, and stored. The Shipyard’s port side already had some LH2 and LOX premade. In general, though, they tended not to keep a lot of them on hand, because they were tiresome substances to work with. The demand was supplied by the naval reactor on the Shipyard’s long arm, which was brought up to full power for the first time since it had been launched, piece by heavy piece, from Cape Canaveral on a series of heavy-lift rockets. Pumping juice down heavy cables to the splitters, it was able to turn twenty-one tons of water into gases and chill the gases to cryogenic temperatures while the other preparations were being made.

This was a lot of water — roughly fourteen liters of it for every surviving human. The Cloud Ark recycled water, of course, and was far from running out of the stuff. Nonetheless, the idea of taking that much of it and spewing it into outer space, never to be recovered, gave many people pause: especially the Dump and Run partisans.

There was a strong counterargument, which was that New Caird’s objective was to take possession and control over a piece of frozen water that weighed as much as Izzy herself, including the giant piece of iron to which Izzy was attached (and would continue to be, if the Big Ride advocates had their way).

Once New Caird had reached her, Ymir could presumably be slowed down, and brought to a rendezvous with Izzy, by firing her engine. And that was a primitive beast, but it had a basically infinite supply of energy in the nuclear reactor, and a vast stock of propellant in the form of ice. The “steampunk” propulsion system had much lower efficiency, however, than a properly engineered rocket motor. Consequently, the mass ratio that would be needed to slow Ymir down from the high-speed elliptical orbit with which it was falling into Earth’s gravity well, to match the much slower, circular orbit of Izzy, was about thirty-four, which meant that 97 percent of the ice currently attached to Ymir was going to be melted, turned into steam, and jetted out its makeshift nozzle just to slow it down. The remaining 3 percent, however, would still weigh as much as Izzy and Amalthea put together. Split into hydrogen and oxygen, it would supply the rocket fuel needed to power the Big Ride, all the way up to Cleft.

“I DIDN’T EXPECT IT TO BE BLACK,” DINAH SAID. SHE WAS HEARING HER own voice as if down a mile-long sewer pipe. She was pretty sure she had lost consciousness a minute ago. Maybe she wasn’t all the way back yet.

Markus was slow in responding. Maybe he had blacked out too. Maybe he was just distracted. “Comet cores are covered in—”

“Stinky black stuff, yeah, I know that, Markus. Remember who I am?”

“Sorry. Not enough blood in brain.”

“But this is just a shard that Sean broke off of Grigg-Skjellerup. Why’s it all covered?”

“I don’t know,” Markus said.

They were looking at Ymir from a distance of ten kilometers and closing. They were viewing her on their tablets, through a zoomed-in video camera. Vyacheslav Dubsky, floating closest to New Caird’s forward end, put his face to the vessel’s tiny window and searched the black sky for the black ship, but the squint on his face suggested it was still too far away for naked eyes to be of much use.

“Maybe he was doing us a favor,” Dinah said. “The black stuff has all kinds of goodies on it. Carbon, obviously. But also nitrogen, potassium—”

“Micronutrients,” Markus said, “that the Cloud Ark will be needing.”

“So maybe he used the robots to scrape some of it off Greg’s Skeleton, and loaded up on the gunk,” Dinah speculated.

“We will know soon,” Vyacheslav said. “Presumably he left a document.”

“Which we will not be alive to read, unless we stick the landing,” Markus pointed out, “so no more chatter from now on, please. Slava—” and he broke into a string of bad Russian meaning something like I trade places with you now. Vyacheslav responded in equally bad German. Both men were perfectly fluent in English. But they made a private joke of butchering each other’s languages, ostensibly as part of a project to preserve Old Earth’s linguistic heritage. Markus then added, “The rest of you, buckle up.”

With the deft movements of one who had been in space for two years, Vyacheslav glided aft. He was one of the veteran Russian spacewalkers who had come up to Izzy way back on A+0.17, in the first launch after the moon had blown up. He had been a mainstay of the Scout and Pioneer eras, racking up more time in a suit than anyone, wearing out three Orlans. He was a little worn out too, being sallow and gaunt compared to the strapping hero who had emerged two years ago from the same Soyuz that had carried Rhys and Bolor-Erdene. Markus replaced him at the forward window and buckled himself into the pilot’s seat.

Behind that was a row of three acceleration couches, mounted on a frame that spanned a diameter of the arklet’s hull. Dinah was loosely belted in on the port side. A few minutes ago she had been tightly belted in. She had not adjusted the straps. The entire couch, and its supporting truss, had been deformed by the same burst of gee forces that had left her so woozy. To starboard was Jiro Suzuki, a nuclear engineer who had been involved with the design of Ymir’s nuclear reactor core. It wasn’t clear whether he was conscious; but then it never was with Jiro. Vyacheslav, the fourth member of New Caird’s complement, settled into the middle position and pulled the top straps of the five-point harness over his shoulders.

A staccato burst sounded from the gamma spectrometer — the modern equivalent of a Geiger counter — floating in front of Jiro’s face. Then the Eenspektor, as they called this device in butchered Russian, dropped back into the normal sporadic patter.

Radiation was striking the Eenspektor — and their bodies — all the time, at random moments and in no particular pattern. Sometimes there would be a little burst, and that part of the mind that liked to see meaning in everything would identify it as an event. But then it would die away and be forgotten. That was just the way of the universe, and of the human psyche. There was a lot more radiation in space than there had been down on the ground, but all the survivors had long since come to terms with that, and Jiro had dialed down the sensitivity on his Eenspektor so that it wasn’t screaming at them all the time.

If it started screaming in the next few minutes, it wouldn’t be because of some faraway cosmic event. It would be because of a radiation leak from Ymir.

“Starting to see the exhaust trail,” Markus commented. “Can you see it on video? It’s faint. The sunlight hits it perfectly a few hundred meters aft of the nozzle bell.”

He was referring to a thread of steam that emerged from Ymir all the time, even when her engine wasn’t powered up. This was how Konrad and Doob and the other astronomers aboard Izzy had been able to track the ship’s course using their optical telescopes, and to verify that the params encoded in Sean’s last transmission had been accurate. Wispy as it was, the steam trail reflected more light than did the ship itself.

It was created by the slow, steady boiling of ice caused by the latent radioactivity in the ship’s fuel rods. When the control blades were pulled out and the reactor operated full blast — which was almost never — it produced four gigawatts of thermal power by splitting uranium and plutonium into smaller nuclei, many of which were themselves unstable isotopes. As these fission fragments decayed into “daughters” and “granddaughters,” heat continued to be generated even when the reactor had been shut down. There was nothing that could stop it, so some loss of ice, in the form of this tenuous trail of steam, was unavoidable. It was okay. Ymir had plenty more where that came from, and Sean would have allowed for it in his calculations.

Sean, never the most emotionally sharing kind of guy, further throttled by his makeshift radio, had not supplied details as to what had killed him and his crew. Had it been some kind of disastrous problem with the reactor core, he probably would have given them a heads-up. For that matter, Ymir wouldn’t have made it this far if the system hadn’t been basically working. So Jiro was not coming into this thing expecting a total nightmare. But there was no telling.

No one spoke for several minutes as Markus monitored their approach and occasionally touched the controls, spanking New Caird into a slightly different course.

They had gotten here by means of two large burns. The first, and smaller, had placed them into an ellipse that had shot beyond the orbit of the former moon. After several days of weightless coasting away from the Earth, they had succumbed to the force of gravity, looped lazily around, and begun to fall back again toward the burning planet. This had been timed in such a way that, about a day later, they would be overtaken by Ymir, coming in on a roughly parallel track. But Ymir was traveling much faster — coming in hot, as Sean had told them — basically because it had been falling in toward the Earth from an extremely high starting place, gathering speed relentlessly for weeks. Left unmolested, Ymir would come screaming in toward Earth with a relative velocity of some twelve thousand meters per second, make a hairpin turn just a few kilometers shy of a catastrophic encounter with the glowing atmosphere, then go hurtling back outward again, not to return for a couple of months. Eventually her orbit would decay to the point where she would get dragged down by the atmosphere and destroyed.

At any rate she would have flashed by New Caird too quickly even to be seen, her relative velocity faster than that of a rifle bullet, had New Caird not just matched her velocity by making a long, precisely timed burn of her main engine. The four members of the crew were still recovering from this. The vehicle’s main engine was oversized — the kit of MIV parts from which she’d been built only had so many options — and so the gee forces had been impressive at the beginning of the burn and brutal toward the end, as the lavish expenditure of propellant had made her lighter and lighter compared to the engine’s formidable thrust. If Dinah had blacked out for a few seconds, why, maybe that was just as well, given that they had aimed themselves almost squarely at the Earth and then launched themselves at it as if on a suicide run. This was necessary in order to go where Ymir was going, but added up to maybe a little more excitement than she really was in the mood for at this moment in her life.

Earth was, of course, completely unrecognizable. From this distance it was about the size of a tangerine held at arm’s length, and about the same color. Formerly a cool blue-and-white lake in the cosmos, it now hung there like a blob of molten steel thrown out by a welder’s torch. In the belt between the tropics, where most of the Hard Rain was falling, it glowed orange. The color faded and reddened to a kind of sullen brown around the poles, and the whole planet continually sparkled with the bluish light of vaporizing and exploding bolides. In a few days it would blot out half of the sky for a hectic few minutes while they slingshotted around it. By that time, they needed to have Ymir’s main propulsion up and running so that they could execute the huge braking burn that would slow her down to the same velocity as Izzy.

It was crazy. It was a crazy plan. The crushingly high acceleration that they had survived at the end of the big burn a few minutes ago was a physical reminder that they had only taken enough propellant to sync New Caird with Ymir. If they failed in their basic mission — if they couldn’t dock with Ymir and get her engine working — they had no way of getting back to Izzy, save perhaps by the utterly insane measure of diving into Earth’s atmosphere on their next pass and using the air to slow them down.

Dinah had been a little slow to absorb the full meaning of the little ship’s name. The James Caird was a small boat that Shackleton had used to make a desperate run for help to save the remnant of his failed South Pole expedition. They had aimed it at South Georgia Island, a speck on the map, in the knowledge that if they didn’t hit it spot on, the prevailing winds would never allow them to turn around and make another try.

She wondered if that very craziness wasn’t Markus attempting to make a point. The overall situation of the human race was, of course, ludicrously desperate. Doob had been the first to point this fact out in public, two years ago. Planning and preparation had consumed the time since. The work had been hasty, improvised, and politically inflected, but fundamentally it had been a well-ordered and methodical engineering project. As it had to be. But its plodding bureaucratic nature had a kind of lulling effect. How many times in the last two years had Dinah leaned back from a screen full of code and forcibly reminded herself of what was going on, and how bad it was? Unable to keep that squarely in their minds, the fifteen hundred or so survivors tended to live from one day to the next and keep doing what they had done the day before. Of all people, Sean Probst had been the least susceptible to that; he had seen what needed to be done almost immediately, and he had made efforts to do it that had been fantastically strenuous and, in the end, fatal. With his final transmission he had passed that responsibility on to Markus. Dinah suspected that Markus had stepped away from his position at the top of the org chart, and set out on this mission, partly to set an example for everyone else.

And if that was true, bringing Dinah along was to make a point as well. He would spare no one, play no favorites.

Markus broke the silence once during the approach: “Definitely a shard. As you said. Not snowball. Not candle.”

“I agree,” Dinah said. She could see its shape clearly enough, now, on the screen of her tablet.

Unlike normal ships, which carried their propellant in tanks, Ymir was a big chunk of solid propellant — ice — inhabited by a sort of parasitical infestation of equipment whose purpose was to convert that propellant into thrust. Not knowing exactly what he would find on Comet Grigg-Skjellerup, Sean had come equipped with more than one alternative architecture for putting Ymir together. If the comet core had turned out to be a loose ball of ice-dust, then he’d have had to scoop out what he needed and pack it into something like a snowball, giving Ymir a spherical shape with the reactor embedded in its center. Another option would have been to fashion a long cylinder of ice and plant the reactor in one end of it, then “burn” it forward, consuming the ice en route, like a candle. What they were seeing now looked more like the third architecture, which was the shard. It suggested that, upon his rendezvous with Grigg-Skjellerup, Sean had found it made up of at least one fairly hard and solid crystal that could be relied on to hold itself together structurally during the maneuvers to come. He had split the shard off from the main body of the comet and planted the reactor system somewhere near its middle, then embedded the rest of his ship — the part where the humans lived — in what would become its nose. If the equipment had worked as planned, then executing the “burns”—i.e., pulling out the control blades to place the reactor into operation and make steam — had been a matter of sending signals to actuators embedded in the core: motors that would move the rods, valves that would control the flow of steam and water, and so forth.

Implicit in all of this was a hell of a lot of robot activity, which was why Sean had taken the extraordinary step of traveling personally to Izzy to clean out Dinah’s supply of them before proceeding to his rendezvous with Ymir. The reactor had to be fed with ice. Because ice was a solid, it couldn’t flow through tubes. Robots had to mine ice from the shard and transfer it to a feed system: a set of augers that would move it into the reactor chamber to be melted and vaporized. A Siwi robot could move a lot of material in a hurry by embedding its “tail” in the ice and then using a whirring mill on its “head” to throw off a fountain of fine shavings that could be collected and carried off by Nats. The long intervals of time between burns could be used to store up a supply of shredded ice in hoppers that would feed the augers.

Downstream of the engine, robots were also needed to maintain the shape of the rocket nozzle. This was a long duct with a wide mouth on the aft face of the shard, tapering to a narrow throat near the reactor. The throat had been constructed on Earth and launched up with the reactor. It was made of a corrosion-resistant alloy called Inconel. Any other material would rapidly wear out from the hot steam blasting through it. Conditions in the long spreading bell of the nozzle, however, were more benign, and so it worked fine for that to be sculpted from ice. Nonetheless, it changed its shape as it was used. Deeper in, where the exhaust was hot, it grew wider as its walls were melted by the torrent of steam. Closer to the exit, where the exhaust had cooled to below freezing, it accumulated on the walls and narrowed the passage. So robots had to scuttle around reshaping the nozzle. This was a fine task for the Nats that Larz had experimented with in Seattle.

Finally there was a third “crew” of robots living on the exterior surface of the shard, trying to keep it from falling apart by embedding fibrous reinforcement in the outer layer of ice and wrapping cables and nets around it, somewhat like a butcher tying up a roast to prevent it from collapsing in the oven. This was a good match for the capabilities of the Grimmed (steel-armored) robots, which were mostly Grabbs.

All of these robots needed power, of course. They could store a little of it in batteries, but those had to be recharged. Some of them collected energy from sunlight; others had to converge from time to time on one of Ymir’s little nuclear generators to sip electricity.

The general picture was that Ymir would not be anything like the traditional idea of a spaceship, in the sense of an orderly, symmetrical piece of architecture. It would be more like a flying robotic anthill, constructed out of a natural found object. The robots crawling around on and in it had general instructions as to what they were supposed to be doing, but could make their own judgments from moment to moment to avoid collision with other robots, or from hour to hour as to when they needed to recharge their batteries.

Or that had been the general scheme, anyway. Since there’d been no guessing what Sean would find, there’d been no way of coming up with any plan worthy of that name. Instead they had sent him up with tools, resources, and ingenuity. Dinah, Markus, Vyacheslav, and Jiro were about to inherit the tools and the resources.

Jiro’s Eenspektor made steadily more noise as they approached, but the growth was slow enough that their minds didn’t quite register it. Jiro did not seem alarmed by the level of radioactivity, but Dinah didn’t know how to interpret that. Earlier in the mission, she had probed him for some general background about what to expect. “If it’s very bad, we all just lose consciousness and the mission fails,” he’d said. “The flux of radiation just shuts down our nerves, our sphincters open, we never even know it’s happening.”

“In that case,” Markus had pointed out, somewhat testily, “there is little point in discussing that scenario.”

“If all four of us throw up,” Jiro had continued, “and, say, one or more of us gets diarrhea, then we have hours to live. In that case we should just transmit a warning to Izzy and encourage them to send a second mission. In the meantime, maybe we can transmit some useful information to them. Eenspektor data, pictures, et cetera.”

“Noted,” Markus had said.

“If, say, one of us throws up, then it means that half of us will probably die, and so we have some chance of accomplishing the mission. If no one is barfing, then none of us is likely to die, at least over a time span of weeks.”

“Thanks for that,” Dinah had said, and tried to put it out of her mind. Now that they were actually approaching Ymir, however, it was coming back to her, and she was trying to convince herself that she wasn’t feeling any nausea.

“I am going to traverse the nozzle mouth in about thirty seconds,” Markus announced.

“Roger,” Jiro said, and then switched off his Eenspektor altogether. He pulled up a window on the screen of his tablet. “Switching to the external gamma spec now.”

Suddenly Ymir was filling the window. It was dead ahead of them. The glowing Earth, a third of a million kilometers away, “set” below its black horizon as they sidled in behind it. Markus had placed them on a trajectory that would slowly cross that of Ymir, bringing them laterally across the ice ship’s aft end.

Dinah’s older relatives might have described Ymir as having a sugarloaf shape, meaning a cone with a blunted tip. If so, this sugarloaf had been splashed with boiling water and attacked with a screwdriver in several places, giving it a scarred, irregular form. But it clearly had a fat end and a narrow end. These were about half a kilometer apart. The fat end, which was beginning to swing across their field of view, was a couple of hundred meters wide. It had a big circular hole in it, which was the outlet of the ice nozzle. New Caird could have flown into that hole and followed it almost all the way up to the throat before running out of room. And perhaps they would do so later, if they could find no other way in. But for now they were just going to make a lazy swing across it. The edge of the hole was blurry because of the evanescent steam cloud leaking out of it. This looked not so much like rocket exhaust as like breath emerging from someone’s mouth on a cold day. It didn’t so much block their view as soften it. But the visual landscape of space was one of intense contrasts, and so it was impossible to see down into the nozzle bell, even when they were squarely in the middle of the cavernous hole. It was just a black disk — like staring into the muzzle of a rifle. Hair-thin needles of frost grew on the window as the steam condensed.

Jiro focused intensely on his tablet until they had drifted past the midway point, then seemed to draw back into himself. He switched his Eenspektor back on. It was making a lot more noise than it had a few minutes ago, but this gradually diminished as they traversed beyond the nozzle exit and across the wide base of the sugarloaf. With a tap on the thrusters Markus got them moving forward with respect to Ymir. Earth “rose” on her other side. New Caird moved up alongside the shard, headed for her forward end.

“What’s the verdict, Jiro?” Markus asked, when he was satisfied with how things were going.

“Based on the gamma spec,” Jiro said, “I would say that at least one of the fuel rods ruptured. Not at the beginning, when the rods were new, and not recently, when they were full of fission fragments and daughters, but somewhere in between. Could be worse, could be better.”

A memory came back to Dinah. “One of Sean’s last messages said he was thrusting at full power.”

Jiro shrugged. “This reactor contains sixteen hundred fuel rods, grouped in assemblies of forty, so the failure of a single rod wouldn’t measurably affect performance. Even the ruptured rod still makes power, remember. It’s just that it would be spewing fuel fleas, fragments, and daughters into the rocket exhaust. We would expect to see a mixture of alpha, beta, and gamma — which is just what the Eenspektor is reporting.”

Dinah was no nuclear physicist, but she’d had enough radiation facts drilled into her to get the gist. Gamma was high-energy light. It would pass through just about anything. So, bad news, good news: It was hard to shield against the stuff. But most of it passed right through your body without interacting — that is, without doing damage. It made scary noises on the Eenspektor.

Betas were free-flying electrons. They were easy to shield against. Good news, bad news: You could stop them with a little bit of water or plastic. But by the same token, if they came into contact with your body they were certain to break something inside of you.

Alphas were helium nuclei, four thousand times as massive as betas, moving at relativistic speed. They could not pass quietly through matter any more than cannonballs could, but they did a lot of damage to whatever they hit.

In order to detect anything other than gamma, Jiro had been forced to switch over to equipment mounted on the outside of New Caird, since alpha and beta couldn’t penetrate the hull. And by looking at the energies of the various particles striking that equipment he had been able to diagnose conditions inside the reactor.

Since she could no longer see Ymir out the front window, Dinah focused on the slivers of frost that had grown on the glass. These were rapidly sublimating into space, and would be gone in a few more minutes. She’d found them beautiful until Jiro had made them aware that they were probably contaminated.

“Any residual beta now?” she asked.

“We are well clear of the nozzle and the plume,” Jiro said, a little taken aback.

“I mean, did we pick up any contamination on that flyby?”

“It is back down to background levels,” Jiro said. “But the detector would only ‘see’ sources on its side of the hull. We will have to do a more thorough survey later.”

“Get a load of this,” Markus said, and punched in a maneuver that swung New Caird around ninety degrees. They were now flying “sideways,” their nose aimed directly at Ymir, which was only about a hundred meters away from them. She more than filled the window. Her narrow end — her bow, if you wanted to think of her as a ship — was a hill of dirty ice. A few fine structures suggested that humans had been at work there: some structural netting, some cables, a glinting wire that might have been the radio aerial. But it wasn’t obvious, yet, where they were actually going to dock.

“It is really buried,” Markus observed. He didn’t have to explain that “it” was the command module — the part of Ymir that had life support systems. It ought to be reachable through a docking port. But they weren’t seeing anything. They had known — because it was part of the plan — that Sean and his crew would have buried it in the ice, to protect them from radiation and from rocks. They looked to have buried it deep.

Dinah’s tablet was running a terminal window, a simple programmer’s interface that just displayed lines of text. For the last little while, this had shown only a blinking cursor, but now it came alive and began to display cryptic, one-line messages.

“Picking up some new bot sigs,” she reported. These were the digital signatures of robots, pinging the universe to find out what, if anything, was listening. New Caird had shipped with a complement of robots of various types, but she knew all of their sigs and was filtering them out of this terminal window. Anything that showed up here was, by process of elimination, from Ymir’s complement of robots.

Like the clicks on Jiro’s Eenspektor, these came up sporadically and in bursts.

“At least twenty. . so I am going to filter out the Nats,” she said, typing in a command. Being so numerous, Nats tended to overload the screen. “Okay, in addition to a pretty well-developed Nat swarm I have half a dozen Grabbs and at least that many Siwis.”

“Any clues in their names?” Markus asked. It was possible to give each robot a unique name, which would show up on its sig. By default these were just automatically generated serial numbers, but they could be manually changed.

“Well,” Dinah said, “here is a Grimmed Grabb whose name is ‘HELLO I AM RIGHT ON TOP OF THE DOCKING PORT,’ which seems promising.”

“Can you make it flash?”

“Hang on.” Dinah established a connection to HELLO I AM RIGHT ON TOP OF THE DOCKING PORT and, after quickly checking its status, told it to blink its LEDs until further notice. Before she even looked up from her screen she could tell, by subvocal exclamations from the others, that it had worked.

“I see it very clearly,” Markus said. Some pops and bangs sounded from the thrusters as he adjusted New Caird’s attitude. They were now flying in nearly perfect sync with Ymir, looking at the flashing Grabb from a distance of maybe five meters. It was anchored into the surface of the shard in an area that was relatively free of the black stuff.

“Aim the light down into the ice, please? And put it on continuously?” Markus requested.

The Grabb’s LEDs were mounted on snaky stalks that could be aimed. Dinah made it happen. When next she looked up through the window, she could see the silhouette of the Grabb centered in a nimbus of white light, produced by its aiming its lights directly into the ice. A sharp white disk was visible in the center of that silvery cloud. It was blurred by the ice, but they all recognized it for what it was: a docking port, buried at least a meter deep.

“Did anyone bring an ice pick?” Jiro asked. It was not like him to make a joke, but Dinah was happy to take humor from any quarter at this point.

“Slava,” Markus said, “you’re up. Dinah, maybe you can help by bringing more of the robots to the area.”

By entering a fairly simple command, Dinah was able to summon every Grabb and Siwi in range, telling them, in effect, “Figure out a way to get closer to HELLO I AM RIGHT ON TOP OF THE DOCKING PORT and don’t bother me with the details.” By the time Vyacheslav was suited up, enough of these had drawn near that she was able to clinch several of them together and form a temporary construct that “reached” up from the surface of the ice to grapple New Caird, first in one location and subsequently in two more. So, even though they had not been able to dock yet, they at least had a mechanical link to Ymir that would prevent them from drifting away.

Other robots, including HELLO, meanwhile busied themselves carving a hole in the ice “down” toward the buried docking port. Vyacheslav exited through New Caird’s airlock, clambered down a stack of robots to the surface, and then made his way toward the site. Since the gravity of Ymir was negligible, Vyacheslav’s “weight” here was about half a gram, and the faintest contact with the surface would send him rebounding off into space. So instead of walking he had to rely on some sort of anchor fixed into the ice. Dinah was able to send two of New Caird’s Grabbs scuttling along ahead of him. These had been engineered for movement on ice, and could rapidly anchor themselves by melting and refreezing it with their footpads. All Slava had to do was follow them and hold on to them. Once he had reached the mouth of the hole he was able to embed anchors and carabiner himself into place. Then he speeded up the work of the robots by scooping out more ice, more quickly, than they were capable of moving with their little claws.

Not knowing what to expect, they had brought with them a small arsenal of improvised ice-mining tools, including a Craftsman garden shovel that had mysteriously made its way up from a Sears, Roebuck in an Old Earth mall. Slava put it to work.

Meanwhile Markus was sending a status report back to the Cloud Ark, and Jiro was doing more typing than seemed necessary just for taking notes. He was communicating with someone, or, more likely, something. Dinah was tempted to ask what, but there was only one plausible answer: he had established contact with the computer that controlled the reactor core.

Markus seemed to have come to the same conclusion. “Jiro?” he asked. “News from the belly of the beast?”

“It’s alive,” Jiro said, in what might have been either awkward phrasing, or a second consecutive joke. “I am trying to make sense of the logs. There is a lot of repetitive material.”

“Error messages?” Markus asked, making the obvious guess.

“Not so much. It is robot stuff. Status reports.”

Dinah moved over one seat and had a look. Though she couldn’t tell exactly what was going on, her general read tallied with Jiro’s. Lots of robots had been working away, executing variations on the same small set of programmed behaviors, pumping out occasional status reports — and, yes, some error messages — that had generated a log too vast for any human to read. They would have to sort it out later by writing a computer script that would crawl through it, accumulating statistics and looking for patterns.

“Could you scroll to the top, please?” she asked. She wanted to know the date and time of the first log entry.

“I checked it,” Jiro said. “Right around the time of Sean’s last transmission.”

So Sean, probably knowing that he was at death’s door, had told the robots to do something, and to keep doing it, until they were ordered to stop. Since the outer surface of the shard was pretty quiet, this probably related to some internal work hidden beneath the surface. “Mining fuel, probably,” Dinah guessed. Then, before Jiro could object to the incorrect choice of words, “Propellant, that is.”

Vyacheslav exposed the docking port. Using a combination of taps on New Caird’s thrusters, some pushing and pulling by the robots, and Vyacheslav simply grabbing the spacecraft and nudging it this way and that, they inserted her “front door” docking port into the little crater that Vyacheslav and the robots had excavated, and mated it with that of Ymir’s buried command module.

Slava then had to reenter New Caird through its side airlock. By sounds conducted through the hull they could track his progress as he climbed into the chamber, closed the outer hatch, and activated the system that would fill the lock with air.

In the meantime, Markus was able to make contact with the computers on the other side of the port, and verify that there was breathable air and other amenities.

It was damned cold, though: about twenty degrees below freezing.

“That was Sean doing us a favor,” Markus said. “He turned the thermostat down before he died. His body will be frozen solid.” For Ymir had no lack of power from its nuclear generators, and its electrical systems were still working.

Markus entered a command that would turn the command module’s environmental systems back on and bring the temperature back up. He pressurized the tiny space between Ymir’s hatch and New Caird’s. Then he opened the latter.

They were all looking now at the slightly domed exterior surface of the hatch that would lead into Ymir’s command module.

Someone had written on it with a felt-tipped marker. He had drawn the trefoil symbol used to warn of radiation hazards and beneath it had written the Greek letters alpha, beta, and gamma. Then, as a darkly humorous doodle, he had added a crude skull and crossbones.

Markus was the first to recover. He spiraled out of the pilot’s chair and propelled himself aft to the inner hatch of the airlock. There he punched a virtual button on a screen, which had the effect of locking the inner hatch. He was not letting Vyacheslav come in. He reached up with one hand and adjusted his headset. “Slava,” he said, “can you hear me? Good. Listen. We have contamination. You may have picked some of it up on your space suit. Before you come inside, I would like you to go over to Jiro’s external radiation detector and see if we pick anything up.”

Jiro was already scanning the hatch with his Eenspektor, fortunately without results.

Outside they could hear Vyacheslav cycling the airlock again and clambering back out. Using external handholds on the hull he made his way to the place where the external gamma spec was mounted, and devoted a couple of minutes to turning this way and that, directly in front of them, paying particular attention to his gloves, his knees, his boots — anything that had come into contact with the ice. No bursts of radiation were noticed, and so he was given clearance to go back to the airlock and enter New Caird.

They had brought warm clothes, which seemed advisable when going on a journey to a huge piece of ice. Jiro put his on. Dinah reached for the stuff sack in which she had stored hers, but Markus held up a restraining hand. She noticed he was making no effort to dress for the occasion. Jiro was going down there alone.

“I am going to overpressurize us a little bit,” Markus said, working with an interface on his pad. Dinah felt pressure building against her eardrums. Markus didn’t explain himself, and didn’t have to: they wanted clean air from New Caird to waft into Ymir, as opposed to potentially contaminated air coming in here.

Jiro then pulled a disposable one-piece bunny suit over his cold-weather gear. For they had come prepared to find the ship contaminated. He slung his Eenspektor over the outside of the bunny suit. Dinah handed him a respirator mask, so that he wouldn’t breathe radioactive dust into his lungs, and he pulled it on over the bunny suit’s hood and checked it for a good seal against his face. He pivoted into the space between the ships, operated the external latch on Ymir’s hatch, and jerked forward slightly as the overpressure in New Caird pushed it open. He let himself drift into the command module, then got himself turned around so that his feet were oriented toward the “floor.” Meanwhile Markus pulled the hatch closed behind him.

Vyacheslav by now had emerged from the airlock. He, Dinah, and Markus were listening to Jiro’s breathing on their headsets.

“Sean bled to death,” Jiro announced.

YMIR’S COMMAND MODULE WAS ARKLET-SIZED. OF COURSE, THAT went for almost everything now in space, since an arklet was just the biggest object that could be launched into orbit on the top of a heavy-lift booster. Some arklets were “tunnel,” meaning that they were laid out in a “horizontal” orientation, meant to lie flat, as it were, like railroad tank cars, with a single long floor running from end to end. This was good if you wanted a large open space, but tended to be a less efficient use of available volume. The command module of Ymir, like that of New Caird, was “silo,” meaning that it was oriented in a “vertical” way, diced into a number of round stories — typically four or five — joined by a ladder. Each story was a fat disk of space about four meters in diameter, big enough for one room that would be considered large by space travel standards, but more often divided into smaller compartments.

Ymir was a five-story silo, meaning that it had low ceilings that must have made it a claustrophobic place in which to spend a two-year journey. The first story Jiro had entered, being closest to the surface with its cosmic ray and bolide hazards, was a single room. On the plans, it was supposed to be used for storage of things like food, scrubber cartridges, robot parts, and tools.

After a few minutes Jiro was able to set up a video link from a camera mounted to his head. They watched it on their tablets.

The frozen body of Sean Probst was floating in a sleep sack that had been zip-tied to the ceiling. The porous fabric was stained dark brown. Very little of it had not been soaked with blood.

Bumping lightly against him was an old-school Geiger counter, tethered by another zip tie. The word BUSTED had been written on it with the same felt-tip pen used to make the sign.

After sweeping Sean’s body and the rest of the level with his Eenspektor, Jiro floated down the gangway to the next level “down.” The noise of the Eenspektor built steadily.

“Oh, turn the fucking sound off,” Markus said, and it went quiet. It would now display the counts per minute on its little screen, which only Jiro could see, but they wouldn’t hear the clicks.

The next story was a sort of general meeting, dining, and muster room, mostly open space lined with storage lockers. The third, or middle, story was divided into sleeping compartments, toilets, and showers. The fourth was a laboratory and workshop space. Those functions continued down into the fifth and bottom-most story.

“Cold here,” Jiro said, as he reached the bottom level. “Suddenly a lot of beta.”

“Okay,” Markus muttered, “so the contamination is there. On the fifth level down.”

It was cold, as they soon saw, because someone had left the door open: a manhole in the middle of the floor, big enough for a person in a space suit to climb through it and into a round shaft leading straight down into the ice. The entire length of the shaft was illuminated by white LEDs.

“That is remarkable,” Markus said.

Jiro descended into the tunnel headfirst and began to propel himself along it by the simple expedient of pulling on a knotted rope that had been fixed into its wall by ice anchors. He moved tentatively at first, then more rapidly. “There is a hatch at the far end — a hundred meters away, maybe,” Jiro said.

“Radiation?” Markus asked.

“Not so much,” Jiro said. “I do not think this was the route of the contamination.”

The hatch at the end was adorned with a more formal rendering of the radiation hazard symbol. They all knew what was on the other side of it: a small pressurized module that was physically connected to the guts of the reactor. Jiro elected not to go through, instead turning around for a return to the command module.

Then he turned back suddenly, and swept the beam of his headlamp across the ice wall of the tunnel. Some long slender object was embedded in the ice.

Two long slender objects.

Two human bodies. Dinah gasped as she recognized Larz’s strawberry-blond hair.

Without making any comment, Jiro made his way back “up” the tunnel to the lower level of the command module. He turned his attention to a locker near the hatch. Its door was open. Mining tools and space suit parts were floating around in it. Others had spilled out into the room and were drifting around aimlessly, pushed by currents of air.

“Jiro,” Markus said, “talk.”

“Strong beta from here,” Jiro said. “This is where the contamination came from.”

He drifted back up to the common room and found a garbage bag in a cabinet, then returned to the bottom level and went to work sorting through the tools and the clothing, holding each of them in turn up to the Eenspektor as he focused on its screen. From time to time he would grimace at the results and push the item into the garbage bag.

Dinah, Markus, and Vyacheslav waited in New Caird for an hour, pretending to pass the time with tasks on the screens of their tablets.

Then they heard Jiro’s voice again: “Prepare to put something out the airlock!” he was shouting.

It took them all a few moments to understand Jiro’s thinking. New Caird and the command module of Ymir now formed a closed system. Since the latter was completely embedded in ice, the only way to remove something from that system — to take out the radioactive garbage — was to put it out New Caird’s airlock.

There were some distant thuds. Dinah floated forward and opened the hatch to be greeted by a garbage bag, filled to the dimensions of a beach ball, and all wrapped up in duct tape. Propelled by a shove from Jiro, this entered New Caird. Dinah pushed it up to Markus, who intercepted it and tapped it sideways into the airlock. Vyacheslav slammed the hatch behind it. Then they heard a hiss, indicating that the lock had cycled. The bundle was now adrift in space.

Jiro’s head, then the rest of him came through the port. He had stripped off the bunny suit and the respirator and presumably stuffed them into the garbage bag. He was sweaty and exhausted.

“Just like old times, my friend?” Markus said, referring to Jiro’s earlier career running cleanup at Fukushima.

“I don’t miss it,” Jiro said.

It was warm in the command module now, so they didn’t need the parkas. But they all used bunny suits when they went into Ymir, and stripped them off before going back into New Caird. Contamination was “sneaky,” as Jiro put it. The beta emitted by a microscopic speck of fallout could be hidden from the Eenspektor’s view by just about any random obstacle — and the command module was cluttered with those. So Jiro’s initial sweep was no guarantee that tiny beta-emitting particles weren’t still hidden in there. If such particles found their way into a lung, or the digestive tract, fatal radiation damage was likely to result. He had, though, identified a space suit glove on the lower level as being heavily contaminated, and found lower levels of contamination on some other odds and ends that had gone into that garbage bag and out the airlock. With luck all serious sources of contamination had now been removed.

BEFORE IT HAD TIME TO THAW, VYACHESLAV TOOK SEAN’S BODY down from the ceiling. Slava wasn’t a life scientist, but he was a jack-of-all-trades. Bundled up in parka and moon suit, he cut the sleep sack open as Jiro stood over him with the Eenspektor. He performed a cursory exam, then wrapped the body back into the sleep sack. He maneuvered it to the lower level, threaded it through the manhole in the middle of the floor, and then pushed it down the tunnel to the end, where Larz and the other crew member had been buried. There, he stashed Sean’s body against the ice wall.

Somewhat ruining their appetite, he reported on the findings of this impromptu autopsy as they got ready to eat a meal in the common room.

“Sean bled to death out of his asshole,” he reported. “He had an internal rupture of the bowel.”

“I picked up some beta through his belly,” Jiro added. “He was very emaciated at the end.”

“Meaning?” Markus asked.

“He swallowed a particle of fuel. Probably a fuel flea that got loose and somehow was tracked in here.”

“Fuel flea?” Jiro had used the term before. No one else knew what it meant. It had gone in one ear and out the other, just another bit of the tech jargon that was so ubiquitous on Izzy. Now that fuel fleas were killing people, it was time to learn about them.

“A tiny piece of uranium or plutonium that has gotten loose from a ruptured rod. As it throws off alpha particles, it zigs and zags around the room — conservation of momentum. So it hops around like a flea. The point is, it is small and it makes a lot of alpha. It lodged in a diverticulum in his bowel. It burned through his bowel wall and started a bleed that could not stop.”

Everyone pushed back their food.

“Okay,” Markus said. “We eat in New Caird.”

Once they had finished their meal, Markus told everyone that they needed to sleep, since they had a busy few days ahead of them. Jiro volunteered to take the first watch, and so the rest of them slept while Jiro stayed up going through logs and notebooks, assembling a picture of all that had happened on Ymir’s journey.

Suddenly they had a lot of space to spread out in. Dinah was tempted to retreat to the far end of the New Caird and get some privacy, but Markus insisted that everyone sleep down in the command module. New Caird might be free of radioactive contamination, but it was exposed to the direct hazards of space. A bolide strike would kill anyone in it. Whereas a beta-emitting particle, inhaled into the lung, would take days or weeks to incapacitate the victim — time during which they could do useful work.

