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==Obstacles== ===Biosphere=== Such a ship would have to be entirely self-sustaining, providing [[life support system|life support]] for everyone aboard. It must 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 [[Biosphere 2]], have been built in an attempt to examine the engineering challenges of such a system, with mixed results.<ref>{{cite web |url=https://www.vice.com/en/article/biosphere-2-how-a-sci-fi-stunt-turned-into-the-worlds-biggest-earth-science-lab/ |title=Biosphere 2: How a Sci-Fi Stunt Turned Into the World's Biggest Earth Science Lab |last=Merchant |first=Brian |date=June 10, 2013 |website=Motherboard |publisher=Vice Media LLC |archive-url=https://web.archive.org/web/20160819124432/http://motherboard.vice.com/blog/biosphere-2-how-a-sci-fi-stunt-turned-into-the-worlds-biggest-earth-science-lab |archive-date=August 19, 2016 |url-status=live }}</ref> ===Biology and society=== Generation ships would have to anticipate possible biological, social and morale problems,<ref name="Malik20020319">{{cite web|last1=Malik|first1=Tariq|title=Sex and Society Aboard the First Starships |url=http://www.space.com/scienceastronomy/generalscience/star_voyage_020319-1.html |url-status=dead |archiveurl=https://web.archive.org/web/20020407132242/http://www.space.com/scienceastronomy/generalscience/star_voyage_020319-1.html |archivedate=2002-04-07 |work=Space.com |date=19 March 2002}}</ref> 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, without [[genetic testing]] of people before boarding the ship, [[social control]] and / or [[social engineering (political science)|social engineering]] (such as requiring people to wait until their thirties to have children), nor [[cryopreservation]] of eggs, sperm, or embryos (as is done in [[sperm bank]]s), a minimum of 160 people boarding the ship 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]]. If the people who board the ship are couples, presumably in their early twenties, and everybody who lives in the ship is required to wait until their mid to late thirties before having children, then the minimum would be just 80 people. However, many variables are not accounted for in the estimate, including the higher chance of health problems for both the woman who is pregnant and the fetus or baby because of the [[advanced maternal age|pregnant woman's age]].<ref>{{cite web |url=https://www.newscientist.com/article/dn1936-magic-number-for-space-pioneers-calculated/ |title="Magic number" for space pioneers calculated |date=15 February 2002 |author=Damian Carrington |work=[[New Scientist]] |access-date=6 August 2019}}</ref> 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.<ref>{{cite journal|title=Smith, C.M., "Estimation of a genetically viable population for multigenerational interstellar voyaging: Review and data for project Hyperion" |journal=Acta Astronautica |volume=97 |pages=16β29 |date=2013-12-13 |doi=10.1016/j.actaastro.2013.12.013|bibcode=2014AcAau..97...16S|last1=Smith |first1=Cameron M. }}</ref> 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 [[evolution|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 learn scientific and technical skills needed to maintain, operate and pilot 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. ===Size=== 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.<ref >{{cite arXiv | last1=Marin | first1=F. | last2=Beluffi | first2=C. | title=Computing the minimal crew for a multi-generational space travel towards Proxima Centauri b | date=2018 | class=astro-ph.IM |eprint=1806.03856}}</ref> A spacecraft of such a size would require much 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.<ref>{{cite web |author1=Kim Stanley Robinson |title=What Will It Take for Humans to Colonize the Milky Way? |url=https://www.scientificamerican.com/article/what-will-it-take-for-humans-to-colonize-the-milky-way1/ |work=[[Scientific American]] |access-date=January 31, 2019 |language=en |date=January 13, 2016}}</ref> ===Social breakdown=== Generation ships traveling 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:<ref name="budrys196608">{{Cite magazine |last=Budrys |first=Algis |date=August 1966 |title=Galaxy Bookshelf |url=https://archive.org/stream/Galaxy_v24n06_1966-08#page/n185/mode/2up |magazine=Galaxy Science Fiction |pages=186β194 }}</ref> {{quote|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){{r|budrys196608}} and [[Brian Aldiss]]'s [[Non-Stop (novel)|''Non-Stop'']] (U.S. title: ''Starship'') discussed such societies. ===Cosmic rays=== {{Main|Health threat from 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]] (GCRs). Like other [[ionizing radiation]], high-energy cosmic rays can damage DNA and increase the risk of cancer, cataracts, and neurological disorders.<ref>{{cite web|url=http://spaceflight.nasa.gov/spacenews/factsheets/pdfs/radiation.pdf |archive-url=https://web.archive.org/web/20041030191015/http://www.spaceflight.nasa.gov/spacenews/factsheets/pdfs/radiation.pdf |url-status=dead |archive-date=2004-10-30 |title=NASA Facts: Understanding Space Radiation |access-date=2010-04-01}}</ref>
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