Free How to Write Science Fiction and Fantasy by Orson Scott Card

There comes a point where the fuel to accelerate any more will add enough weight that you either can’t lift it or can’t design a sturdy enough ship to hold it. Furthermore, because it takes just as much fuel to slow you down at the end of your voyage so you don’t just sail right on past your destination, you have to save exactly half your fuel for the slowdown, plus any fuel required for maneuvering into orbit. That means that the fuel must be able to accelerate more than twice its own mass. Worse yet, if there isn’t any more fuel at your destination, you’re either not coming home again or you’re going to have to carry more than four times the fuel needed to accelerate you to your traveling speed.
    So that you don’t waste fuel trying to lift a huge ship out of the gravity well of a planet like Earth, such ships are usually assumed to have been built out in space and launched from a point as far as possible from the Sun. Thus, when they arrive at the new world, they put their huge ship into orbit and use landing vehicles or launches or (nowadays) shuttles to get down to the planet’s surface.
    Using the technology I’ve just described, you’ll be lucky to get to ten percent of lightspeed. That’s pretty fast- about 67 million miles an hour but at that rate, it will take your ship more than three hundred years to get to a star system thirty lightyears away. And that doesn’t even allow for acceleration time!
    That’s why such ships are called “generation ships.” Assuming that the ship is a completely self-contained environment, with plants to constantly refresh the atmosphere and grow food, a whole human society lives aboard the ship. People are born, grow old, and die, and the elements of their bodies are processed and returned to the ecosystem within the ship. This idea has been well-explored in many storiesparticularly stories about ships where the people have forgotten their origin, forgotten even that the ship is a ship-but it has a lot of life left in it.
    The problem with this (besides the fact that a completely self-contained ecosystem would be almost impossible to create) is that none of the people
    who reach the new world have any direct memories of their home planet. Their whole history for generations has been inside a ship-why would they even want to go out onto a planet’s surface? The fact of living inside a ship for so long is so powerful that it almost takes over the story. If your story is about that, like Rebecca Brown Ore’s brilliant debut story, “Projectile Weapons and Wild Alien Water,” then that’s fine-but if your story is about something else, a generation ship is hard to get over.
    Cryo-travel. Another alternative is to have the crew travel for all those years in a state of suspended animation-either frozen or otherwise kept viable until the ship itself, or a skeleton crew, wakens the sleepers at the voyage’s end. This has the advantage of not requiring living space and supplies for so many people for so many years, and it still achieves the result of malting frequent voyages between the new world and the home planet unthinkable-or at least impractical.
    The drawback is that if suspended animation is possible at all in your future universe, then you have to let it be used for anything it’s needed for. Characters who get sick or critically injured or even killed must be rushed back to the ship and popped into a suspended animation chamber until a cure or repair can be worked out. Also, there are bound to be people who try to abuse the system to prolong their lives beyond the normal span of years. You can’t have a technology exist for one purpose and then ignore it for another-not unless you want to earn the scorn of your more critical and vocal readers.
    A variation on cryo-travel is to send colony ships that contain no human beings at all, but rather frozen human embryos; when the ship’s computer determines that the starship has reached a habitable planet, some of