So Vyacheslav ended up sleeping in one of the berths on Ymir’s crew accommodation level, while Dinah and Markus shared another. Somewhat to her surprise, they actually managed to have sex, a thing that had occurred only once since the White Sky. It was a sly surprise, not the athletic banging around that they had enjoyed the first few times they had done it, back in the good old days when the Hard Rain had seemed far in the future and the Cloud Ark had still felt like an isolated research colony. Ymir, now separated from the rest of the human race by millions of kilometers’ distance and several thousand meters per second of delta vee, now had some of that old feel to it. And despite the ghoulish scene that had greeted them on arrival, Dinah liked it here — it was the space equivalent of one of Rufus’s old mining camps — and didn’t really want to go back.

But they were supposed to be saving the human race, not enjoying an exotic holiday, so she tried to get some sleep. When Markus’s alarm went off five hours later she peeled out of the bag they’d been sleeping in and did her best to clean up and get into some fresh clothes. Ymir had long ago turned into a smelly bachelor pad, short on toiletries, and, as they discovered while rooting around in the common area, on food. Sean had definitely been killed by the fuel flea in his gut, but he had likely been weakened before then by malnutrition, and even by lack of oxygen. For the systems that the crew had been using to replenish Ymir’s air supply were not in the best condition. The new arrivals were awakened twice during their sleep cycle by alarms from the life support system, which Jiro silenced and dealt with.

When they were all awake, they ate food from the stores they had brought with them and listened to a briefing from Jiro.

“Let me tell you what happened to this expedition,” he said. And then he told them the story as he had pieced it together from the logs left behind by the dead.

The failure of the radio, shortly after the beginning of the mission, had been caused by a defective part for which there was no replacement: a simple, stupid oversight. The longest leg of the trip — the year and a half spent coasting from the L1 gate to Grigg-Skjellerup — had consisted of lengthy stretches of boredom interrupted by occasional panics, most of which had to do with the life support system. This was based on using sunlight to grow algae, a process that worked well in the lab but had turned out to be difficult to sustain on Ymir. The newest arklets in the Cloud Ark had benefited, in this respect, from lessons learned operating such systems in the time since Zero, but Ymir had been built and launched very early, using systems that now seemed painfully out of date.

Once they had reached “Greg’s Skeleton” and thereby gotten access to vast amounts of water, they’d been able to make oxygen by splitting H₂O, and life had improved. Until then, however, they’d been oxygen hungry and tense, trying to keep their consumption of air and food to a minimum by floating listlessly in their sacks watching the same DVDs over and over again. Health, and mental status, had suffered.

They broke the shard from Grigg-Skjellerup using small mining charges planted by hand, or by robots programmed by Larz. Into its nose they embedded the command module, making themselves comparatively safe from cosmic radiation and bolides for the first time since the beginning of the mission. Life began to improve. They started excavating the access tunnel into the core. Into the aft end of the shard they inserted the reactor system, letting it melt its way into the ice. Around it, in the heart of the shard, they began to excavate a cavity and sculpt out hoppers: containers designed to hold broken-up ice produced by the mining robots. Twelve augers — long, spiraling ice movers, like the ones used to transport grain into elevators — were set up to convey that loose ice from the hoppers into the space surrounding the warm reactor vessel, where it would melt and be pumped into the core itself. Meanwhile, a separate corps of robots worked on the outside of the shard, melting the ice a little bit at a time, mixing it with the fibrous material they’d brought with them, and letting it refreeze into the much tougher material known as pykrete.

The “steampunk” propulsion system had basically worked as planned — though not without a lot of tinkering and head scratching — on the first “burn” that had put it on the course back to L1. There had, however, been some problems with the augers that were used to feed ice into the reactor chamber. The augers received their inputs of ice from hoppers that had to be filled up by “mining” solid ice from the inside of the shard, a process for which robots were well suited, and so nothing worked at all without the assistance of a small army of robots conveying flakes of ice from mine head to hopper like ants dismantling a loaf of sugar. This part of it had actually worked. But some of the pieces of ice being mined by the robots had little rocks in them. These jammed the augers. Jams could often be repaired by operating the auger in reverse for a short time, but sometimes a robot, or even a person in a space suit, had to be sent to pry a rock out of the mechanism. An auger accident had led to the death of one member of the crew.

During the months between that first burn and their arrival at L1, Larz did some programming work on the robots, trying to teach them not to collect rocky ice. They conducted a number of system tests intended to make sure that the problems they’d experienced the first time around wouldn’t be repeated during the critical second burn. These ranged from small-scale tests on individual robots all the way up to full dress rehearsals where the entire system would be energized and the reactor turned on to generate thrust for a few minutes.

It had been during the first of those dress rehearsals when something had gone wrong in the core, resulting in damage to the jacket of a fuel rod.

Jiro had an idea as to what had gone wrong. Ymir’s reactor used water — the melted ice of the comet core — as its moderator. In nuclear engineering, that meant a medium that slowed down the neutrons hurled out by fission reactions, making them more likely to stick around long enough to trigger more such reactions. In the absence of an effective moderator, the neutrons would mostly escape from the system without doing anything useful.

Between being as dead as a doornail and running out of control was a narrow band of normal and healthy power output in which basically all commercial reactor operations happened. The essential problem with Ymir’s reactor was that its moderator — being a naturally occurring substance — was impure and unpredictable. The water that flooded into the chamber for the first dress rehearsal had been melted from ice a few months earlier, around the time of the initial “burn,” and had been sitting in the plumbing system ever since then. There, it had been in contact with rocks and grit that had made it through the augers. It had leached various minerals out of that rock, and become something other than pure water. When the reactor was started and the pumps turned on, that impure water was drawn through screens and filters intended to exclude all the debris. But it was nonetheless impure water, and when introduced to the core, it failed to perform its function as a moderator. The reactor was sluggish to get going. With the advantage of hindsight, it could be seen that its neutron economy was suppressed, poisoned by the impurities in the water. Overreacting to the slow start, the operators had pulled the control blades out farther than they would have otherwise. But once the first rush of impure water had been flushed through the system and blown out the nozzle, it had been replaced by relatively pure water, only just now melted from the ice. The reactor’s power had surged, producing a sudden buildup of fission products inside the fuel rods. Some of those would have been gases such as krypton and argon. The gases would have created pressure. Fuel rods were engineered to withstand it, but one of them had failed and ruptured. Possibly it had left the factory in excellent condition but been damaged en route by a nanometeoroid that had left a microscopic flaw. In any case, for whatever reason, the rod burst open and began to spill out the highly radioactive “daughters” of nuclear fission, which had become mixed with the steam being blasted out the rocket’s nozzle.

Most of the fallout had, therefore, dissipated into space. But the whole point of a rocket nozzle was to convert the thermal energy in the gas — its heat — into velocity. The faster the steam went, the colder it got, until the steam near the nozzle exit was so cold that it actually began to condense into snow. Tiny particles of fallout made excellent nuclei around which a snowflake might begin to form. Some of that snow had stuck to the ice walls of the nozzle bell.

The most likely explanation for what had happened next was that one of the robots crawling around in that area maintaining the shape of the nozzle had become contaminated with a mixture of alpha-emitting fuel fleas and beta-emitting daughters, and tracked the material to a location where it had been transferred to the glove of a space suit — possibly by a mechanism as simple as a spacewalker reaching down to brush some ice from the claw of a Grabb, or planting a foot in a location where a contaminated Grabb had stepped. The contamination had then been brought into the command module when the spacewalker came indoors. They might not even have known about the burst fuel rod, so they might not have been checking for contamination. Or, as suggested by Sean’s note, their Geiger counters might have broken down, one by one, rendering them blind to the presence of radioactivity in their environment. In any case, the particles had spread around the command module. Some men had inhaled them, some had swallowed them. They hadn’t been healthy to begin with.

IN ANY CASE, THE GOOD NEWS, IF IT COULD BE SO CALLED, WAS THAT the reactor and the engine basically worked. The improvements Larz had made to the robots’ mining programs had led to fewer rocks in the hoppers, and fewer jammed augers, during the L1 burn. Since then, Nats had been crawling around in the hoppers identifying rocks that had sneaked in anyway, and pushing them away from the augers. The damage to the fuel rod would have been a major catastrophe by Old Earth standards — had it happened on an earthbound reactor. Here, it was messy, and had already been fatal to a few. But everything still worked. Yes, the New Caird expedition would be bringing a radioactive disaster right into the middle of the Cloud Ark, but once they drew close enough they would jettison the reactor and let it fall into the atmosphere.

Forty-eight hours, give or take a few minutes, now remained before Earth would loom huge below them, and the nadir surface of the shard would sweat and steam as the radiant heat shining up from the incandescent air softened, melted, and vaporized the ice. It was then that they would have to pull out the control blades and execute Ymir’s next big burn. First they would have to spin the whole ship around so that she was flying “backward,” her nozzle bell pointed in the direction of movement. For the delta vee they needed was a negative one — a braking, as opposed to accelerating, burn.

For spin moves, all spaceships were equipped with thrusters, not powerful enough to impart big delta vees but capable of rotating the ship as a whole into the desired attitude so that the main engine was pointing in the right direction. As a rule the thrusters were more effective when they were situated out toward the “corners” of the vehicle, where they could exert more leverage and crowbar the thing around with minimal thrust. Not knowing what they were going to find at Grigg-Skjellerup, the mission planners for Ymir had packed aboard a collection of modular thruster assemblies that basically consisted of little rocket engines, propellant tanks, wireless control links, and hardware for anchoring them into ice. A cursory survey of Ymir and a look at the dead crew’s records made it clear that Sean and his crew had embedded those packages into the ice at suitable locations: one complex up at the nose with nozzles aiming in four perpendicular directions, and four more spaced around the fattest part of the shard.

Now that New Caird was docked, her engine could also be put to use in getting Ymir spun around. But this one maneuver — a 180-degree flip, which would have seemed comparatively simple in a small craft such as an arklet — was fraught with difficulties and complications in something as huge and asymmetrical as Ymir. Anticipating a need to use the thrusters, Dinah sent robots out to inspect them during that first “morning,” and Vyacheslav suited up and went out to do a bit of troubleshooting on a propellant line that had somehow become kinked. But so ponderous were the shard’s movements that the actual rotation, end-over-end, consumed eight hours, and tweaking it into precisely the right orientation then took another six.

Whereupon Markus announced that all of their assumptions were probably wrong anyway.

“The atmosphere is too big,” he said. He had been staring pensively, for a long time, at a string of emails from Izzy.

Dinah felt a spear go through her heart. After all that had happened in the last couple of years, it was remarkable that she still had it in her to react in that way to bad news. It seemed to be some kind of built-in psychological program, triggered by phrases like “your mother has cancer,” “there’s been an explosion in the mine,” or what Markus had just said.

They had known, from very early in the planning of the Cloud Ark, that the Hard Rain would heat up the air—all of the air, all over the world. When air got hotter, it expanded. The atmosphere had only one direction in which it could expand: out into space. So, whatever drag Izzy felt from the traces of air at its accustomed altitude of some four hundred kilometers was bound to get worse as the atmosphere reached upward. How hot the air would get, how much it would expand, and how heavy the drag would get were questions of colossal import that, however, simply could not be answered until the Hard Rain actually started. As Doob always put it, the experiment of blowing up the moon had never been attempted before. The most they could do was wait and perform observations. Which was exactly what they had been doing ever since the Hard Rain had begun. But Markus had been distracted for most of that time, and was only now absorbing the latest results.

For the Cloud Ark, of course, there were plans to cover various contingencies. In the easy case where the atmosphere didn’t expand that much and the drag wasn’t too bad, they didn’t have to do much. In the more difficult case — which was apparently the way the experiment was now shaping up — they had no choice other than to raise the orbit of every vessel they had — Izzy herself, and each arklet. The delta vees involved were not that large; three hundred meters per second sufficed to nearly double the orbital altitude and get them well clear of the danger zone. Each arklet had its own engine and enough of a propellant supply to accomplish that. For Izzy, matters were a little more complicated. If they were willing to ditch Amalthea they could get three hundred meters per second pretty easily. Bringing Amalthea along for the ride, however, increased the propellant requirements enormously. All of which had long ago been anticipated by mission planners. This was how the Dump and Run strategy had been dreamed up in the first place.

So it would be easy for the arklets to get clear of the thickening atmosphere by abandoning Izzy, at least for the time being, and jumping to higher orbit. Their drag problems would be solved. But in so doing they would lose the ability to shelter behind Amalthea and begin to take damage from bolides. Exactly how much damage depended on how thick and fast the rocks were coming in, and what the distribution of sizes was — another one of those questions of colossal import that couldn’t be answered until the Hard Rain had actually started and data had been gathered.

And, so far, the data were too thin to make any real determination. With a few spectacular exceptions, bolide impacts and casualties had been light. But this didn’t mean it would remain thus. The White Sky was an ever-changing phenomenon. The explosive uptick in the Bolide Fragmentation Rate that had signaled its onset was still ongoing. The distribution of rock sizes and orbital parameters would continue changing for thousands of years. Trends could be observed, and predictions could be made, but beyond a certain point it was guesswork.

At any rate, Markus had rolled the dice on the gambit that they were now executing. If it worked, and they could slow Ymir down enough to mate her with Izzy, then the Big Ride strategy became possible, and the arklets could climb to higher and safer altitudes behind the shelter of Amalthea’s metal and Ymir’s ice.

The one part of that plan that Markus apparently had not considered until now was that the atmosphere was too big.

In truth, it wouldn’t have made a difference if he had considered it. The crucial decision had been made and executed weeks ago by Sean Probst, when he had laid in a course at L1 and executed the burn that had placed Ymir on its current trajectory. This was an ellipse with a very low perigee. That was a sound idea from an orbital mechanics point of view in that the steam engine would have maximum leverage at that point — it was the natural place to make a burn and effect a transition to a low circular orbit matching Izzy’s. But, sick and exhausted from his two-year odyssey, isolated from the latest scientific discourse by radio failure, Sean might have overlooked the expansion of the atmosphere when making his calculations.

“Are we going in?” Vyacheslav asked. This being a polite euphemism for the scenario where Ymir got so deep, and slowed down so much, that it burned up and became just another streak of blue light against the lambent background of the pyrosphere.

“We are more likely to skip, I think,” Markus said. Meaning that Ymir might bounce off the atmosphere like a flat rock skimming across a pond. “With unpredictable results. But I cannot be sure. All I am saying is that this is not going to be the mission plan that Sean had in his mind. It is going to be something else. Something maybe a little more exciting.”

ANTICIPATING THAT CAMILA MIGHT BE ON HER TOES — SHE HAD ALREADY survived one attempt — the gunman had crouched behind the rear of her school bus, sawed-off shotgun at the ready, and waited for her to emerge. A narrow stretch of pavement separated the vehicle’s side door from the entrance of her school, so he didn’t have much margin for error. He jumped the gun, as it were, springing out into the open while Camila was still negotiating the descent to the street; the long hem of her burqa was apt to get caught on her foot as it probed for the running board, so she had to take it slowly. The delay saved her life. Alerted by a schoolteacher standing in the building’s doorway, Camila turned back into the bus. Rather than hitting her full in the face, the shotgun blast raked the left side of her jaw, removing eleven teeth, tearing away much of her cheek, and causing massive structural damage to her jaw. Surgeons in Karachi and, later, London had saved most of the functions of her tongue, rebuilt her mandible from pieces of bone carved from her pelvis, and fitted her with a set of artificial teeth. After a world tour raising money for girls’ education in Afghanistan and the tribal regions of Pakistan, Camila had been granted permanent asylum in Holland. Dutch plastic surgeons, funded by charitable donations from all over the world, had gotten to work repairing the cosmetic damage. This was a long-term project that had been interrupted by Camila’s selection as one of the Dutch candidates for the Cloud Ark. No one believed that this was a random outcome of the Casting of Lots. Clearly the Dutch authorities had placed their thumb on the scale and seen to it that she was chosen, as a rebuke to some conservative Muslim countries who had refused to nominate female Arkers unless given assurance that arrangements would be made for them to live in orbital purdah. Camila was well suited to serve that symbolic purpose, since she had not adopted Western ways. She dressed conservatively and wore the head scarf and the face veil. She was coy, however, as to whether the purpose of the face veil was submission to the demands of religion or to hide her disfigurement. She had pulled it down several times to display the scars to television cameras, and when she had dined at the White House she had gone uncovered in the dining room, by prearrangement with her hostess, the president of the United States.

Julia’s startling arrival in the Cloud Ark had therefore led to a reunion between the forty-four-year-old ex-president and the eighteen-year-old refugee. To call it joyous or even happy would have been wrong given the circumstances. It was a fact of human nature, though, that some people just got on well with each other. This had clearly been the case during that dinner at the White House and was no less true in Camila’s abode, Arklet 174, which was where Julia ended up lodging after she had recovered from her eventful flight up and gone through a bit of basic training in how to live in space.

Arklet 174 belonged to a heptad, or a cluster of seven arklets all connected to a hexagonal frame; it and five other arklets surrounded a seventh, which was positioned in the middle of the hexagon, where it served as a twenty-four-hour-a-day common room and working area for the people who lived in the others. Four to five people were assigned to each of those, and two more had been shoehorned into small private cabins at the boiler room end of the central arklet, so the total population of the heptad, including Julia, was twenty-nine. This increased to thirty when Spencer Grindstaff managed to hitch a ride on a Flivver that was bringing a spare part and a technician from Izzy to fix a problem in one of the arklets’ thrusters. The technician returned to Izzy when he was finished, but Spencer stayed, and talked his way into a berth on Arklet 215. There was a tendency for arklets within a group to become segregated by sex over time, as the populations sorted themselves out; 215, which was predominantly male, ran on the same shift as 174, which was all-female. They both ran on second shift, which, for reasons that were now purely historical, tended to be culturally American. They slept from dot 8 to dot 16. First shift was Asian and third shift was European. The cultural shadings were perpetuated by food: the warm odors that greeted one’s nostrils upon entering the common space first thing in the “morning,” the tastes one could expect to savor in the “evening.” Since space food was lacking in variety, this was largely a matter of spices. The second shifters had their little bottles of Tabasco, the first shifters had plastic packets of curry powder, and so on.

“Ganging” was the term used by the Arkitects to denote this clustering of arklets into formations of three or seven: triads and heptads. It helped simplify Parambulator’s job by reducing the total number of separate objects that needed to be tracked. It gave Arkies more living space to roam around in, and provided some redundancy in the case of a bolide strike. They didn’t like to form anything larger than a heptad, though.

“Spencer, I am fully aware that I am out of my depth here,” Julia said, “but I don’t understand the upper limit of seven. I was assured, during my early briefings, that any number of arklets could in principle be ganged. Limiting it to seven seems arbitrary. Which suggests that some deeper agenda might be behind it.”

“One moment please, Madam President,” Spencer said. He was doing rather a lot of typing.

“You really shouldn’t call me that,” Julia returned, though her tone of voice was indulgent.

Spencer smacked his laptop’s Enter key, then leaned back slightly and adjusted his glasses. His eyes jumped around to various parts of the screen. Then he looked up, and, in a clearer tone of voice, announced, “It’s all shut down.”

“The surveillance, you mean.”

“Situational Awareness Network,” he corrected her, and winked.

“Surveillance to you and me. It’s like living in the Nixon White House. Old reference. You wouldn’t understand. Now, where was I?”

Camila knew. She hadn’t taken her eyes off Julia the whole time; she was on top of it. “The agenda behind the seven-arklet limit?”

“Yes, thank you, Camila. I don’t buy their arguments. To me it feels, rather, like a way of atomizing the population. Keeping the Arkies from cohering into their own polity — a polity that might serve as a wholesome and desirable counterweight to the central dominance of the power structure on Izzy. Speaking of which, Spencer, I want to say how much I appreciate the work you have done in. . managing things. . on the IT front. As just now. Giving us the freedom to talk among ourselves without the SAN recording our every word and gesture.”

Spencer nodded as if to say all in a day’s work.

It was dot 18, the beginning of the workday for second shifters. They were in Arklet 215, home to Spencer, three other men, and a woman. The others had gone to breakfast in the common area, to exercise, or to work. Spencer, Julia, and Camila had been joined by a guest: Zeke Petersen, who had arrived by space suit and was still clad in his thermal coverall. He looked mildly agog. Sensing this, Julia turned toward him with a smile. “Major Petersen,” she said, “it is so good you were able to join us. Though I am new to space, I have some understanding of how difficult it is to simply drop by and say hello, as it were.”

“Well, technically I am no longer a major, since that would imply the existence of a military,” Zeke said, “but if we are going to use extinct titles as a courtesy, then I’ll just thank you for your hospitality, Madam President.”

Madam President was a little while parsing that and wasn’t sure if she liked it. Nervous at the silence, Zeke went on: “I’ll apologize in advance that I can’t stay for very long. I’m here with a specific job to do, and once it’s done, I need to move on.”

“Inspecting Arklet 174 for possible damage from a microbolide strike,” Julia said.

“Yes, ma’am.”

“I called it in yesterday. I could have sworn I heard a loud banging noise. It scared me to death. But there doesn’t seem to be any damage. And the more time goes by the more I wonder if I just imagined it. Space is a noisy environment. I hadn’t expected that. The thrusters are so loud when they come on. Maybe it was nothing more than that. I would feel so embarrassed if I summoned you all the way out here to no purpose.”

“Summoned me?” Zeke asked, a little bewildered. “The Incident Report System is an automatic queue; the assignments are handed out at random.”

Julia exchanged a mischievous look with Spencer. “You and Spencer have been together on Izzy for more than two years,” she said. “I’m sure you’ve come to appreciate his skills — as have I.”

Zeke looked just a bit queasy. “So you got in and manipulated the queue?”

“Old habits die hard,” Julia said. “I’m accustomed to working with people I know and trust. If an inspection of my arklet is required, and someone has to do it, then why not have that someone be a person I have met before? Since the assignments are handed out at random, as you say, it might as well be you.”

“Well,” Zeke said, “since you put it that way, I’m glad to be able to catch up with you for a few minutes, Madam President. Just saying that I’ll have to complete the full inspection anyway, so we can close the loop on your report.”

“Of course, and I’ll bet it will go quickly,” Julia answered with a wink. “Zeke, you are a member of the General Population, are you not?”

“Of course,” Zeke said. “As an original member of the ISS crew, that’s naturally. .” but then his eyes strayed toward Spencer and his voice trailed off.

Julia smiled. “An awkward topic has come up, and it’s best to face it with absolute transparency. Despite being a longtime, trusted member of the ISS crew, Spencer here has been removed from the General Population and demoted to the status of an Arkie.”

“I wouldn’t look at it as a demotion,” Zeke began.

Julia silenced him with a dismissive fluttering of the fingers. These were still manicured. Camila had been doing her nails for her. “We all know it was a demotion. Markus sprang it on Spencer when he got news of the Eight Ball and saw what was coming. Oh yes, I’ve been filled in on all of the carefully laid plans that Markus set into motion when his sweetheart so conveniently gave him the news. Had word of it reached us at the White House, I don’t know how I would have reacted — but we were busy protecting Kourou, and supporting Markus as best we knew how. Spencer here, after all those years of patient service, was replaced by that hacker boy—”

“Steve Lake?” Zeke asked.

Julia’s eyes darted to Camila, who nodded.

“Yes,” Julia said, “Steve Lake. I guess he’s quite clever, but obviously no competition for Spencer.”

“Are they in competition?” Zeke asked.

“In a sense yes, when we Arkies are exposed to the all-seeing eye of SAN, and the GPop is permitted to have some semblance of privacy.”

“It depends on where you are in the space station,” Zeke began, but then trailed off.

“I wouldn’t know, since I’ve been permitted to spend very little time there. Oh, I know the official justification. I’m not qualified to be a member of the GPop. By process of elimination, that makes me an Arkie. Fine. But that doesn’t mean I can’t maintain a degree of social connection with old friends who are so privileged.” Julia reached out and clasped Zeke’s hand briefly.

“To be sure,” Zeke said, “and I think that as time goes on those two populations will cease to be thought of as separate groups.”

“I know that is the official dogma,” Julia said, amused.

“But most of that social interaction is not going to be through face-to-face visits.”

“So I’m told. Hard to envision how the populations will merge as long as that is the case.”

“Most of it is going to be happening through Spacebook and Scape and whatnot,” Zeke went on, referring to the Cloud Ark versions of popular Internet communication apps. “At least until—”

“Until we all ascend into heaven and live happily ever after as one big friendly Ark,” Julia said. “Zeke, you know space operations better than anyone. What is your opinion of the strategy that Markus has been foisting on us? The Big Ride? Even the name seems a bit suggestive, doesn’t it, of. . I don’t know what.” She exchanged a look with Camila, who giggled at the witticism.

Zeke looked around.

“You don’t need to worry about that,” Julia reassured him.

“About what?”

“Markus’s surveillance network.”

“SAN? I wasn’t worried about it,” Zeke protested. “Just thinking.”

“About what, pray tell? Major Petersen, all kidding aside, I really am quite keen to hear your opinions as an expert.”

“To tell you the truth, I’m thinking about how thin the walls of this pressure hull are,” Zeke said. “When you called in that bolide strike yesterday, you sounded pretty alarmed — I heard the message. Well, you had every reason to be alarmed. I do this for a living now — I go out and inspect these craters, big and small, that are piling up on our equipment. I patch holes, repair stuff that’s broken, and twice now I’ve had to handle fatalities. It’s no joke. If Markus sees an opportunity for us to ascend into heaven, as you put it, behind the shelter of Amalthea, well, I think it’s worth a try.”

“Is Amalthea going to shelter us from the thickening atmosphere? Camila here has been reading the technical reports for me, which Spencer has been so good as to download from the server. She tells me it’s quite serious.”

“The expansion of the atmosphere? It’s damn serious,” Zeke said. “But Izzy’s ballistic coefficient, with Amalthea attached, is huge. She can plow through some pretty thick air, and the rock will absorb all the heat. And arklets can ride along in her wake, like bicyclists drafting behind a truck.”

“All of the arklets?”

Zeke swallowed. “No. She doesn’t make a big enough bow wave to shield all of the arklets. Unless they fly so close together that Parambulator goes nuts.”

“This is the part of Markus’s plan that I can’t understand,” Julia said. “What is to happen to all of the arklets that are not afforded the privilege of nestling into Amalthea’s wake?”

“I don’t know all the details of the plan,” Zeke said. “It is fluid.”

“Meaning, it’s not really a plan,” Julia said.

“It depends on when Ymir gets back. What kind of condition she’s in. How much ice she has. Then we’ll make a plan.”

“And is that to be a dictatorial process? Under the, whatever it’s called — the martial law thing?”

“PSAPS,” Camila said.

Zeke shrugged. “I don’t think Markus is going to put it up for a vote. He’ll get together with his brain trust and they’ll decide.”

“Why bother consulting the brain trust?” Julia asked, as if the idea were a fascinating novelty.

“To bring in different perspectives. . make sure they’re not missing anything.”

“Are there any Arkies in this brain trust, or are we expected to meekly accept its verdict?”

Zeke was flummoxed. Had he been given the ability to rewind and replay the conversation, he would see that he’d been outmaneuvered. Lacking that perspective, he was tongue-tied for now.

Julia wasn’t. “I ask only because I’ve been getting to know a lot of Arkies. I have nothing else to do. No duties. No applicable skills. I find that many of them crave a bit of society. It’s a natural human need, just as much as sleep and exercise. So I talk to them — in person here on our little heptad, or through the channels you mentioned, the Spacebook and the Scape. These young people find it at least a novelty to have a conversation with a lonely and bored ex-president. My point being, Major Petersen, that our system worked. The Casting of Lots and the training camps produced the brightest collection of young talent it has ever been my privilege to encounter. They are brimming over with energy and ideas. These are the scarcest resources in our universe right now — scarcer than water, scarcer than living space. And as such I’d consider it a shame if their energy was wasted and their ideas were not taken into account by whatever smoke-filled room Markus assembles to make his plan — assuming he even survives what sounds to me like a somewhat harebrained endeavor.”

THE CREW OF THE ORIGINAL JAMES CAIRD HAD USED CELESTIAL NAVIGATION to find their way across hundreds of leagues of stormy seas to the coast of South Georgia Island. The crew of the New Caird would have to do something similar. It was easier for them. The navigator on James Caird had had no choice but to await breaks in the ever-present cloud cover and snatch observations when he could, comparing them against a mechanical chronometer that he hoped was still telling true time. New Caird had better timepieces and a better view of the sky. In place of a sextant, they had a device consisting of a wide-angle lens and a high-resolution image sensor that could tell what direction it was aimed in just by comparing what it saw to an astronomical database stored in its memory. So they knew precisely how they were oriented in space, and how that orientation was shifting as the giant shard of ice to which they were attached progressed through the inexorable mathematics of its long ellipse. That, combined with direct measurements of Earth’s position, enabled Markus to calculate the parameters of their orbit and to reckon, with precision that grew each time he rechecked the figures, exactly how low they were going to go. Whenever Izzy was on their side of the planet, which was about half the time, they were able to get the latest figures from Doob concerning the expansion of the atmosphere.

It was in combining those two sets of figures that pure Newtonian mechanics began to break down. For, in a traditional calculation of a space vehicle’s trajectory, one assumed no atmosphere and no extraneous forces resulting from it. But there was now no denying that Ymir would be going low enough to scrape the air. At a minimum, this meant it would experience some drag that would throw it off the course that Sean Probst had laid in. As these things went, drag wasn’t that difficult to calculate. Its effect on their course could be estimated. But because the ice shard wasn’t a symmetrical body, coming in straight, it was also going to generate some lift. Not a lot of lift — nothing like an airplane wing — but some. If that lift got aimed in the wrong direction it would make Ymir veer downward, like a stricken airplane going into its death spiral. But if they aimed it up, it would ease their passage by pushing them away from the Earth into an altitude where air was thinner. They would lose the benefit of lift then and drift back downward, but as the air got thicker, the lift would resume and push them back up. They might skip off the atmosphere several times during the hectic half hour when they were slingshotting around the world. The results would have been difficult to predict even if Ymir had been a traditional vehicle with a fixed and regular shape. But the shard was irregular. They didn’t have time to measure it and to feed the data into an aerodynamics simulator, so they could only guess how much lift it was going to produce. And when its leading edge and its underside began to plow through the air — even though the air might be so thin as to be indistinguishable, for most purposes, from a vacuum — it was going to heat up. Steam would rise from it, producing some amount of upward thrust, and its shape would change. So even if they had been able to simulate the shard’s aerodynamics, its lift and its drag, those numbers would quickly have become wrong during its first encounter with the upper air.

Compared with all of those complexities, the fact that Ymir would be flying backward while operating a damaged, experimental nuclear propulsion system at maximum power seemed like a mere detail.

Faced with so many imponderables, a well-managed aerospace engineering project would have called a halt to all further work and devoted several years to analyzing the problem down to its minutest detail, exposing pieces of ice to the blast of hypersonic wind tunnels, building simulations, and war-gaming possible alternative strategies. But by the time Markus understood the general shape of the problem, they had twenty-four hours remaining to perigee. The tangerine Earth had grown to the size of an orange. No power wielded by humans could prevent Ymir from passing around it and scraping the atmosphere. They couldn’t even ditch. New Caird, detached from Ymir, didn’t have enough propellant in her tanks to materially change her course and would end up going on the same ride anyway. So Markus made a reasonable guess as to what would be a good angle of attack — the orientation that Ymir would adopt vis-à-vis the atmosphere — and initiated a program of thruster firings that, over the course of half a day, swung the ponderous shard around into the position he deemed best.

Ymir’s “stern” was now aimed in the direction of movement, the huge mouth of the nozzle aimed forward so that it could make the all-important braking burn. But she was now twisted about her long axis in such a way that New Caird, still docked up near the “bow,” and projecting from the side of the shard at approximately a right angle, was on the zenith. This meant that during the passage of the high atmosphere her view of the Earth below would be blocked by Ymir, the nozzle of her engine pointed “up” toward the stars. Firing that engine would therefore tend to rotate the bow downward and the stern up, an attitude likely to produce more lift and help get Ymir out of trouble. Were the shard to tumble the other direction, it could end up in a position that would yield much more drag and much less lift, pulling the whole contraption deep into the atmosphere. In effect New Caird had been reduced to the status of a small attitude control thruster. It was a thruster that could only push in one direction, and so Markus had picked out the direction most likely to be useful if things began to go sideways. Vyacheslav would ride it out in New Caird’s pilot’s seat, from which he would enjoy an arm’s length, tunnel-vision view of some dirty, five-billion-year-old ice. He would wait for a verbal command from Markus, ensconced in Ymir’s common room, to fire New Caird’s main propulsion if needed.

All of this was mere background noise to Dinah, who was entirely consumed with coordinating the efforts of robots. The number of Nats was in the tens of thousands. They could only be talked to collectively, as swarms. Trying to address and control them one by one, while theoretically possible, was a mug’s game. Their general task was to morph the shard.

One swarm would be working on the inner surface of the nozzle bell. At the moment, all of those were out on the back end of the shard, sunning themselves to build up their internal reserves of power. At a signal from Dinah they would all converge on the circular maw of the nozzle, climb down into the bell, and spread out to reshape it as needed during the burn. They’d be running a program that Larz had developed and tweaked. So all Dinah really needed to do was to turn them on.

Likewise, the smallest of the three swarms was down inside the ice hoppers, running Larz’s program for keeping rocks away from the augers. Working in the dark, these had to sip power from electrical taps that Ymir’s crew had installed for that purpose.

The largest of the three swarms, though, was responsible for sculpting the interior of the big shard as it was hollowed out. By the time the journey to Izzy was finished, most of the ice would have been fed into the hoppers and blown out the nozzle, leaving a hollowed-out shell with just enough internal structure to hold the reactor in place and maintain some semblance of a nozzle bell. This was not as crazy as it sounded, for two reasons. First, it was what miners had done since time immemorial. They didn’t just hollow out mountains, since that would lead to collapse. They sculpted the mountains into structurally sound architectural systems, complete with pillars, arches, and vaults. This was just that, except that the material was ice, and the forces in general were not as large. Secondly, most of the shard’s interior was of little consequence from a structural engineering standpoint. There was a reason why airplanes and race cars had been hollow shells — all skin and no bones. Most structural forces were naturally transmitted through the outermost layer of the vehicle, so that was the best place to put the strength. Enough strength on the outside made it possible to leave the inside hollow.

Ice, of course, wasn’t the best material to work with. It was brittle. But the Ymir expedition had shipped out carrying a large supply of high-strength plastic cord, net, fabric, and loose fiber. And during the months that it had been coasting in from Grigg-Skjellerup, Larz’s robots had been at work converting ice into pykrete. That outer layer of visually black ice was no longer ice per se, but a synthetic material with much better structural properties. Frozen, it could stop bullets. Melted and strained, it would separate into water, artificial fibers, and black crud from the dawn of the solar system. In any case the larger robots — the Grabbs and the Siwis — responsible for doing most of the heavy material removal on the inside could scrape to within a few meters of that outer skin without compromising Ymir’s structure. It was the responsibility of the third Nat swarm to clean up after them and maintain the internal pillars and webs that would keep the reactor and the hoppers suspended in the middle of the hollow shard. This swarm-based ice-sculpting algorithm had been Larz’s invention, and he’d had a couple of years in which to perfect it, but Dinah was in charge of it now, and had a lot of learning to do between now and when she became fully responsible for it.

Outnumbered by the Nats, but responsible for moving a much larger tonnage of ice, were the hundred or so Grabbs and Siwis, now mostly stationed at the ready around the shard’s interior. Most of these were general-purpose robots with some added-on bits that made them good at moving on ice, but there were also half a dozen Leatherface machines: upsized Grabbs with shovel-studded chain saws for limbs, made to move a lot of ice in a hurry. These were so good at their jobs that they tended to destroy their surroundings, so they had to move frequently. Each one had to be followed around by an entourage of smaller robots cleaning up its mess and getting it anchored to fresh locations.

In theory it was all just a big computer program that, when executed, would smoothly convert the solid hill of ice into something like a walnut with the meat removed: a thick, pockmarked outer shell with an organic internal system of ribs, veins, and webs. As with any other computer program, it might run perfectly when Dinah started it. But it might just as well go sideways, perhaps in a manner that wasn’t obvious at first. So situational awareness was going to be a big part of her task. Interesting as it might be to look out the window and watch the Earth screaming by at twenty-four thousand miles per hour, she would need to keep her head down, searching through a roar of weak and ambiguous signals for signs that something was going awry. She liked to imagine that her days as a little girl in a mining camp, sitting in front of a radio console trying to pick out Morse code signals from far away, through static and crosstalk, might have prepared her for it in some way.

A FEW MINUTES INTO HIS SCAPE CONVERSATION WITH J.B.F., DOOB realized that, two years ago, he had done his job too well.

He’d gone into that meeting at Camp David with the mission of getting the president to understand the exponential breakup of the moon that was going to wipe out life on the Earth’s surface. Putting on his Doc Dubois hat, he had coined the terms White Sky and Hard Rain as easy-to-grasp handles on phenomena that, truth be told, were much more complicated. Dr. Harris was now wishing that the late Doc Dubois had never opened his big fat mouth.

He was in a corner of the Farm that, since the departure of New Caird, had developed into a sort of bullpen where he and Konrad and some of the other orbital mechanics geeks hung out. The Farm had always operated something like a high school cafeteria, with different cliques habitually sitting in certain areas, and now those choices were hardening, becoming a part of Izzy’s unwritten procedure manual. Anyway, they had printed out charts and plots representing, in more or less abstract form, everything that they knew about the ongoing development of the lunar debris cloud and what it might mean for the future of the Cloud Ark. The expenditure of paper and printer ink had been somewhat lavish. Two generations from now, if any humans survived, they would look on this heap of documents with some combination of disgust and amazement. Because paper was going to be scarce by then, and they would view its use for such purposes in roughly the same way as Americans of the twenty-first century had viewed the use of sperm whale oil to fuel streetlamps.

But then life would get better, forests of genetically engineered trees would grow in vast rotating space colonies, paper would become plentiful, and these sad yellowed scraps would be displayed in a museum as evidence of the privations suffered by the Arkers.

Assuming they didn’t screw it up. Which was really the topic of this Scape call with Julia. She was floating in her arklet. She seemed to have adjusted to zero gee; she’d figured out how to pull her hair close to her head, the moon face had abated, she wasn’t visibly nauseated. People were drifting to and fro in the background. The only one Doob recognized was Camila. A couple of other kids were doing what looked like work: prodding and massaging their tablets purposefully, looking up from time to time to engage in brief conversations. A South Asian lad, an African girl, another girl who was probably Chinese.

