Generation ship
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A generation ship is a theoretical starship that would travel much slower than light across great distances between stars (see interstellar travel). Since such a ship might take thousands or tens of thousands of years to reach even nearby stars, the original occupants would die during the journey, leaving their descendants to continue traveling.
It is estimated that in order to assure genetic diversity during a centuries-long trip, any generation starship would require at least 500 inhabitants (though this could also be achieved for a much smaller crew through the use of sperm banks or egg banks brought along for the journey). Additionally, the ship would have to be almost entirely self-sustaining (see Biosphere, life support) so as to provide food, air, and water for everyone on board. It must also have extraordinarily reliable systems that would not fail even over long periods of time, or alternately that could be repaired by the ship's inhabitants if they did.
People have advocated that before humans send generation ships to the stars, we should create large self-sustaining space habitats first. Each space habitat would be isolated from the rest of humanity for a century, but near enough to Earth for help. This would test if thousands of humans can survive a century on their own before sending them beyond the reach of any help.
Generation ships are often found in science fiction stories. A common theme is that inhabitants of a generation ship have forgotten that they are on a ship at all, and believe their ship to be the entire universe. Famous examples of this include Brian Aldiss's novel Non-stop, Robert A. Heinlein's novella Orphans of the Sky, Gene Wolfe's four novel series Book of the Long Sun, the early 1970s TV series The Starlost, the novel Colony by Rob Grant, the anime OAV Megazone 23, and the Star Trek episode "For the World is Hollow and I Have Touched the Sky".
Some think of planets with life as generation ships; this idea is usually called "Spaceship Earth".
Other means of colonizing space
Theory of relativity is often misinterpretted to infer that (barring discovery of faster than light travel) generation ships are the only way of traveling to distant star systems, because no ship can travel faster than light, and it takes more than person's lifetime for light to reach distant planets.
Such reasoning is however fundamentally incorrect, because time wouldn't pass at the same rate on Earth and on the ship. If the ship travelled at speeds very close to speed of light (as observed from Earth), its clock would run at "different speed" than clocks on Earth, making one ship-day equivalent to many days or even years on Earth.
For example, one day on a starship travelling at 99% of light speed would be equivalent to 7 Earth days. At 99.9999% of speed of light one day would be equivalent to almost 2 Earth years, and at 99.99999999% c to 61 Earth years.
Thus a starship travelling 2.9 million ly to Andromeda Galaxy at 99.99999999% c would reach it in about 41 years, according to ship's clock, and the original human travellers could still be alive. On the other hand slightly over 2.9 million years would of course pass on Earth, making it completely unlike the Earth they've left.
Travel to distant stars in our galaxy is possible at much more modest speeds, without generation ships.