A generation ship is a hypothetical type of interstellar ark starship that travels across great distances between stars at a speed much slower than the speed of light. Since such a ship might take thousands of years to reach even nearby stars, the original occupants of a generation ship would grow old and die, leaving their descendants to continue travelling.
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Such a ship would have to be almost entirely self-sustaining, providing energy, food, air, and water for everyone on board. It must also have extraordinarily reliable systems that could be maintained by the ship's inhabitants over long periods of time. Large, self-sustaining space habitats would be needed. For gaining experience before sending generation ships to the stars, such a habitat could be effectively isolated from the rest of humanity for a century or more, but remain close enough to Earth for help. This would test whether thousands of humans can survive on their own before sending them beyond the reach of help. There are also the concerns of immune systems atrophying in the ship's environment. Small artificial closed ecosystems, including Biosphere 2, have been built in an attempt to work out the engineering difficulties in such a system, with mixed results.
Some have compared planets with life (in particular Earth) to generation ships. This idea is usually called "Spaceship Earth".
Generation ships would also have to solve major biological, social and moral problems,[1] and would also need to deal with complex matters of self-worth and purpose for the various crews involved. As an example, a moral quandary might exist regarding how intermediate generations (for example, those destined to be born, reproduce, and die in transit, without actually seeing tangible results of their efforts) might feel about their forced existence on such a ship.
Estimates of the minimum viable population vary. The results of a 2005 study from Rutgers University theorized that the native population of the Americas are the descendants of only 70 individuals who crossed the land bridge between Asia and North America.[2] However, anthropologist Dr. John Moore estimated in 2002 that a population of 150 to 180 would allow normal reproduction for 60 to 80 generations, equivalent to 2000 years.[3] Careful genetic screening and use of a sperm bank from Earth would also allow a smaller starting base with negligible inbreeding.
Generation ships are based on the human life span not changing dramatically. Even though people are living longer and longer it would take a lifespan extension beyond anyone's forecasts for any one individual to live throughout the entire trip.
The radiation environment of deep space is very different from that on the Earth's surface or in low earth orbit, due to the much larger flux of high-energy galactic cosmic rays (GCRs), along with radiation from solar proton events and the radiation belts. Like other ionizing radiation, high-energy cosmic rays can damage DNA, increasing the risk of cancer, cataracts, neurological disorders, and non-cancer mortality risks.[4] The only known practical solution to this problem is surrounding the crewed parts of the ship with a thick enough shielding, say a couple meters of ice.
If ever mankind is so enthusiastic about interstellar travel that a generation ship is sent to a star system 20 light years away in 200 years, it is quite unlikely that those remaining at home will just wait 220 years to hear the news of the success of the mission, and would not build any further starships meanwhile. So, if 50 years after launch they build a better spaceship that can make the same travel in just 50 years, when the first generation ship arrives they might find a 100-years old human colony at their destination.
Generation ships are often found in science fiction stories. Perhaps their earliest description is in the 1929 essay "The World, The Flesh, & The Devil" by J. D. Bernal.[5] The first fiction dealing with one is the 1940 story "The Voyage That Lasted 600 Years" by Don Wilcox.[6]
Beginning with the 1941 novels Universe and Common Sense by Robert A. Heinlein, 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. French writer Léon Groc actually wrote the first complete novel about this theme in the 1950 book L'Univers Vagabond. In the anglophone world, Brian Aldiss is attributed with the first complete novel dealing exclusively with the theme in the 1958 book Non-Stop. Edmund Cooper's Seed of Light (1959) is already criticized because it again deals with the old subject, but counted the author's best novel.[7] Harry Harrison's novel Captive Universe (1969) deals with similar themes.
In the Star Trek: The Original Series episode For the World Is Hollow and I Have Touched the Sky (1968) the Enterprise encounters a computer-led generation ship, whose inhabitants do not know they are within a ship traveling through space but instead believe themselves to be on a solid world and that the created sky is real. Harlan Ellison's The Starlost, a 1973 Canadian TV series, is set aboard the giant spaceship called The ARK. In the Space: 1999 episode Mission of the Darians (1975) the Alphans encounter the heavy damaged but still populated generation ship Daria, bearing an evil secret.
Gene Wolfe's tetralogy The Book of the Long Sun (1993) deals directly with the challenges facing the inhabitants of the starcrosser Whorl and the continuing challenges after planetfall in The Book of the Short Sun (1999). The 2008 Pixar film WALL-E contains a subplot in which a generation ship containing humans returns to Earth after many centuries. Toby Litt's 2009 novel Journey into Space is about people living on a generation ship who return back to Earth after an onboard conflict. This method of slow interstellar travel is hinted at in Arthur C. Clarke's Rendezvous With Rama (1972) as the spacecraft Rama is analyzed, and this theme continues in the book's sequels. The Mass Effect series features a large caravan of jerry-rigged generation ships, referred to as the Migrant Fleet. The 1999 Star Trek: Voyager episode The Disease features a generation ship of a species called the Varro. The 2010 "Doctor Who" episode, "The Beast Below", centers around a generation ship known as "Starship UK", which contains the entire future population of the United Kingdom (except for Scotland, which opted for its own ship) fleeing the deadly solar flares on the Earth in the 29th century.