Girl, kid, lad, girl. His politically correct superego, cultivated during long years of service in academia, was trying to light up his shame neurons. Doob felt no shame — he was way past that — but he was struck by just how young the Arkies were, how different they were demographically from the General Population. It gave him a vaguely troubling sense of being out of touch. It had been decades since he had been young, but he had always been one of the cool kids anyway, with a big following on Facebook and Twitter. Now he was stuck on Izzy and Julia was stuck in an arklet. The two of them were hanging out with completely different populations. GPoppers saw each other all the time and talked face-to-face. Arkies were isolated in their arklets and had to use social media to reach out. Doob hadn’t looked at his Spacebook page since the White Sky, and this call with Julia had been delayed for fifteen minutes while he’d tried to figure out the user interface on Scape — something with which Julia was obviously familiar and comfortable. She used it all the time, and if it didn’t work, one of those kids in the background would help her with it.

Another straw in the wind: while Doob had been fumbling with Scape, he had overheard a brief snatch of conversation from the other end in which the South Asian kid had addressed Julia as “Madam President.” This seemed so odd that he was tempted to bring it up in conversation. But he knew what the answer would be: It was just a courtesy. Former presidents were always addressed thus. It didn’t mean anything. Why was he making a big deal about it? He would come off as some combination of uncouth and comically hypersensitive.

“Dr. Harris, as you know, I’m something of a fifth wheel around here, and so I want you to know I appreciate your taking any time at all out of whatever it is you are busy with to touch base,” Julia began.

“Not at all, Madam. . Julia,” Doob said, and then, because this was a video connection, resisted the urge to slap himself.

She found that interesting but decided to overlook it. “I feel like a camp counselor here,” she said. “Of course, I was on top of every detail of the Arkitects’ work during the run-up. But to sit in the White House looking at PowerPoints is one thing. Actually to be here is quite another.”

This was quite obviously bait. Fully aware of what a sucker he was being, Doob said, “How so?”

“Well, of course the range of cultural perspectives is vast,” Julia said, “but, modulo that, I find a lot of uncertainty. A sense that all of the Arkies’ talents and energies are bottled up — like so many genies just waiting for someone to rub the lantern. They all so keenly want to help.”

“It is, of course, less than two weeks since the Hard Rain began,” Doob pointed out. “We have five thousand or so years left to go.”

“The Arkie Community is well aware of those numbers,” Julia remarked.

The Arkie Community. Wow. He had to admire the way she’d slipped that in.

“Julia, what’s the purpose of this call? Am I to understand that whatever answers I give you will then be somehow disseminated to the Arkie Community? Because we have an email list for such purposes. An email list that includes every living human being.”

“That list was most recently used two days ago. An eternity for bottled-up Arkies.”

“We have been just a tad busy with the New Caird expedition.”

“There is a lot of curiosity about that in the Arkie Community.”

“There’s a lot of curiosity about it here.”

“I mean about its purpose,” Julia said.

“How could its purpose be any clearer?” Doob asked. “Anyone who made it through the screening and the training required to become an Arkie”—which doesn’t include you, Julia—“will understand exactly what we are trying to do from an orbital mechanics standpoint.”

“Obtain the stupendous amount of water that will have to be expended in order to attempt the Big Ride gambit,” Julia said. “Yes, Dr. Harris, even I understand that.”

“Gambit? Really?”

“Do representatives of the GPop ever make much of an effort to reach out and acquaint themselves with the thoughts and perceptions of the AC?” Julia asked.

“The what?”

“The Arkie Community,” Julia explained, with the slightest roll of her eyes.

“At any given time, about ten percent of the Arkies are rotating through Izzy. You know this. It is the largest number we can accommodate.”

“I’ve talked to several who have experienced that rotation. They all report the same thing. As soon as one enters the privileged environment of Izzy, with safer conditions, more room to move about, better food, and greater exposure to senior staff, the GPop worldview seems so sensible. Which only accentuates the reentry shock upon being deposited back into one’s arklet.”

Doob bit his tongue.

Julia continued. “What about reversing the roles a little — sending members of the GPop on temporary home stays in randomly selected arklets?”

“What about it?” Doob asked. “What purpose would it really serve?”

“From a purely technocratic standpoint, perhaps none whatsoever,” Julia said. And left the rest of her thought unsaid.

“If I went on a ‘home stay’ in a random arklet, what would I learn that I can’t learn from Scape or Spacebook?”

“A great deal, since you don’t actually use those applications,” said Julia, her voice deepening in amusement.

“I’m a little busy trying to get New Caird home. Go ahead, tell me. What am I missing?”

Movement caught his eye across the table.

He looked up to see Luisa shaking her head. Then Luisa clamped her face between her palms, closed her eyes for a moment, and opened them again. Doob felt his face warming, and once again resisted the temptation to slap himself.

“There is a lot of ferment in the AC around alternative strategies,” Julia said, speaking briskly and authoritatively, as befit a woman who had just been anointed the spokesperson for said Arkie Community. “A fascinating school of thought is developing around the idea of making a passage through clean space to Mars.”

“Clean space?”

“Oh, I forget you haven’t been following the relevant discussion groups. Clean space is just what Tav has been calling the translunar zone, relatively free of bolides.”

“Tav? Tavistock Prowse?”

“Yes, you should have a look at your old friend’s blog occasionally.”

Tav had been sent up to Izzy a month before the White Sky, when someone on the ground had decided that social media was going to be the glue that would hold the Cloud Ark together and that Tav was just the man for that sort of thing.

“I’ve been busy,” Doob said. “But Tav ought to know that we have simulated and war-gamed the Mars option to within an inch of its life, and it’s just not a good idea.” He could see Julia formulating an objection that he didn’t have the patience to listen to. “Anyone who is seriously advocating we go to Mars is—” He didn’t want to say what he was thinking, which was smoking crack, and so he settled for “—not taking some of the practical realities into account. One solar flare at the wrong time could kill everyone.”

“Only if everyone goes.”

“If you’re talking about just sending a contingent to Mars, then you have to consider how much of our equipment and supplies they’ll be allowed to take with them.”

“I think many talented Arkies would volunteer to be part of a small, lean advance party. The lure of clean space is strong.”

“Well, we are not in what I guess Tav considers clean space,” Doob said. “We are in dirty space, and we have to focus on that reality, rather than woolgathering about trips to the Red Planet.”

“You needn’t remind me,” Julia began.

“Yes. You saw your friend and colleague Pete Starling get chowdered by a bolide. I did actually see your Spacebook post about that, Julia. It was most affecting. I sense a ‘but’ coming, however.”

“As the days go by without a serious incident, people begin to wonder how dirty space really is. Interest grows in the Dump and Run option. The White Sky now feels like ancient history. The Hard Rain is upon us. Every day brings a course correction or two, to avoid a major bolide, and a litany of minor events. But the death toll stands at—”

“Eighteen, as of ten minutes ago,” Doob said. “We just lost Arklet 52. See, I am keeping my ear to the ground.”

“I am sorry to hear that,” Julia said, “and I’ll bet the rest of the AC will feel the same way, once that news has been distributed to them.”

“It’s on a fucking spreadsheet, Julia. All you have to do is look at it. We don’t distribute news. This is not the White House.”

“But in many respects it behaves like the White House,” Julia said. “An orbiting White House unfettered by constitutional checks and balances. But at least the White House had a briefing room, a way of reaching out. I would be happy to. .”

“Why are you even talking to me about this?” Doob asked. “I’m a fucking astronomer.” Then, a thought. “How many conversations like this one have you been having with other members of the GPop?” He’d been assuming that Julia had singled him out as special, but for all he knew she had a call list as long as her arm, organized by those assiduous youngsters in the background. “Ivy is temporarily in charge.”

“I am familiar with the chain of command that has been improvised,” Julia returned. “To answer your question, Dr. Harris, I am talking to you precisely because you are an astronomer, and well positioned to answer the questions and concerns among the AC about the exact nature of the dirty space threat. This news from Arklet 52 is going to raise questions about the effectiveness of Ivy’s current strategy.”

“It is a statistical problem,” Doob said. “On about A+0.7 it stopped being a Newtonian mechanics problem and turned into statistics. It has been statistics ever since. And it all boils down to the distribution of bolide sizes, and of the orbits in which they are moving, and how those distributions are changing over time — which we can only know from observation and extrapolation. And you know what, Julia? Even if we had perfect knowledge of every single one of those statistical parameters, we still wouldn’t be able to predict the future. Because we have an n of 1. Only one Cloud Ark, only one Izzy to work with. We can’t run this experiment a thousand times to see the range of different outcomes. We can only run it once. The human mind has trouble with situations like that. We see patterns where they don’t exist, we find meaning in randomness. A minute ago you were casting doubt on whether dirty space was really that dirty at all — obviously arguing in favor of Dump and Run. Then I told you about what just happened to Arklet 52 and now you’re swinging around to the other point of view. You are not helping, Julia. You are not helping.”

Julia did not look to be accepting Doob’s remarks in the spirit intended. Instead she squinted through the screen at him and shook her head slightly. “I don’t understand the intensity of your reaction, Dr. Harris.”

“This conversation is over,” Doob said, and hung up on her. He then fought off a temptation to slam the tablet down on the table. Instead he sat back in his chair and looked Luisa in the eye for the first time in a while. On one level he’d wanted to watch her face the whole time. But Julia would have noticed that, would have figured out that someone else was in the room, silently listening.

Just as someone had probably been doing at Julia’s end.

Luisa just sat there in her listening shrink mode.

“It would be easier,” Doob said, “if I could figure out what the hell she wanted.”

“You’re assuming,” Luisa said, “that she has a plan. I doubt that she does. She is driven to seek power. She finds some way to do that and then backfills a rationalization for it afterward.”

Doob pulled his tablet closer and started trying to find Tav’s blog. “To what extent do you imagine she really is reporting facts about the AC? As opposed to creating the reality she describes?” Doob asked.

“What’s the difference?” Luisa asked.

DINAH LOOKED UP AND SAW THAT THE EARTH WAS THE SIZE OF A grapefruit. She took a nap, ate some food, and buckled down to work again, then looked up to see that it was the size of a basketball. Still not that big; and yet such was their speed that it was only an hour away.

They had a final briefing session in Ymir’s common room, which had become a makeshift bridge for this ungainly ship.

Dinah had scrounged three flat-panel monitors from various parts of the command module and zip-tied them to the common room table. These were covered with overlapping windows of various sizes. Some were terminal windows showing log entries or editors showing code, but most were video feeds showing different robots’ points of view on the mining operation. Only one of them looked outward: she’d positioned a redundant Siwi on the stern, toward the nadir, and aimed its camera at Earth. Other than that, her only “situational awareness” would come from a celestial navigation program that would display, in a small window, a three-dimensional rendering of the Earth with Ymir’s trajectory superimposed on it as a geometric curve. Along the bottom of that window was a series of graphs plotting velocity and altitude versus time. Their velocity at the moment was some six thousand meters per second, up from four thousand just a couple of hours ago; within the next hour it would double if they took no action, then begin to drop again as they left perigee behind and coasted away into space.

That velocity would take them all the way back out to L1 again unless Jiro succeeded at his task, which was to slow them down. He had satisfied himself with but a single flat-panel screen, which he’d set up directly across the table from Dinah’s triptych. From here he would be managing the reactor. He had already begun to pull some of the control blades, just to get a sense of how quickly it would come to full power when he did it for real. A miscalculation on that front had led to the fuel rod breach that had indirectly killed Sean, and Jiro didn’t want any surprises this time around.

Some minutes before perigee, if Markus felt that things were otherwise going to plan, Jiro would issue commands that would bring the reactor up to its full thermal power output of about four gigawatts. Ice would melt to superheated water, and steam would howl through the Inconel throat of the nozzle, expanding and cooling in the bell until it turned into a hypersonic blizzard, a white lance of cold fire pushing against the great ship’s movement, slowing her down. Not so much as to let her fall into the atmosphere and die, but enough to reduce her orbit to something more like Izzy’s. Ymir would experience acceleration, which would feel to its inhabitants like gravity. All the stuff that was now floating around loose aboard Ymir and New Caird would fall “down.” Dinah and Jiro would drop into the chairs that they’d positioned in front of their monitors. So would Markus, for he had built his own nest of tablets and monitors at the head of the table, mostly occupied with navigational data. Up in New Caird, Slava would find himself pressed into his acceleration couch at an awkward sideways angle. The gee forces would be modest enough — even a four-gigawatt nuclear propulsion system could only exert so much force against the momentum of such a large chunk of ice. If their “weight” remained steady over time, it was a sign that things were going well. If it increased, it probably meant that they were going to die. For the only thing that could slow them down and increase their perceived weight beyond a certain level was contact with the atmosphere. The more they slowed down, the lower they dropped. The lower they dropped, the thicker the air got. The thicker the air got, the more force it exerted on the ship. They would read this as a sense of increased weight. It was an exponential spiral that, beyond a certain point, would lead to the inevitable destruction of Ymir, New Caird, and everyone on board. The only real question would be the manner of their death. In a smaller, lighter craft they might be burned alive. Here, being surrounded by ice, it was more likely that they would lose consciousness from the gee forces first — a relatively painless way to go. Dubois Harris and Konrad Barth, gazing down on them from a few hundred kilometers above, would see them go out as a blue streak over the southern hemisphere, and would give the news to Ivy so that she could issue a statement to the Cloud Ark that, if Dinah was any judge of her friend, she had already written, just in case she needed it.

It was strange to be so close to them in distance, but so far away in the nonintuitive space of delta vees. Bandwidth between New Caird and Izzy was excellent now, and Dinah had to make a conscious effort not to get distracted by the availability of text messaging and even Spacebook. See you in a few xoxo, Ivy had texted her, and Dinah had sent back something in the same vein, then closed the window.

Vyacheslav was donning one of the blue thermal garments worn beneath space suits. This, she knew, was just a precaution, in case he had to “go outside” on short notice for some reason. Slava had stationed his space suit in New Caird’s airlock, so that he could exit to the outside of the shard if needed, and Dinah had pre-positioned two Grabbs there, to help him get around.

Markus could be pretty unceremonious about things, which was just his leadership style — his way of implicitly telling people he expected them to do their bloody jobs without pep talks beforehand or congratulations afterward. It didn’t work for everyone. Some people liked ceremony. But he hadn’t invited any of those along on this expedition. So there was no particular moment when it all started. They just kept getting closer to the Earth. Slava darted up the companionway to the top of the command module, and a minute later announced that he was positioned before the controls of New Caird. Jiro called out milestones in the startup of the reactor, occasionally proffering hints as to how the figures should be understood: “That is a little faster than I expected. . settling down now. . this is according to plan. . ready to proceed on your command. .” and so on. Markus’s participation consisted largely of chewing his thumbnail while staring fixedly at his screen. From time to time he would reach out and type something, or swipe and tap on his tablet. Dinah’s work was almost entirely abstract, several layers removed from what obviously mattered. She tried to focus on it and to ignore the sounds of a thousand loose objects settling to the “floor” of Ymir as gravity “came on” due to a combination of Ymir’s increasing thrust and the steady buildup of atmospheric pushback.

“Now,” Markus said.

“Acknowledged,” Jiro responded. “Control blades are responding to program. . and. . we have criticality.”

Four gigawatts of thermal power — enough to supply Las Vegas — came online in the next few seconds. Dinah felt it as a massive increase in weight and heard it as a cacophony of creaks, groans, and crashing noises as the command module, and the ice surrounding it, came under structural load. She saw it on her screens as sudden and frantic change in windows that had remained frustratingly static for the last hours. The ice hoppers, which had been brimful for weeks, began to empty at a shocking pace as the augers spun. A couple of her “point of view” robots fell or skidded from their points of anchorage, events that showed up as sudden and unhelpful shifts in camera angle. She hit “go” on a program that tasked every robot in the shard to deliver more ice into the hoppers as fast as possible, and tried to keep one eye on that while monitoring the structural integrity of the shard as a whole. In the traditional scheme of things, miners had done all their work under gravity, so structural mistakes had manifested themselves soon, and dramatically, as cave-ins. Ymir was a mine that had been slowly delved under zero gee and only subjected to “gravity” for short periods when the engine came on, and so there was a certain nervous feeling of not knowing whether it might all collapse. So far, it looked fine.

“We are losing velocity nicely,” Markus muttered through what was left of his thumbnail, and Dinah permitted herself a glance at the graphs to verify that this was so. Time had gone by faster than she had known; they were just minutes away from perigee. “Nicely” in Markus’s phrasing meant “enough to make a difference but not so much as to kill us.”

Then Markus said, “Slava. A three-second burn, please.”

“Da,” answered the Russian. Then, a few seconds later, he said, “It is beginning.”

They wouldn’t have known Slava was doing anything, save for an external camera that Dinah had positioned on the surface of the shard at some distance from New Caird, looking back at it. This showed a ghostly blue flare emerging from the small ship’s nozzle bell, shoving Ymir’s nose down and swinging its stern up slightly.

Ymir shuddered faintly. Dinah didn’t know what to make of it. She feared it might be a cave-in until she identified it as a sensation she had never expected to feel again: atmospheric buffeting. She had not been this close to the surface of the Earth since she’d been launched into orbit almost a year before Zero. And if the next few minutes went well, she’d never be this close again.

The shuddering didn’t last. The graphs on her screen had all picked up little wiggles that were steadily receding into the past. “We skipped,” Markus said. “I think we will do it at least one more time.”

“Augers four and eleven are down,” Jiro announced. “I will try reversing them to clear the jam.”

This brought Dinah’s attention back. At a glance she checked all of the levels in the hoppers and saw them dropping quickly, as expected, despite the robots’ efforts to replenish them. The two that Jiro had identified were overfull, since there was no way to get the ice out of them. Dinah activated a subprogram that would put some Grabbs to work transferring surplus ice from hoppers 4 and 11 to nearby ones that could use it.

“The Caird burn worked,” Markus said, “but it gave us a little too much rotation. I am counteracting using thrusters — and this will take a little while.” He entered some commands that, presumably, turned on those of Ymir’s attitude thrusters that pushed in the opposite direction from New Caird’s big engine. “By the way, we just passed through perigee — I hope.”

Dinah glanced at the plots and saw that they had indeed passed the midway point of the maneuver. Somewhat paradoxically, though, their altitude was dropping — headed for their second, and hopefully their last, “skip” off the atmosphere.

“We’re on some weird new course now,” she said.

“It is true,” Markus said. “If we survive the next few minutes, we can fix it later.”

“Auger eleven works again,” Jiro reported, “but two and three are down. We may have a critical propellant shortage.”

“Damned thrusters are not powerful enough. We have overcorrected,” Markus said, “and now we are coming in for another skip. We are flying not only backward but upside down.”

So the burn from New Caird’s engine had done its job. It had depressed the ship’s “nose,” which was pointed backward, and prevented the stern, currently pointed forward, from digging in. They’d grazed the atmosphere with the shard’s broad side and gotten a nice skip out of it — a bounce that might’ve saved their lives. But once the shard began to rotate, it was difficult to make it stop, and now it had gone too far. The nose was pointed too steeply downward, the nozzle bell was aimed up toward space.

“So we’re thrusting down toward the planet now?” Dinah asked.

“Not enough to hurt us. Maintain thrust,” Markus ordered.

“I am running out of ice,” Jiro said, and glanced over his monitor at Dinah.

Dinah had already warned them that supplying enough propellant to do all of this in one huge burn would be a close-run thing, assuming everything went perfectly. Everything hadn’t. She met Jiro’s eyes, shook her head, and went back to work.

“Get ready to shut it down, Jiro,” Markus said. “We are descending into thick air and I don’t know what is going to happen.”

Their inner ears told them that something was happening. The powerful thrust was still driving them into their seats, but some force had taken Ymir by the nose and was torquing it around.

“We hit nose first,” Markus said, “and we are spinning back. Main engine shutdown in three. Two. One. Now.”

A nuclear steam engine didn’t shut off quickly. The thrust faltered and tapered off in response to whatever commands Jiro had entered. It was the better part of a minute, though, before they were back in zero gravity — meaning in a free orbit with no thrust pushing them around.

“I’ll give you our new orbital parameters in a minute,” Markus said. “It is complicated because we are tumbling.”

In the sudden silence that followed the engine’s shutoff, Dinah could hear distant, tinny shouting. She realized it was an open audio channel from Izzy, coming from a pair of headphones she had ripped off her head during the maneuver. It was the sound of people in the Tank. When she pulled the phones back onto her head she could tell that they were celebrating.

“THAT WAS A BIG-ASS DELTA VEE YOU GUYS JUST RIPPED OFF!” DOOB said when he heard Dinah’s voice on the other end of the link. “You deserve congratulations.”

Dinah’s response, after a few seconds’ delay, was guarded. “But not big-ass enough?”

It was strange hearing the voice of one you knew well modulated through this old-school audio tech. Like hearing Dinah doing a Buzz Aldrin impression at a party. The emotional nuance came through more clearly than the actual words.

“Konrad is still calculating your params,” Doob said, “but just on visual inspection we can see how much you slowed down. Fantastic.”

“Sounds like we’ll be needing another pass then,” she said. Meaning that they would have to wait for Ymir to loop once more around the Earth, and do another burn at her next perigee, in order to slow down enough to rendezvous with Izzy.

“This time you can work with a higher perigee,” he pointed out, “so you don’t have to fly that damn piece of ice through the pea soup again.”

“Flying this damn piece of ice kind of stresses me out,” Dinah allowed.

“The glass is half full, baby,” Doob said. “The glass is half full. You lit that candle. It worked. You bounced off the atmosphere. You’re a hell of a lot closer to us — Konrad is saying your apogee is definitely sublunar.” Meaning that Ymir would turn around and start falling back toward the Earth before reaching the orbit of the former moon. “This is huge,” he added. “It is going to change the picture politically.”

After a lengthy pause, Dinah asked, “Politically?” as if she couldn’t quite believe what she had heard.

“I’M AWARE OF THE FACT THAT IVY HAS TURNED A DEAF EAR TO ALL of your ideas,” Julia began, just as soon as Spencer had typed in the commands that disconnected Arklet 453 from the Situational Awareness Network. “I presume she also went out of her way to place obstacles in the path of your coming here for this meeting.”

The Martians — Dr. Katherine Quine, Ravi Kumar, and Li Jianyu — looked somewhat nonplussed. It was always difficult to travel between arklets. The waiting time for nonurgent Flivver trips was about two days, and emergencies could rearrange the queue at the last minute. As a member of the General Population, Dr. Quine had the most Olympian perspective on this — she was an urgent care doctor frequently called upon to make excursions to arklets. She was about ten years older than Kumar and Jianyu, who were Arkies chosen in the Casting of Lots from India and China respectively. Those two had ended up together in Arklet 303, which had turned out to be a hotbed of Martian agitation. It was part of a triad with a total population of eighteen, half of whom currently had the flu, and so Katherine Quine had had a legitimate excuse to go there. She’d made the most of the opportunity by scooping up Ravi and Jianyu and coming here with them. Of the people in this conversation, she was probably the least inclined to see dark deeds by Ivy in the slowness of inter-arklet transport. It was a different story with Ravi and with Jianyu, who, for a number of reasons, were receptive to Julia’s suggestions on that front. In another time and place, Dr. Quine might have quibbled. But time was short, and trying to raise Julia’s opinion of the Cloud Ark’s current management did not seem like an efficient way to use it. So she let it go. And by the time she had processed all of that, Julia had moved on anyway.

“Given that, I’m all the more appreciative that you made the arduous and risky journey to meet with me in person,” Julia said. “It is my firm conviction that, centuries from now, young Martians sitting in classrooms on the Red Planet will read in their history books — or whatever they have in place of books — about this meeting and what came of it.”

Ravi Kumar raised an index finger. “Instead of educating the young in classrooms,” he said, “why not do away altogether with the traditional structure of mass education and take a personalized, individualized approach? There’s no reason to repeat Earth’s mistakes on Mars.”

“I could not agree with you more,” Julia said, “and these sorts of fresh ideas only make me more eager to find a way of getting as many people there as soon as possible. How do we get started? What would be entailed in sending a forward advance party to Mars?”

For the second time in as many minutes, Dr. Quine looked a bit unsettled. She glanced around Arklet 453. This was the central, common-space arklet of the heptad that included numbers 174—the abode of Julia and Camila — and 215—that of Spencer Grindstaff. Or at least that was what it said on the official records. Some reshuffling had occurred. All the men and women who lived in those two arklets now seemed to conceive of themselves as members of J.B.F.’s personal staff. They had taken over 453 and turned it into a sort of West Wing.

Katherine Quine said, “Presuming we had authorization to send such a mission—”

“Let me just cut you off there, if you would indulge me, Dr. Quine. What you just raised is a matter of politics. I consider that to be my ‘superpower’ and I would like to place it at the disposal of you and the other members of the Martian Community — the ones you already know of, the ones who sympathize with you in secret, and others who may sign on once it becomes clear to them what a fundamentally sensible idea the Mars trip really is. So I would propose that we assume, for purposes of this little chat, that authorization is not a problem. I would like to see you three using your own ‘superpower’ of designing this mission in a way that makes sense without letting the political dimension interfere at all. Once we have designed a coherent plan, we can then move on to questions of implementation.”

“In a perfect scenario we would dump the rock and simply take everything, all at once,” Jianyu said. It was the first time he had spoken, but he seemed to have been emboldened by Julia’s talk of superpowers.

“There are powerful forces that would have to be convinced before such a thing could happen,” Julia said. “Let’s think in terms of an advance party: lean, efficient, smart, but big enough to get the job done. That means landing on Mars and reporting back to the remainder of the Cloud Ark.”

“We’ve been talking about such a mission. We think we could do it with a bolo consisting of a heptad and a triad,” Katherine said.

“Ten arklets,” Julia said. “That doesn’t seem all that many, does it?”

“During the initial delta vee,” Ravi Kumar said, “the arklets would be stacked. Once they were on course for Mars they would form a bolo, so that the members of the expedition could experience Earth-normal gravity during the six-month journey.”

Jianyu added, “Propulsion and other components could come from the MIV kit. Most of the design work has already been done for us.”

Katherine said, “Aerobraking would be needed at the end, to slow it down. Before that, the bolo could be reeled in, the arklets could restack into a unified ship, and there would be time to survey the surface from orbit and decide on a landing place.”

Julia nodded. “And if I may put a hard question to you all, what would be the survival time of this isolated colony, once it had landed? How long before it ran out of provisions?”

This caused the three Martians to clam up and look at one another.

“I only ask,” Julia said, “because politics — my department — once again rears its ugly head here. Once your heroics have been accomplished, the burden falls to me to seal the deal, as it were. The advance party lands and sends back its joyous message. A ticking-clock element enters the picture. Which I do not mean in a negative way — this can be a powerful incentive to mobilize people’s energies, as we saw in the case of the buildup to the Hard Rain. It is at that point when I can address the people of the Cloud Ark and say, ‘Here is the opportunity — will we seize it? Or will we shrink away from it and let these brave people slowly expire?’ That is a speech that I think I could deliver to great effect. I just need to have some sense of the time element.”

“A year for sure,” Katherine said. “Beyond that, it becomes a medical question. A statistical question.”

“Statistics,” Julia repeated, and sighed. “I have been hearing a lot about that from Dr. Harris.”

“SO, YOU’RE TELLING ME WE’VE LOST TRACK OF WHO IS EVEN IN J.B.F.’S heptad?” Ivy asked.

There was silence around the big table in the Banana. Ivy had begun to hold important meetings in this old familiar space, closer to the central axis of the Stack and farther forward in Amalthea’s cone of shelter. It wouldn’t do to have the Cloud Ark’s command structure decapitated by a single unlucky bolide strike — a disaster much more likely to happen whenever they met in the big T3 spaces like the Tank and the Farm.

Present for this meeting were Doob, Luisa, Fyodor, and three handpicked members of Markus’s staff who had become a sort of executive troika: Sal Guodian, the one-man judicial system. Tekla, the head of security. And Steve Lake, the dreadlocked ginger who was responsible for network and computer matters.

“The default system for keeping track of who is where,” Sal began, “is based on the assumption that people will actually cooperate with it.”

Ivy held up a hand. “Stop. Before you go into explanations, I need a yes or no.”

“Yes,” Steve Lake said, “we have lost track of who is in J.B.F.’s heptad.”

“Thank you,” Ivy said. “And somehow the SAN isn’t helping us fill in the gaps?”

Steve said, “One of the people who is definitely in that heptad is Spencer Grindstaff.”

Ivy nodded.

Sal said, “Steve, when Markus pulled you into his office, just before the White Sky, and put you in charge of the network — replacing Spencer — you made some remark to the effect that Spencer might know of back doors into Izzy’s systems. Back doors that would be impossible for you to know about until he used them.”

“Yeah,” Steve said. “Almost by definition, we can’t find something like that until it’s used. Not without manually reading through every line of code.”

“You think he has a back door into the SAN?”

“We know he’s doing something,” Steve said, “because as soon as he turned up there, the arklets in J.B.F.’s heptad began dropping off the network from time to time. Whenever she’s having a meeting she doesn’t want us to know about, he turns everything off.”

Ivy considered this for a moment, then looked across the table at Tekla and nodded. Tekla rose — carefully, for the gravity was quite weak here — and went to the door. She opened it to reveal Zeke Petersen waiting outside, and waved him in.

“Thanks for joining us,” Ivy said, breaking a silence during which Zeke took a seat at the foot of the table. Ivy was at the head of it. She was looking “up” a long ski jump ramp at him, and he was doing likewise at her.

“Just like old times, Commander Xiao,” Zeke said.

“Well, I appreciate your loyalty,” Ivy said. “I know this must be awkward for you.”

“Not at all, actually,” Zeke said. “The announcement that Markus made, when he called it, at the onset of the Hard Rain — declaring all existing nations to be dissolved — I took that to heart. Julia didn’t hear that announcement. She didn’t get the memo.”

“We’ve been hearing a little about Spencer’s ability to disconnect from SAN.”

Zeke nodded. “Confirmed. I was there for one such incident. We had a very strange conversation. I think they were testing the waters to see whether I might be recruited. She spoke to me as if I were already on her side — as if it were unthinkable that I wouldn’t be. It’s a pretty good persuasive technique — she had me going for a little bit. But once I got out of there and slept on it, I saw how crazy it was.”

“Did you have the sense that this was a one-off? Or was she working her way down a list of possible recruits?”

“If I had to guess, I’d say there was a list,” Zeke said, “but not a long list.”

Ivy nodded. She didn’t have to spell it out: J.B.F. might have recruited some others — others they didn’t know about yet.

“It tallies with what I saw,” Doob said, and glanced at Luisa for a confirming nod. “I think she is just being opportunistic. She reaches out to people, draws them into conversations, drops hints, probes for vulnerabilities.”

“Is she nuts?” Ivy asked Luisa.

“In a sense it doesn’t matter,” Luisa said. “If she’s making trouble, she’s making trouble. Tracing that to a diagnosable psychiatric condition doesn’t really change anything.”

“It might change the approach that we take.”

“She was narcissistic to begin with,” Luisa said. “This isn’t a formal diagnosis, mind you. But according to what we heard from you and Dinah, her trip up to Izzy was pretty traumatic. She lost her husband and her child and blood was spilled along the way. It doesn’t take a trained professional to guess that she is suffering from some level of PTSD. Connected with that we might expect her to have a dark, paranoid vision of the world. But she may have been that way to begin with.”

“She’s cagey,” Ivy said. “As long as all she’s doing is talking to people, there’s not much I can or should do.”

“Agreed,” Luisa said. “She is building a political base among the Arkies. If you take some action against her, with your sole pretext being that she’s talking to a lot of people, then you’ve given her just what she wants. But some outreach on your part to the Arkies might be a good idea.”

Ivy sighed. “The only answer to politics is more politics,” she said, “and that’s where I’m most useless.”

“Deeds, not words,” Zeke said. “That’s what really matters. And when Ymir pulls in, you and Markus will have accomplished something that’s going to make J.B.F. and her clique look puny.”

“I MAY BE OUT OF MY DEPTH HERE,” JULIA SAID, AFTER A LONG AND thoughtful pause, “but we seem to have a striking coincidence that is staring us right in the face.”

“Go on, Madam President,” Camila urged her. “It might be obvious to you, but I for one cannot see it.”

Julia looked at Katherine Quine. “As I understand it, the key elements of the proposed Mars ship are a heptad, for the humans to live in during the voyage, and a triad, for bulk storage of propellants and whatnot. And this matches up quite neatly with our strengths.” She managed a self-deprecating chuckle. “I say ‘our.’ What do I mean by that? I suppose I’m going out on a limb by imagining that there might be some sort of a natural alliance between the Arkie Community and the Martians. A sort of ragtag rebel coalition, if you will. Here in this heptad we have rapidly assembled a social hub for advocacy of concerns relevant to the AC. In a sense we have our own heptad now. And in a like sense, you, Ravi and Jianyu, have developed your triad into an intense focus of Martian advocacy. You have your own triad. So the two largest components of the Mars expedition have already been acquired. They just need to be put together.”

Ravi was nodding. “Two of the engineers on the MIV team are keen on it. They helped build New Caird and are eager to tackle a new problem. Of the two, one might even come with us. Paul Freel. He has been a strong advocate of Mars colonization since long before Zero.”

Katherine had been listening intently, and now broke in: “I don’t mean to sound a skeptical note, Madam President, but in what sense do you really ‘have’ this heptad, or do our friends here ‘have’ the triad where they live? It might be true in the sense of having majority rule. But—”

“But what does ownership really mean in this context? Hmm, yes, it is a very profound question, Dr. Quine, and I’m glad you raised it. So many things we took for granted before, such as property rights and individual liberty, are clouded by Markus’s declaration of PSAPS. Or martial law, if we want to call a spade a spade. But as a first step toward answering your question, I would suggest that the ability to come and go at will is inextricable from ownership — that’s what it would really mean to ‘have’ an arklet, or a triad, or a heptad.”

“Well, in that sense we’re really all subject to the collective dictates of the swarm,” Katherine said. “Parambulator is what decides where we go when.”

“It truly is one of the most insidious instruments of social control ever devised,” Julia said.

Katherine looked mildly aghast. “But without it, we have a disaster.”

“That is what makes it so insidious,” Julia said. “One can always justify it by making the safety argument. We will all be slaves of Parambulator until and unless someone decides that some things are more important.”

Jianyu was looking alert and curious. “If someone did decide that,” he said, “it would change nothing unless the arklet in question was switched over to manual control.”

“It’s my understanding that this can be done at any time,” Julia said. “Was I misinformed?”

“No,” Jianyu answered, “but it would show up very prominently on Parambulator. It would set off alarms all over the Situational Awareness Network.”

“In that case,” Julia said, “we shall have to deal with the SAN when and if the time comes to take decisive action.”

IVY’S GRANDMOTHER, A GUANGZHOU-BORN, HONG KONG — RAISED woman who spoke only a few words of English, had ruled the family from a mother-in-law apartment over a garage in Reseda. Enthroned on a duct-taped La-Z-Boy and swaddled in crocheted afghans, she had handed down a series of diktats, pronunciamentos, and fatwas that had taken on the force of law within her family of three dozen direct descendants and in-laws scattered across the San Fernando Valley. While not indifferent to money, love, security, and other common psychological drives, she seemed to have been motivated by another need that was obscure and hence mysterious to most of those who paid fealty to her. Anglos might have Orientalized this as “face” or Confucian respect for one’s elders. Ivy came to understand it as a simple need for attention. Anyone who entered or left the house had to check in with Grandmother. And it was not enough just to poke one’s head in the door and say hello or goodbye; one had to sit down in the rattan side chair next to the La-Z-Boy and spend a few minutes and say a few words. Grandmother had no power to enforce this regulation other than finding arcane and baroque ways to wreak long-term revenge on those who flouted it.

Julia Bliss Flaherty, as Ivy now realized, was of the same stripe. Pinned down and obliged to justify herself, she would explain her actions in terms of some altruistic plan. And she might even believe it. But it wasn’t that at all. She was like Ivy’s grandmother. If you paid fealty to her, she would favor you, and your reputation and power would grow among all the others who did likewise. If you sent her off to an arklet and ignored her, you became an enemy of her and of her network. She wielded no power other than that. But, ignored long enough, she could become a mighty foe. Her status as an ex-president — and not just any old ex-president, but the one who had overseen the construction of the Cloud Ark and even used nuclear weapons to protect it — gave her credibility among the Arkies. It had become common to think of those as scattered and demoralized, just waiting for a leader to bring them identity and purpose. Ivy had lost track of whether that was an accurate perception or a self-perpetuating myth spread by J.B.F. In any case, it had taken on the force of reality.

She was sitting across the table from Tekla, wondering whether it would be productive to explain all of these thoughts to her. Would this Russian heptathlete care about, or understand, Ivy’s dead Cantonese grandmother in Reseda?

Maybe. But Tekla came from a tradition in which details were hoarded and dispensed on a need-to-know basis. Presented with too much information, she became baffled, bored, and finally irritated. Toward those who talked too freely, she felt the same sort of contempt as a businessman might feel toward a spendthrift. She just wanted to know what her job was.

The same quality made it difficult to get inside Tekla’s head. But that was okay. In a big organization with a military-style chain of command, you didn’t have to be everyone’s friend and treasured colleague. Markus understood as much, which was why he had ended up running the place. More Ivy’s speed had been the boutique operation that had been Izzy at Zero. Markus would have been terrible at that.

“This thing with Julia is a distraction. Nothing more,” Ivy said. “Much more important things demand my focus. Making a big deal out of it will backfire — give her more power than she deserves. But we can’t ignore what she is doing.”

Tekla was nodding. Good.

“I want you to go and visit her heptad,” Ivy continued. “You will go there in your capacity as Markus’s security chief. Do you understand? It is an official visit. You will explain that there have been problems with the Situational Awareness Network that could have dangerous consequences unless they are fixed. Beyond that, I just want you to listen to her. Because I think that she will try to bring you over to her side. It’s what she does with everyone. You would be a prize catch.”

“If she does as you predict,” Tekla said, “what should be my response?”

It was a measure of Ivy’s naïveté that she didn’t even follow Tekla’s question at first. Then she understood that Tekla was suggesting she might pretend to become one of Julia’s followers. She was volunteering to become a mole in Julia’s network.

Tekla stolidly watched Ivy’s face as Ivy figured it out.

“I would suggest taking no immediate action,” Ivy said. Which, in truth, was Ivy being not so much cagey as timid.

“Of course,” Tekla said, “to show eagerness is poor tactics, it will only arouse her suspicion.”

