A generation ship is a hypothetical type of 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. The invention is credited to J. D. Bernal in his 1929 novel The World, The Flesh, & The Devil. A common theme is that inhabitants of a generation ship have forgotten 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 Original Video Animation Megazone 23, Arthur C. Clarke and Gentry Lee's Rama series of books, and the Star Trek episode "For the World is Hollow and I Have Touched the Sky".

A book in which a sleeper ship goes wrong and becomes a vastly different type of generation ship was written by James White.

Some have compared planets with lifeforms to generation ships; in Earth's case this idea is usually called "Spaceship Earth" or "Spacecraft Earth".

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