The Rolling Stones

VII - IN THE GRAVITY WELL

A gravity-well maneuver involves what appears to be a contradiction in the law of conservation of energy. A ship leaving the Moon or a space station for some distant planet can go faster on less fuel by dropping first toward Earth, then performing her principal acceleration while as close to Earth as possible. To be sure, a ship gains kinetic energy (speed) in fall­ing towards Earth, but one would expect that she would lose exactly the same amount of kinetic energy as she coasted away from Earth.

The trick lies in the fact that the reactive mass or 'fuel' is itself mass and as such has potential energy of position when the ship leaves the Moon. The reactive mass used in accelerating near Earth (that is to say, at the bottom of the gravity well) has lost its energy of position by falling down the gravity well. That energy has to go somewhere, and so it does - into the ship, as kinetic energy. The ship ends up going faster for the same force and duration of thrust than she possibly could by departing directly from the Moon or from a space station. The mathematics of this is somewhat baffling - but it works.

Captain Stone put both the boys in the power room for this maneuver and placed Hazel as second pilot. Castor's feelings were hurt but he did not argue, as the last discussion of ship's discipline was still echoing. The pilot has his hands full in this maneuver, leaving it up to the co-pilot to guard the auto-pilot, to be ready to fire manually if need he, and to watch for brennschluss. It is the pilot's duty to juggle his ship on her gyros and flywheel with his eyes glued to a measuring telescope, a 'coelostat', to be utterly sure to the extreme limit of the accuracy of his instruments that his ship is aimed exactly right when the jet fires.

In the passage from Earth to Mars a mistake in angle of one minute of arc, one sixtieth of a degree, will amount to - at the far end - about fifteen thousand miles. Such mistakes must be paid for in reactive mass by maneuvering to correct, or, if the mistake is large enough, it will he paid for tragically and inexor­ably with the lives of captain and crew while the ship plunges endlessly on into the empty depths of space.

Roger Stone had a high opinion of the abilities of his twins, but on this touchy occasion, he wanted the co-pilot backing him up to have the steadiness of age and experience. With Hazel riding the other. couch he could give his whole mind to his delicate task.

To establish a frame of reference against which to aim his ship he had three stars, Spica, Deneb, and Fomalhaut, lined up in his scope, their images brought together by prisms. Mars was still out of sight beyond the bulging breast of Earth, nor would it have helped to aim for Mars; the road to Mars is a long curve, not a straight line. One of the images seemed to drift a trifle away from the others; sweating, he unclutched his gyros and nudged the ship by flywheel. The errant image crept back into position. "Doppler?" he demanded.

"In the groove."

"Time?"

"About a minute. Son, keep your mind on your duck shooting and don't fret."

He wiped his hands on his shirt and did not answer. For some seconds silence obtained, then Hazel said quietly, "Unidentified radar beacon blip on the screen, sir. Robot response and a string of numbers."

"Does it concern us?"

"Closing north and starboard. Possible collision course."

Roger Stone steeled himself not to look at his own screen; a quick glance would tell him nothing that Hazel had not repor­ted. He kept his face glued to the eyeshade of the coelostat. "Evasive maneuver indicated?

"Son, you're as likely to dodge into it as duck away from it. Too late to figure a ballistic."

He forced himself to watch the star images and thought about it. Hazel was right, one did not drive a spaceship by the seat of the pants. At the high speeds and tight curves at the bottom of a gravity well, close up to a planet, an uncalculated maneuver might bring on a collision. Or it might throw them into an un­tenable orbit, one which would never allow them to reach Mars.

But what could it be? Not a spaceship, it was unmanned. Not a meteor, it carried a beacon. Not a bomb rocket, it was too high. He noted that the images were steady and stole a glance, first at his own screen, which told him nothing, and then through the starboard port.

Good heavens! he could see it!

A great gleaming star against the black of space... growing growmg!

"Mind your scope, son," said Hazel. "Nineteen seconds."