Ivy said nothing. Tekla explained, “I know many people with such minds.” And you obviously don’t, honey.

“My suggestion is that you report to me in person first and then we will come to a decision.”

“We?”

“I. I will come to a decision.”

“It is good that we meet here. In the Banana,” Tekla said.

“You like it?”

Tekla looked nonplussed. “It is not that I like it. The Banana is more secure.”

“From bolides, you mean.”

Tekla shook her head. “From Grindstaff.” Then she stood up — carefully, so as not to fly up and bang her head on the ceiling — and departed, leaving Ivy alone with a head full of questions. Had she really just embarked on the project of setting up an internal espionage network within the Cloud Ark? How was she going to explain that to Markus? Would he be horrified, or impressed? In either case, how would she feel about his reaction? When the hell was Dinah going to get back so that they could discuss this kind of thing over distilled spirits?

And what had Tekla meant by that last comment, that the Banana was more secure from Grindstaff? It was old, pre-Zero, and so its connection to the SAN was retrofitted and kludgy. Tekla seemed to be suggesting that if Spencer could hack the SAN to the extent of disconnecting Julia’s arklets from it, then maybe he could also hack it to the extent of placing other parts of the Cloud Ark — including the Farm, the Tank, and Markus’s office — under surveillance.

I know many people with such minds, Tekla had said. She was talking about Russian military and intelligence types, accustomed to the byzantine thoughtways of those professions. Perhaps Tekla herself had once been groomed as an intelligence asset. If Tekla really did become a mole in Julia’s network, then how could Ivy be sure that she was a straight-up mole, loyal to Ivy, and not a double agent, loyal to Julia?

THE SCRAPE WITH THE ATMOSPHERE HAD LEFT YMIR TUMBLING slowly as it hurtled away from the Earth on its new orbit. Calculating exactly what that orbit was took them fifteen or twenty minutes, and told them that they had fewer than four hours in which to take actions needed to save their lives.

If all had gone perfectly, the nuclear burn would have slowed Ymir down to the point where a rendezvous with Izzy could then have been achieved with a few small additional delta vees. They had hoped this might happen, but not seriously expected it. The best they could really hope for was to shed some velocity and reduce the height of their apogee.

That figure — the distance separating the Earth and the ship at the top of her orbit — was directly related to how much velocity she had at the bottom. Because Ymir had “fallen” in from an extremely high apogee, far beyond the moon’s former orbit, she had come in screaming hot for her skip off the atmosphere. Every bit of velocity that was killed by the huge nuclear retro-rocket burn, or by friction with the air, translated into a lower altitude at the succeeding apogee, which — depending on how the numbers had worked out — would occur weeks, days, or hours later.

The answer, once they had run the numbers, turned out to be hours.

In one sense, Ymir had missed her target by a mile; the total delta vee she had achieved had been less than a third of what they’d hoped for. And yet this had been enough to bring her apogee down from far beyond the moon’s orbit to a figure only about thrice the altitude at which Izzy circled the Earth.

Likewise, the period — the amount of time it took to complete an orbit — had dropped from seventy-five days to a mere eight hours. The lesson being that huge alterations in those figures could be purchased for comparatively small amounts of delta vee.

Bringing Ymir the rest of the way down to Izzy’s orbit, on the other hand, would require twice as much delta vee as they’d wrung out of the “burn” just completed.

Long before worrying about that, however, they would have to survive the next eight hours.

Ymir’s apogee might have been radically altered, but her perigee altitude was unchanged — meaning that it was still dangerously low. If they took no action, the next go-round would therefore bring them roaring and bouncing across the top of the atmosphere again.

On one level, raising the perigee a bit, so that they’d never have to worry about the atmosphere again, was an easy task. They could do it with a small but precisely calibrated burn at apogee. In a normal space mission, such a thing would have been straightforward. Here, it was complicated by two factors. First of all, their success in lowering the apogee, and shortening the period, had imposed a tight deadline — four hours after perigee — when that burn needed to occur.

The second complication was the ship’s slow tumble. This meant that their nuclear rocket engine was never pointed in the right direction, save by lucky accident. During the big burn at perigee, they had wanted the nozzle pointed forward, so that it would serve as a huge retro-rocket. The upcoming burn at apogee was intended to speed her up a bit, and so they needed the nozzle pointed aft. But as long as she tumbled, it was aimed in no particular direction.

So their task now was to stabilize Ymir’s attitude by using her thrusters to push back against the unwanted rotation. And, as they had discovered the first time they’d tried it, her thrusters were small and weak compared to the momentum of the big ice shard. In aerospace lingo, they lacked control authority. Ymir was like a truck skidding on a patch of oil, responding only faintly to the steering wheel. That problem had been alleviated somewhat by the large expenditure of mass during the burn. Many tons of ice had been hurled out the nozzle in the form of steam. Ymir was lighter and more wieldy as a result. Calculating exactly how much more wieldy she was, and what it meant for the thrusters’ control authority, was, in itself, a significant task that consumed another half an hour just for a rough estimate.

The result was not encouraging. In the three hours remaining, there was simply no way that Ymir’s attitude control thrusters — designed for tiny adjustments over long spans of time — could neutralize her tumble. The tumble wasn’t especially fast — the crew in the command module could barely sense that they were rotating — but it was enough to make the next rocket burn impossible. And if they couldn’t make that burn in three hours, they’d scrape the atmosphere again in an additional four, and again eight hours after that. They might survive one more ride like the first one, but they couldn’t survive two.

Once all of this had become clear to Markus, he had divided the crew in half, leaving Dinah and Jiro in the command module’s common room to look after the propulsion system and going “above” with Vyacheslav to consider the problem of attitude control.

Dinah’s task was, comparatively speaking, routine. During the perigee burn, they had expended most of the ice stored in the hoppers. Some of the augers had jammed, and the whole ice-mining operation had been thrown into general disarray as she had improvised solutions to problems that came at her from every direction. Robots were in the wrong places; some hoppers were overfull while others were empty. New ice needed to be mined and old ice needed to be rearranged. Fixing all of that in time for another burn in three hours was not an insuperable task, but it would require her full attention. Likewise, Jiro had a few reactor issues to think about. Both of them would have to toil diligently between now and the apogee burn in order to be ready.

Assuming, that is, that the other half of the crew had, in the meantime, figured out a way to get Ymir aimed in the right direction. Markus had moved that job to another part of the ship where it wouldn’t pose a distraction to the propulsion crew. Or such was his intent; but in moments when Dinah lost focus briefly, while compiling some code or foraging for a snack, she found herself wondering what they were doing up there.

By process of elimination, it had to be something involving New Caird. They had already demonstrated that Ymir’s thrusters weren’t up to the task. Only New Caird’s main engine had enough thrust to make a difference. The problem was that it was pointed in one fixed direction, which didn’t happen to be the one in which they actually needed to push.

Following that chain of reasoning to its logical conclusion made her nervous, to the point where she was almost more distracted than she would have been had Markus and Slava been working in the same room with her.

She held her curiosity and her trepidation at bay until she was certain that the engine would have enough ice to achieve the apogee burn. Her work was finished. Half an hour remained. Jiro seemed to have his side of it under control.

A sharp thud, resounding through the walls of the command module, gave her an excuse to pull up some video and to eavesdrop on the audio channel that Markus and Slava were using. Robots salted all over Ymir’s exterior gave her eyes that she could turn in any direction. Even so, it took her a few minutes to obtain a picture of what was going on.

New Caird had undocked from Ymir and was nowhere to be seen. Presumably Markus was at her controls.

A man in a space suit was visible on the outside of Ymir, “walking” toward the stern by using a pair of Grabbs as mobile anchor points. This had to be Vyacheslav. His feet had sprouted thick white whiskers. It took Dinah a few moments to make sense of the image: he had zip-tied each foot to the back of a Grabb, and the “whiskers” were the protruding ends of the zip ties. It was the kind of improvisation that would have made old-school NASA engineers turn over in their graves, had the Hard Rain not eliminated that possibility. But in the last two years, and particularly the last two weeks, this kind of hillbilly engineering had become routine.

Which only made the question of what the hell Markus was up to more compelling. If Slava was being that creative with two robots and a sack of zip ties. .

She finally spotted New Caird on a camera belonging to a Bucky that was attached to the stern of the shard, about halfway between its edge and the cavernous maw of the nozzle. The little ship was hanging in space maybe a hundred meters away, white jets erupting from her attitude thrusters every few moments as she tried to keep station behind the slowly rotating shard. Markus was flying her by hand, and it was some fancy flying indeed.

The geometry was difficult to visualize, but Dinah convinced herself that Vyacheslav was “walking” toward the same general location that New Caird was aimed at. In their own ways, the two men were focused on the same part of the shard: one of its outermost corners, where the widest part of the sugarloaf terminated and connected to its base, along a sharp but irregular edge. There, embedded in the ice, was a scrap of structural framework about the size of a car. It served as the anchor for a cluster of small conical rocket nozzles: one of those thruster systems that had proven so miserably underpowered for the current job. Aiming another camera at it, Dinah saw a steady jet of blue-white fire emerging from two of the nozzles. They were burning continuously, full blast. They weren’t designed to do that. But Ymir’s attitude control system had calculated that thrust, and a lot of it needed to be applied in those two directions if its programmed objective — getting the ship’s “nose” pointed forward and her nozzle aft — were to be achieved.

Dinah got it. Her thinking was confirmed by the chatter she could now hear, in a mix of English, German, and Russian, between Markus and Vyacheslav. But she could see in her mind’s eye what Ymir must look like, right now, to Markus, viewing it through the front window of New Caird: a huge drifting arrowhead of black ice, generally dark, but decorated at the nose and “corners” by twinkling white lights, and streaks of hot gas: the exhaust from the thrusters, running an automatic program controlled from within. Sometimes they flashed on and off. Occasionally, though, when a lot of thrust was called for in one place, they ran for a long time. Those long steady burns would stand out clearly against the dark of space.

Markus didn’t need to calculate Ymir’s rotation in his head. He didn’t need to know her spin rates about her three axes or the torque needed to counteract them. He didn’t even need to pull up the user interface on his tablet. All he had to do was fly around the shard and look for places where thrusters were staying on continuously. Those were the ones that were overloaded and underpowered. Those were, therefore, the ones where New Caird’s big engine could be used most effectively.

But how?

Her view of the thruster system was interrupted by a blurry gray form: Vyacheslav moving in front of the camera. He then came back into focus, groping for a carabiner along his waist and snapping it onto a structural member that protruded from the ice. Dinah could hear him breathing. Bracing himself with his left hand, he reached into the network of struts with his right. After a bit of groping he seemed to find something, then worked for a minute, his arm reciprocating slightly.

The thruster jets faltered and winked out.

“Done,” Vyacheslav said. “Apologies. Valve was sticking.”

“Get clear, tovarishch,” Markus said.

“Getting,” Slava returned. He unhooked the carabiner and bent away from the framework, trusting himself to the Grabbs zip-tied to his feet, and began to move away with the painfully slow gait of a man walking in hot caramel. “Just do it,” he said, then added a phrase in German that Dinah was pretty sure meant If it doesn’t work we are all dead anyway.

New Caird drifted out of frame. Dinah spent a few moments reacquiring her view. The smaller ship was closing on Ymir, headed directly for the thruster system that Slava had just shut down, and coming in on an angle between the two nozzles that had been burning.

The logic was clear; the method was insane. New Caird was going to do the job that the tiny thrusters couldn’t. Markus had to get her big nozzle aimed in about the right direction, namely, about halfway between the two that had been doing all the work. Fine. But he was also going to have to make a mechanical connection between New Caird and the shard, so that the thrust of the big engine could be transmitted into the mass of ice.

And it looked like he was going to achieve this by ramming the little ship into the big one. It was a slow ramming, like a tugboat shoving its nose against the side of an oil tanker to nudge it into a berth. But it was ramming nonetheless: not a thing for which spacecraft were generally designed.

She relaxed her painfully tight grip on the edge of the table just a bit when, moments before the collision, Markus fired the retro-thrusters, slowing New Caird at the moment of impact. But still she felt and heard the crunch resounding through the walls of the ice palace. She’d heard it before over the last couple of hours and wondered what it was; apparently Markus had done this several times already.

He had aimed for the place where the structural framework emerged from the ice, forming a sort of angle into which New Caird’s nose could trap itself, as long as the thrust stayed on. Right now that force was being delivered by her aft thrusters. But Dinah, watching Markus’s face through the front window, saw him working at the touch screen that served as New Caird’s control panel, and had a pretty good idea of what was coming next.

She pulled up the interface for Ymir’s attitude control system and saw craziness: thrusters firing all over the shard, lit up by angry icons warning of too little propellant, not enough time, overheated nozzles. The thing that Markus had just rammed was flashing red, indicating that it wasn’t even connected to the system anymore. Graphs at the bottom of the screen, and a three-dimensional rendering of the shard in space, showed just how far off they were from where they wanted to be.

She heard a little symphony of grinding, groaning, and popping, and felt the ship rotating around her.

The video feed showing New Caird was awash in white light as her main propulsion came on full blast. A quick glance at the attitude control plots showed good things happening.

“It is good,” Jiro said, “but we are going to over-rotate now.”

“Not if I got the timing right,” Markus said. “We should rotate through the correct attitude just at the time of the apogee burn. Afterward, yes, we’ll over-rotate. But we’ll have plenty of time to fix it.”

Then his transmission was cut off by an exclamation and a thud. He cursed in German, and then the audio went dead.

Dinah looked at the video feed to see New Caird canted over at the wrong angle. The flame from the engine flickered out.

The framework against which New Caird had been pushing had given way under the thrust of the big engine and crumpled, causing her to slew around. She now lay almost sideways against the ice, the crushed remains of the thruster system sandwiched between her hull and the stern of Ymir.

“Some kind of gas escaping,” Jiro observed quietly. “Or smoke.”

He was right. The eye didn’t pick it up right away because smoke behaved differently in space than in an atmosphere, under gravity. But something was burning, or at least smoldering, along the side of New Caird’s hull, no more than an arm’s length from where Markus sat.

Vyacheslav said, “The hot nozzle of the thruster is melting through the hull.”

Markus came back on the air. “Jiro and Dinah, you must be ready to fire the main propulsion at apogee—” The word was cut off by a constriction of his throat, and he coughed several times. When he resumed speaking, his voice had a strangled timbre. “About two minutes from now. Focus on that — initiate the startup procedures. Vyacheslav can help me with this little problem.” He was coughing convulsively. “Switching off,” he said.

Dinah, against orders, made a last glance at the video feed showing the nose of New Caird. Through its front window she could no longer see Markus. She could see only smoke, and the flickering, lambent light of a fire within it.

The realization of what was happening struck her like a two-by-four across the forehead. She grabbed the edge of the table and closed her eyes for a few moments, felt them fill with hot water, felt the snot flooding into her nose.

“Dinah,” Jiro said. “The auger startup checklist begins now.”

She opened her eyes and saw glowing blurs where user interface widgets ought to have been.

“If it is to mean anything,” Jiro said. “Please.” Then he reached up to enclose his headset’s little microphone with one hand, muffling the sound, and added: “He can probably hear us.”

She reached out and typed a command. “Auger one,” she said. “Go.” And she slapped the Enter key.

And so on down the list. It got easier as she went. Jiro did his part of it delicately, quietly, efficiently. And when the nuke came on to full power, right on schedule, she made sure to mention it. Loudly. In case Markus could hear them.

Only then did she look at the video feed. She was expecting to see Markus’s final resting place, a tomb of acrid smoke.

But nothing was there except a crumpled framework and Vyacheslav, standing with one hand braced against it, gazing away aft. In the background, a spreading plume of steam the size of Manhattan as Jiro’s engine fired.

“Slava?” she said. “Where is—”

“She fell off,” Vyacheslav answered. “When the engine came on, and we began to accelerate. New Caird did not come along for the ride.”

“Is she—”

“She was entrained in the plume of steam and thrown back. I can hardly even see her now.”

“Oh.”

“Dinah?”

“Yes, Slava?”

“Markus was already dead.”

“IT WOULD READ ALMOST AS SLAPSTICK COMEDY IF IT WERE NOT SO tragic — the consequences so dire,” Julia said. She was mesmerized by a video loop, the final transmission from New Caird before radio contact had been lost.

The people hovering around her in the White Arklet — as Julia’s unofficial base of operations had come to be known — all nodded, or made agreeable-sounding murmurs. They were all reading Tav’s blog post about the Ymir catastrophe, which had been posted only seconds ago.

The one exception was Tekla, who had become distracted by a detail. Attached to the wall of the arklet with strips of blue masking tape was a sheet of paper on which had been printed the seal of the president of the United States. Only two printers remained in existence, and both of them were on Izzy. So, by process of elimination, this must have been printed on Old Earth, prior to the Hard Rain, by a device that had been running a little low on cyan ink. It had seen hard service: it was torn in two places and repaired with clear tape. It had been creased and crumpled, then smoothed out. Its edges were fuzzy where previous applications of tape had been peeled away. And in the white space below and to the right of the presidential seal there was a brown smudge, oval, the size of the ball of a person’s thumb. As a matter of fact, Tekla was certain that it was actually a thumbprint, and the more she looked at it the more certain she became that the brown substance was blood.

She looked Julia in the eye, and became aware that the former president was awaiting some form of reaction from her. Unlike most people, Tekla felt no pressure, no obligation to fulfill any such expectations. Julia, a bit unnerved by that, broke eye contact and continued: “I don’t quite understand the story they are telling us anyway!”

“It is pretty convoluted,” said one of her aides, a young male Arkie with an American accent. He was one of the MIV engineers. His tone suggested that he was amused by the sheer cheekiness of the powers that be in trying to get people to believe such a yarn, and that he was much too clever to be taken in. “It kind of hinges on the idea that the hull was made out of what amounts to plastic. If it gets too hot, it—”

“It’s like plastic on a stove burner, I understand that,” Julia said. “It melts and it stinks.”

“New Caird shifted in a way that caused the hull to come into contact with a nozzle that was extremely hot.”

“But according to the story they are putting out, Vyacheslav had shut that nozzle off beforehand.”

“They stay hot for a long time. Anyway, the nozzle melted right through the hull. First it would have made a lot of toxic smoke. That would have been enough to kill him. Then, when the melting proceeded to the point where the hull was perforated, all the air would have escaped through the hole.”

“Well, that is horrible — if it is true,” Julia said, and then swiveled her eyes toward Tekla, looking for some sign in the visitor’s face that it might not be true. Tekla stared back at her in a manner that betrayed nothing. “What sort of pass have we come to, one wonders, when such crazy improvisations are called for — ramming one ship with another!”

More murmurs of agreement.

Julia was on a roll. “And as far as I can make out, it didn’t even solve the problem!”

“Problem is solved,” Tekla said. She was fluent in English and was perfectly capable of saying “The problem is solved,” but sometimes dropped the article for effect. Anglophones found this mysterious and impressive. It was also an implicit statement of Russian pride. The language of the Cloud Ark, by default, was English. That was never going to change. But the dialect was going to evolve over time, and Russians could bend it in their direction by finding ways to inject their grammar and vocabulary into everyday speech. “Burn is complete,” she went on.

“But the ship is still tumbling out of control!” said the American boy who so fancied his own intelligence.

“Slow tumble,” Tekla said. “Not problem. Plenty of time to fix now that perigee is raised.”

“Fix it how?! Markus demolished three of the external thruster packages by ramming them! Who does that? Anyway, there are only two of them left. It is a basic reality of physics that you can’t control a three-axis tumble using only two thrusters!”

“Thank you for explaining basic reality,” Tekla said. “Tumble can be eliminated by making scarfed nozzle.”

This silenced them for a few moments. One of Julia’s followers — Jianyu, a Chinese Arkie, very passionate about going to Mars — looked like he understood it. Tekla nodded in his direction. “This man will explain later. My time here is limited.”

“Yes, Tekla, and we do appreciate that you’ve been able to make time for us at all,” Julia said.

Tekla wanted to slap her so much that her hand actually twitched. The sentence Julia had just spoken, had it been delivered in a different tone, might have actually meant what it said. Instead of which, it meant I am being callously ignored and it’s about time someone important came out to talk to me. Tekla had an almost physical sense of how that mentality was radiating outward from Julia to infect the other Arkies.

Like almost everyone else in the Cloud Ark, Tekla was wearing a coverall with many pockets, compartments, external holsters, and the like. One of them contained a knife with a four-inch, double-edged blade. Its tip could find J.B.F.’s heart easily. Tekla faded from the conversation briefly as she considered how to manage this. Julia probably wouldn’t be expecting a frank assassination attempt — though you never really knew, with people who had such minds.

Tekla said, “Would you like to report any difficulties with the SAN? Repeated outages have been observed.”

Julia pressed her lips together in a satisfied way and looked toward Spencer Grindstaff.

“First I’ve heard of it,” Spencer said. The statement was met with perfect, deadpan silence.

Tekla just waited. Soon the temptation to boast would get the better of them. Her training in tradecraft — in how to be a spy — had not been all that extensive. A few basic courses, some assigned reading. The reason was simple: She was too conspicuous to be useful as a spy. Too similar to the Hollywood profile. Real spies went unnoticed. So they had kicked her out of the program and put her to work in roles, such as being an Olympic athlete, where her conspicuousness was an asset. But she had picked up a few general precepts. And she knew that this one thing — the urge to boast of one’s accomplishments — had betrayed more secrets and destroyed more careers than anything else.

She looked at Grindstaff. Unlike most people, who soon broke eye contact, he looked right back at her, grinning.

“Unusual,” Tekla said, “for one of your background.”

“Sources and methods,” he said.

“Then I will confine my remarks to what I came here for,” Tekla said. This produced an immediate exchange of glances between Julia and Spencer. Tekla ignored it. “For security reasons it is imperative that we have accurate census of which person is in which arklet. Some people like to move around. To trade places. We understand. Fine. But safety and security problems are created when, for example, arklet is struck by bolide, air is leaking, we do not know how many people are in it, their medical requirements, et cetera. Small person needs less air than big person.”

Julia was nodding. “I take your point very clearly, Tekla. Speaking for the Arkie Community, I can confirm that a more informal mind-set prevails out here on the outskirts. The perception of neglect by the powers that be on Izzy leads to a bit of a chip-on-the-shoulder attitude. Reshuffling of people between arklets seems like a harmless form of rebellion. But it’s easy to overlook the safety issue that you are pointing out. Which is a mistake. I will say that the confusion as to the real threat level we are under, as long as we—”

“As long as we confine ourselves to dirty space,” Ravi Kumar threw in.

“Yes, thank you, Ravi. It just seems that one day we hear one thing, the next day we hear another.”

“Statistics,” Tekla said.

“Yes, that is what we are told again and again, but—”

“I can say no more,” Tekla offered, and flicked her eyes at one of the small cameras mounted to the hull of the arklet.

Julia held her gaze this time, and, after a few moments, threw a glance Spencer’s way. “Tekla, a minute ago we were dancing around the topic of the Situational Awareness Network and Spencer was being a bit lighthearted — his sense of humor at work. But I feel comfortable telling you that, thanks to Spencer, we do have a way to disconnect from the SAN when we want to just have a normal conversation without wondering who might be listening in. And we have done so now. Anything you say here and now will not leave this arklet.”

Tekla favored the circle of hangers-on and admirers with a long, slow panoramic look, then actually rolled her eyes.

“Everyone out!” Julia commanded. “You too, Spencer. Just Tekla and me.”

“Your tradecraft is of low quality,” Tekla said, when all the others had dispersed through the hamster tubes to the other arklets in Julia’s heptad.

“I know,” Julia said. “It is so difficult rebuilding an intelligence community from scratch. One must make do with the materials at hand. Their youth, their inexperience, and the openness they’ve come to expect from living their whole lives on the Internet — all are inimical to doing things as they ought to be done. That is why we need more experienced hands — people who have learned the right instincts.”

“It is not just that,” Tekla said. “That is obvious.”

“Oh?” Julia narrowed her eyes. “What have I missed that is not so obvious?”

“You should not trust Zeke Petersen with further information,” Tekla said. “Unless you wish to plant false intelligence, in which case he will be an effective channel.”

Ivy and Zeke and Tekla had discussed it beforehand, and Zeke had cheerfully volunteered to be given up by Tekla as a supposed turncoat. It made little difference to him personally. And it would go a long way to cementing the idea, in Julia’s mind, of Tekla as a master double agent. By Cold War standards it was an obvious and amateurish gambit, but this was not the Cold War. This was a small town of fifteen hundred people with a former mayor who was trying to stir up trouble.

Julia narrowed her eyes and nodded slowly. She was fascinated. “I had wondered about him,” she said. “He seemed like he was just playing along. Just being polite.”

“Not a problem with Tekla,” Tekla said.

Julia liked that. She had drifted closer, and now reached out to touch Tekla’s forearm briefly. “I like that about you, Tekla. What I see is what I get.”

“Yes.” Then, after a somewhat uneasy silence, Tekla added, “You play long game. Patient.”

“To a degree,” Julia said, and suddenly her face and attitude had changed, as if her face had been recast in painted steel. “We cannot afford to be patient for very long. Markus’s death has changed everything. Until that tragic event, the members of the Arkie Community could look forward to the return of the great leader. Ivy was a mere caretaker. Her shortcomings could be overlooked. Now, awareness is spreading through the swarm that Markus is not coming back. Ivy is back in power. Sal will quote from obscure clauses in the Constitution to legitimize her status. But true legitimacy comes from the support of the governed. She’ll be moving now to solidify her hold on the reins. It’s at such a time that small, symbolic gestures can have the greatest effect. And that, Tekla, is why the next few days are such a critical time for us. Perhaps Ymir will pull through, perhaps not. We can’t afford to wait. Preparations are afoot. Three days from now, arklets will begin to break free of the Cloud Ark and begin their epic trek to high orbit. The powers that be might fear to implement the Pure Swarm strategy, for the loss of control it will mean for them. But the Arkie Community, tired of huddling behind an ineffective shield, slowly being decimated by the Hard Rain, knows no such limitations.”

“Survival of the breakaway group will demonstrate the falsity of the GPop’s predictions of danger,” Tekla said, nodding. “The power of the center will be broken.”

“For the first time, the Cloud Ark Constitution will truly come into effect,” Julia said, “notwithstanding the sophistry of the apologist Sal Guodian. That Constitution, Tekla, as I’m sure you know, calls for the formation of a security force. Not the Praetorian Guard that Markus cobbled together, but something real. I can think of no one better qualified than you to command it.”

“HOOK, LINE, AND SINKER,” SAID SPENCER GRINDSTAFF AS HE AND Julia watched Tekla’s Flivver depart with a staccato series of thruster burns.

“Oh, she definitely bought it,” Julia admitted, “but I don’t like the note of triumphalism in your voice, Spencer. What we have really learned is that Ivy is a formidable opponent. Somehow she has managed to get people like Tekla on her side. And they have come up with a fairly elaborate strategy for penetrating our organization.”

Grindstaff shrugged. “As these things go, it’s not that elaborate. Kind of obvious, really.”

“Easy to say,” Julia said, “given that you have a bug hidden in the Banana, and we knew everything they were going to do. But lacking that information, Spencer, do you really think we’d have seen through it? I thought Tekla did a marvelous job.”

“You need to look out for her. She really hates you. And she’s carrying at least one weapon.”

“Thanks to Pete Starling,” Julia said, “so am I.” She reached into her bag and drew out a small revolver, just far enough that Spencer could see the butt of its grip, then slid it back in.

“At the risk of insulting your intelligence,” Spencer said, “I would like to remind you of the consequences of firing that thing inside of a space vehicle.”

“No offense taken. I’ve actually seen those consequences. And you know what? The air doesn’t leak out that fast. Anyway, I’m told that the rounds in this weapon are designed to mushroom on impact, so they are less likely to exit the body.”

“That’s great,” Spencer said, “provided you actually hit a body.”

“If it comes down to me and Tekla,” Julia said, “I’m not going to miss.”

ALL DINAH WANTED TO DO WAS SLEEP. SINCE NEW CAIRD HAD DEPARTED from Izzy she had never gotten more than four consecutive hours, and the numbers for the last day or so were even more dismal. In a weird way, she wanted to sleep so that she would be able to grieve properly. She knew Markus was dead, but it hadn’t really sunk in. Nor would it, as long as she was running from one crisis to the next.

The burn had worked. Ymir’s perigee altitude had been raised to the point where it would never again be troubled by the atmosphere. But the ship was still tumbling, albeit slowly. And Vyacheslav was still trudging around on its outer surface with his feet zip-tied to Grabbs.

At the start of this extravehicular activity, Slava had exited through the airlock on the side of New Caird—a ship that was no longer with them. His supplies were running low. He had to get inside the command module before he ran out of air. This could be achieved using an airlock built in for that purpose. It was located adjacent to the docking port in the “nose” of the ice-buried command module. Passing through it, he would enter the uppermost level of the module, where he could breathe the same air as everyone else. But he had taken the precaution of checking himself out with an Eenspektor, and found powerful radiation coming from several locations on his suit — basically, wherever he had come into contact with the surface of the shard.

“I was worried about this,” Jiro said, “but there was nothing to be done.”

“Worried about what?” Dinah asked. “I thought the surface was reasonably clean.”

“It was,” Jiro said, “until we did the perigee burn. The nozzle was pointed forward. Some of the steam was blown back over us by the wind — by the atmosphere we were passing through. It condensed and stuck to the surface of the shard. So, now there are little pieces of fallout all over the outside of Ymir. And some of them have gotten stuck to Slava’s space suit.”

“He’s got to get out of that thing.”

Jiro shrugged. “The suit will block most of the beta.”

“I mean, he’s got to get out of it before he runs out of oxygen.”

“That is true.”

“Which means he has to come in here.”

“Also true.”

“He’s going to bring that radiation inside with him.”

“It will take weeks to kill us. By that time we will have accomplished our mission. Or not.”

In the end, though, they came up with a workaround that did not involve dying, which was that they taped some plastic over the companionway that joined the command module’s top level to the one below it. Before doing so, they moved a generous supply of food and water to that level, along with toiletries, a sleep sack, and other items Vyacheslav would be needing. Slava passed through the airlock with some minutes to spare, doffed his suit, and closed it up in the chamber of the airlock, which would block most of the beta radiation coming off it. He then stripped off his clothes and went through several repetitions of decontaminating himself with premoistened towelettes, throwing all of it into the airlock chamber before slamming its hatch shut.

Then he threw up.

The upper level of the command module, along with Slava himself, now had to be treated as contaminated, but they didn’t need it anymore. Jiro and Dinah would be confined to the lower levels, separated from Vyacheslav and the possible contamination by a sheet of plastic, until they reached Izzy or died. A common air supply circulated through all the levels in ducts, but it had a filter system, which they hoped would catch any floating motes of fallout.

Having seen to all of those matters, they turned out the lights and slept. Dinah slept through her alarm, in fact, and finally woke up to realize she’d been out for twelve hours.

Her next thought was to wonder where Markus was. Then she remembered, with a kind of astonishment, that he was dead. It came as a stinging slap, followed by grief. But on the heels of the grief came a feeling of deep fear that she had rarely experienced in all the time since Zero. It was not the sharp bracing fear one felt on an adventure, such as the ride through perigee, nor the kind of intellectual, abstract fear that had been with them ever since Doob had predicted the Hard Rain. This was a kind of morbid panic that was second cousin to depression. It was how a child might feel upon learning that she had been orphaned. Not a child, rather, but an adolescent, the oldest sibling, on whom responsibility for the family now fell. Markus was gone. He wasn’t going to shoulder any more burdens for them. Others would have to take up those burdens. And some of those — the ones, perhaps, most eager to step into Markus’s place — would certainly make the wrong decisions. And so, as sad as Dinah felt about the fact that she would never see Markus again or feel his embrace, the thing that really made her want to contract into a fetal position was this knowledge that it was on her now. On her, and Ivy, and Doob, and the others who could be trusted.

She went “up” into the common room and found Jiro, as usual, lost in contemplation of arcane plots on his computer screen, reflected in miniature on the lenses of his bifocals. During the year he had been in space, his prescription had changed, and so he had been the first consumer of the optical lens-grinding machine that had been sent up and installed in Izzy. Without it, much of the Cloud Ark’s population would gradually have been rendered nonproductive as their eyeglasses were broken or wore out. It was a military machine capable of making glasses in one style, and one style only. At some point a few years down the road, everyone who needed glasses would be wearing this style. It was interesting to contemplate how many decades or centuries would have to go by before the population had grown, and the economy developed, to the point where it could support an eyewear industry with different styles.

He looked up at her through the milky reflections. “I let you sleep,” he said. “Your robots seem to be working fine. There is nothing to do until I finish these calculations.”

“And then what?”

“We have to eliminate the last of the tumbling,” Jiro said, “before we make the final braking burns.”

For Dinah, that was all clear. Ymir was in a safe orbit now and would not fall into the atmosphere anytime soon. But she was still going much too fast, and way too high, to rendezvous with Izzy. They needed to finish the execution of the same plan they’d had all along, which was to make one or two more braking burns as Ymir passed through her perigee, slowing her down to the point where a rendezvous with Izzy was feasible. This required getting the nozzle pointed forward again, and keeping it that way.

“How much damage was done to the—”

“They were destroyed,” Jiro said. “We have two remaining.” They were both referring to the thruster packages, embedded in the ice, that would normally have been used to manage the shard’s attitude. “It is fine. It was necessary,” Jiro added, almost as if worried that Dinah would think poorly of him for criticizing the decisions made by a dead commander.

“Can more be sent out from—”

Jiro nodded. “It is possible to assemble a MIV that could rendezvous with us and assist with the problem. Just in case my idea does not succeed. But since our radio link was lost with New Caird, we cannot coordinate this.”

“And what is your idea?”

“We can use your robots to alter the shape of the nozzle exit,” Jiro said. He held up one hand like a blade, pointed toward the ceiling, and then flexed his knuckles slightly, indicating a shallow bend. “Make it asymmetrical.”

“Like a scarfed nozzle?”

“Exactly. When we fire the main propulsion it will give us an off-axis thrust. If the scarf is oriented correctly, it will have tremendous control authority.”

“Maybe too much,” Dinah said. “We’ll end up overcorrecting.”

“One thing at a time,” Jiro said. “We scarf the nozzle, we do a little correction, we scarf it the other way, we kill the rotation. It might take several repetitions. It can be done. I have been modeling it.”

Dinah pulled herself into position before her triptych of flat-panels, and began opening windows, checking on the activities of her menagerie of robots: some sunning themselves on the outside to soak up power, others sipping juice from the reactors, some mining propellant for the next burn, others mending the nozzle. The latter group, mostly Nats, would be responsible for the nozzle-sculpting program. Until now, having an asymmetrical nozzle, delivering thrust at an off angle, had been a problem to be avoided, not a feature to be encouraged. Jiro had emailed her some diagrams of what the nozzle bell would need to look like. The alterations were surprisingly minor. In an engine that produced that much power, a little bit of off-axis thrust went a long way. “When’s our next perigee?” she asked.

“We just went through one. So, about eight hours from now.”

THE PARAMOUNT DUTY OF THE CLOUD ARK’S COMMANDER — SO IMPORTANT that Markus had broken character and described it to Ivy as “sacred”—was to stay on top of the Bolide Scan, which was a feed of information synthesized out of all of Izzy’s long-range radars and optical telescopes. A disproportionate number of GPop members devoted their lives to managing this, or to maintaining the equipment used to produce it. Because the stream of data was continuous, it needed to be broken down, for the consumption of someone like Markus or Ivy, into reports that showed up on their screens at regular intervals. A Triurnal Bolide Scan was issued at the top of each shift: dot 0, dot 8, and dot 16. Ivy read one of them when she woke up, another in the middle of her “afternoon,” and the third just before going to sleep. Each of these summarized what was known about bolides that might come near them during the next eight hours, and made recommendations as to what maneuvers the Cloud Ark should execute in order to avoid them. Typically they made small burns several times a day for this purpose. The policy was “default go,” which meant that the maneuver would be promulgated to the Cloud Ark via Parambulator, and carried out automatically, unless the commander vetoed it. The only reason this was ever likely to happen was in cases where two dangerous rocks were headed their way at about the same time and they needed to make a decision. Such events had occurred twice during the Hard Rain so far, but they had been simulated and war-gamed hundreds of times before that. The name of the game was to avoid getting cornered.

To be detected eight hours in advance, a bolide had to be pretty big. Smaller ones came along all the time, and weren’t picked up on the radars until minutes or even seconds before collision. Accordingly, smaller reports were issued at the top of each hour, listing all noteworthy rocks that had been detected during the last sixty minutes. This covered most of them, so the commander — or whoever covered for her while she was sleeping — could discharge most of her responsibilities in re the Bolide Scan by dropping everything else she was doing at the top of each hour and reading it. From time to time, however, they would become aware of a “hot rock” or “streaker” that had surprised them by coming in from a weird angle, or at unusually high speed, and then the commander would be notified immediately so that an alert could be issued and evasive action could be seen to. The Streaker Alerts combined elements of the small-town midwestern tornado siren with the red alert from Star Trek. All sleeping people were awakened, all nonessential personnel were evacuated from the larger tori, which were considered most vulnerable, and hatches were closed between different sections, in case of a breach. Similar precautions were taken by the Arkies. Arklets were, of course, more vulnerable to bolide strikes, but they were also more maneuverable. As the hot rock drew closer and its orbital parameters were determined more precisely, the data would be fed to Parambulator. Any arklets in danger of being hit would be identified, and a collective solution would be calculated that would enable them to move into safer trajectories without banging into anyone else. These events happened, on average, between one and two times a day, but as always the devil was in the statistical details. They had once gone three days without a single streaker. On another occasion they’d had five of them within a twelve-hour period. The first of these events had caused an upwelling of chatter on Spacebook to the effect that the powers that be were overstating the threat in order to cow the Arkies into submission, and the second had generated a hard-hitting blog post from Tav Prowse calling the GPop on the carpet for systematic incompetence.

It was in the wake of such an alert, while cleaning up her desktop of postaction chatter, that Ivy’s attention was drawn to a post that had just come up on Tav’s blog: an interview with Ulrika Ek.

“Ulrika has a lot to learn about bloggers” was Ivy’s verdict, after she’d finished reading it. She shook her head. “You’d think she of all people would know — she’s been through PR training.”

These remarks were delivered to a Banana that had been slowly repopulating itself, during the last few minutes, with people who had been called away on other duties during the Streaker Alert. Tekla was the last to arrive, bringing Tom Van Meter and other members of Markus’s security detail in her wake. Luisa and Sal were already present in the room. Doob had just texted his regrets, explaining that he needed to crunch some numbers about what had just happened.

