Generation ship

Such a ship would have to be 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. This would require testing whether thousands of humans could survive on their own before sending them beyond the reach of help. Small artificial closed ecosystems, such as the Biosphere 2, have been built in an attempt to work out the engineering difficulties in such a system, with mixed results.[6]

Generation ships would have to anticipate possible biological, social and morale problems,[7] and would also need to deal with matters of self-worth and purpose for the various crews involved.

Estimates of the minimum reasonable population for a generation ship vary. Anthropologist John Moore has estimated that, even in the absence of cryonics or sperm banks, a population capacity of 160 people would allow normal family life (with the average individual having ten potential marriage partners) throughout a 200-year space journey, with little loss of genetic diversity; social engineering can reduce this estimate to 80 people.[8] In 2013 anthropologist Cameron Smith reviewed existing literature and created a new computer model to estimate a minimum reasonable population in the tens of thousands. Smith's numbers were much larger than previous estimates such as Moore's, in part because Smith takes the risk of accidents and disease into consideration, and assumes at least one severe population catastrophe over the course of a 150-year journey.[9]

In light of the multiple generations that it could take to reach even our nearest neighboring star systems such as Proxima Centauri, further issues on the viability of such interstellar arks include:

• the possibility of humans dramatically evolving in directions unacceptable to the sponsors
• the minimum population required to maintain in isolation a culture acceptable to the sponsors; this could include such aspects as
  • ability to maintain and operate the ship
  • ability to accomplish the purpose (planetary colonization, research, building new interstellar arks) contemplated
  • sharing the values of the sponsors, which may not be likely to be empirically demonstrated to be viable beyond the home planet unless, once the ship is away from Earth and on its way, survival of one's offspring until the ship reaches the target star is one motivation.

In order for a spacecraft to maintain a stable environment for multiple generations, it would have to be large enough to support a community of humans and a fully recycling ecosystem.[10] However, a spacecraft of such a size would require a lot of energy to accelerate and decelerate. A smaller spacecraft, while able to accelerate more easily and thus make higher cruise velocities more practical, would reduce exposure to cosmic radiation and the time for malfunctions to develop in the craft, but would have challenges with resource metabolic flow and ecologic balance.[11]

Generation ships travelling for long periods of time may see breakdowns in social structures. Changes in society (for example, mutiny) could occur over such periods and may prevent the ship from reaching its destination. This state was described by Algis Budrys in a 1966 book review:[12]

The slower-than-light interstellar spaceship, pursuing its way through the weary centuries, its crew losing touch with all reality save the interior of the vessel ... Well, you know the story, and its unhappy downhill round, its exciting struggles between the barbarian tribes which develop in its disparate compartments, and then, if the writer is so minded, the ultimate flash of hope as the good guys win out and prepare to meet their future on some noble, if erroneous basis.

Robert A. Heinlein's Orphans of the Sky (the "impeccable statement of this theme", Budrys said)[12] and Brian Aldiss's Non-Stop (U.S. title: Starship) discussed such societies.

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 influx of high-energy galactic cosmic rays (GCRs). Like other ionizing radiation, high-energy cosmic rays can damage DNA and increase the risk of cancer, cataracts, and neurological disorders.[13] One known practical solution to this problem is surrounding the crewed parts of the ship with a thick enough shielding such as a thick layer of maintained ice as proposed in The Songs of Distant Earth, a science fiction novel by Arthur C. Clarke (note: in this book the ship's mammoth ice shield is only in the forward part of the ship, preventing micrometeors from damaging the ship during its interstellar journey).

If a generation ship is sent to a star system 20 light years away, and is expected to reach its destination in 200 years, a better ship may be later developed that can reach it in 50 years. Thus, the first generation ship may find a century-old human colony after its arrival at its destination.