== Eksterne henvisninger ==
* [http://www.nas.nasa.gov/Services/Education/SpaceSettlement/CoEvolutionBook/Interview.HTML "Is the surface of a planet really the right place for expanding technological civilization?"] {{Webarchive|url=https://web.archive.org/web/20090720131743/http://www.nas.nasa.gov/Services/Education/SpaceSettlement/CoEvolutionBook/Interview.HTML |date=20. juli 2009 }} Interviewing Gerard O'Neill
* [http://www.faqs.org/docs/Newtonian/Newtonian_91.htm Biological Effects of Weightlessness]
* [http://www.daviddarling.info/encyclopedia/S/spacecolony.html space colony – The Encyclopedia of Astrobiology, Astronomy, and Spaceflight]
* [http://www.hudsonfla.com/spaceviewinner.htm Visualizing the Steps of Solar System Colonization]
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