“She probably dropped her guard,” Luisa suggested, “thinking she was just chatting informally.”

“You’ve read it then?”

“I scanned it.”

“Referring to what?” Tekla asked.

“Ulrika made a few off-the-cuff remarks about swarm theory, and which strategies we might wish to pursue in the future, and Tav is blowing them up into a cause célèbre,” Ivy said.

“What if anything would you like to do about it?” Sal asked.

“Nothing,” Ivy said. “Look, the longer this thing with Ymir continues, the more anxiety people have about the Big Ride. Every time a hot rock comes in it juices up that anxiety for a little while. Well, either it’ll work or it won’t. If it doesn’t work then we have very little choice anyway — we have to Dump and Run.”

Sal nodded. “But if it does work, it’ll change everything about the way people think.”

Ivy nodded. “Yeah. And I am growingly certain that it will. Even if the scarfed nozzle gambit fails, we still have that MIV we can send out as a backup plan. I think that in a week we’ll have a successful rendezvous with Ymir and we’ll be prepping for the Big Ride.”

Ivy made a gesture indicating that the new arrivals should find places for themselves around the table. “Which brings me to the topic of this meeting,” she continued. “We know what J.B.F.’s plan is. She’s recruiting some number of Arkies willing to strike out on their own. The general scheme seems to be that they’ll get a few arklets stocked with provisions for a few weeks’ journey. Then, on a signal, they’ll break away from Izzy and make burns that will take them to a higher orbit. An orbit that we can’t reach without expending a lot of propellant. We don’t know what their long-term plan is — or if they even have one — but I think Julia is basically playing the odds that these people will survive long enough to send back messages saying ‘Come on in, the water’s fine!’ and encourage other Arkies to follow suit. They all know that they can’t really be pursued once they have departed the swarm. Membership in the Cloud Ark is, under the current state of affairs, voluntary.”

“I infer you mean to change that?” Luisa asked drily, casting her gaze over Tekla and the members of the squad.

“They can’t make a break for it without hoarding certain critical supplies,” Ivy said. “We can’t allow people to just ransack our storage facilities for whatever they want. And we have clear evidence that this has been happening. There’s traffic on Spacebook about where to look if you want to score a box of fresh batteries or scrubber cartridges. So, our basic approach to this is going to be simple. We’ve identified the worst offenders, where hoarding is concerned. I’m going to make an announcement in an hour, explaining how the Cloud Ark Constitution works when it comes to theft of public supplies, and I’m going to offer a twenty-four-hour amnesty during which anyone can turn in stuff that they have been hoarding. As soon as that time is up, Tekla and her team are going to move on one arklet that we know is being used as a storage dump for contraband, and they are going to restore order. And then Sal will step in, as prosecutor, and take whatever action he deems justified.”

“How can you put people in jail when they are already confined to tin cans?” Luisa asked. “How can you fine them when there is no money?”

“We will have to evolve solutions as we go,” Sal said.

Tekla stared him down, then drew her thumb across her throat.

“WELL, THAT SEEMS DEFINITIVE,” JULIA SAID.

She and Spencer Grindstaff were hovering in the middle of the White Arklet. Drifting near them was a laptop whose speakers had been playing the audio feed from the Banana. They could hear the sounds of the meeting breaking up, and people separating into smaller conversations as they moved out of the room. Spencer pulled it closer and whacked the volume button a few times to mute the sound.

“As I said before: hook, line, and sinker.”

“Unless,” Julia said, “they are somehow aware of the fact that we have surveillance on the Banana, and everything we just heard was a sort of radio play staged for our benefit.”

Spencer beamed. “Now, that is paranoid! I thought I had it bad, but—”

“Just kidding,” Julia said, a little too quickly. “This is actionable, Spencer. I believe we are justified in taking everything we just heard at face value. Which means I am comfortable giving good news to the Martians. Are they ready?”

“Yes, they’ve been waiting,” Spencer said, and thumbed out a text message to summon them.

The Martians all had to come via the same hamster tube, so it took a few moments for the core members of the first human expedition to the Red Planet to filter in: Dr. Katherine Quine, whose professional role was obvious; Ravi Kumar, who would be the expedition’s commander; Li Jianyu, who would act as a general science officer; and Paul Freel, an American MIV expert, the head engineer. They, as well as a score of other Arkies waiting in the wings, had sworn an oath that they’d not spend the rest of their lives sealed up in tin cans, but would walk on the surface of Mars or die in the attempt. In their wake came several other members of Julia’s “staff.”

Julia opened the meeting with a few words of greeting and a solemn announcement to the effect that the Mars mission was a go. Once the ensuing round of zero-gee high fives and embraces had trailed off into an awkward silence, she singled out Paul Freel. “Paul, no doubt you’ve briefed these others on the very latest while you were so patiently cooling your heels, but might I know what has been going on with the MIV?”

“Of course, Madam President. As you know, they’re trying to stabilize Ymir with—”

“A Rube Goldberg scheme in the form of an ice sculpture. Yes, I know about that.”

Paul chuckled, showing a lot of gum. “Not surprisingly, the powers that be are a little nervous about that and so word came down from on high that we ought to be preparing a backup plan, so we can go pull Ymir’s fat from the fire if need be. Well. That couldn’t be better, from a Martian point of view! As you know, we have been planning this mission for years. After Zero, I kept it going as a side project all through the development of the MIV program, and we managed to grease it in as one of the use cases.”

“Use cases?”

“One of the hypothetical uses to which the MIV kit might eventually be put,” Spencer explained.

“Basically it just gave us an excuse to include a few components, like throttleable landing engines and aeroshield material, that might not have made it in otherwise,” Paul went on. “So, stabilizing Red Rover’s design has been a piece of cake.”

“Red Rover?”

“Yeah, that’s what we’re calling her.”

“I would like to propose something a little more suggestive of a higher purpose,” Julia said. “Spearhead or some such thing.”

This led to an uneasy silence terminated by Camila, who said, “I’ll draw up a list of options and submit it to you right away, Madam President.”

“Thank you, Camila. You understand, Paul, that this mission will have symbolic as well as scientific value, and we want to send the right message to the other Arkies so that they will feel inspired to follow in her wake.”

“Of course! Consider it a working title only,” Paul said. “A code name.”

“It’s not even good as a code name,” Spencer pointed out, “because anyone can—”

“Let’s move on,” Julia said. “You were talking, Paul, about the design.”

“Done. It took, like, a man-day. We just had to make a few tweaks to a preexisting use case to reflect the materials and supplies we actually had on hand.”

“Excellent.”

“But a design is not a ship, of course,” Paul continued, “and until a couple of days ago it would’ve been pretty darn hard to put the actual propulsion system together without bringing down the wrath of Ivy!”

“That is not a phrase calculated to strike fear into the heart of anyone except those most worshipful of her authority,” Julia remarked, speaking with the gravity that could only be summoned by one who had recently used nuclear weapons on live targets.

Paul cackled. “You know what I’m saying, though — everything happens in a fishbowl here! So, you can imagine the grin on my face when we got the order to begin assembling the Ymir rescue MIV.”

“Are the specifications similar?”

“Similar enough. They can both use the same main engine. The thruster packages, the control systems, life support — all of that stuff is completely standardized; it doesn’t change from one use case to another, it’s just a matter of punching different parameters into the code. It’s just a config file!”

Seeing that Julia didn’t necessarily know what a config file was, Spencer put in, “They can essentially download the DNA, if you will, of Red Rover, or whatever we end up calling it, into the Ymir rescue vehicle with a few keystrokes.”

Satisfied with that, Julia asked, “What of the arklets? The heptad and the triad?”

“Well, they’re already functional, independent space vehicles. Way more than enough space for twenty-four Martians and their vitamins. Obviously, we’ve been stocking up,” Paul said, waving his hands around at the bags of food and other supplies crowding the White Arklet.

“Yes,” Julia said, “but the critical part of the operation is going to be moving them from their default positions in the swarm — which will seem unremarkable as far as Parambulator is concerned — to the propulsion stack that you have been assembling. And that’s going to make a hell of a stink, is it not?”

The smile on Paul Freel’s face became a bit frozen. “We could just go for it,” he said.

“I have a workaround,” said Spencer Grindstaff. “I think we can make this happen. A Streaker Alert is all we need. It’ll go down tomorrow.”

“How do you know there’s going to be a Streaker Alert?”

“Such an event is nothing,” Spencer said, “other than a particular configuration of bits.”

DINAH HAD BEEN DREAMING OF MARS.

As an asteroid miner, she had never been that interested in the distant and inhospitable Red Planet. The politics of the pre-Zero space exploration world had obliged her to show skepticism, even disdain toward those who wanted to go there and to build colonies and terraform the planet. Mars colonists were siphoning attention and resources away from the asteroid miners, who wanted to use easier-to-get resources to make much more human-friendly habitats: space colonies, rotating to provide full gravity, with plenty of water and fresh air.

In any event, it had been a dead issue for two years. But that didn’t prevent Mars from showing up in her dreams, and now infiltrating her daydreams. Almost three years had now gone by since she had walked on the surface of a planet, looked up into a sky, seen a horizon. Intellectually she knew that death would take her, sooner or later, before she did any of those things again. She and everyone else in the Cloud Ark would live out their lives in environments resembling bomb shelters, hospital basements, and research labs. The best they could hope for was to look out a small window at the starry sky. The view of the blue, green, and white Earth had once provided fascination and solace. The orange ball of fire they now circled was such a disagreeable sight that most people actively avoided looking at it. No one was ever going back there. For those who still aspired to go for a walk before dying of old age, Mars was the only hope, be it ever so impractical. People had been talking about it on Spacebook, and on some of the blogs that had been cropping up on the Cloud Ark’s miniature Internet. Before the loss of New Caird had severed Ymir’s data link to the Cloud Ark, some of it had trickled through to her tablet, and Dinah read it in idle moments.

At least she had some idle moments now. Since the decision to try the scarfed-nozzle approach, they had executed two burns, about twenty-four hours apart, each with a slightly different configuration of the ice nozzle: a canted lip, constructed by the Nat swarm, projecting almost imperceptibly above the aft surface of the shard and bending the torrent of steam slightly. The first of those burns had gotten them spinning the way they wanted to go, though “spinning” might be too strong a word for a rotation that took the better part of a day. During that day the Nats had decamped to the other side of the nozzle’s rim and built a lip there. The second burn, then, had stopped the rotation that the first one had started, and brought them close enough to their desired attitude that the surviving thrusters could handle the details.

Another perigee was coming up soon. This time the nozzle would be aimed the way they wanted it — forward, once again turning the nuclear engine into a powerful retro-rocket. The robots on the inside of the shard had been at work scooping it out, sculpting the walnut-shell architecture that, according to the structural engineering simulations, would enable the whole thing to hold together during the last round of maneuvers. The hoppers were full of ice, with more on the way, and they’d finally learned how to make the system work consistently. Part of that lesson was not to try to accomplish too much with any one burn. It was better to take it easy, set a reasonable delta vee target, get it done and lock it down, then take stock of the situation and plan the next burn at leisure. Consequently their rendezvous with Izzy looked to be happening much later than they’d first expected, and almost every day brought a further postponement. But at the same time it came to seem more and more of a sure thing, less of a wild chance, and this began to affect Dinah’s thinking. Her robots were doing their work almost entirely on autopilot, leaving her somewhat bored. Vyacheslav, sealed up on the other side of a wall of plastic, could be talked to, but preferred keeping to himself. Jiro, on the other hand, had been working almost around the clock and had been showing signs of strain. Dinah would find excuses to float behind him and look over his shoulder at his screen. Was he playing solitaire? Running orbital mechanics simulations? Writing his memoirs? He seemed mostly to be looking at video feeds of machinery. By process of elimination, this had to be near the core of the reactor.

In the floor of the “bottom”-most level, three stories “below” them, was a manhole giving way to a shaft sunk into the ice. At the far end of that shaft was another hatch providing access to what, on an oceangoing ship of Old Earth, would have been called the boiler room. A small pressurized compartment housed control panels and access ports connected to the reactor, which was only a few meters away, on the other side of a heavy wall. The wall was a radiation shield, at least in theory. But sending up a huge piece of lead hadn’t been an option for the hastily assembled Ymir expedition, and so the “boiler room” got washed with neutrons and gamma rays whenever the reactor was used. The radiation detectors that Sean and company had left behind, the last time they’d closed that hatch, didn’t leave much to the imagination. The place was a hellhole now. Fortunately, all the systems connected there had been designed to be operated remotely, from the safety of the command module, so there was no need to go down that ice tunnel and open that hatch.

Their instruments told them they were nearing perigee again. Jiro, assisted by Dinah, executed what they hoped would be the second-to-last burn of the big engine. This went on longer than Jiro had predicted, but it seemed to work. Ymir shed most of her excess velocity. Her orbit, at apogee, was now only a few hundred kilometers higher than Izzy’s. In spite of attrition suffered by the robots as they wore out, broke, or succumbed to radiation damage, Dinah still had enough of them to restock the hoppers for the final major burn, which they calculated would be happening at a perigee a few hours later.

“If you are satisfied with the disposition of your robots,” Jiro said, “I would like to show you how to operate the main propulsion.”

She had grown up in mining camps where older men liked to amuse her, and themselves, by teaching her how to operate heavy machinery, blow things up with dynamite, pilot airplanes, and the like. So Jiro’s offer didn’t seem unusual to her, at first. Teaching people how to do stuff was, among other things, a way to alleviate boredom. But over the course of the next hour it slowly became clear to her that Jiro really was expecting her to operate the engine during the upcoming burn. It might have been the language barrier; but his English was pretty good, and he was being quite persistent in saying things like “you will keep an eye on this thermocouple” and “you might see some flutter in this valve.”

“If you don’t hear from me beyond the thirty-second mark,” he said at one point, “then you are on your own and you will have to initiate shutdown based on observed delta vee.”

“Why would I not hear from you?” Dinah asked. “Where are you going to be?”

“In the boiler room,” Jiro said.

“Why would you go there?”

“Some of the control blade actuators have stopped responding,” he said. “I think that the electronics have been damaged by radiation. It’s okay. We have replacements. But they will have to be installed manually.”

“So you’re going to go down there?”

“Yes,” Jiro said. “And that is where I am going to stay.”

“IT IS FOR ALL PURPOSES EMPTY,” TEKLA REPORTED OVER AN ENCRYPTED voice link to Ivy. “Empty of people. Empty of supplies.”

She, Tom Van Meter, and Bolor-Erdene had spent the last ten minutes searching Arklet 98 from front door to boiler room, under the eye of Sal Guodian. They had arrived via Flivver, docked, and entered 98 without incident. Sal had gone through first, carrying a tablet on which was displayed the first search warrant ever issued under the provisions of the Cloud Ark Constitution. He had been ready to show it to the first person who challenged him. But no one was here.

Tekla, Tom, and Bo had then come in, wearing orange vests improvised from survival kits that, since they’d been designed for use on Earth, had no practical utility anymore. These would serve as police uniforms until something else could be stitched together. With any luck, they wouldn’t be needing a lot of cop gear. But Ivy had been clear, and the others in her ad hoc council had agreed, that if they were executing what amounted to a police action, they couldn’t beat around the bush — couldn’t try to palm it off as an informal visit. A new constitution had to be exercised, or it was just words.

“Can you get it back on the SAN?” Ivy asked, over the voice link. “I’d like to see what’s happening.”

“I’ll reboot everything,” Sal said, pulling himself up into the control couch. “But it depends on what Spencer did — whether he broke it permanently, or just entered a temporary command.” He reached around in back of a panel, felt for a connector, pulled it out, and jacked it back in.

“We had estimated that you were going to find ten person-years worth of nonrenewables in that thing,” Ivy said. She meant, not bulk food (which could be grown in the outer hull space of an arklet) or air (which was renewed by the life support system), but generally smaller items like toiletries, vitamins, medicine, and specialty food. “That was based on circumstantial evidence — the amount of stuff that’s gone missing, the number of Flivver trips and EVAs that have touched that arklet. We always knew it was only a guess. But for it to contain nothing at all is. . odd.”

“More than odd,” Tekla said. “Surprise attack.”

“You think there’s going to be an attack?”

“Maybe not in sense of violent assault,” Tekla said, “but something.”

“And Arklet 98 was a decoy?”

“Obviously.”

A musical tone sounded from the arklet’s PA speakers, and the white LEDs changed their hue to red. “Alert,” said a synthesized voice. “All personnel should now be awake and at stations for urgent swarm maneuver. This is not a drill.”

They’d heard it before. It was a Streaker Alert.

Normally, though, they took it at face value. “Remarkable coincidence,” Tekla said.

“I think you guys had better get back in the Flivver,” Ivy said. “Follow the usual procedures for one of these, but keep your eyes open.”

“STEVE, DO YOU HAVE ANYTHING YET ON THE BOLIDE?” IVY ASKED. They were about five minutes into the alert, which had obliged them to move down into the Banana. As much as Ivy wanted to know what was happening with J.B.F., and what Tekla had characterized as a “surprise attack,” her responsibility in a case like this one was clear: all of her attentions had to be focused on the evasive maneuvers being carried out by the Cloud Ark and their possible consequences. Those might include collisions between arklets or the separation of one or more arklets from the swarm. In dire cases it might be necessary to send out rescue teams, which was why her first act had been to get Tekla and the others into their Flivver. For the normal role of that makeshift police service was not to serve warrants on hoarders; it was to respond to emergencies. As keenly as the space geek in Ivy’s soul wanted to pay attention to the scientific phenomenon of the incoming rock, it was a task she had to delegate; and she’d delegated it to Steve Lake as soon as the alert had sounded.

Thus far, the alert had been proceeding as most of them did, which meant that most network activity had been shut down to leave open bandwidth for Parambulator. That system swung into action without human intervention, calculating courses, making suggestions, and gathering data about what the motes in the data cloud were doing. The Parambulator screens were looking pretty angry, but that was normal as almost every arklet fired its thrusters and shunted into a new trajectory. In time it would get sorted out. It always did. But part of the sorting-out process was refining what they knew about the trajectory of the incoming streaker. The closer it came, the more precisely they could track it. By the time it passed through, or near, the swarm, they’d have its parameters dialed in to high precision. And once it had flashed by, all Parambulator had to do was clean up the mess.

Ivy had asked Steve about the bolide for a couple of reasons. One was that hot rocks, by definition, tended to come and go rapidly. This one had been approaching for several minutes — a long time to wait. Another was that Parambulator looked more chaotic than usual. Normally there would be a spray of red in the first couple of minutes. Presently it would begin to fade as the arklets reported that they were out of harm’s way. But in this case, it never seemed to get any better. “Are we having trouble with bandwidth, or—”

“The rock is weird,” Steve said. “Normally I’d expect to see a stream of packets from SI, refining the params as they gathered more data.” He meant Sensor Integration: the department that managed the radars and telescopes.

“And you’re not?”

“Well, I am — but with different numbers.”

“What do you mean, different numbers?”

“It’s like we have two different Streaker Alerts happening at once. The packets are stepping on each other. There’s some kind of crosstalk going on.” Steve sat back from his screen for a moment and tugged his beard. “Just a sec,” he said. “I think that these packets are coming from different sources.”

“But they should all have the same point of origination,” Ivy said. “SI.”

“They claim to,” Steve said, “but I think that some of them are forgeries.”

Feeling his chair shift subtly beneath him, he reached out involuntarily with one hand and held the edge of the table. Izzy was firing her thrusters, coming about to a new orientation, trying to put Amalthea between herself and the bolide — real or imagined.

“You think this whole alert is them spoofing us?”

“It would fit in with Tekla’s theory of what’s going on,” Steve said.

“I’ll try to voice with Doob,” Ivy said. “Work on that forgery hypothesis.”

“MADAM PRESIDENT,” CAMILA SAID, PULLING A HEADPHONE AWAY from her ear. “As you requested, I am informing you that Ivy has figured it out.”

“She knows?” Julia asked.

“Not quite, but Steve Lake has detected the forged packets and is running further analysis.” Camila’s eyes were big and her voice — which was always somewhat impaired by her facial injuries — was thick and dry.

Julia threw her a shrewd look, then turned to Spencer Grindstaff, who shrugged. “Sooner or later a man of Steve’s talents was bound to—”

“I don’t care about that,” Julia cut in. “I want to know whether our gambit has bought us enough time.”

“There’s—” Camila began.

Spencer ran Camila off the road. “It has bought us enough confusion. We should be in a position to dock this heptad at the Shipyard in twenty seconds.”

“There’s another bolide!” Camila squeaked. “I think.”

Julia shook her off, keeping her focus on Spencer. “Where is the triad?”

“Already there,” Spencer said.

“The spacewalkers?”

“Suited up, out of the airlocks, in position.”

“Still. The assembly. The integration. It will take time.”

“Madam President, if I may,” Paul Freel broke in. “All we need is to slap her together — with zip ties, if that’s what it takes — and achieve separation from the Shipyard. A small thruster burn will do it. Izzy doesn’t have phasers to blast us out of the sky! They could send a Flivver after us, but what are they going to do? All we need is to get clear. Then we can spend days prepping Red Hope before we embark on the mission in earnest.”

“I wouldn’t put anything past that Tekla.”

“Say what you will about her, she’ll follow orders,” Paul said.

“Well, as a stay-behind supporter of your expedition, I will be happy to run interference for you until you can get cleanly away,” Julia said.

Through the heptad’s structure, a programmed series of whirrs and clunks resonated as it docked with a port on the long truss projecting to the side of the Caboose: the heart of the Shipyard, rich in airlocks and anchoring points. Docked at the next port along was a glinting, angular framework: the skeleton of Red Hope, awaiting its final components. It sported four large propellant tanks clustered around a knot of pumps, valves, actuators, and sensors that fed a rocket engine centered below.

“Madam President?” Ravi asked. “I’m afraid the time is now. Unless you want to go to Mars. Which you would be welcome to do.”

Julia snapped to attention. She had been checking herself in the mirror of her compact. Hardly glamorous, but by Cloud Ark standards, her appearance would do.

“It is tempting,” Julia said, “but I have responsibilities here, I’m afraid.” She snapped the compact shut and glanced over, verifying that Camila was ready to shoot video on her phone. She was, but she still had that rattled look on her face. What had come over her? They’d have to have a heart-to-heart later.

“Very well,” Ravi said, with a note of regret that sounded only a little forced. “Perhaps you’ll be wanting this.”

He held out a sheet of paper. Taking it from him, Julia recognized it as the presidential seal, much the worse for wear. Ravi had carefully peeled it from the wall, bringing most of its rectangle of blue tape with it. Julia smoothed it out and tucked it under one arm.

Slowly drifting away from her, Ravi snapped out a salute.

Julia returned it. “Godspeed, Ravi. I look forward to hearing your first transmission from the surface of Mars.”

“And I look forward to sending it, Madam President.”

“We shall meet again, I feel. Somehow the intrepid people of the Cloud Ark will find a way, in spite of all opposition, to win through to the realm of clean space and follow Red Hope to a better place.”

Ravi was one of those who could never quite tell when he was dismissed. He began to mumble out a stirring response, but Julia glanced at Camila to let her know that she could stop recording, then propelled herself toward the nose of the White Arklet. Camila followed in her wake.

After a few moments of squirming through tubes, they emerged from the port into one of the modules that made up the Shipyard. It was something of a madhouse. The total roster of the Red Hope expedition was two dozen. Most of those were already aboard the heptad or the triad, waiting to be mated with the vehicle’s frame, but a few were “outside” in space suits and several were in here, engaging in hasty conferences or shoving bundles of supplies about.

Adding a bizarre note were four members of the General Population — apparently Shipyard workers — who had been zip-tied, hands behind backs, to convenient attachment points around the inside of the module. Most looked fine, but one man had a stream of small blood globules drifting away from a laceration on his eyebrow. Paul Freel had mentioned in passing that several of the MIV team had become unwitting accomplices, helping to assemble the frame of Red Hope on the understanding that it was part of a backup plan to rescue Ymir. Apparently they had changed status to witting, and raised objections.

The bleeding man was staring at Julia through the eye that hadn’t swollen shut. “Julia!” he called out.

In an odd way Julia had nothing to do. The other Martians were busy shoving their hoarded supplies through the port into the heptad. One by one the Martians were following suit, and so the space was rapidly clearing out. She ignored the bleeding man at first. But it got to the point where only one Martian — Paul Freel himself — was remaining. Lacking Ravi’s feel for ceremony, he was checking off items on the screen of his tablet, paying Julia no attention whatsoever.

“Julia!” the zip-tied man said again. He wasn’t shouting. His tone was almost conversational.

“Yes,” she finally said.

“What’s your friend’s name?” he asked, nodding toward Camila.

Julia bridled for a moment at the impertinent request, then remembered that it was never too late to turn an enemy into a friend. “Her name is Camila,” she said. “And let me say, sir, that I am shocked and dismayed to see what has occurred to you. Let me assure you that—”

“Hey, Camila!” the man said.

“Yes?” Camila answered, sounding very much the scared eighteen-year-old girl.

“Your friend is crazy,” the man told her.

“Madam President?” Paul asked, before Julia had time to react.

She turned toward Paul, her face burning.

“If you would do the honors?”

“What honors?” Honestly, these engineers. Was she supposed to break a bottle of champagne over it?

“Close the hatch when I have gone through. Then we can undock.”

“Happy to.”

“See you on Mars.” He stuck his hand out. She grasped it lightly and gave it a little shake. Camila, rattled by the exchange with the bleeding man, had forsaken her duties as camera operator.

Paul Freel reached into the portal joining Earth to Mars, pulled himself through, turned about, and closed the hatch on his side. Julia followed suit on hers. Immediately she felt, as much as heard, the hisses and clunks that signaled the undocking of Red Hope. Unfamiliar noises radiated through the module’s hull too, very close to her, and she realized that these were the boots of space suits moving around.

“The alert is canceled,” announced a synthetic voice. The color of the lights changed.

Camila emitted a short, explosive scream. Then she pointed down the length of the Shipyard, toward where it connected with the Stack.

Down in the Caboose, some thirty meters away, a few people could be seen, dressed in orange vests. One of them looked directly at her.

It was Tekla.

The synthesized voice spoke out again, sounding a second alert.

That wasn’t part of the plan.

Tekla must have gathered her legs against something down in the Caboose capable of pushing back, because all of a sudden she was flying toward them like a rocket. Her arms were in motion, reaching this way and that to slap at anything that could help her correct her course, but her eyes were fixed on Julia and she was coming straight for her. Something gleamed in her hand, a thin arc of silver light: the honed edge of a dagger.

A crisp metallic noise resounded through the module as Julia pulled back the hammer on Pete Starling’s revolver.

“Gun!” shouted the bleeding man. “Gun! Gun!”

If Tekla heard, she did not care, but only pushed back harder against a strut in the neighboring module and came on faster.

To Julia the weapon’s recoil came too soon, as if it had gone off accidentally. She’d been in space long enough to know that it would knock her back, and it did; but she also saw things she could not explain. Camila had entered the picture, flying in from the side with an arm outstretched. The wall of the Shipyard itself reached out to body-check Tekla. A moment later it struck Camila, then Julia. She had expected the high-pitched hiss of a bullet-sized hull puncture; but what followed was more like a roar. Like the crowd in a football stadium when a pass is intercepted. Camila’s arm had turned into a wing of fire. Something took Julia from behind and hurled her toward the Caboose. She looked around, thinking, crazily, that the bleeding man had somehow gotten loose and tackled her. But the force pushing her along was no human being. It was a torrent of escaping air.

“JIRO, CAN YOU HEAR ME?” DINAH ASKED FOR THE FOURTH TIME.

She conjectured that he could, but that he was simply too weak to answer. So she went ahead and delivered the good news. “We made it,” she said. “I have Izzy on optical. We’ll converge with them in about half an hour.”

“Good,” he said, “good.” She was startled to hear anything at all. But the second “good” was a lot fainter than the first one, and she reckoned it was all he could get out.

She decided not to tell him anything further. Entombed in Ymir’s boiler room, simultaneously freezing to death while being cooked alive by radiation, he didn’t need to hear a description of what Dinah was seeing through the telescope.

They had been calling this the Cloud Ark for two years. The name had been meant somewhat poetically. Today, however, it really did look like a cloud. Her view of Izzy, which usually was so crisp and sharp in the high-contrast light of outer space, was cloaked in a glinting and winking shroud of what might be clinically described as particulate matter.

It went without saying that Izzy had taken a direct hit from a bolide. Beyond that, it was difficult to make out details.

Ymir’s final burn — for Jiro, a suicide mission — had settled her in an orbit quite similar to Izzy’s: the same plane, the same average altitude. The only difference was that it was a little bit more oval, calculated to cross Izzy’s path twice during each revolution around the Earth. They were approaching one of those crossings, and so from Dinah’s point of view the space station kept getting closer, filling the window on her computer screen, obliging her to zoom out, giving her a progressively sharper, more detailed picture. As minutes ticked by, she was able to piece together a guess as to what had happened.

The rock must have come in from an angle, missed Amalthea cleanly, and struck somewhere near H2, the hub that anchored Tori 2 and 3. Both of the tori had huge bites taken out of them, and both had stopped rotating. Aft of that point, Izzy’s spine — her central Stack — was actually bent. The spreading wings of the Shipyard were still attached to the Caboose, but they were askew, and leaking debris. The original torus — the one that contained the Banana — was still rotating, and looked whole, but as she drew closer she saw it had taken damage, perhaps from shrapnel.

A faint thud resounded through the ice hull. They’d probably struck a piece of jetsam. No matter, it wouldn’t be moving very fast. Ymir could nuzzle her way through a cloud of that stuff and never feel it.

One of the windows on her screen flashed up a video feed, triggered by the motion detector on a Grabb’s camera, and she saw a human body drifting away into space. She swallowed against a sharp contraction of her throat.

Part of her wondered if she would find Izzy a ghost ship — if she were the only human remaining alive. For Vyacheslav had stopped communicating with them yesterday. Before then, he had mentioned that he had been suffering from diarrhea. If this was caused by radiation exposure, it was a death sentence. He might simply have committed suicide rather than wait for the inevitable.

Alone at the controls of Ymir, she coasted toward Izzy, silent and adrift in the cosmos, and entertained the thought, just for a while, that she might be the only human being left in the universe.

Then a red light — a laser, aimed right at her — began to flash from the Mining Colony, and her mind began to pick out Morse code.

SENDING FLIVVERS TO EFFECT FINAL MANEUVERS

DISREGARD STRAY ARKLETS

WELCOME HOME

Lacking any way to respond, she waited, and watched. Shreds of insulation, scraps of structural material, spilled vitamins, and the occasional body tumbled across the window as she panned and zoomed over various details. Everything forward of Zvezda looked pretty good. The Mining Colony and Moira’s stash of genetic equipment appeared to be unscathed. Good.

Three Flivvers had separated themselves from the cloud and established trajectories that would bring them to Ymir within a few minutes. She guessed that they would act as tugboats, butting their heads against the shard and using their main engines to effect the final delta vee needed to achieve rendezvous. So, the first part of the transmission made sense. DISREGARD STRAY ARKLETS, however, was something of a mystery. Why would there be such a thing? And what did it mean, anyway, for an arklet to be “stray”? And yet as Dinah panned the telescope across the arc of space forward and aft of Izzy — the realm where most of the arklets normally parked themselves — she found it curiously underpopulated. It was just a general visual impression. She couldn’t verify it scientifically without access to a Parambulator screen.

It occurred to her, then, that all she needed to do was switch on her tablet’s connection to the mesh network. Shortly after New Caird’s departure from Izzy, she had turned it off because, once they got out of range, it was a useless battery drainer. And indeed the tablet soon brought up the little icon announcing it had found a connection, perhaps relayed through one of those Flivvers. A minute or two went by as the device downloaded all of the email and message traffic that had been piling up in her inbox during her “vacation.”

She passed the time playing with the telescope. A detail caught her eye as she panned across the scene, and she went back and zoomed in on it for a closer look.

It was a MIV, an unusually big one. Basically a five-layer stack, wasp-waisted. The bottom layer was an engine of the most powerful class in the MIV toolkit. Above that was a fat cluster of propellant tanks. The third layer — the narrow waist — was a single arklet with an airlock on its side — a command module, she guessed, similar to New Caird’s. Above that was a triad, and on top, forming the fat head of the vehicle, was a heptad. All of it was shrouded in structural webbing. Snared like little bugs in the edges of that web were small modules that she recognized as attitude control thrusters. The most notable thing about the vehicle was her outsized propellant tanks, hinting at a long journey — to where? The thing was keeping station several kilometers forward of Izzy, in a region that had been largely denuded of arklets.

Her tablet finally finished downloading messages, most of which were long out of date by this point. She sorted them by age, newest first, and scanned the headings. Very few had come through in the last several hours. That stood to reason, given that the Cloud Ark had other things on its mind. But close to the top was one that caught her eye: OPEN COMMUNIQUÉ FROM PRESIDENT JBF TO THE PEOPLE OF THE CLOUD ARK.

Merely seeing those words gave her a feeling as if she’d been socked in the solar plexus. She tapped it anyway, and read it:

Today’s shocking tragedy has left us all bereaved, and seeking answers. I was in a Shipyard module when it happened, having just bid farewell and godspeed to the brave explorers of the Red Hope expedition. Thanks to the automatic closure of a hatch, I experienced only minor injuries and discomfort from partial decompression. As we all know, many members of the General Population were not so lucky. I join with all humanity in mourning their sacrifice. By its nature, the Arkie Community was less affected by this disaster. As I had envisioned from the very beginnings of the Cloud Ark project, the distributed architecture of the swarm prevented serious damage. We did lose three arklets, I am sorry to say, and several more sustained damage from minor collisions or debris impacts. But overall the system worked as we had planned from the beginning. Many members of the AC are now, quite naturally, asking themselves whether it is safe to remain in low Earth orbit, clustered around a heavy, aging space station that lacks the ability to maneuver out of harm’s way. The open vista of clean space beckons above us. Red Hope will soon fire its main engine and begin its trek across that unexplored frontier to a planet that will one day have room for us all. The Cloud Ark cannot follow her — yet. But as all members of the AC know, having gone through extensive training in space operations and orbital mechanics, it is well within the capability of any arklet to raise its orbit substantially by making use of its engines and its onboard propellant supplies. Alone, a single arklet, triad, or heptad will not long endure. As part of a swarm, however, it has a fighting chance. Many members of the Arkie Community who have been watching the desperate trials and tribulations of the Ymir expedition, and who have now witnessed the damage inflicted upon Izzy by a single bolide, are now asking themselves whether it is safe to remain, and to trust themselves to the agonizingly slow climb toward clean space envisioned by the partisans of the Big Ride faction. I am a politician, not a scientist, and so I cannot pretend to render a technical opinion. Some may question whether I should be making a public announcement at all. The simple fact of the matter is that my past career as President of the United States has given me prominence in the Arkie Community, whether or not I deserve it. Many have been asking me what I shall do now. Rather than wait for rumor to sow confusion, I am therefore issuing this communiqué. For what it is worth, I have, with the assistance of some loyal friends, escaped from the wreckage of Izzy and found safe haven aboard Arklet 37, currently part of a triad. Shortly after I transmit this message, we will initiate a burn of our main propulsion that will lift us clear of the drifting debris that surrounds what once was the International Space Station, and move us in the direction of clean space. Our orbital parameters will be posted openly on the network so that like-minded members of the AC may join us in creating a swarm-based solution to the acute problems currently imposing themselves on the human race. From a safe position in higher orbit, we will look for ways to extend a helping hand to our surviving friends marooned in the General Population. Working together as a community, we will preserve what we have and build a stable way of life in the sky as we await with breathless anticipation the results of Red Hope’s inspiring venture to the welcoming surface of Mars.

“She’s right about the ‘breathless’ part of it,” Dinah muttered to herself, closing the window and looking at the time stamp again. It had been transmitted three hours ago. Then, only half an hour ago, Ivy had responded with a counter-communiqué. Dinah didn’t read it, but based on the subject heading she knew what it would say: don’t listen to J.B.F., stay in formation, we need you and you need us.

But from what Dinah was seeing, both through the optical telescope and on Parambulator, Ivy’s message had come too late to forestall the departure of a large number of arklets. Somewhere out there, up above them in higher orbit, a new swarm was taking shape, running its own, independent instance of Parambulator, and looking to J.B.F. for leadership.

Dinah had been through many emotional ups and downs while retrieving Ymir. More downs than ups, of course, given the fatality rate. In a strange way, however, the emotional high point was just a few moments ago when she had scanned the word “desperate” in J.B.F.’s communiqué. She rather liked being described as desperate, particularly when she was just on the verge of succeeding.

Parambulator was working on her screen now. She used it to check the status of those three Flivvers. They were still closing. Messages were starting to come in from their pilots, trying to make out whether anyone was still alive in the shard, whether it was safe to approach.

Dinah texted back: One survivor. Stand clear for a sec while this thing takes a big glow-in-the-dark crap.

Then she pulled up the window she used to communicate with her network of robots and typed in a single-word command: JETTISON. It was the name of a program that Sean had started, Larz had improved, and Dinah had recently finished. It was a program meant to be run simultaneously by every robot in the shard, as well as some other systems down in the boiler room.

A prompt came back: ARE YOU SURE Y/N

Y, she typed.

CONGRATULATIONS!!! came back. The dead crew of Ymir had sent her a message from the void.

She pushed herself over to the companionway, got her head aimed “down” through the hole in the floor, and pulled herself straight to the bottom level of the command module. The hatch in the floor — the one that led to the ice tunnel that terminated in the boiler room — had already been closed, as a basic safety measure. But Dinah verified that one last time and made sure it was sealed. Because in a few seconds, there would be nothing but vacuum on the other side of it.

Ymir had begun grumbling. Dinah felt as if she were trapped inside the belly of a frost giant with indigestion. What she was hearing, she knew, was the collective noise made by thousands of Nats, and hundreds of larger robots, as they moved to safe positions on the inner surface of the hollow shard and gnawed away at the structural webbing that connected it to the reactor core.