He put his eye back to the scope; the images were steady. Hazel continued, "It seems to be drawing ahead slightly."

He had to look. As he did so something flashed up and obscured the starboard port and at once was visible in the portside port - visible but shrinking rapidly. Stone had a momen­tary impression of a winged torpedo shape.

"Whew!" Hazel sighed. "They went that-a-way, podnuh!" She added briskly, "All hands, brace for acceleration - five seconds!"

He had his eye on the star images, steady and perfectly matched, as the jet slammed him into his pads. The force was four gravities, much more than the boost from Luna, but they held it for oniy slightly more than one minute. Captain Stone kept watching the star images, ready to check her if she started to swing, but the extreme care with which he had balanced his ship in loading was rewarded: she held her attitude.

He heard Hazel shout, "Brennschluss!" just as the noise and pressure dropped off and died. He took a deep breath and said to the mike, "You all right, Edith?"

"Yes, dear," she answered faintly. "We're all right."

"Power room?"

"Okay!" Pollux answered.

"Secure and lock." There was no need to have the power room stand by, any correction to course and speed on this leg would be made days or weeks later, after much calculation.

"Aye aye, sir. Say, Dad, what was the chatter about a blip?"

"Pipe down," Hazel interrupted. "I've got a call coming in." She added, "Rolling Stone, Luna, to Traffic - come in, Traffic."

There was a whir and a click and a female voice chanted:

"Traffic Control to Rolling Stone, Luna - routine traffic precautionary: your plan as filed will bring you moderately close to experimental rocket satellite of Harvard Radiation Laboratory. Hold to flight plan; you will fail contact by ample safe margin. End of message; repeat – " The transcription ran itself through once more and shut off.

"Now they tell us!" Hazel exploded. "Oh, those cushion warm­ers! Those bureaucrats! I'll bet that message has been holding in the tank for the past hour waiting for some idiot to finish discussing his missing laundry."

She went on fuming: ""Moderately close!" "Ample safe margin!" Why, Roger, the consarned thing singed my eye-brows!"

""A miss is as good as a mile"."

"A mile isn't nearly enough, as you know darn well. It took ten years off my life - and at my age I can't afford that."

Roger Stone shrugged. After the strain and excitement he was feeling let down and terribly weary; since blast-off he had been running on stimulants instead of sleep. "I'm going to cork off for the next twelve hours. Get a preliminary check on our, vector; if there's nothing seriously wrong, don't wake me. I'll look at it when I turn out."

"Aye aye, Captain Bligh."

First check showed nothing wrong with their orbit: Hazel followed him to bed - "bed' in a figurative sense, for Hazel never strapped herself to her bunk in free fall, preferring to float loosely wherever air currents wafted her. She shared a stateroom with Meade. The three boys were assigned to the bunkroom and the twins attempted to turn in - but Lowell was not sleepy. He felt fine and was investigating the wonderful possibilities of free fall. He wanted to play tag. The twins did not want to play tag; Lowell played tag anyhow,.

Pollux snagged him by an ankle. "Listen, you! Weren't you enough trouble by being sick?"

"I was not sick!"

"So? Who was it we had to clean up after? Santa Claus?"

"There ain't any Santa Claus. I was not sick. You're a fibber, you're a fibber, you're a fibber!"

"Don't argue with him," Castor advised. "Just choke him and stuff him out the lock. We can explain and correct the ship's mass factor tomorrow."

"I was not sick!"

Pollux said, "Meade had quite a bit of sack time on the leg down. Maybe you can talk her into taking him off our hands?"

"I'll try'."

Meade was awake; she considered it. "Cash?"

"Sis, don't be that way!"

"Well ... three days' dishwashing?"

"Skinflint! It's a deal; come take charge of the body." Meade had to use the bunkroom as a nursery; the boys went forward and slept in the control room, each strapping himself loosely to a control couch as required by ship's regulations to avoid the chance of jostling instruments during sleep.