A generation ship, or generation starship, is a hypothetical type of interstellar ark starship that travels at sub-light speed. Since such a ship might take centuries to 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 traveling.
Origin
The concept of a generation starship is a good example of how science and fiction influence each other. Many space scientists and engineers who contributed to the concept of a generation starship were also science fiction writers. Rocket pioneer Robert H. Goddard was the first to write about long-duration interstellar journeys in his "The Ultimate Migration". In this he described the death of the Sun and the necessity of an "interstellar ark". The crew would travel for centuries in suspended animation and be awakened when they reached another star system. Konstantin E. Tsiolkovsky, father ofastronautic theory, first described the need for multiple generations of passengers in his essay, "The Future of Earth and Mankind", a space colony equipped with engines that travels thousands of years which he called "Noah's Ark". Another early description of a generation ship is in the 1929 essay "The World, The Flesh, & The Devil" by J. D. Bernal. Bernal's essay was the first publication to reach the public and influence other writers. He wrote about the concept of human evolution and mankind's future in space through methods of living that we now describe as a generation starship, and which could be seen in the generic word "globes". The Enzmann starship is categorised as "slow boat" because of the Astronomy Magazine title “Slow Boat to Centauri”. Gregory Matloff's concept is called a "colony ship" and Alan Bond called his concept a "world ship". These definitions are essentially based on the velocity of the ship and population size.
Obstacles
Biosphere
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.
Biology and society
Generation ships would have to anticipate possible biological, social and morale problems, 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 throughout a 200-year space journey, with little loss of genetic diversity; social engineering can reduce this estimate to 80 people. 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. 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 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.
Size
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. 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.
Social breakdown
Generation ships travelling for long periods of time may see breakdowns in social structures. Changes in society 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: Robert A. Heinlein's Orphans of the Sky and Brian Aldiss's Non-Stop discussed such societies.
Cosmic rays
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. Like other ionizing radiation, high-energy cosmic rays can damage DNA and increase the risk of cancer, cataracts, and neurological disorders. 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.
Technological progress
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.
Ethical considerations
The success of a generation ship depends on children born aboard taking over the necessary duties, as well as having children themselves. Even if their quality of life might be better than, for example, that of people born into poverty on Earth, this raises the question of whether it is ethical to severely constrain life choices of individuals by locking them into a project they did not choose. A moral quandary exists regarding how intermediate generations, those destined to be born and die in transit without actually seeing tangible results of their efforts, might feel about their forced existence on such a ship.