She returned to her seat in the command module and pulled up a video feed from the interior of the shard. Its walls were now thin enough to admit some sunlight, and so it had become a sort of vast pellucid amphitheater where all of those robots could look inward to the smooth beryllium pod — a neutron-reflecting shroud — surrounding the reactor core. Formerly this had been buried in ice; the recent excavations made to fuel the big perigee burns had left it exposed, also revealing the smaller pod of the boiler room mounted on its side, and the system of hoppers and augers that fed it. Aft of that was what remained of the ice cavern of the nozzle bell, now mostly melted away to expose the blackness of space beyond. The only thing now holding the reactor chamber in place was the massive central thrust pillar, a tree trunk of ice that grew from its forward end and extended straight up to the solid nose of the shard, where the command module was embedded.

JETTISON did her the courtesy of showing a countdown on the screen, so that she could plug her ears. When it hit zero, a sickening crack resounded through the whole structure. The video feed showed a brilliant spray of ice blown free from the central pillar, just above where it connected to the reactor vessel. Demolition charges, placed there long ago by Sean’s crew, had detonated and severed the connection. For a moment she feared nothing further would happen, but then jets of white steam lanced from the reactor vessel’s rounded top. JETTISON had opened valves, releasing pressure that had built up in the chamber from the reactor’s residual heat, and those valves were now acting as makeshift rocket engines, pushing the whole reactor, and everything attached to it, down toward the vacancy of the nozzle.

The entire reactor chamber dropped out the bottom of the shard and was gone.

If JETTISON continued to do its work, the reactor, now a free-floating vehicle, all brawn and no brains, would execute a few clumsy maneuvers to kill its own orbital velocity and drop itself into the atmosphere.

“Bye, Jiro,” Dinah said. “Thank you.”

One of the Flivver pilots texted her: Wow.

Dinah gave it all one more thorough scan, using several cameras. But there wasn’t much to see. Ymir was now a hollow, sugarloaf-shaped shell, crawling with robots, and helplessly adrift in space.

She texted, Did someone place an order for a megaton of propellant?

INSTINCT HAD HERDED THEM TOGETHER IN THE SCRUM, CLOSE TO Amalthea and far away from the parts of Izzy that had been damaged or destroyed. That was where Dinah found them, after she’d been brought aboard, scrubbed clean, checked and checked again for contamination. Pink and raw, she embraced Ivy first, for a long time, and then made the rounds to Doob, Moira, Rhys, Luisa, Steve Lake, Fyodor, and Bo. Konrad Barth and many others were dead. Tekla was still in surgery. One of her breasts had been damaged by a fragment and was being surgically repaired.

Curled up in very nearly a fetal position at one end of the SCRUM was a woman who was quietly weeping. She hid her face from the room with an arm swathed, from fingertips to shoulder, in white gauze. Dinah recognized her as Camila, Julia’s sidekick.

Ivy insisted that they all move back down the Stack and meet in the Banana. It took some gentle persuasion to get Camila to come with them, but eventually Luisa talked her into it. Out of habit she kept reaching for the veil she normally drew across the lower half of her face, but it wasn’t there anymore. She was dressed like everyone else, in a shapeless coverall.

“What is Camila doing here?” Dinah asked Moira, as they maneuvered down the Stack.

Moira had obviously been crying and seemed badly shaken up. She and Tekla had become a couple at some point, and Moira was taking the news of her partner’s injury hard.

“Tekla came for J.B.F.,” Moira said, “and J.B.F. tried to shoot her. Camila reached out and grabbed for the gun, I guess. She was always wearing that gauzy wrap, as a veil. The fabric caught fire from the flash of the gun, and burned her arm before she could get it off.”

“But she saved Tekla?”

“Who knows? The bullet struck something else and fragmented, apparently.”

The holes where shrapnel had struck T1—the first, oldest, and smallest torus — had been patched, and it had been repressurized. They had always considered it a safe place before; they needed to begin thinking of it in that light again, which was why Ivy had insisted they come here. They took seats in the Banana.

The numbers had come in. Ivy opened the meeting by reciting them.

At the onset of the Hard Rain, the human population — not counting any who might still be alive on Earth — had been 1,551, or 1,553 if you counted the two late arrivals, Julia and Pete Starling. Starling hadn’t even made it out of his space capsule, so the initial number had been 1,552.

At the same time there had been 305 occupied, free-flying arklets plus 11 spares that were attached to Izzy but not occupied. The free-flying ones had housed 1,364 people; the remaining 188 humans had lived aboard Izzy as members of the General Population. But at any given time, 10 percent of the Arkies had been rotating through Izzy, bringing its population on a typical day up to 324.

Prior to today’s disaster, 26 people had been killed in various mishaps, mostly smaller bolide strikes. Another 24 were now aboard the stolen MIV calling itself Red Hope, and if their claims were taken at face value, they would soon be en route to Mars.

Of the persons who had been aboard Izzy at the time of the disaster, 211 had been killed outright and another two dozen or so remained in critical condition. The number of living people aboard Izzy had therefore been reduced to 113. The General Population — the older, more experienced, highly trained specialists — had been reduced from 188 to 106.

At the moment of the disaster, 1,178 persons had been living in arklets. The distributed nature of the swarm, combined with the fact that many arklets had flown the coop with Julia, made it difficult to estimate casualties. The best estimate they currently had was that seventeen arklets had fallen victim, with assumed 100 percent loss of life, reducing that population to about 1,100. If that was correct, then the day’s full death toll had been close to 300.

In terms of arklet count, they’d started the day with 299 surviving, occupied arklets, a figure that had been reduced to 282 by the collision. Ten of them — a heptad and a triad — were attached to Red Hope, leaving 272. Approximately 200 were missing and presumed to have flown the coop with J.B.F. The remaining 70 or so had elected to stay behind and were still reporting in as members in good standing of the Cloud Ark. The 11 spares were still attached to Izzy and would be inspected for damage later.

The arklets still with them probably had a population of some 300. That plus the survivors aboard Izzy added up to a bit over 400. The population of J.B.F.’s breakaway swarm must then be something like 800 souls. She had taken two-thirds of the human race with her.

“God forgive me,” Doob said, “but right now I don’t even care about head count. The number I’m after is engines. Arklet engines. Until Dinah showed up with all of that ice, they were useless. Now, we have a way to fuel them. If we get them all pointed in the same direction, all pushing on Izzy, we can go on the Big Ride.” He paused to look at his notes. With his reading glasses down on his nose he suddenly looked a lot older to Dinah. She could only imagine how she looked. “Based on what you just told me, am I right in thinking we have—”

“About seventy,” Ivy said, “plus the eleven spares. We haven’t checked those yet, but on visual inspection they seem undamaged.”

“Eighty-one,” Doob said. “I like that number. A perfect square.”

“A perfect square of perfect squares,” Rhys put in.

“If we could come up with a structural system for ganging them in clusters of nine — just a three-by-three grid, with shared propellant feeds — and make nine of those clusters, and integrate them into Izzy’s structure somehow — that being the hard part — then we’d have an array of eighty-one engines. If those things all come on at full power when we pass through perigee, it’ll give us enough combined thrust to make a difference. I think we can make the Big Ride work with that level of power.”

“It’s a lot of structure,” Fyodor pointed out. “A lot, a lot, a lot.”

“We have a lot of raw materials to work with, don’t we?” Luisa asked. “I’ve seen rolls and rolls of that aluminum ribbon for feeding into the extruder machines.”

“It is a question of time,” Fyodor said. “Yes, we have a lot of material. But to assemble it with so few people is difficult. Atmosphere is growing, drag is increasing, orbit is decaying.”

Dinah looked across the table at Rhys. Rhys the biomimetic engineer, the man who had perhaps saved Ymir with his idea of turning robots into little, radiation-resistant Ben Grimms.

“We’ll build it out of ice,” Dinah said.

Rhys looked up at her, pondered it for a second, and nodded.

“Too brittle,” Fyodor said.

“I don’t think Dinah’s speaking of regular ice,” Rhys said. “She means the pykrete stuff they used on Ymir. Fiber-reinforced ice. It worked to hold the shard together. We can make it work here.”

Moira spoke up. “Perhaps I’m missing something, but I was under the impression that the ice was our propellant. Aren’t we going to melt it, and consume it, as we go along?”

“Yes,” Doob said.

“And doesn’t that mean that we’ll be consuming the structure that’s holding everything together?”

“Yes,” Doob repeated, “but it’s okay. Because the more of it we use, the lighter we get, and the less thrust we need. So it’s okay to sacrifice some structure as we go along.”

Sal had been listening intently. “I don’t mean to throw cold water on the idea,” he said. The pun — assuming he meant it as such — elicited a few groans. “But we’ve been hearing about radioactive contamination.”

“On the outer surface of the shard, yes,” Dinah said. “Microscopic motes of stuff that is super radioactive, stuck to the ice. Beta won’t penetrate our living spaces. We’ll have to be careful, though, not to track it inside. We can program robots to crawl around, look for those hot motes, and get rid of them over time.”

Sal looked unconvinced.

“I won’t lie to you,” Dinah said. “People are going to die of it.”

“But the trade is as follows,” Rhys said. “Izzy already has a massive battering ram of nickel-iron on her snout. Her flanks are vulnerable, as we learned today. Now we have the ability to shroud everything — the entire space station — in reinforced ice. Oh, it will dwindle over time. But through most of the Big Ride we’ll be living deep inside of a gigantic iceberg with a steel nose. I submit that the death toll from possible contamination will be minor compared to what we would experience if we went on the same journey unprotected.”

“What do you need to make it happen?” Ivy asked.

“Permission,” Dinah said.

“When did you ever ask for that before?”

The joke elicited a high-pitched laugh from the corner of the conference room. Heads turned toward Camila.

“Camila,” Ivy said, “we’ve hardly heard a word from you since we found you in the Shipyard. One of our witnesses there claims you may have saved Tekla’s life. You had the opportunity to escape with Julia. Instead you stayed behind to free the bound Shipyard workers. You saved their lives. Now you’re here among us. You must know how this looks.”

The look on Camila’s face made it clear that how it looked had never crossed her mind. She didn’t even get what Ivy was saying.

“Dear,” Luisa said, “people are going to say you are a spy who volunteered to be left behind.”

Camila held up a closed fist and opened it to reveal a small white plastic box, loose tape still dangling from it. “Julia’s bug,” she announced. “It was here.”

No one looked very convinced.

“She invited me to dinner at the White House,” Camila said. “She helped me pick out a dress. She introduced me to generals, ambassadors, movie stars. She wrote me letters on White House stationery. I was — I was in love with her. You can call me naive if you want. All right. I was naive. Until this morning. And then all of a sudden I saw. I saw what I was dealing with. I hate her now. And I hate myself for having been in love with her.”

“Best remember that, sweetheart,” Moira said. “Because she made the wrong choice today. And sooner or later, she’ll be coming back.”

“I’ll be ready,” Camila said.

Endurance

SEEN BY HUMAN EYES, THE HOLLOW HULK OF YMIR’S ICE SHARD WAS as dead, brittle, and gleaming as the discarded carapace of a beetle. Captured through the electronic eyes of cameras, then speeded up a hundred-thousandfold, so that the events of one day were compressed into one second of video, it looked like an amoeba pursuing, capturing, and swallowing Izzy. A person with no preconceptions of what they were watching would perceive Izzy as a steel-headed insect, all legs and pods and antennas, twitching and kicking in an effort to defend itself from the slow, relentless, liquid onslaught of the ice monster.

In truth, of course, the four hundred survivors, moving at lightning speed compared with the slow evolutions of the ice, were reconfiguring the space station in preparation for the Big Ride. The crippled Caboose was cut free and the components of the Shipyard moved forward. The big power reactor was brought in close to the Stack; from now on they would rely on ice to shield the rest of Izzy from its radiation. The eighty-one arklets arranged themselves into nine groups of nine and were tacked into place at the aft end, nozzles aimed backward. The structural works holding them into place at first were flimsy trellises on which spacewalkers could string cables, propellant lines, and hamster tubes. As soon as those were in place, the ice caught up with them, driven forward by the ceaseless operations of a giant Nat swarm, and the arklets were gradually cemented into place within a solid matrix of the fiber-reinforced ice known as pykrete.

Forward the ice flowed. It was like watching video of a melting iceberg played in reverse. The Nats, blindly following a simple collection of rules, packed it into every vacant space they happened upon. In the few minutes out of each day when the crew could take some rest and eat some rations, they would try to top each other telling funny stories about where they had found a living infestation of ice, and what they had done to beat it back.

Within a month, the remnants of Ymir had all been consumed, and Izzy had seemingly ceased to exist. The two of them had merged into an orbiting mountain. Its summit was a battered and scarified lump of nickel-iron, hazy with angular scaffolding where antennas and sensors were mounted. Its slopes were a smooth rampart of black ice, interrupted here and there by outcroppings of thrusters or other equipment, observation domes peeking out like hermits’ huts. Its base was a plane decorated with a neat grid of eighty-one small holes from which blue-white fire erupted from time to time as the ship passed through her perigee.

They couldn’t make out what to call the thing. People tried and failed to combine the words Izzy and Ymir. The closest they came was Izmir, but that had been the name of a city in Turkey. Sentiment was in favor of naming her after the martyrs of the Ymir expedition, but there had been several. In honor of Markus it was likened to the Daubenhorn, later shortened to the Horn. Which was not a bad nickname. But the name that stuck was a continuation of the Shackleton theme that Markus had established with New Caird. Shackleton’s big ship had been called Endurance, and was famous for having gotten stuck in the ice. So Endurance it was, and Fyodor christened her thus by getting into his battered Orlan, climbing out onto the surface of Amalthea, and dashing a bottle of champagne against the metal.

A more distant camera, looking down on Earth from high above the North Pole and watching the career of Endurance over the next years, would have seen a nail-biter of an opening, followed by endless grinding tedium, slowly building to a dramatic final reel.

Prior to Ymir’s arrival, the Cloud Ark’s pilots had put no small amount of attention into the problem of keeping Izzy out of the expanding atmosphere. This produced greater drag, which Amalthea, with its high ballistic coefficient, was well made to resist. But the decay of her orbit had to be mended from time to time with burns of the big engine that in those days had lived on the aft end of the Caboose, fueled by the Shipyard’s reactor-powered splitters.

The Break — as they called the event when the big bolide had smashed into Izzy, and the Swarm and Red Hope had gone their separate ways — had put an end to all that. Between then and the day about a month later when Endurance was christened, she spiraled gradually downward. Had the featherweight arklets tried to keep formation with her, they’d have been pushed back by the wind. They were forced to creep into the lee of Amalthea and ride along within her bow wave, like bicyclists slipstreaming behind a truck, until they could be integrated into the framework of the ship. Down and down she spiraled, and the SI team had to send Grabbs out onto the forward trusswork and remove the fragile antennas and sensors mounted there, lest they be slowly burned off by a rarefied but white-hot windblast. Fyodor’s champagne space walk was a brief one, and when he got back inside he reported that he could see the spraying foam of the champagne being blown backward by the atmosphere.

Their mission was to move their apogee from where it was now — just a few kilometers higher than the altitude of the perigee — all the way out to the altitude of Cleft, some 378,000 kilometers more distant. It was a reversal of the maneuvers that Markus, Dinah, Jiro, and Vyacheslav had executed in order to bring Ymir into orbital sync with Izzy. The way to achieve it was to burn the engines for brief intervals as Endurance made her regular swings through perigee.

The first of those burns happened about thirty minutes after she was christened, and yielded a delta vee of four meters per second. The acceleration was so mild that most of the crew could not even sense it. For the combined thrust of eighty-one arklet engines was nearly powerless against the bulk of Endurance, with her roughly equal masses of iron and ice. Nonetheless it was enough to boost her apogee, which occurred some forty-six minutes later, by 14.18 kilometers. And forty-six minutes after that, another burn during another scrape with the atmosphere gained them another four meters per second that, at the ensuing apogee, added 14.21 kilometers on top of that. The result of Endurance’s first day of operations was a boost in apogee altitude of more than one hundred kilometers, enough to get them clear of the expanded atmosphere except during the few minutes each orbit when they swung through perigee.

After that, however, they had to suspend operations, since they’d used up all the propellant stored in the Shipyard’s ice-buried tanks. They needed to give the reactor and the splitters some time to catch up. Even a nuclear power plant could split water only so fast.

Not long after, the operation was shut down for a week by problems in feeding clean water to the system. For another month it could only operate at about a quarter of its planned capacity. But over time they worked the bugs out and began to burn the engines more and more at each perigee, gradually extending Endurance’s reach toward Cleft.

If they could keep it up, that reach would get less gradual over time. The first delta vee had gained them 14.18 kilometers. The second, equivalent delta vee had reaped 14.21 kilometers — an improvement of about thirty meters. These gains were tiny in comparison to the distances of outer space, but from a mathematical standpoint the trend was extremely significant. It meant that the higher they went — the more elongated the orbit became — the more leverage they could obtain from each one of those tiny delta vees. That difference of thirty meters would grow and grow until it spanned many kilometers, and each of those improvements would feed back into the equations and amplify the next result a little bit more. It was an exponential sort of phenomenon, and this time humanity was on the right side of it.

This didn’t even take into account another piece of good mathematical news, which was that Endurance grew a little bit lighter with each one of those burns. She had less mass with which to resist the force of the thrusters, and so it gradually became possible to produce more than a piddling four meters per second of delta vee on each turn around the planet.

So everything was going to get better, if they could stay alive and keep Endurance working. But these gains accrued painfully slowly at the beginning.

IT ENDED UP TAKING THREE YEARS.

They had planned for one. It took longer because things kept breaking and needed to be fixed. The tools and supplies needed to fix them weren’t always available. Sometimes they had to be improvised. Elaborate workarounds had to be devised through the force of human ingenuity, hard work, and, when all else failed, the risking and the sacrifice of lives.

The human capital of Endurance dwindled. They were always short on food. Arklets were designed to grow their own food supplies in their translucent outer hulls. But Endurance’s arklets were buried in ice to protect them from the Hard Rain. The ones near the outside got enough sunlight to produce some food, but not enough compared to the mouths that had to be fed. She began her journey well stocked with emergency provisions, which were rationed out on a schedule that assumed a mission length of one year. As it became clear that the journey would go on much longer, the rations were cut back. Endurance also had abundant stockpiles of vitamins, most of which had survived the Break. These were sought after by the people of the Swarm, who had flown the coop without stockpiling enough of them. Trade began to happen between Endurance and the Swarm, but it wasn’t the free market that the Swarmamentalists had once envisioned. Deals were negotiated over the radio and consummated by exchanges between MIVs and arklets, difficult to arrange because of the need to match orbits that had now become very different.

As they had done with Ymir, they mined ice from the interior volume of Endurance, leaving the outer “walnut shell” as a structural support and as a first line of defense against bolides. But as J.B.F. and other Dump and Run partisans had never tired of pointing out, such a heavy craft lacked maneuverability. When a big rock was seen far enough in advance, they could use her engines to make small course changes that would have large outcomes by the time the rock came close. Doing so was the full-time occupation of most of Endurance’s complement, which worked at it three shifts a day. But below a certain threshold they could not see rocks soon enough, or maneuver out of their way quickly enough, and then they just had to hope that the bolide would strike Amalthea. Most did, but some hit the icy lower slopes, and of those, some struck with enough power to penetrate and kill.

Suicide took about one in ten over the course of the three-year journey. Sometimes this was for traditional reasons. After a great burst of creative activity during the weeks when Endurance was being designed and crafted, Rhys fell into a black depression and took his own life a month into the voyage. In other cases a spacewalker agreed to go on what was clearly a suicide mission, or a patient suffering from cancer decided to end her life rather than create a drain on limited resources of food, air, and medical supplies. And there was quite a bit of cancer, for Dinah’s prediction on the day of the Break had been borne out. In spite of all precautions, particles of fallout made their way into the air and the food chain and lodged in lungs and guts. Even without this, the environment of space, with its ambient radiation, lack of exercise, poor diet, and exposure to various chemicals, tended to jack up the cancer rate. Endurance’s medical facilities were not up to the job of detecting and treating cancer in the way people had been used to on Earth.

Periodic crises in the supplies of food and air, caused by blights in the greenhouses or breakdowns in equipment, took away people whose strength had been sapped to begin with. The journey entailed thousands of traversals of the Van Allen radiation belts. Rather than passing through these but one or two times, as might have been the case in a more traditional space voyage, they had to do it twice on each orbit; and during the first year they were, for all practical purposes, never out of them. They sheltered as much as they could in shielded parts of the ship. But no shelter was perfect. Some of the crew were obliged by duty or by happenstance to remain in exposed locations. And the mere fact of being crammed together in a confined space for a significant fraction of the time was a drag on health.

The gender ratio began to skew even more toward females. The General Population, whose Break-surviving members had made up roughly a quarter of Endurance’s original complement, had been predominantly male. This was a simple consequence of the fact that they had been drawn from traditionally male-dominated professions such as the military, the astronaut corps, and science and engineering. The other three-quarters had been Arkies. The original Arkie population had been 75 percent female and 25 percent male. The ones who had elected to stay with Endurance at the time of the Break were more strongly skewed toward women.

The men tended to be older — in many cases two or even three times the age of the Arkies. Compared to the Arkies, who had mostly been sent up at the last minute, they tended to have been in space, and subject to its health effects, for much longer. They had been picked for brains, not for physical fitness. At least at the beginning, while the Arkies were still learning the ropes, they tended to draw the most hazardous duty, such as space walks. And men simply were not as well suited to life in space. They were more biologically vulnerable to radiation. They needed more air and more food. And, whether it was the result of cultural upbringing or genetic programming, they simply were not cut out psychologically for the idea that they were going to spend the rest of their lives in crowded indoor spaces. Many of them felt an urge to go outside and get away from people that manifested itself as a tendency to volunteer for more space walks. People who went on space walks were much more likely to die of radiation exposure, bolide strikes, equipment malfunction, misadventure, or contamination by reactor fallout.

As well, there was an understanding, widely shared but rarely spoken of, that men were not the scarce resource. Women — to be specific, healthy, functional wombs — were. Acting on that belief, or perhaps just electing a more socially constructive form of suicide, men continued volunteering for hazard duty, instinctively herding the women toward the protected interior spaces of the ship; and when the women objected to it, as some did, they were apt to be shut down by the simple, hard-to-argue-with assertion that their lives and health had to be preserved at all costs.

Communication with the Swarm was sporadic and tended to come in bursts, when the Swarm needed something. The groups had separated under conditions that would have been deemed a state of war had the Break not happened in the midst of a catastrophe more deadly than anything that one side could have inflicted upon the other with force of arms. Neither side was likely to begin trusting the other anytime soon. Free, Internet-style communication between them was forbidden on both sides, since it could have been used for purposes that were mischievous or worse. The channel between Swarm and Endurance was more akin to a hotline linking two Cold War capitals. It went unused for months at a time. This was not so much a matter of the two sides snubbing each other as it was that they were both fully occupied trying to stay alive. Ivy and J.B.F. were like the captains of two damaged ships, many miles apart in stormy seas, with other things on their minds. When the channel was used, it was to negotiate terms of exchanges between the two groups. Neither side was of a mind to share much information about its status. But much could be inferred from the things that the Swarm urgently asked for: mostly propellant, but also the sort of medicine used to treat radiation sickness, blight-resistant strains of food crops, nutrients, spare parts for CO₂ scrubbers and for the Stirling engines that supplied power to arklets. In exchange they offered mostly food, which was the only thing they could make that Endurance didn’t already have.

Eleven weeks following the Break, a solar flare had occurred, followed by an event known as a coronal mass ejection: a vast release of charged particles hurled out from the sun into the solar system. With its array of sensors, some of which were always pointed toward the sun for just this reason, Endurance had seen the storm coming and had sent a warning message to the Swarm. In those days Endurance had been well inside the protection of the Earth’s magnetosphere. That plus the shielding provided by iron and ice had enabled her crew to ride out the storm with little exposure to its radiation. They had no way of knowing, though, whether the Swarm had even received or understood the warning. The danger of coronal mass ejections had been well understood by the Arkitects, who had provided “storm shelters” in each arklet: sleeping bags, in effect, made so that water could be pumped into the space between their inner and outer walls, surrounding the occupant with molecules that were good at absorbing high-energy protons. The arklets were also stocked with doses of a drug called amifostine, which protected DNA from damage produced by the free radicals generated in the body by radiation exposure. The scheme was a good one provided the Arkies had at least half an hour’s advance warning and enough water in their arklets’ tanks to fill up all the shelters. They practiced it every so often, as sailors would perform lifeboat drills. But there was a lot that could go wrong, and it seemed unlikely that all eight hundred Arkies had made it through the storm unscathed.

In the ensuing three years there had been ten more coronal mass ejections big enough to worry about. Endurance had transmitted a warning to the Swarm in each of those cases but never received an acknowledgment.

It was worrisome that the Swarm always seemed to want more water. Since the water of an arklet’s ecosystem was recycled, the only way the arklet could lose it was by expending it as propellant: splitting it into hydrogen and oxygen and feeding it to a thruster. All the arklets in a swarm would have to do this from time to time, simply in order to remain in formation. That was true even if they never dodged a rock and never changed their orbit around the Earth. But it seemed that they had changed their orbit on several occasions, making it higher and more circular to keep it clear of the Van Allen belts. Presumably they had their reasons for doing so. But if they ran so low on water that they couldn’t fill their storm shelters when needed, they were open to a disaster that might kill most or all of them at a stroke. Ivy could only assume that they were still reasonable people and that if things got that bad, they would call for help. In the meantime, she tried to guard against the seductive idea that Endurance had all the water it could ever need. There weren’t going to be any more Ymir expeditions. The water they carried with them might be all that the human race had to live on for hundreds of years.

She had already made up her mind what she would say if J.B.F. ever contacted her with an urgent demand for storm shelter water: nothing doing, come to us, rejoin the crew of Endurance and take shelter here. She wondered, sometimes, if J.B.F. had anticipated that Ivy would make such a demand, and just how far she was willing to go to avoid such an unconditional surrender.

“WELL, THAT WAS HARD,” DOOB CROAKED, THEN WETTED HIS WHISTLE with a swig of the Ardbeg, mixed with a few drops of five-billion-year-old asteroid water.

He was in the Banana, speaking to an empty room, staring up at a projection screen on the wall. His reading glasses no longer worked; zero gravity had changed the shape of his eyeballs. The people who knew how to operate the lens-grinding machine were all dead or missing, so there was no way to make new eyeglasses until someone figured out where the machine had been squirreled away and read the instruction manual. Since only twenty-eight people remained alive on Endurance, this didn’t look like it would happen anytime soon. His distance vision was still pretty good, but because of the problem with the glasses he didn’t like to use his laptop for long periods of time. Instead he would come here to the Banana, soak up a little gravity, plug the computer into the projector cable, and work at long range.

He had been here for an hour, because he didn’t want to miss the big moment. He knew exactly when that moment would occur, plus or minus a few seconds, but in the meantime he couldn’t concentrate on anything else. The other twenty-seven were asleep or busy. So he was celebrating alone.

The display in front of him was dominated by a single large window displaying six numbers in fat, easy-to-read block letters. These were the orbital parameters of Endurance. They were updated several times a second, the numbers blurring and twitching. The one he was focusing on was labeled R, short for Radius. It was the distance separating Endurance from the center of Earth. At the moment, it was the highest it had ever been, at 384,512,933 meters and still climbing, slowly, in the last few digits. Endurance was creeping toward apogee, the highest apogee she had ever attained, and the height of that apogee was slightly beyond the distance at which the moon had once orbited Earth. For the first time they were as high in the sky, now, as Cleft.

Loose objects shifted position as Endurance’s remaining engines came on. They were down to thirty-seven functioning arklet engines from the original complement of eighty-one. On a good day they could muster thirty-nine. The other half of them had been cannibalized to keep the good ones working. To compensate for the losses, they had jury-rigged all the other engines they could get: the big one from the Caboose, all the propulsion units that had once been part of the Shipyard, and a few spare motors from straggler arklets that had become separated from the Swarm and found a way to rejoin them. Despite the reduction in engine power, Endurance was at least as maneuverable now as she had been at the beginning, when she had wallowed at the bottom of Earth’s gravity well, burdened with years’ worth of propellant. She weighed half as much now as she had in those days.

The burn went on for a while. It concluded with a change in attitude and a burn in another direction. Doob didn’t have to read the numbers on the screen to know what they were doing. They’d been planning it for three years.

They were in a highly eccentric orbit now, a pair of hairpin turns welded together by straightaways a third of a million kilometers long. Earth nestled deep in the crook of one of those hairpins. Endurance’s perigee hadn’t changed in three years; on every one of the thousands of orbits they had made, they had screamed across the top of Earth’s atmosphere while running their engines full blast. On the last such pass, which they’d made about five days ago, they’d topped out at more than eleven thousand meters per second of velocity. The visual symmetry of the orbit was deceptive; at their current location, the opposite hairpin, now slightly beyond the old moon’s orbit, they were crawling along at a speed that could have been matched, back in the day, by a wheeled vehicle on a salt flat. They were like a car on a roller coaster that had been towed all the way to the top and that was creeping along in that moment before it begins the plunge down to the bottom. The Earth was the size of a ping-pong ball at arm’s length. Soon they’d begin falling toward it, building back up toward eleven thousand meters per second during their next perigee pass, five days from now.

In the meantime, though, during these few minutes when they were just inching along, they could work magic. Small changes in velocity out here led to enormous transformations in their orbit down there. Endurance, by dint of enduring for three years and persevering in her plan, had reached Cleft’s distance from Earth. But she’d always been in the wrong plane: the same plane that Izzy had started out in, the plane that had been chosen, seemingly a million years ago, because it was easily reached from the Baikonur Cosmodrome. Down there, deep in the gravity well, changing that plane would have been catastrophically expensive. If they’d had an Earth to go back to, it would have been cheaper to start from scratch and build a new space station than to move Izzy to the plane where the moon had once orbited. Up here, though, by burning the engines at apogee, they could nudge it closer and closer to the desired plane at much lower cost. So they’d been doing little plane change maneuvers at each apogee. It had been going on for months now. It was a thing that had to happen if they were ever to reach Cleft, but it made Doob’s stomach burn, made him wish he hadn’t had a couple of slugs of hoarded single-malt.

For the plane of the old moon — the place they had to go to find safe refuge in Cleft — was where all the rocks were. That was where the rocks had started out, at Zero, and for the most part that was where they had stayed. The ones that had fallen to Earth in the Hard Rain were only a tiny fraction of the lunar debris cloud: just a faint dusting compared to what remained up here. During most of Endurance’s journey, her pilots had, by choice, kept her in that angled Baikonur-compatible plane, well clear of the moon’s debris field. Otherwise they could never have survived for this long.

But the risk that they had to accept, in order to try for Cleft, was to fly through the debris cloud in which Cleft swam. Every time they had reached an apogee in the last few months, and burned their engines to bring their orbit closer to the plane of their destination, they had edged into dirtier and more dangerous space.

Their slowness was part of the problem. If the debris cloud was a fleet of cars roaring around a circular raceway at top speed, then Endurance was a child toddling out into traffic. That extreme disparity in speed would remain until the next apogee, ten days from now, when they would make their biggest and longest burn, expending all of Endurance’s remaining propellant to accelerate her to the same average speed as the debris cloud. In so doing, they’d convert the dual-hairpin orbit into a nearly perfect circle, remaining 384,512,933 meters from Earth forever. Having merged smoothly with the traffic on the circular raceway, they would go hunting for Cleft. Doob had spied it several times on his optical telescopes, gotten a fix on its params, knew how to find it.

This was his life’s work.

If he’d been asked several years ago, before Zero, he’d have said it was something else. But his life until Day 360 had been nothing more than preparation for the mission plan he had laid in and was now executing for Endurance. The day of the Break — the arrival of the propellant needed, the death of his friend and colleague Konrad, and the sundering of the Swarm — had made it clear what needed to be done, and who needed to do it. So he’d been doing it.

Ten days remained until they were swimming in the debris cloud. Perhaps a fortnight before they reached Cleft. He wondered if he would live to see it. Quite obviously, he had cancer. Diagnostic facilities were lacking, but the first undeniable symptoms had been in his digestive tract, and since then his liver had become swollen by metastases. Now he was feeling some weird stuff in his lungs. It had grown slowly. It might have been natural causes — something that had been seeded on Old Earth, before he had even come to space — or it might have been a piece of fallout that had made its way into his food and gotten caught in his gut. No matter. The main question on his mind was whether he would live to see Cleft. He actually didn’t feel that bad, and so the naive answer would be yes, of course; but cancer growth was something of an exponential phenomenon, and he knew how tricky those could be.

Bolor-Erdene was flying the ship, working in the Hammerhead — the deeply sheltered control room that they had built into the lee side of Amalthea. Or at least she was on the duty roster as the nominal pilot. Distinctions of rank and specialty had ceased to matter much. Everyone who had survived — nine men and nineteen women — knew how to do everything: fly the ship, fix an arklet engine, go on a space walk, program a robot. The Doob of a few years ago would have ridden it out in the Hammerhead with her, looking over her shoulder, checking the params, swapping witty remarks in the occasional moment of downtime. The Doob sitting in the Banana right now had seen it all before, thousands of times, and knew that it was as routine to Bo, or to any of the survivors, as driving to work would have been before Zero. Being there would only have gotten his stomach riled up. He needed to conserve his energy.

He realized that he had dozed off. Opening his eyes and focusing, with some effort, on the screen, he saw that nearly an hour had passed since apogee. They were falling toward Earth for the last time.

His phone rang. Held at arm’s length it was blurry, but some vestigial part of his brain could still recognize the smear of pixels as a snapshot of Bo, taken years ago. He swiped it on and answered it.

“We are being contacted by the Swarm,” Bo said.

“Are we really?” he answered. Suddenly he was awake. “What does J.B.F. want?”

“It’s not J.B.F. It’s someone named. .” Bo paused. “A-ida. Or something. Two dots on the i.”

Doob tried to place the name. Aïda. He had a vague memory of her from his early days on the Cloud Ark. An Italian girl. Young. Arkie, not GPop. Socially a little weird. Hyperacute in a way that could be exhausting.

“It’s pronounced ‘I-yeeda,’” he told Bo.

“Anyway, they send congratulations on the successful completion of our maneuver, and request a parley. Should I wake up Ivy?”

“I’ll be there in a minute,” Doob said. “Let her sleep.”

He hated to think this way, but the Swarmers well knew what time it was, and which shift Ivy slept on, and that she was sleeping now. Rousting her out of bed would send the wrong message, making the crew of Endurance seem overeager.

Which might have been an excess of caution — a J.B.F.-style exercise of byzantine thinking — he reflected, as he pushed himself up the middle of the Stack. This had become a dingy place, sort of yellowed and shiny with human exhalations, condensed on its ice-cold walls and never really scrubbed off. He was glad he couldn’t see it very well.

They knew so little about the Swarm. From the straggler arklets they’d picked up over the last three years, they knew that J.B.F. had moved swiftly to consolidate her power, exploiting the crisis of the first coronal mass ejection — which had killed something like 10 percent of the population — to set up her own version of martial law. From there the trains had run more or less on time, albeit with a steadily dwindling population, until about a year ago, when some Arkies had begun to rebel and the Swarm had divided into two Swarms, coexisting with each other — as they had little choice — but not talking.

The people of Endurance had paid surprisingly little attention to matters Swarm related, because, in the end, it didn’t really matter that much. The die had been cast on the day of the Break. Not so much on the level of politics as on physics. Those who had stayed behind on Izzy had committed themselves to following Doob’s plan, his life’s work: the Big Ride. You were either aboard Endurance, simultaneously trapped and protected by her mass, or you weren’t. If you were, there was no getting off. If you weren’t, you had to find a way to survive as part of the Swarm, which meant moving to a completely different orbit and following a plan that was incompatible, on an orbital mechanics level, with the Big Ride. Once those orbits had diverged, the only way to reconnect was by effecting a big delta vee. That meant spending a lot of water that you were never going to get back. Less water meant less shielding from coronal mass ejections, limited food production, and hobbled maneuvering when bad rocks came at you. Getting a whole Swarm to agree on that course of action was impossible, and might actually have been a bad idea, since Endurance couldn’t accommodate a lot of refugees. Her mission plan was predicated on her ability to absorb significant bolide strikes without taking serious damage. A bunch of naked arklets following in her wake would soon get beaten to death. So on a physics level alone, the Break had been irrevocable, even had the two groups badly wanted to get together.

But apparently what was left of the Swarm had been watching Endurance. Biding their time, waiting to see whether she would win through.

This Aïda person must understand Doob’s plan. She knew what was at stake now. If the remnants of the Swarm could rejoin Endurance in the next ten days, before she disappeared into the maelstrom of the debris cloud, they had a hope of reaching the comparative safety of Cleft. Otherwise they were condemned to circle Earth in some relatively clean and safe orbit as their population and their water supply dwindled.

Doob swam into the Hammerhead. Three other people were in here: Bo, Steve Lake, and Michael Park, a former Arkie, a gay Korean-Canadian from Vancouver who had found six different ways to make himself indispensable.

“Aïda Ferrari, according to our records,” Bo said, before he asked. “A leader of the anti-J.B.F. faction. Sounds like J.B.F. lost.”

Steve seemed busy. It was good to see him active. He had come down with some kind of long-running bowel complaint, an imbalance in the bacteria that lived in his gut. He had kept the dreadlocks, but they were now bigger than he was. He must weigh less than a hundred pounds. But his fingers still flew over the keys of his laptop.

Bo had already turned her attention back to the business of running the ship, but Michael explained, “Steve’s getting a video feed going. No one’s done it in years.”

He meant that no one had recently been doing it over the old-school S-band radios used for long-range communication between space vehicles. Of course, on the short-range mesh network that the Arkitects had set up to knit the Cloud Ark together, people did it all the time using Scape. But depending on where they were in their orbit, the remnants of the Swarm might be hundreds of thousands of kilometers away from Endurance, far out of mesh range, and so they had to use the same sort of pre-Internet technology that the Apollo astronauts had used to send television signals back from the moon.

Eventually Steve did get it going, and then they were treated to a full-face image, in blocky pixels, of a dark-eyed woman with a fine-featured head that had been buzz-cut a few weeks ago and little tended since.

Once Steve did him the favor of throwing it up on a big screen where he could actually see it, Doob saw the obvious signs of malnutrition that had been affecting everyone on Endurance. He was mildly surprised by that. They had tantalized themselves by imagining the Swarm as a cornucopia of agriculture. But maybe it was low on water. The woman’s gaze was downcast, which, as everyone understood, meant that she was focusing on the screen of a tablet below the camera. Once she understood that the link was up and running, she raised her chin and seemed to stare directly into the Hammerhead with a pair of huge dark eyes. The low quality of the video made these seem pitch black, with no distinction between iris and pupil, and starvation had given them a sort of hot gleam.

