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Pluto, Charon, Nix, and Hydra

Pluto and Charon are a double-planet system, tidally locked to each other like two ends of a dumbbell, same sides always facing each other, and their center of gravity out there between them. They rotate out of the plane of their orbit around the sun, and their days are a bit over six days long, with their years 248 years long. Pluto is ten K colder than it would be if it didn’t have its atmosphere, which in freezing at apogee and subliming at perigee creates a reverse greenhouse effect and cools the surface. The atmosphere is as thick as Mars’s original atmosphere, around seven millibars—in other words, not very thick. Surface temperature is forty K.

Charon, half the size of Pluto, has a surface temperature of fifty K. The next closest moon-to-planet size ratio is Luna to Earth, with Luna one-fourth the size of Earth. Pluto has a 2,300-kilometer diameter; Charon, 1,200 kilometers. Both have rock cores and mostly water ice shells.

Two much smaller moons orbit the bigger pair: Nix and Hydra, at 90 and 110 kilometers in diameter. Nix, at 80,000,000,000,000,000,000 (eighty quintillion) kilograms, mostly ice with some rock, is currently being disassembled and processed into four starships, which are to be sent roughly as a group, though the first is scheduled to go ahead, in part to test the systems being built. The interiors of the starships are typical terrarium cylinders, spinning to create an interior gravity effect. They are being stocked with a very large number of species, spanning several biomes. The four ships are intended to stay in contact, and will reduce the genetic impacts of their islanding by occasional exchanges. The engines installed in the sterns will combine mass drivers with antimatter plasmas to run for a century, followed by a powerful Orion pusher plate, eventually reaching speeds at which ramjets will work, and these will be deployed in their turn. All together these engines will accelerate them to 2 percent the speed of light, a truly fantastic speed for a human craft, thus reducing their trip time to only two thousand years. For the stars are far away. And the nearest ones to us have no Earthlike planets.

Sorry, but it’s true. It has to be said: the stars exist beyond human time, beyond human reach. We live in the little pearl of warmth surrounding our star; outside it lies a vastness beyond comprehension. The solar system is our one and only home. Even to reach the nearest star at our best speed would take a human lifetime or more. We say “four light-years” and those words “four” and “years” fool us; we have little grasp of how far light travels in a year. Step back and think about 299,792,458 meters per second, or 186,282 miles per second—whichever you think you can grasp better. Think of that speed as traversing 671 million miles in every hour. Think about it traversing 173 astronomical units a day; an astronomical unit is the distance from the Earth to the Sun, thus 93 million miles—crossed 173 times in a day. Then think about four years of days like that. That gets light to the nearest star. But we can propel ourselves to only a few percent of the speed of light; so at 2 percent of the speed of light (ten million miles an hour!) it will take about two hundred years to go those four light-years. And the first stars with Earthlike planets are more like twenty light-years away.

It takes a hundred thousand years for light to cross the Milky Way. At 2 percent of that speed—our speed, let us say—five million years.

The light from the Andromeda Galaxy took 2.5 million years to cross the gap to our galaxy. And in the universe at large, Andromeda is a very nearby galaxy. It resides in the little sphere that is our sector of the cosmos, a neighbor galaxy to ours.

So. Our little pearl of warmth, our spinning orrery of lives, our island, our beloved solar system, our hearth and home, tight and burnished in the warmth of the sun—and then—these starships we are making out of Nix. We will send them to the stars, they will be like dandelion seeds, floating away on a breeze. Very beautiful. We will never see them again.