A generation ship is a starship that travels at sub-lightspeed. Before all those convenient FTL (Faster-than-light) drives, wormholes and warp speeds were invented by writers to reduce the interstellar journey to more human proportions.

Since such a ship might take centuries or more to reach even the nearest stars, the original occupants of a generation ship would grow old and die, leaving their descendants to continue travelling and reproducing again and again until their descendants eventually reached their destination. The ship would have to be self sustaining in respect of food, air, water and fuel.

Many stories start with an apocalyptic scenario on Earth which necessitates the need to build an "ark" type ship to rescue the remnants of humanity and take them into space to find a new home.

A key element of the generation ship sub-genre, is the nature of the society which develops during the voyage.

Robert Heinlein's "Orphans of the Sky" (originally published in two parts in 1941) is generally considered a landmark work. Over the generations, the Vanguard inhabitants have forgotten both their origins and their purpose, and lapsed into a pre-technological society, ruled by superstition and fear of mutants. Their ship is the only universe they know. Another early book along these lines, and worth a read, is "Non-Stop" by Brian Aldiss.

Other generation ship stories deal with the theme of returning to Earth hundreds or thousands of years later (for example, the animation film WALL-E) or else arriving at a planet which has already been colonised by humans who came earlier, having travelled by a faster method (for example – The Voyage That Lasted 600 years by Don Wilcox).

However, my favourite example from a generation ship is that classic TV series, Red Dwarf. How can you top the character of the Cat, a new cognizant species evolving over millions of years from the ship's cat?

As in many sci-fi subgenres, there is a strong connection between science and science fiction for the generation ship. Two early scientists, rocket pioneer Robert Goddard (USA) and the father of astronautic theory, Konstantin Tsiolkovsky (USSR) both put forward the idea of a generation ship in the early part of the twentieth century. And given our present technology, a generation ship would be the only spaceship we might be capable of launching for the foreseeable future. Let's just hope we don't reach the apocalyptic scenario first.

An interesting reference for those of you who might want to explore this sub-genre is:

Caroti, Simone (2011). "The Generation Starship in Science Fiction: A Critical History, 1934-2001" Mcfarland.ISBN 978-0-7864-6067-0