“Aïda,” the woman said, by way of self-introduction. “I see you, Dr. Harris.” She began to smile, offering a glimpse of bad teeth, then thought better of it. Her eyes changed direction momentarily to someone or something off-camera, then came back to them. She raised her tablet up closer to the camera so that she could look at the feed from Endurance. Her hand passed briefly in front of the lens and they caught a glimpse of dirty, ragged fingernails, the frayed and shiny cuff of a sleeve. Faint murmurs in the background suggested that other people were in the same arklet with her, off-camera. She was in zero gee, therefore, not part of a bolo. Her eyes were exploring the feed on her tablet, trying to make sense of what she was seeing. The Hammerhead had not existed at the time of the Break, so it was a new thing to her. “Steve Lake,” she muttered, as she recognized him.

“Bo,” Bo said.

“Michael,” Michael said.

“Who is in charge?” Aïda asked. “Is Ivy. .”

“Ivy’s still alive and she is still the commander as per CAC,” Doob said. “She’s off shift. We can wake her up if you need to speak to her urgently.”

“No. Not necessary,” Aïda said, recoiling slightly and narrowing the eyes just a bit. The distance between her and Endurance introduced a time lag in the video, which made conversation halting and awkward.

“How many do you have?” Doob asked.

“Eleven.”

Doob, accustomed to working professionally with extremely large numbers, couldn’t quite process one so small. Eleven. One plus ten.

A thought came to him. “Do you mean eleven arklets?” That would imply scores, maybe a hundred people.

Aïda looked amused. “Oh no, of arklets we have many more. We have twenty-six.”

“Ah. So what is it you have eleven of?”

“People,” Aïda said.

“Aïda,” Bo said, “just to be clear. So there is no misunderstanding. You are speaking for the entire Swarm. And you are saying that, of the entire Swarm, there are eleven survivors.”

“Yes. Plus one. .”

“One what?”

A look of amusement came over Aïda’s face. She broke eye contact. It almost seemed that she rolled her eyes a little. Doob was reminded, hardly for the first time, that the Arkies had been sent up as teenagers. “It is complicated. Let’s just say there is one more who might as well be dead.”

Those in the Hammerhead still could not quite process it. Something occurred to Michael: “We know that the Swarm broke up into two factions. One led by J.B.F. You were part of the opposing group?”

“Yes.” Aïda laughed. Again she reminded Doob of a teenager going through the pretense of talking to a clueless parent about something they would never understand.

Michael, a little wrong-footed, went on haltingly: “And so when you say that there are eleven. . plus one who is, I take it, in a bad way. . anyhow, are you referring just to the anti-J.B.F. faction?”

“They were defeated a long time ago. Months.”

“When you say that, do you mean that there was some kind of a conflict? A war?” Doob asked.

Aïda shrugged. “There was some fighting.” She didn’t see it as important. “Call it a war if you wish. More like some brawls. The real battle was, you know, on the Internet. Social media.”

Silence ensued. Aïda waited for them to respond. When no one did, she shrugged. “What were we going to do? Smash our arklets into each other? There is no way to have, like, actual violence in this setting! So we just had a war of words.” She held her hands up in front of her, making them into little pantomime mouths, aimed at each other, thumb-jaws flapping up and down. “Trying to, you know, persuade others to join our side. Trying to make the other side look bad. Just like the Internet always was.” She chuckled, put one hand to her cheek, rubbed her eye. “Look, it is very complicated and I cannot explain everything right now — how it all came out.”

“But you said that J.B.F.’s faction was defeated,” Michael said. Of all the people in the Hammerhead, he seemed most committed to the proposition that there was a reasonable and logical explanation for all of this.

“Her and Tav, yes.”

“By which you mean, you defeated them with words. Ideas. A social media campaign.”

“We were more persuasive,” Aïda said. “I was more persuasive. Arklet by arklet, they came over to my side. The White Arklet held out for a while, then they gave up.”

“What became of them?”

“J.B.F. is fine. Tav, not so good.”

“He’s the one you mentioned. The twelfth one who might as well be dead.”

“I am afraid so, yes.”

“So getting back to the earlier question,” Doob said, “the number you quoted is for the entire Swarm. Both factions.”

Aïda, finally seeming to understand what they were getting at, sat up straighter and got a more serious look on her face. “Yes. There are no other survivors whatsoever. Of the eight hundred, eleven remain.”

There was a long silence as the four in the Hammerhead took this in. They had all harbored fears that the Swarm might go terribly wrong, but this was worse than anything they had imagined.

Finally Doob raised his hands in front of him, palms up, and shrugged. “What happened?”

“Agriculture crashed.” Aïda turned her head and stared off-camera for a few moments. “I mean, I could say many things, but that is basically it. Between the CMEs, algae blights, lack of water. . very few arklets produce food anymore.”

“What have you been eating?”

Aïda snapped her head around, as if surprised by the question, and looked quizzically into the camera. “Each other. Dead people, I mean.”

There was a long silence during which Doob, Bo, Michael, and Steve all exchanged looks.

The terrible thing was that they had considered doing the same thing, many times. Every freeze-dried corpse that they jettisoned was a big collection of protein and nutrients that, from a certain point of view, could seem mouthwatering.

Seeming to read their minds, Aïda went on: “And you?”

“You mean, have we resorted to eating dead people? No,” Doob said.

“Tav started it,” Aïda said. “He ate his own leg. Soft cannibalism, he called it. Legs are of no use in space. He blogged it. Then it went viral.”

No one had anything to say to that. After a few moments had gone by, Aïda continued. “But Endurance is better stocked with MREs and so on. Plenty of water. You would not have gone there.”

“No, we did not go there,” Doob said. He could tell from the body language of the others in the Hammerhead that they were too shocked to be entrusted with speaking at the moment.

“As for us,” Aïda said, “you should also know that supplies were conserved. Even as people died and we lost arklets. We moved what we had into the arklets that survived. Our twenty-six arklets are well stocked.”

“With everything except food,” Doob said.

“Yes.”

“Do you have enough water to match our trajectory?”

“Yes,” Aïda said. She was a beautiful young woman, Doob thought, with a fierceness about her that helped explain her success in the social media campaign against Tav and J.B.F. “We have performed all of the calculations. If we jettison mass and pack all we have into a heptad, we can make the rendezvous around the time of your next apogee. But we will need to know your exact params.”

“We will discuss your proposal,” Doob said, “and make any necessary preparations.” He looked over at Steve Lake, who severed the connection just as Aïda was about to say something.

THEY SAT IN THE BANANA AND DISCUSSED IT AS IF THERE WAS ANYTHING really to discuss. They all registered their rote shock and disgust at what the Swarm had been reduced to. It all sounded hollow to Luisa. Finally she spoke up. It was what Luisa did. They expected it of her. They relied on it.

“Seven billion died. Next to that, this is small. And God knows we’ve all thought about eating the dead, so let’s not pretend to be shocked that they actually did it. The real reason we’re all freaked out by this is that our hopes have been dashed. We thought that the Swarm was going to contain hundreds of healthy people, lots of food, lots of good company. Oh, intellectually we knew it wouldn’t be the case, but we were all hoping for it. Now we learn it’s eleven carrion eaters. Are we going to leave them to die? No. We’re going to make room for them and for their heptad full of scarce vitamins.”

“I am terrified of the woman Aïda,” Michael Park said.

Luisa sighed. “Let me throw out an idea, which is that you’re terrified because you wonder, at some level, whether you could turn into Aïda if you got hungry enough.”

“Still — to let her on board Endurance—”

“And J.B.F. too,” Tekla said. She and Moira were sitting next to each other as they always did, hands clasped, fingers intertwined.

“I hoped I would never see Julia again,” Camila put in. “I know it is small and selfish of me, but. .”

“I understand all of your misgivings,” Ivy said, “because I share them. The question, now, is whether those misgivings are going to have any effect on the decision we make. Are we really going to let one-third of the surviving human race die because Aïda’s creepy and we hate J.B.F.? Obviously not. So, we transmit our params and our burn plan. And during the remainder of this orbit we make arrangements to accommodate some new arklets.”

THE REMAINDER OF THE ORBIT WAS BUSY INDEED, TO THE POINT where they broke out hoarded rations and upped their calorie intake to fuel their brains and their bodies. In the middle of that ten-day stretch, however, was an intermission. Dinah and Ivy had wordlessly agreed to spend it together in what Doob had once called the Woo-Woo Pod, and what they now called the Kupol.

After the Break, when Rhys had reengineered Izzy and Ymir into a single moving sculpture of metal and water, he had moved this module to a different location in the Stack and then let the living ice flow around it, completely surrounding its inboard hemisphere and later building up in a protective brow that shielded part of its windowed half. It projected from the side of Endurance like an eyeball and gave people a place to go when they wanted to look at the universe. As such it had no legitimate function from an engineering point of view. In fact it was a liability, since it got hit by little rocks from time to time, depressurized, and had to be repaired. Anyone in it was getting directly exposed to cosmic radiation, and so it was a no-go zone when they were passing through the Van Allen belts, which was often. But people loved it anyway, and kept patching it up when it was broken, and went there when they wanted to be alone or when they wanted to share some special time with another person. Putting it there had been one of Rhys’s best moves as a designer, and Dinah silently thanked him whenever she used it. Doob’s old term for it had begun to seem a little tasteless after the Hard Rain. For a little while, people had instead referred to it as the Dome. But dom had a different meaning in Russian, and so they’d settled on Cupola or Kupol, whose meanings in English and Russian respectively were not too far apart. In the latter language it carried a vaguely religious connotation, having to do with cathedral domes.

Ivy and Dinah didn’t have to worry too much about cosmic rays during the intermission, because they had so arranged it that the Kupol was on Endurance’s nadir side, facing toward Earth. And Earth was close enough to fill their view. Useless as the planet might be for the support of life, it still acted as a very effective cosmic ray absorber. Nothing was getting through that, short of another mysterious Agent that could pass all the way through a planet and keep on going. So, Dinah and Ivy hovered in the middle of the sphere, arms linked so that they wouldn’t drift apart, and sucked bourbon from plastic bags, and looked at their old planet for the last time. In their six years of hurtling around this world, they’d grown accustomed to the steep angle that Izzy’s orbital plane made with the equator, and the views it afforded them of the high latitudes. Because of the changes they had lately been making to Endurance’s plane, however, they were now confined to a belt around the tropics.

Not that it mattered a hell of a lot with Earth in its current state. The sky was still on fire, streaked with the bluish-white incandescence of the Hard Rain. The ground, where they could see it through smoke and steam, was a mottled terrain of dully glowing lava: some of it the hot impact craters of recent big meteorites, some of it spewing up out of the Earth’s fractured crust. Oceans were dark at night, hazed with steam in daylight, their coasts difficult to make out, but clearly shallower than they had been. Florida was reaching out toward the Keys but being battered down and chipped away by bolides, and washed away by tsunamis, even as it did so. A year and a half ago, a big rock had torn the lid off the long-dormant Yellowstone supervolcano. That had been cloaking most of North America with ash ever since then; glimmers of yellow light in the northern extreme of their view hinted at a vast outpouring of magma. A long-suppressed habit told Dinah, absurdly, that she should go and turn on her radio in case Rufus was transmitting. This made the tears come, and that in turn made Ivy’s tears come, and so they spent the last half of the intermission, from perigee onward, gazing at Earth through water. It didn’t really affect the view much. But Dinah tried to register the memories as best she could. Humans would not again look on Earth from such a close vantage point for thousands of years.

The burning planet started to drop away from them. It would only ever get smaller from now. They needed to get back to work. But they found it difficult to let go of each other. Back in the old days, before Zero, they’d had the occasional heart-to-heart about their shared, secret fear that they weren’t qualified to carry out the missions for which they’d been sent up, at vast taxpayer expense. That they’d screw it up, fall on their faces, and embarrass a lot of people on the ground. By now, of course, they had long since put those fears to rest, or at least seen them overwhelmed and buried by much greater fears. Ever since the beginning of the Cloud Ark project, however, and especially since they had made the irrevocable decision to build Endurance and go on the Big Ride, it had frequently come back to them in a bigger and more dreaded form. What if they were completely getting it wrong? They could scarcely remember, now, the great civilization that had once spread across the planet below them. But the contrast between it and its orbiting residue was painful. The dirty, beat-up kludge that was Endurance was an embarrassment to the human race. Could they really have done no better than this? And now, after a voyage of three years — three years that had been an unrelieved spiral of decline, punctuated by catastrophes — they were reduced to a maneuver, coming up in five days’ time, that seemed more and more desperate the more they thought about it.

If they screwed it up, it would be their fault, more than anyone else’s.

Of course, no one would be left to blame them for it.

They went through these crises of confidence frequently, but usually at different times, so that one could pull the other out of despair. Right now they were both feeling it together, and so they had to pull themselves out.

Dinah was thinking about Rufus’s last transmission:

BYE HONEY DO US PROUD

“Okay,” she said. “Come on, sweetie. Let’s get to work.”

WORK GAVE THEM SOMETHING TO DO DURING THE FINAL ORBIT OF the Big Ride besides worry about what was going to happen at the end of it. The huge burn that they would make at apogee, combining a final plane change with an acceleration into the “fast lane” where Cleft rolled around the world like a ball bearing in a tire, contained so many unfathomable chances that it beggared prediction. The new wrinkle, however, was this: since they would be moving into a stream of rocks moving faster than they were, the rocks would be coming at them from behind, where Amalthea had no power to protect them.

Early in the mission Doob had dreamed of reconfiguring Endurance at the last minute, moving the vulnerable stuff around to the asteroid’s other side. With the manpower they’d had in those days, it might have been possible. As it was, reduced to a crew of twenty-eight starvelings, it was out of the question. It took all the people they had to make accommodations for the Swarm’s heptad. They would dock it in the middle of the Stack, lock it in place with a few cables, and hope it stayed attached during the subsequent maneuvers. They would keep the hatch closed. The eleven members of Aïda’s group would simply stay in their arklets until it was all over. The justification was that they’d be safer there. The true reason was that no one wanted cannibals in the shared spaces of Endurance.

The big project for Dinah, and for the small remnant crew of robot jockeys who tended to work with her, was getting ready to dump Amalthea itself.

On one level the idea seemed nearly unthinkable. They had, however, been planning to do it for a long time. Endurance’s final series of maneuvers would have to be accomplished quickly, deftly, in an environment where the rocks tended to be much larger than the ones that made up the Hard Rain. In a sense, the boulders up there were the mothers of the tiny fragments that had destroyed the surface of Earth. Every time two of them collided, a few chips exploded outward from the impact, and a fraction of those ended up falling into Earth’s atmosphere. The Hard Rain would continue until all of them had been reduced to sand and organized themselves into a neat system of rings. In any case, Amalthea’s ability to protect Endurance from the impacts of baseball- or even basketball-sized rocks was of little interest in a place where a rock the size of Ireland would be considered unremarkable. The entire ship, Amalthea and all, would be a bug on the windshield of such a thing. Their only way to stay alive, once they had entered the slipstream of the main debris cloud, was to maneuver around the big rocks and hope that they didn’t get hit by too many little ones during their dash to Cleft. And that sort of maneuvering was impossible as long as Amalthea — which weighed a hundred times as much as the rest of Endurance—was attached to her.

In addition to Amalthea, Endurance was still burdened by a considerable mass of ice, which they had hoarded both as shielding and as propellant. It weighed a significant fraction of what Amalthea weighed. But unlike Amalthea, they could burn it. The basic plan was to split most of that ice into hydrogen and oxygen, then burn it during the final speed-up at apogee. Over the course of a few hectic minutes, Endurance would expend most of her water weight by using it as propellant. Between that and the ditching of Amalthea, her total weight would drop by a factor of more than a hundred in the course of an hour. After that, she truly would be like a bug flitting around in heavy traffic, dodging big rocks and taking hits from little ones until she made her way to Cleft.

In any case, they had anticipated all of this long ago. Dinah and the other surviving members of the Mining Colony had had three years in which to reshape Amalthea from the inside. Seen from the forward end, the asteroid looked the same as ever. Internally, however, most of it had systematically been whittled loose. In a sense the process had begun around Day 14, when Dinah had set one of her Grabbs to work carving out a niche in which she could stash her electronic parts. Since then it had proceeded in fits and starts. They’d moved a lot of metal in order to make a storage bay for Moira Crewe’s genetic equipment, which in a sense was the raison d’être of everything else they had done in the last three years. Once that was safe they had begun tidying up, enlarging the pockets of protected space, breaking down walls, and joining them together into a cylindrical capsule scooped out of Amalthea’s back, called the Hammerhead for the way it lay athwart the top of the Stack.

Thanks to a lot of ticklish work by robots in the last couple of years, the Hammerhead was now separated from the rest of Amalthea—99 percent of the asteroid’s bulk and mass — by walls of nickel-iron that were only about as thick as the palm of a person’s hand. This still made them extremely massive by the standards of space architecture — more than strong enough to hold in atmospheric pressure and to stop small bolides. But the additional tens of meters’ thickness of metal beyond those walls was now physically detached from the hand-thick walls, and could be pushed away by a puff of compressed air.

Or rather, given the disparity in masses, Endurance could be pushed away from it. Most of Amalthea would stay where it was, and the radically lightened Endurance would back away from it like a grasshopper springing off a bowling ball.

When the time came, they would have to shatter the remaining structural links with demolition charges. One of Dinah’s duties on the last lap, as they soared back up out of Earth’s gravity well toward their rendezvous with Cleft, was to go out in a space suit and inspect those charges, make sure they were packed in where they needed to be and wired up correctly. She was the only person remaining who knew much about explosives, and so she was the only person who could be sure. Just another of those duties that would have left her half paralyzed six years ago and now seemed routine.

“I KNOW THAT WE DON’T NEED ANY MORE BAD NEWS,” DOOB ANNOUNCED to the 25 percent of the human race seated around the conference table in the Banana, “but here’s some for y’all.”

No one said anything. Nothing could make much of an impression on them at this point.

Forty-eight hours remained before apogee, the final burn, the ditching of Amalthea, the dash to Cleft. If Aïda’s transmission of half an hour ago was to be believed, the remnants of the Swarm would rendezvous with them shortly before all of those things happened.

“Let’s have it,” Ivy said.

“I’ve been keeping an eye on a certain sunspot,” Doob said. “Kind of angry looking. Well, about twenty minutes ago it kicked out a huge flare. Not the biggest we’ve ever seen, but pretty big.”

“So, we’re expecting a CME?” Ivy asked.

“Yeah. Somewhere between one and three days from now. I’ll provide better estimates as soon as I have more data.”

They all considered it. Until recently, coronal mass ejections had been of little concern to them except insofar as they made them wonder how the people of the Swarm were getting along. As for the tiny faction that had split away on Red Hope, it was assumed that they had long since been wiped out by one or more of the hazards and calamities that had inflicted such a death toll on the Swarm. For the crew of Endurance, Amalthea and ice had provided plenty of shielding. Even the comparatively thin walls of the Hammerhead would protect anyone inside of it against the kind of radiation that would envelop them in a CME. But Endurance’s flanks were now exposed. Grabbs had been at work carrying away the last of the ice and feeding it to the splitters to be made into rocket fuel. They were storing the cryogenic gases anywhere they could now, pumping them into empty arklet hulls and disused modules. Parts of the Stack were seeing the light of day for the first time since the Break.

“It’ll affect our operations,” Ivy concluded. “But this is a drill we know pretty well. Take amifostine. Get your space walks finished before it hits. We should make arrangements to accommodate all nonessential personnel in the Hammerhead. Some of us will have to be down farther in the Stack, but we’ll have storm shelters ready.”

“What about. . them?” Michael Park asked.

“They are a problem,” Ivy admitted. “They’re in plastic arklets. They’re gonna get cooked. Even if they have any amifostine left, even if they have enough water to fill their storm shelters, they’re going to take damage. Ethically, we need to bring those eleven aboard Endurance and get them to safer places.”

“The original plan was to send three people out on an EVA to lock down their heptad, make it fast to the stack, so that we could maneuver,” said Zeke Petersen. Of all the crew of Endurance he looked the most similar to his pre-Break appearance. He was skinnier, of course, with a bit of gray around his temples, but his health was still good, and he’d managed to keep his electric shaver working, so he was beardless. After the deaths of Fyodor, from an accident, and Ulrika, from a stroke, Ivy had designated him Endurance’s second in command.

He referred to the fact that Endurance was about to shed 99 percent of her mass, which meant that the same complement of engines, producing the same thrust, could make her accelerate a hundred times faster. The gee forces still would not be extreme — well within the range that humans could tolerate — but the maneuver would impose stresses on the ship’s frame the likes of which it had never experienced before. This was another of those eventualities that they had foreseen long ago and built into Endurance before covering her with ice.

So most of Endurance came prerigged for higher acceleration. Provided that nothing had broken during the last three years, she’d hold together, albeit with a lot of loose junk sliding around her interior spaces the first time they hit the throttle.

They hadn’t planned, though, for the last-minute addition of the heptad from the Swarm. This was awkward. It would be connected to the Stack by a docking port, which wasn’t designed to take a lot of mechanical strain. It was heavy, because Aïda and her crew had crammed it full of supplies and strapped even more to the outside of it. For the same reason, Ivy didn’t want to just ditch it — they could use those supplies. So the plan had been for three spacewalkers to greet the heptad and lash it into place with cables as soon as it arrived.

“We’ll just have to see what we can do with robots,” Ivy said, looking toward Dinah and Bo. “Just about everything we have out there is Grimmed, correct? So it can operate even in heavy radiation.”

“We’ll get ready for that,” Dinah agreed.

“As soon as the heptad docks, the robots get to work,” Ivy said, “locking it down as best we can. We open the hatch and get the eleven down through the hamster tubes as fast as possible — they’ll have no protection whatsoever while they’re moving through those tubes. We’ll have storm shelters waiting for them. They can climb into those and ride out the rest of the journey. The flight crew will operate out of the Hammerhead.”

THE NEXT TWO DAYS REMINDED DINAH OF THE NEW CAIRD EXPEDITION, in that there was a lot to do but no way to affect the schedule. They were at the mercy of astronomical events. Part of her wanted to pull all-nighters until this thing was finished, but she knew she had to be well rested and fed when it counted, and so she forced herself to eat and sleep on the usual schedule. When awake, she worked on preparations for the arrival of the heptad, pre-positioning Grabbs near the docking port that it would use, connecting cables to suitable anchor points, tuning up the programs that the robots would execute when it came time to snap the other ends of those cables onto the heptad, rehearsing them to check for places where the cables might snag.

The timeline gradually came into clearer focus. Aïda sent out a sharp request for amifostine and water to fill their storm shelters. Of course, it was impossible for Endurance to comply. They had plenty of both, but they had long since cannibalized all of their MIV parts and so they had no way to transport them.

Aïda decided to roll the dice by committing all the water she had left to a large burn that would bring them to the rendezvous with Endurance a little earlier than they’d originally planned. Meanwhile Doob’s space weather forecast was becoming more precise; he had a better idea now of when the radiation storm would break over them and thought that the timing was looking favorable. The heptad might arrive before things got bad. It might be okay after all to have some spacewalkers out there to cooperate with Dinah’s robots.

Dinah didn’t know how to feel about that. The schedule had been accelerated, and now she had to take into account the vagaries of human spacewalkers. If Aïda’s heptad docked soon enough, Doob pointed out, they might be able to transfer many of her heavy supplies through the docking port into the Stack and thereby reduce the awkward strains that all of Dinah’s cables were meant to take up.

Meanwhile Ivy and Zeke, the pilots, were addressing similar last-minute convolutions in their mission plan. As they got nearer, they got better information about the part of the debris cloud that they’d be maneuvering through. They could clearly make out Cleft’s radar signature, as well as those of many other big rocks that traveled in its vicinity. A clutter of faint noise and clouds on the optical telescope gave them data about the density of objects too small and numerous to resolve. All of it fed into the plan.

Doob looked tired, and nodded off frequently, and hadn’t eaten a square meal since the last perigee, but he pulled himself together when he was needed and fed any new information into a statistical model, prepared long in advance, that would enable them to maximize their chances by ditching Amalthea and doing the big final burn at just the right times. But as he kept warning Ivy and Zeke, the time was coming soon when they would become so embroiled in the particulars of which rock was coming from which direction that it wouldn’t be a statistical exercise anymore. It would be a video game, and its objective would be to build up speed while merging into a stream of large and small rocks that would be overtaking them with the speed of artillery shells.

The details, the sudden distractions and improvisations, piled up and thickened in a way that made Dinah think of a sonic boom on Old Earth: the onrushing stream of air thickening and solidifying in the path of the airplane, turning into a barrier that must be broken through or succumbed to. They seemed to break through it at the point when Michael and two other spacewalkers pulled on their cooling garments, much patched and mended, and donned their space suits. Doob had the incoming heptad on radar, then on optical, and verified that it was on course to rendezvous with them. This meant, of course, that the heptad was on a collision course with Endurance; the difference between a collision and a rendezvous was the final burn of the heptad’s thrusters that would slow it down at the last minute and bring its params into nearly perfect synchronization with the larger ship’s. Endurance herself, still burdened with Amalthea and with many tons of stored propellant, had next to no maneuverability, and so it would all be up to Aïda, or whoever was at the controls of her heptad.

The reunion of Endurance and the Swarm began, as it turned out, with a collision. It was not a catastrophic high-speed collision, but it certainly was no orderly and controlled rendezvous. Aïda had the presence of mind to give them about thirty seconds’ warning. Until then it had all been going well. The heptad had approached, using its thrusters to kill most of its velocity relative to the larger ship, and executed some little burns intended to bring it home to the docking port. Then Aïda announced, in a barely controlled tone of voice, that one of the thruster modules had run out of propellant and could no longer perform its function.

“It’s too heavy,” Zeke muttered. “They loaded in too much cargo; the thrusters are eating too much fuel trying to push all of that crap around.”

The heptad came in too fast and at the wrong angle and crashed into Caboose 2, which was a module, recycled from the wreckage of the Shipyard three years ago, that they had plugged into the back of H1 to serve as the aft-most thing in the Stack. They saw it happen on their screens, they felt it in their bones, and they heard the three spacewalkers exclaiming and cursing. A little storm of debris emerged from a hole that had evidently been torn in the skin of Caboose 2.

“C2 depressurized,” Tekla reported. “Sealed off from Stack.”

The debris cloud included one large object that had two arms, two legs, and a head. The limbs were flailing. Everyone watched silently.

“We lost Michael Park,” one of the other spacewalkers announced.

“We need more people back there,” Ivy announced to the crew in the Hammerhead.

Ivy’s message was clear. Later we will mourn for Michael. Now we have other things to worry about.

“Moira, you stay,” Ivy added.

Moira hadn’t even moved. She was accustomed to being treated, against her own will and instincts, like a cherished and fragile child.

“Maybe you could talk to Michael on the radio. He’ll be alive for a while.”

Moira nodded, swallowed hard, and focused on her laptop, entering the commands needed to establish a private voice link to Michael.

“Dinah, you stay here — run the robots. We are going to have to do some improvising. Bo, go back. Steve too. Luisa, deal with Aïda over voice — for me it’s too much stress and distraction. Stay in the Hammerhead and make that problem go away for me. Doob, stay here. Zeke, go back.”

Ivy looked around. “If I haven’t mentioned your name yet, go back and see what you can do. Doob, you’re the weatherman. Your job is to make announcements about the storm and when it’s going to hit.”

“Half an hour,” Doob said. “But yes. I will do that.”

Moira, headphones on, had retreated into the quietest corner of the Hammerhead and was engaging in a murmured conversation with Michael. She was holding a cloth over her eyes to absorb tears before they broke loose in the cabin. Luisa had already gone into her assigned role and had been listening to a voice transmission from Aïda. “She says she is going to try again.”

“I thought her thrusters were empty,” Ivy said.

“She can transfer propellant from some of the other thruster modules to the empty one. It’ll take a few minutes. She requests instructions on where to make the next attempt, since the docking port on Caboose 2 has been rendered unusable.”

With a bit of deliberation they agreed that the heptad should make its next attempt on a docking port in the old Zvezda module.

Dinah, who had spent most of the last couple of days preparing for the docking to occur on Caboose 2, sent her robots scrambling forward along the outside of the Stack, bringing their cables with them. That caught her up in a stew of minor complications that more than filled the time it took for the heptad to get its dead thruster up and running again.

They watched the second approach, and the docking, in silence. It took about ten minutes. Doob interrupted once to give an update on the approaching radiation storm.

Unexpectedly, it was Moira who broke the silence. “Don’t let them dock,” she said.

“What?!” Ivy said.

“It’s a trap.”

Zeke’s voice came over the PA: “Positive docking achieved. Getting ready to open the hatch.”

Moira added, “Michael figured it out.”

“Fifteen minutes before the storm breaks,” Doob announced.

Dinah had entered into a state of intense focus on the problem to be solved, seeing through the eyes of ten different robots performing ten different tasks, occasionally blurting out terse requests to the two surviving spacewalkers, asking them to shake a stuck cable loose or pull a wriggling Grabb out of trouble. She tried to filter out the conversation between Moira and Ivy.

“What do you mean, it’s a trap?”

“Aïda’s heptad joined the mesh network as soon as it got within range,” Moira said. “If you check your email right now, or your Spacebook, you’ll see stuff flooding into it. Terabytes of old messages and posts that have been bottled up in the Swarm. Mailing list traffic that’s three years old.”

“So?” Ivy asked.

“Michael saw some weird stuff just now, and drew my attention to it.”

“He’s floating in space!”

“He’s floating in space and checking his email.”

“What weird stuff did he notice?”

“They’re cannibals, Ivy.”

“We already know that.”

“A few hours ago,” Moira said, “they slaughtered Tav and ate what was left of him.”

Dinah was having difficulty focusing on her work.

“They wanted to be well fed for today.”

The time was approaching when the spacewalkers would have to go to their airlocks and get indoors ahead of the storm. Dinah had to focus on them. There was nothing she could do about what Moira was saying. She began speaking to one of them but was interrupted when Zeke came over the PA again: “Ten survivors aboard. Waiting for J.B.F. to emerge from the hatch.”

“Zeke, be on your toes,” Ivy said. “We have indications they may be up to no good.”

“Get inside,” Dinah said to the spacewalkers. “Head for the nearest airlock. Stay away from the new people, we don’t trust them.”

“Ditching the rock,” Ivy announced. A sharp hiss came through the walls as compressed air flooded the hair-thin gap between the outer surface of the Hammerhead and the surrounding cavity of Amalthea. “Plug your ears.” Then, before anyone could comply, a shattering, sickening bang as Dinah’s demolition charges went off, destroying the structural connections that joined Amalthea to Endurance. They felt a sharp jostle — more acceleration than they had experienced in three years — as the Hammerhead sprang free, pushing the rest of Endurance along with it.

“Three minutes before the storm hits,” Doob said.

“J.B.F. is aboard,” Luisa announced. She was on voice to Zeke and the rest of the crew aft, relaying what they said to the others in the Hammerhead. Her brow wrinkled. “Something’s wrong with her — I don’t quite follow.”

“Burning hard,” Ivy announced. Meaning that they were near their apogee, entering the fringe of the main lunar debris cloud, and that all the surviving engines had just come on full force. She had inaugurated the big burn that would, with a delta vee of some twelve hundred meters per second, inject them into the debris cloud.

Every loose object in the Hammerhead dropped to what was now the floor. At the same time they could hear all manner of percussion, from all over Endurance.

Zeke’s voice came in over the voice link. “We are in combat,” he said.

“Combat?” Ivy asked.

“They shot Steve Lake.”

“We are now experiencing very high levels of high-energy proton radiation from the CME,” Doob announced. “Everyone who is not in the Hammerhead should be getting into a storm shelter.”

“Shot him?” Ivy asked.

“With J.B.F.’s revolver. I suggest you try to lock down the network, they are trying to backdoor it.”

After that, communications were hectic and confused for a minute, and seemed to suggest that adversaries in different parts of the ship were all trying to use the same channel.

Then their communications went dead. The equipment still worked; they’d simply been locked out of the network. Ivy could still fly the ship, but none of them could talk to people outside the Hammerhead.

They were startled by a metallic rapping on the hatch that sealed the Hammerhead off from the SCRUM. Dinah’s ears soon read it as Morse code.

“‘Chocolate,’” she said. “That’s kind of a code word between me and Tekla. I think we should open the hatch.”

They did so, not before arming themselves with whatever makeshift weapons they could find, and found Tekla, suffering from a knife wound to the hand; Zeke, looking flustered but unharmed; and a woman, barely recognizable as Julia Bliss Flaherty given that most of her hair was gone and she had both of her hands firmly clamped over her mouth. Tekla vaulted into the Hammerhead and pulled Julia along behind her.

“What is going on?” Ivy demanded.

Zeke held up both hands. “I got this,” he said. “We have killed four of them already. Two more are casualties. We have them outnumbered. We just have to keep fighting.”

“You need to get into a storm shelter,” Ivy said.

Tekla, unaccustomed to working in even weak gravity, had gotten her footing enough to drag Julia into a corner of the Hammerhead and sit her down on the floor. She then turned back toward the hatch. Dinah had never seen Tekla in this state before, and feared her greatly in that moment. Moira had a different reaction; peeling off her headphones, she lurched across the space and threw her arms around Tekla’s neck. It looked like a greeting but soon developed into something else as Tekla began dragging Moira toward the hatch and Moira began trying to prevent her from returning to the fray.

“Sweet one,” Tekla was mumbling into Moira’s ear, “you want me to use wrestling moves on you? Then you should let me go, because I am going to kill that bitch Aïda.”

“Zipping into storm shelters is exactly what they wanted us to do,” Zeke explained. “Their plan was to take the ship as soon as we did so. Good thing you warned us.”

Tekla by now had peeled herself loose from Moira’s grip and advanced toward the hatch with a full stride.

Zeke, waiting for her, reached out with one hand. He was holding a small black plastic box. He pressed it against Tekla’s thigh and pulled a little trigger on its side. The device erupted with a sharp ticking, buzzing noise. Tekla’s leg collapsed and she floated to the floor, glassy-eyed.

“Sorry, Tekla,” Zeke said. “You stay here. Get your hand fixed. Keep Moira company — she needs you. And if you have a little boy, name it Zeke.”

Then, before any of them could respond, he slammed the hatch shut.

In the silence that followed, a sharp crack resounded through the structure of Endurance. Everyone knew the sound: they’d just taken a hit from a bolide.

“Aren’t you supposed to be flying the ship?” Doob shouted to Ivy.

Wordlessly, Ivy went back to her screen.

Dinah rounded on Julia. “What the hell is going on?” she demanded.

Julia’s hair had been cropped. In the last three years it had gone silvery. Her hands still obscured the lower half of her face. Her eyes were clearly recognizable, though without benefit of cosmetics they seemed to be staring out of a face two decades older.

Slowly she removed her hands.

She was sticking her tongue out. It looked like a piece of metal was caught in her teeth.

On a closer look, it was clear that J.B.F. now had a pierced tongue. It had been done cleanly and professionally; there was no bleeding, no apparent signs of infection or discomfort. A stainless steel bolt about two inches long had been inserted vertically through the piercing, fixed in place with nuts and washers above and below the tongue. It was too long to fit into Julia’s mouth, so it kept her tongue stretched out. Above and below, the rod pressed against her lips.

“Oh Jesus Christ,” Dinah said.

Julia tapped at the bolt with one finger, then made screwing and unscrewing gestures with both hands. The nuts had been doubled, and torqued tightly against each other. Dinah took a multitool from a holster on her belt and unfolded its needlenosed pliers, then borrowed Ivy’s. By twisting gently in opposite directions she was able to loosen the nuts. Julia pushed her away and unscrewed the nuts with her fingertips, then gently extracted the bolt. Her tongue retracted into her mouth. She put one hand over her lips and leaned back against a bulkhead for a few moments, moving her jaw to work up some saliva and get limbered up.

When she finally spoke, Julia sounded weirdly normal, as if delivering remarks from the White House briefing room. “When we surrendered,” she said, “they took my gun, and they tortured Spencer Grindstaff until he spilled everything he knew about the IT systems here. All the passwords, all the back doors, all the details as to how it all works. Exactly what they would need in order to take the place over. Then they killed him, and. .”

“Ate him?”

Julia nodded. “They have a sort of hacker type among their group. When he came on board just now, he went to a terminal and began to execute this plan. Steve Lake tried to stop him. One of the others had the gun — shot Steve to death. That was always part of their plan. They knew that only Steve could stop them.”

“How many bullets are left in that thing?”

“I’m sure it is empty now. Most of them are using knives and clubs. They didn’t expect a real fight, because. .”

“Because they thought we’d all be zipped up in our storm shelters,” Dinah said, “like lambs to the slaughter.”

THE BIG BURN LASTED FOR THE BETTER PART OF AN HOUR. BY THE end of it, they’d consumed so much propellant and made Endurance so light that acceleration made the blood fall out of their heads and pool in their feet. Ivy piloted the ship lying flat on her back, lest she lose consciousness. The journey was punctuated by a few terrific bangs, and those who could stand to watch Endurance’s status readouts could observe various modules turning yellow, then red, then black as they succumbed to damage. Dinah watched through the eyes of several cameras as a ten-mile-long piece of the moon tumbled past them, overtaking them and zooming by just a few hundred meters to their starboard side. Nor was that the last such encounter; but with Doob acting as her wing man, calling out the biggest threats, and with Dinah making such use as she could of Parambulator, Ivy was able to steer them clear of the big stuff.

They had no way of knowing the progress of the combat aft. Zeke had spoken optimistically of their odds, but there was no telling how the damage they’d taken from bolides might have swung the course of the battle to one side or the other. Endurance’s automatic sealing off of damaged parts of the ship had now partitioned her into a number of separate zones between which movement was impossible.

Zero gee returned, meaning that the engines had shut off. They were now traveling as fast, on average, as the rest of the debris cloud. Dinah had only just adjusted to the steady acceleration of the big burn and now felt a wave of sickness come over her as the inner ear readjusted. Her eyes closed and she sank into a sort of catnap, floating loosely around the Hammerhead, thudding gently against a wall every so often when Ivy used the thrusters to avoid a rock.

Then she realized she’d been fully asleep for a time.

Part of her wanted to stay that way. But she knew that big things were happening, so she opened her eyes, half expecting to find herself alone, the last person alive.

Ivy was the only person awake, her face lit up by her screen. And for the first time in a long time, it looked the way it had used to when she’d been on the track of some fascinating science problem: alive, intent, fiercely joyful.

