There is a terrible problem with interstellar travel. That problem is distance.
At the speed of light, faster than which no material object can dream of traveling, Earth’s nearest neighbors lie years away, and the truly interesting ones, decades or centuries. But even if the, quite frankly, absurd energy requirements to accelerate a spaceship even close to that fast were tractable—which, do not forget for a moment, they are not—there are other obstacles with which to contend.
One of these is dust. At relativistic speeds, dust particles start looking an awful lot like mountains. And hitting one of them starts to look an awful lot like detonating a nuclear warhead, point-blank, against your hull.
So you can’t cover that inconceivably vast distance by going fast. Which means you need to go slow. And there, you have another, tremendous problem: time. In some sense, this is the same problem—which is why distance and time are the same thing to a rocketeer.
To put this in perspective, the U.F.P. Discovery left low Earth orbit in the year 2401. At its (destination-relative) ludicrous speed of 0.00114c, its target Gliese 667 Cc lay 23.62 light years—and nearly 21,000 years—ahead. That’s like the empires of ancient Egypt, ancient Mesopotamia, ancient and imperial China, the Mayans, the Romans and Greeks, Mongols, Ottomans, and the entirety of modern world history all concatenated together end-to-end.
Distance and time are the same thing to a rocketeer.
How do you build an airlock door that lasts that long? You can’t. Let alone a nuclear reactor, a computer, a rocket engine, a 3D fab, or any of the other necessities of the 25th century. You probably can’t even build a wrench.
So the Discovery really is just a tremendous steel cylinder, with walls some 90 meters thick at points—and the people and resources were just welded permanently inside. It has no guidance, no senors, no engines, no nothing. It’s the only way the ship itself could possibly survive. It was accelerated by Mercury’s laser launching grid, beaming maximum power clear across the system for ten full months.
So there’s a self-contained biosphere, plus raw building material, out in that speeding hulk. Someday, in Earth’s distant future, they will arrive, and the Discovery, still on utterly passive guidance, will spontaneously be captured into a wide and long elliptical orbit around the system’s central two suns.
The hope is that, if any of the humans’ descendants survive tens of thousands of years of cultural isolation, they will be able to devise a way to slice their way out of their steel imprisonment—that protective eggshell—to seek their futures on the unknown worlds they may find.
Assuming, of course, that their remembered origins are not lost to the relegation of legend.