“Why’s it so quiet?” Dinah asked. For it seemed to her that it had been a long time since she had heard the crack of a bolide or sensed the thrust of one of Endurance’s engines.

“We’re in the shadow,” Ivy said. “A new Cone of Protection. C’mere.” She tossed her head.

Dinah came around behind and hooked her chin over her friend’s shoulder. The monitor had several windows open. Ivy enlarged one of them to fill most of the screen. A legend superimposed at the bottom identified it as AFT CAMERA.

The field of view was entirely filled by a close-up image of a huge asteroid.

Dinah was an asteroid miner. She had looked at many pictures of asteroids in her day. She had learned to recognize them by their shapes and their textures. She had no difficulty in identifying this one. “Cleft,” she said.

Ivy reached out and touched the screen. Red crosshairs appeared beneath her fingertip, which she dragged across the big rock’s surface until centered on a vast black crevasse that looked like it nearly split the asteroid in two: the canyon that had given this rock its name. She pulled her finger away, leaving the crosshairs in the middle of the cleft. “I was thinking there,” she said.

“How about a little below, where it gets wider?”

“I don’t think we want a wide place. Too much exposure.”

“Go there, then,” Dinah suggested, reaching out and dragging the crosshairs to a slightly different location. “Then we can snuggle into the narrow part once we’ve gotten inside.”

“You ladies enjoying yourselves?” Doob rasped.

“Not as much as you’re going to in about an hour,” Ivy said.

“I’ll try to hold out that long.”

THERE WAS NO PROBLEM GETTING INTO IT. IVY FLEW ENDURANCE into the great crevasse like a Piper Cub into the Grand Canyon. Within minutes the walls were reaching far above them. The bottom was still lost in shadow.

Following Dinah’s general suggestion, Ivy then nudged the ship toward a part of the canyon, several tens of kilometers distant, where the walls converged and the radioactive sky became a narrow, starry slit above. Still she kept pushing onward, occasionally scraping the ship’s outlying modules against the walls, until she reached a place where she could go no deeper.

Looking both directions along the crevasse from this place, they could see spots where the sun was shining in. Here, though, they were protected from rocks and radiation alike. Ivy set Endurance down on the floor of the canyon. Cleft’s gravity was exceedingly faint, but it was enough to give words like that a little meaning, and it was enough to keep the ship lodged in one place until they decided to move it.

Which they never would.

Cleft

ON THE SURFACE OF CLEFT, A HUMAN WEIGHED ABOUT AS MUCH AS three pints of beer would weigh on Earth. Endurance weighed about as much as a couple of semi-trailer rigs.

Ivy lit the ship’s attitude control thrusters one last time and pivoted her tail up until it was vertical. Endurance was standing on her head, the torus aloft, iron Hammerhead nose-down on the iron floor of the crevasse. Dinah sent out some Grabbs to weld the ship to the asteroid. Ivy shut down the thrusters.

Endurance was no longer a ship but a building.

From the Hammerhead, now one piece of metal with Cleft, the Stack ran straight up like the trunk of a tree. Various structures ramified outward from it like boughs. Its widest part was the array of eighty-one arklets that had formerly made up the stern of the ship. These now projected upward like leaves.

Or so they imagined. They couldn’t actually go outside and look at it until they got out of the Hammerhead. During the battle, they had sealed the hatch. By the time they had brought her to rest and welded her down, the rest of Endurance had been quiet for a long time. Finally they opened the hatch and began to explore it one module at a time. They sent Buckies and Siwis out ahead of them to illuminate dark spaces and aim cameras into hidden corners. Tekla then went in, taking point, with Dinah and Ivy watching her back. They were armed with cudgels made from lengths of pipe. But they never had to use them.

It was some combination of crime scene, battleground, and disaster zone. Only about half of the modules were still pressurized. Some of them had become completely isolated and could only be reached by a person in a space suit. It took days to get to them all.

In one of them they found Aïda, the only other survivor from the heptad. Two days had passed since she had eaten the last of Tavistock Prowse, so she was very hungry, but otherwise in good shape. After becoming trapped by a combination of combat and bolide strikes, she had holed up in a water-filled storm shelter, then begun drinking its contents as she awaited rescue.

The total number of living humans was now sixteen. Several had suffered injuries from combat or from the consequences of bolide strikes. Anyone who had not taken shelter in the Hammerhead, or in a storm shelter, was suffering from radiation sickness. The healthy ones patched holes, repressurized modules, got the torus spinning again, and turned it into a sick bay, which filled up immediately.

Dinah managed to get Doob out for one last space walk. He had been failing for days. Once they got him into the suit, though, his energy flooded back. Dinah took him out on the floor of the crevasse where he could walk, light-footed, with magnetized Grabbs latched onto his boots to keep him from floating away with every step. They rambled for about a kilometer, turning around every so often to look back at humanity’s new home. Above the spinning torus, where Moira was even now unpacking her genetics lab, Tekla was inspecting the arklets on the top level, learning which were whole, which were beyond repair, and which could be patched up for future occupancy. On the floor of the crevasse, Grabbs and Siwis were at work, rooting Endurance to her final resting place with spreading cables and struts.

Where they walked, it was dark most of the time. That was the price of being sheltered from cosmic rays and coronal mass ejections. Looking up, however, they could see sunlight gilding the edges of the crevasse above them. They talked about how to set up mirrors that would bounce sunlight downward onto the arklets, which could grow food and scrub air in their translucent outer hulls. Doob spoke of Endomement, the idea that, in time, a ceiling could be thrown over the top of the crevasse and walls built to keep in the air, whereupon a whole section of the valley could be given an atmosphere and turned into a place where children could go “outside” without the need for space suits.

Then he walked home and died.

They stored his body with the others, in a damaged arklet that would serve as a mausoleum until such time as they could cut a grave out of Cleft’s surface. That would take a long time, but the survivors all shared the conviction that, having sacrificed so much to make it here, they should be interred and not burned. Doob would share a grave with Zeke Petersen, Bolor-Erdene, Steve Lake, and all the others who had died at about the same time.

Some of those remained conscious long enough to relate the stories of what had happened to them during the conflict with the people who had come in from the Swarm, and Endurance’s final, hectic passage through storm and stone. Their accounts were recorded and archived. One day some historian would piece the story together, comparing it with data logs to figure out who had slain whom in combat and which module had gone dark when.

Aïda, of course, might have been their best source of information, had she felt like talking. But she didn’t. She had sunk into a profound depression, emerging from it at seemingly random moments to chatter about whatever stray thoughts were flitting through her head. No one wanted to talk to her. When she talked to you, she watched you too carefully with those avid, penetrating eyes, as if she saw, or imagined she saw, too deeply. It was impossible to be the object of that gaze without thinking of what she and the others had done, and without imagining that you were being sized up as food.

An epic tale was told by the three-year backlog of email, Spacebook posts, blog entries, and other ephemera that filled up all of their inboxes as soon as the network of the Swarm recombined with that of Endurance. The general arc of the story seemed to be a growing detachment from reality that had afflicted J.B.F. and some of her inner circle. Luisa likened it to the growth of spiritualism after the First World War. During the 1920s, many who had not been able to bring themselves to accept the loss of life in the trenches and the subsequent influenza epidemic had fallen prey to the belief that they could communicate with their lost loved ones from beyond the grave. They had, in effect, sidestepped grief by convincing themselves that nothing had happened.

The analogy was a loose one. The loss of life in the Hard Rain had, of course, been much worse. And few of the Arkies had adopted spiritualist beliefs per se. But after a particularly severe coronal mass ejection had slain nearly a hundred Arkies, Tav had written a blog post about the journey he had made to Bhutan with Doob and the conversation that they’d had en route with the king concerning the mathematics of reincarnation. It was a meditative piece, a secular eulogy for those who had fallen, but in retrospect it seemed to mark an inflection point in the survivors’ thinking. The Swarm had always had a sort of quasi-divine status to some, who had perhaps read too much chaos theory too superficially and were prone to believing that its collective decisions, lying beyond human understanding, partook of the supernatural.

The mishmash of techno-mystical ideation that had grown out of that one blog post was unreadable and incomprehensible to Luisa or to anyone else who read it after the fact, with a clear mind, but it seemed to have offered hope and comfort to many terrified young people trapped in arklets. Tav, to his credit, had backed away from any efforts to elevate him to prophetlike status. If anything, though, his modesty might have backfired.

“I have no idea,” Luisa said, “how anyone could read these threads and find hope in them. Or even meaning. But they did. Long enough to distract them from the real problems they were facing. And when Aïda and the others finally came to their senses and began to push back against J.B.F. and the others, the reaction was just that much more severe. Because things had gone too far by that point.”

The backlash had started in a two-triad bolo where a number of like-minded Arkies, including Aïda, had “called bullshit” on the prevailing tone and substance of official statements emanating from the White Arklet and begun to denounce Tavistock Prowse as a puppet blogger for the regime. Dubbing themselves the “Black Bolo Brigade,” they had begun to spread their insurrectionist message to other arklets in the Swarm.

That message — which made perfect sense, as far as it went — was all about the need to face reality and to implement realistic, effective steps to address the Swarm’s problems. That included, if need be, throwing themselves on the mercy of Endurance. They had demanded that J.B.F. open the books and provide a current and accurate account of all stocks of water, food, and other staples, and how those numbers were changing over time. Julia had resisted those demands until the data had finally been leaked by a turncoat on her staff. The food picture had turned out to be bleak. This had led to a variety of responses that had determined the history and politics of the Swarm ever since: among some, a further retreat into mysticism and wishful thinking, based on a belief that the Agent had been some sort of avenging angel sent by God, or by aliens so powerful that they might as well be God, to bring about the end of days and the merging of all human consciousness into a digital swarm in the sky; among others, a frank embrace of cannibalism — in the sense not of killing people for food but of eating those who had died of natural causes — as a stopgap measure until J.B.F. could be toppled and replaced by people who knew what they were doing. The first group, the mystics, had tended to rally under Julia’s banner. The cannibals had ended up under Aïda, who because of her intensity and her charisma had gradually emerged as the leader of the Black Bolo Brigade.

The one Swarm had thus fissioned into two smaller ones, neither of which was as viable, and thereby worsened the same problems that had led to the split in the first place. From there the story had been predictable enough, and had led to the events of the last few days.

Aïda still wasn’t talking, but Julia was. According to her, Aïda and the other Black Bolo survivors had calculated, in the last weeks, that their turn to cannibalism would be so repugnant to the survivors of Endurance as to render them permanent outcasts. Rather than passively await the judgment — which they foresaw as extremely pious, sanctimonious, and punitive — of Ivy and her claque, they would seize part or all of Endurance, beginning with her network, and then negotiate terms from a position of strength.

This explained, at least in a general way, everything that had happened, save possibly for the physical mutilation of both Julia and Tav.

Asked for a theory as to that, Julia shrugged. “We were criminals to them. Criminals need to be punished. It’s hard to punish people who are already starving to death in a confined space. What is really left in the executioner’s tool kit, other than attacking the body? They wanted to silence me, and so they did. And they wanted to give Tav a taste of his own medicine by uploading his physical body into theirs.”

A WEEK LATER, WHEN THE LAST OF THE VICTIMS HAD SUCCUMBED to their wounds or to radiation sickness, eight humans remained alive and healthy.

Ivy called for a twenty-four-hour pause to grieve and to take stock. She then called a meeting of the entire human race: Dinah, Ivy, Moira, Tekla, Julia, Aïda, Camila, and Luisa.

They did not know quite what to do with Julia and Aïda. For years they had dreamed, in idle moments, of one day bringing J.B.F. to justice — whatever that would mean. Then, at the last moment, she had been eclipsed by Aïda. And now it all seemed a moot point anyway. Could six women put two women in jail? What would it mean to be in jail in a place like this? Corporal punishment was at least a theoretical possibility. But Aïda had already gone there, with results that they all found sickening.

J.B.F. was a threat to no one. Aïda still possessed an air of menace. But short of locking her up in an arklet, there was nothing they could do about that save keep an eye on her. And so they did, never letting her out of their sight, never letting her get behind them.

They met in the Banana, sitting around the long conference table. To one side of it was death: the sick bay where Zeke, the last man alive, had given up the ghost a day and a half ago, after making a joke about what a shame it all was: being the only man alive, with eight women to choose from. They had scrubbed the place down with bleach and made the beds with clean sheets in the hopes that none of them would be occupied for a long time. To the other side was life: the series of compartments where Moira had been setting up her genetics lab.

The meeting would later be known as the Council of the Seven Eves. For, though eight women were present, one of them — Luisa — had already gone through menopause. Ivy opened with a report on their general situation. From a certain point of view, this was surprisingly good. They had grown so inured to terrible news that she had to emphasize this more than once. Few places in the solar system were as safe as the one where they had come to rest. No cosmic radiation could touch them here. From coronal mass ejections they were equally immune. Sunlight for energy and agriculture could be had a short distance above them, high on the walls of the crevasse, where the sun shone almost all the time.

In the meantime, their big reactor as well as four dozen arklet reactors were producing far more power than they could ever use, and would continue doing so for decades. Of water they still had a hundred tons left over. While melting and splitting the water they’d used for propellant, they had extracted from it many tons of phosphorus, carbon, ammonia, and other chemicals, left over from the dawn of the solar system, that had once cloaked Greg’s Skeleton in a reeking black carapace. That stuff, as Sean Probst had well known, would be priceless as nutrients to support agriculture.

They no longer had to worry, ever again, about the things that had been their obsessive concerns for the last five years: perigees, apogees, burns, propellant, movement of any kind. No bolide could touch them down here. Even if Cleft banged into an equally huge rock at some point, they would probably survive it.

The vitamins that had been packed into every arklet launched up into the Cloud Ark had been intended to support a population of thousands. Even though many of these had been lost, what remained was still more than enough to keep a small colony in aspirin and toothbrushes for a long time.

They were dependent, in many ways, on digital technology. They could not long survive without robots to do work for them and computerized control systems to keep the installation running. They had no ability to fabricate new computer chips to replace the old. But the Arkitects, anticipating this, had stocked them with a large surplus of spare parts that would last for hundreds of years if husbanded carefully. And they had plans for rebooting digital civilization later; they had tools for making tools for making tools, and instructions on how to use them when the time came.

With immediate needs accounted for, the discussion turned toward the obvious problem at hand. All heads turned toward Moira.

“My equipment made it through perfectly unscathed,” she said. “The last three years have been boring for me. I’ve been treated as a fragile flower. I have spent the time writing up everything I know about how to use that stuff. If I drop dead of something tomorrow, you’ll still be able to work it out.

“Obviously, we’re all women. Seven of us are still capable of having babies. Or, to be specific, of producing eggs. So, where can we get some sperm? Well, ninety-seven percent of what was sent up from Earth was destroyed in the disaster on the first day of the Hard Rain. What survived, survived because it had already been distributed among ten different arklets. All ten of those later ended up going off with the Swarm. None of that material, however, seems to have made its way here.”

Aïda interrupted. Staring across the table at Julia, she announced, “I was in the Swarm, as you know. I can tell you that this fact of the samples in the ten arklets was forgotten. Never discussed. If anyone even knew they were there, they forgot about it soon.”

Julia was construing this as an attack on her record. “We had eight hundred healthy young men and women from every ethnic group in the world.”

“Had,” Aïda repeated. “We had.”

“The amount of effort required to keep a few sample containers deep-frozen wasn’t worth the—”

“Stop,” Ivy said. “If we can start making babies, their great-grandchildren can pore over the records and make judgments and have debates about what should have been done. Now isn’t the time for recriminations.”

“I was in the meeting where Markus called bullshit on the Human Genetic Archive,” Dinah said. She was mildly amazed to hear herself backing Julia’s side of the argument.

“We can’t make the same mistake again,” Aïda said, “of fooling ourselves. Believing in shit that isn’t real.”

Ivy said, “Had we known that it was going to come down, so suddenly, to seven surviving fertile women, we would have had every healthy male masturbating into test tubes for the last three years. We’d have looked for ways to keep it all frozen. But we never imagined it would come to this.”

“It’s not clear what the quality of the results would have been,” Moira put in. “Given the amount of radiation exposure, I probably would have had to do a lot of manual repair on the genetic material in those samples.”

“Manual repair?” Julia asked.

“I should put that in scare quotes,” Moira said, reaching up with both hands and crooking her fingers. “Obviously I’m not literally using my hands. But with the equipment in there”—she tossed her head in the direction of the lab—“I can isolate a cell — a sperm or an ovum — and read its genome. I’m skipping over a lot of details, obviously. But the point is that I can get a digital record of its DNA. Once that’s in hand, it turns into a software exercise — the data can be evaluated and compared to huge databases that shipped up as part of the lab. It’s possible to identify places on a given chromosome where a bit of DNA got damaged by a cosmic ray or radiation from the reactor. It is then possible to repair those breaks by splicing in a reasonable guess as to what was there originally.”

“It sounds like a lot of work,” Camila said. “If there is anything I can do to ease your burdens and make myself useful, I am at your disposal.”

“Thank you. We will all be working at it for months,” Moira said, “before anything happens. We have very little else to do.”

“Excuse me, but what is the point of discussing this, since we have no sperm to work with?” Aïda asked.

“We don’t need sperm,” Moira said.

“We don’t need sperm to get pregnant! This is news to me,” Aïda said, with a sharp laugh.

Moira went on coolly. “There is a process known as parthenogenesis, literally virgin birth, by which a uniparental embryo can be created out of a normal egg. It’s been done with animals. The only reason no one ever did it with humans is because it seemed ethically dodgy, as well as completely unnecessary given the willingness of men to impregnate women every chance they got.”

“Can you do it here, Moira?” asked Luisa.

“It’s not fundamentally more difficult than the sorts of tricks I was just describing in the case of repairing damaged sperm. In some ways, it would actually be easier.”

“You can get us pregnant. . by ourselves,” Tekla said.

“Yes. Everyone except Luisa.”

“I can have a child of whom I am both the mother and the father,” Aïda said. The idea clearly fascinated her. Suddenly she was no longer the prickly, brittle Aïda but the warm and engaged girl who must have charmed the powers that be during the Casting of Lots.

“It will take some tricky work in the lab,” Moira said. “But that is the whole point of having brought the lab safely to this place.”

They all pondered it for a bit. Julia was the first to speak up. “Stepping into my traditional role as scientific ignoramus: Do you mean to say that you can clone us?”

Moira nodded — not to say yes, but to say I understand your question. “There are different ways to do it, Julia. One way would indeed produce clones — all offspring genetically identical to the mother. This isn’t what we want. For one thing, it would not solve our basic problem — the lack of males.”

Camila’s hand went up. Moira, clearly annoyed by the interruption, blinked once, then nodded at her. “Is it really a problem?” Camila asked. “As long as we have the lab and can go on making more clones, would it really be such a bad thing to have a society with no males? At least for several generations?”

Moira silenced her with a gentle pushing movement of one hand. “That’s a question for later. There is another problem with this version of parthenogenesis, which is, again, that all offspring are the same. Exact copies. To get some genetic diversity, we need to use something called automictic parthenogenesis. Look, it’s a long story, but the point is that in normal sexual reproduction there is crossing over of chromosomes during meiosis. It’s a form of natural recombination of DNA. It’s what causes your children to look sort of like you, but not exactly like you. In the form of parthenogenesis that I am proposing to use, there would be that crossing over. An element of randomness.”

“And both boys and girls?” Dinah asked.

“That’s harder,” Moira admitted. “Synthesizing a Y chromosome is no joke. My prediction is that the first set of babies — perhaps the first few sets of them — will all be female. Because we simply need to get the population up. During that time I can be working on the Y chromosome problem. Later on, I hope that some little boys will result.”

“But these little girls — and later the boys — will still be made out of our own DNA?” Ivy asked.

“Yes.”

“So they’ll be quite similar to us genetically.”

“If I do nothing about it,” Moira said, “they’ll be like sisters. Perhaps even more similar than that implies. But there are a few tricks that I can use to create a wider range of genotypes out of the same source material. Perhaps they’ll be more like cousins. I don’t know, it’s never been tried.”

“Are we talking about the inbreeding problem? It sounds like it,” Dinah said.

“Loss of heterozygosity. Yes. I happen to know something about it. It’s why I was chosen as a member of the General Population.”

“Because of your work on black-footed ferrets and so on,” Ivy said.

“Yes. This is a closely analogous problem. But the point I would like you all to keep in mind is that we solved that problem in the case of the black-footed ferrets and we are going to solve it again.”

She said it with force and confidence that silenced the others for a few moments and left them looking at her for more.

Moira went on. “I think we all have at least an intuitive understanding of this, yes?”

That one was aimed at Julia, who looked mildly peeved, and bit off the following: “My daughter had Down syndrome. That is all I will say.”

Moira acknowledged it with a nod, then went on: “Everyone has some genetic defects. When you are breeding more or less randomly within a large population, there’s a tendency for those errors to be swamped by the law of averages. Everything sort of works out. But when two people sharing the same defect mate, their offspring is likely to have that defect as well, and over time we see the usual unpleasantness that we all associate in our minds with inbreeding.”

“So,” Luisa said, “if we follow the plan you have laid out, and begin, a few years down the road, with seven groups of what amount to siblings or cousins—”

“It’s not enough heterozygosity, to answer your question,” Moira said. “If you have a genetic predisposition to any disease, for example—”

“Alpha-thalassemia runs in my family,” Ivy said.

“That’s a fine example,” Moira returned. “As it happens, Old Earth compiled vast databases on such things before its destruction. All of which are in there now.” She gestured in the direction of her lab. “We have a very good idea which defects, on which chromosomes, are responsible for alpha-thalassemia. If you supply me with an ovum, I can find those defects and I can fix them before we begin parthenogenesis. Your offspring will be free of that defect. Barring some random future mutation, it’ll never return.”

Dinah raised her hand. “My brother was a carrier of cystic fibrosis. I haven’t been tested.”

Julia raised hers. “Three of my aunts died of the same form of breast cancer. I’ve been tested. I know I carry that defect as well.”

“The same answer applies in all of these cases,” Moira said. “If there’s a genetic test for it, then it means, by definition, that we know which defects are responsible for it. And knowing that, we can perform a repair.”

A new voice joined the conversation. “How about bipolar disorder?”

Everyone looked at Aïda.

She would live out the rest of her life, and go to meet her maker, without having a friend, or even a friendly conversation. So, no one was in a receptive frame of mind about her question. But the mere fact that she’d asked it suggested a level of introspection they hadn’t seen from her before. Moira considered it.

“I would have to do some research. I think that it does run in families to some extent. To the extent that it can be traced to particular locations on particular chromosomes, it can be treated like any other disease,” Moira said.

“Do you believe it should be?” Aïda asked.

Everyone looked automatically at Luisa, who nodded. “We are long past the point of thinking of mental illnesses as somehow a lesser kind of disease than physical. Such disorders should, in my opinion, be addressed in just the same way.”

“Do you believe it must be?”

Luisa colored slightly. “What is the point of these questions, Aïda?”

“I have done research on it,” Aïda said. “Some say that bipolarity is a useful adaptation. When things are bad, you become depressed, retreat, conserve energy. When things are good, you spring into action with great energy.”

“And your point is. .”

“Will you treat this condition in my offspring against my will? What if I want to have a lot of little bipolar kids?”

In the flustered silence that followed, Camila spoke. “What about aggression?”

Everyone turned to look at her, as if unsure they had heard her correctly.

“I’m serious,” she said. She looked toward Aïda. “I don’t mean to trivialize the suffering that your condition causes. But over the course of history, aggression has caused a far larger amount of pain and death than bipolar disorder or whatever. As long as we are fixing those aspects of the human psyche that lead to suffering, should we not eliminate the tendency to aggressive behavior?”

“That’s different,” Moira began. But she was interrupted by Dinah.

“Hold on a sec,” Dinah said. “I’m aggressive. I always have been. I was on track to be an Olympic soccer player! That’s the only way I’ve ever been able to amount to anything — by channeling my aggression into doing things.” She nodded across the table at Tekla. “Hell, look at her! How many times has she saved our asses by being aggressive?”

Tekla nodded. “Yes. Dinah saved me by taking aggressive action against rules of space station. Problem is not aggression. It is lack of discipline. A person can be aggressive”—she nodded at Dinah—“and still be constructive in society if she controls her passions.” And she threw a significant glare at Aïda, who let out a little snort and looked away.

“So you’re suggesting we breed people for discipline and self-control?” Ivy asked. “I’m not sure if I follow.”

“I believe that Camila was merely saying that certain personality types, taken to an unhealthy extreme, are as bad as diagnosable mental illnesses per se. If not worse,” Julia said.

“I don’t want you to speak for me,” Camila said. “Please do not speak for me anymore, Julia.”

“I am merely trying to be helpful,” Julia said. But where the old J.B.F. would have said it reproachfully, the new one merely seemed exhausted.

Dinah broke in. “Well, what I am trying to say is that I don’t appreciate being labeled as a genetic freak that needs to be eradicated from the human future.”

“No one would say that of you, Dinah,” Ivy said. “Camila’s talking about the knuckle draggers who tried to kill her for wanting an education.”

“And what is your opinion?” Tekla asked Ivy.

“Similar to yours. Aggression is fine. It needs to be controlled. Directed. But the way to do that is through intelligence. Rational thought.”

That elicited a cackle from Aïda. “Oh, sorry,” she said. “I was thinking about the Swarm. Eight hundred people all carefully hand-selected for intelligence and rational thought. In the end, all we could think about was how they tasted.”

“None of us ate each other,” Ivy said.

“But you thought about it,” Aïda said with a smile.

Dinah slammed her palm hard on the table. She sat still for a moment with her eyes closed tight, then stood up and walked out of the room.

“I guess she is not disciplined or intelligent enough to control her aggression!” Aïda cracked.

“It is a form of self-discipline,” Tekla said. “So that she would not kill you. You see, Aïda, thinking about doing such things and doing are different. This is why greater discipline is a requirement.”

“Sweetie, what do you mean when you speak of discipline?” Moira asked. “I’m just trying to cash that word out in terms of genetics. I can find a genetic marker for cystic fibrosis. I’m not sure if the same is true of discipline.”

“Some races are disciplined. Is fact,” Tekla said. “Japanese are more disciplined than. . Italians.”

She gave Aïda a stare that would have frozen most people to their chairs, but Aïda just threw her head back and laughed exultantly. “You are forgetting the Roman legions, but please go on.”

“Men are more disciplined than women. Is just fact. So there must be genes for it.”

This produced yet another silence, eventually broken by Luisa: “I’m seeing a side of you I didn’t know about, Tekla.”

“Call me bad, call me racist if you want. I know what you will say: That it is all training. It is all culture. I disagree. If you do not feel pain, you do not respond to pain. And hormones.”

“What about hormones, lover?” Moira asked. Her affection for Tekla was obvious, and took some of the tension out of the room.

“We all know that when hormones are a certain way, emotions have big impact. Other times, not so much. This is genetic.”

“Or maybe epigenetic. We really don’t know,” Moira said.

“Whatever,” Tekla said. “My point is that for people to live in tin cans for hundreds of years requires order and discipline. Not from above. From within. If there is a way to make this easier with your genetic lab, then we should do it.”

Luisa said, “We never explored Ivy’s point that intelligence was key.”

“Yes,” Ivy said, with a glance at Aïda. “I was interrupted.”

Aïda covered her mouth with her hand and sniggered theatrically.

Ivy went on: “If we are really going to open the door to genetic improvement of our offspring, then it seems obvious to me that we should look to the one quality that trumps all others. And that is clearly intelligence.”

“What do you mean it trumps all others?” Luisa asked.

“With intelligence, you can see the need to show discipline when the situation calls for it. Or to act aggressively. Or not. I would argue that the human mind is mutable enough that it can become all of the different types of people that Camila, Aïda, and Tekla have been describing. But that’s all driven by what separates us from the animals. Which is our brains.”

“There are many different types of intelligence,” Luisa said.

Ivy gave a little shake of her head. “I’ve seen all of that stuff about emotional intelligence and what have you. Okay. Fine. But you know exactly what I’m talking about. And you know it can be propagated genetically. Just look at the academic records, the test scores of the Ashkenazi Jews.”

“Speaking as a Sephardic Jew,” Luisa said, “you can imagine my mixed feelings.”

“We need brains, is the bottom line,” Ivy said. “We’re not hunter-gatherers anymore. We’re all living like patients in the intensive care unit of a hospital. What keeps us alive isn’t bravery, or athleticism, or any of those other skills that were valuable in a caveman society. It’s our ability to master complex technological skills. It is our ability to be nerds. We need to breed nerds.” She turned to look Aïda full in the face. “You ask for realism. Your complaint about her”—she nodded at Julia—“and the people around her was that they were holding out panaceas. Not facing facts. Fine. I’m giving you facts. We’re all nerds now. We might as well get good at it.”

Aïda shook her head in derision. “You completely leave out the human component. It’s why you are a bad leader. It’s why you were replaced by Markus, when wiser people than you were in control. And it’s why we are here.”

“Here, safe and sound,” Ivy said, “unlike the people who followed you. All of whom are dead.”

“So they are,” Aïda said, “and I am alive, and I can see how it’s going to be: you are going to keep me locked up in an arklet making genetic freak babies and taking them away from me.” And she broke down weeping.

“She has what I have, except worse,” Julia explained. “She sees many outcomes — most of which, given the circumstances, are dark — then acts upon them.”

“What an unusual degree of introspection from you, Julia,” Moira said.

“You have no concept of my level of introspection,” Julia shot back. “I have been clinically depressed for most of my life. I once used drugs to fix it. Then I stopped. I stopped because I decided they were making me stupid, and I’d rather be miserable than stupid. I am what I am.”

“Depression is genetically based to some extent. Would you like me to erase it from your children’s genomes?” Moira asked.

“You heard what I said,” Julia answered. “You know, now, the decision I made. Which was to suffer for the greater good. Because society will go astray if there are not those who, like me, imagine many outcomes. Let those scenarios run rampant in their minds. Anticipate the worst that could happen. Take steps to prevent it. If the price of that — the price of having a head full of dark imaginings — is personal suffering, then so be it.”

“But would you wish that on your progeny?”

“Of course not,” Julia said. “If there were a way to have one without the other — the foresight without the misery — I would take it in a heartbeat.”

“We only need a few people of this mentality,” Tekla said. “Too many, and you get the Soviet Union.”

“I am forty-seven,” Julia said. “I have one baby in me, if I’m lucky. The rest of you can punch them out for twenty years. Do the math.”

“It amazes me that we have already gone over to the competitive angle!” Camila wailed. “I am so sorry that I brought this topic up.”

A sharp rapping noise brought the room to attention.

Heads turned toward the Banana’s window. It was not large — about the size of a dinner plate. For three years it had been buried in ice and forgotten about. But now it afforded a clear if somewhat dizzying view of their surroundings.

Outside of it, carabinered to the spinning torus, was Dinah. She had put on a space suit and gone out through an airlock.

Seeing she had their attention, she reached up and slapped a small object onto the glass. It was a lump of clay, some wires, and an electronic gadget. She depressed a button on the gadget and it began to count down from ten minutes.

Aïda screamed with laughter and clapped her hands.

“What on earth is she doing?” Julia asked.

“That’s a demolition charge,” Ivy said. “It’s going to kill us all ten minutes from now if she doesn’t take it off the window.” She turned to survey the room.

“Well, what is her point?” Julia demanded.

“I think my friend is trying to tell us that if we can’t settle this in ten minutes, the human race doesn’t deserve to go on existing,” Ivy said.

They all sat silently for perhaps half a minute before Moira said: “How’s this: every woman decides what is going to be done with her eggs.”

Hearing no objection, she continued: “Oh, let me be clear. If it’s a real disease — something on the books, defined in the medical literature as such — then I will fix it. With no distinctions made between physical and mental disorders. No matter how many of those conditions each of you may be suffering from, I will fix them all before taking any other action. However.” And she smiled, and held up an index finger. “Once all that is done, each of us gets a free one.”

“Free what?” Tekla asked.

“One alteration — one improvement — of your choice, applied to the genome of the fertilized ovum that will grow into your child. And your child only. You cannot force it on any of the others. So, Camila, if you think it would improve the human race to get rid of its aggression, why then, I will search through the scientific literature for a way to reach toward your goal genetically. And likewise for the rest of you, and whatever changes you happen to think will improve the human condition. Your child, your choice.”

They all considered it, glancing at one another from time to time, each trying to gauge the others’ reactions.

Ivy glanced at the timer outside. “Are there any questions? We have eight minutes remaining.”

Luisa said, “I don’t think we need eight minutes.”

Ivy looked each of them in the eye, then turned toward the window and gave a thumbs-up.

Dinah’s eyes, seen through the glass of the window and the dome of her space suit’s helmet, pivoted to focus on that. She nodded.

Moira smiled and put her thumb up. This too was noted by Dinah.

Then Tekla. Then Luisa, Camila, Julia.

All eyes were on Aïda. She would not look back at them. She was, at bottom, very shy. “Whatever,” she mumbled.

“She needs to see your vote,” Ivy said.

“Really? You mean that I could single-handedly destroy the entire human race, simply by not putting my thumb up in the next seven minutes?”

Tekla pulled a folding knife from a pocket on her coverall and flicked the blade open. She kept it low, down in her lap, and pretended to clean a fingernail with it. “Either that,” Tekla said, “or population of human race suddenly goes from eight to seven, and we have unanimous decision.”

Smiling, Aïda thrust her hand out, thumb down.

“I pronounce a curse,” she said.

Luisa let out an exasperated sigh.

“This is not a curse that I create. It is not a curse on your children. No. I have never been as bad as you all think that I am. This is a curse that you have created, by doing this thing that you are about to do. And it is a curse upon my children. Because I know. I see how it is to be. I am the evil one. The cannibal. The one who would not go along. My children, no matter what decision I make, will forever be different from your children. Because make no mistake. What you have decided to do is to create new races. Seven new races. They will be separate and distinct forever, as much as you, Moira, are from Ivy. They will never merge into a single human race again, because that is not the way of humanity. Thousands of years from now, the descendants of you six will look at my descendants and say, ‘Ah, look, there is a child of Aïda, the cannibal, the evil one, the cursed one.’ They will cross the street to avoid my children; they will spit on the ground. This is the thing that you have done by making this decision. I will shape my child — my children, for I shall have many — to bear up under this curse. To survive it. And to prevail.”

Aïda swept her gaze around the room, staring with her deep black eyes into the face of each of the other women in turn, then looked into the window and locked eyes with Dinah.

“I pronounce it,” she said, then slowly rotated her hand until her thumb was pointed up.

DINAH PEELED THE DEMOLITION CHARGE AWAY FROM THE WINDOW. She had no idea what Aïda had just said. Nor did she especially care. It would be the usual histrionic Aïda stuff.

Several minutes remained on the countdown timer. She could have simply turned it off. But she felt like going for a walk. Whatever had just happened in the Banana looked unpleasant. She was tired of being cooped up with these people — even the ones she loved. She felt no great compulsion to rejoin them.

She unclipped the carabiner and let go of the lazily spinning torus. Her momentum carried her toward the wall of the crevasse. Long accustomed to movement in zero gee, she timed a slow somersault and planted her feet on the wall to kill her speed, then turned on the magnets in her boots and began hiking up the crevasse wall. The weak gravity made directions arbitrary. Walking “vertically up” a cliff was little different from walking “horizontally along” the canyon floor.

A tone sounded from the speakers in her helmet, alerting her that a voice connection had been made.

It was Ivy. “Going for a stroll?”

“Yeah.”

“Look, we just realized something.”

“Oh?”

“We all voted — except for you.”

“Mmm, good point.” Dinah glanced down at the countdown timer. The screen was getting more difficult to read, since she was nearing the terminator — the knife-sharp line between sunlight and shadow — and the bright canyon wall above her was reflecting from the screen. Tilting it for a better view, she saw that it was just about to drop through the sixty-second mark. “It’s okay, I still have a minute to make my decision.”

“Well, do you want to know what the rest of us agreed on?”

“I trust you. But sure.”

“We’re all going to try to have babies just like you, Dinah.”

“Very funny.” Dinah crossed over the terminator, and the sun rose. She raised her free hand and flipped down the sun visor on her helmet.

“Moira’s working on it now.”

“Is that why Aïda was being such a drama queen about it?”

“Exactly.”

Thirty-five seconds.

“What did you really decide?”

“One free gene change for each mommy.”

“Oh yeah? So what are you going to do? Make really smart little straight arrow bitches?”

“How’d you guess?”

“Just an intuition.”

“What about you, Dinah?” Dinah could hear the beginnings of anxiety in her friend’s voice. She looked down into the crevasse, saw humanity’s cradle welded helplessly into place, imagined for a moment throwing the demolition charge down on it, like a vindictive goddess hurling a lightning bolt.

She was thinking of Markus. Of the kids she should have had with him. What would they have been like?

Markus had been kind of a jerk in some ways, but he knew how to control it.

Really — she now understood — what had prompted her to slam the table and get up and storm out of the Banana a few minutes ago had not been Aïda at all. Aïda was provocative, yes. But more infuriating had been a slow burn that had started with Camila, and her remarks about aggression. Remarks that Dinah now saw as aimed not so much at Dinah as at Markus. She wished she could grab Camila by the scruff of the neck and sit her down in front of a display and make her watch the way Markus had spent the last minutes of his life.

Markus was a hero. It seemed to Dinah that Camila wanted to strip humanity of its heroes. She’d couched what she’d said in terms of aggression. But by doing so, Camila was just being aggressive in a different way — a passive-aggressive way that Dinah, raised as she’d been raised, couldn’t help seeing as sneaky. More destructive, in the end, than the overt kind of aggression.

It was this that had made her so flustered that she’d had to leave the meeting.

“Dinah?” Ivy said.

“I’m going to breed a race of heroes,” Dinah said. “Fuck Camila.”

“It’s going to be. . interesting. . sharing confined spaces with a race of heroes for hundreds of years.”

“Markus knew how to do it,” Dinah said. “He was a jerk, but he had a code. It’s called chivalry.”

She gave the demolition charge a toss straight up.

“Did you just vote yes?”

“Oh yeah,” she said, watching it dwindle against the stars. The red lights of the LED timer glittered like rubies.

“We’re unanimous,” Ivy said. Dinah understood that Ivy was announcing it to the other women in the Banana.

For the first and last time, Dinah thought.

The red light had shrunk to a pinprick. Like the planet Mars, she thought, except sharper and more brilliant. Then, silently, it turned into a ball of yellow light that darkened as it spread.