Editing Space colonization (section)

==Locations to consider==
Space colonization has been envisioned at many different locations inside and outside the [[Solar System]], but most commonly at Mars and the Moon.

===Near-Earth space===
====Earth orbit====
[[File:Debris-GEO1280.jpg|thumb|alt=Earth from space, surrounded by small white dots|A computer-generated image from 2005 showing the distribution of mostly space debris in [[geocentric orbit]] with two areas of concentration: geostationary orbit and low Earth orbit.]]

[[Geostationary orbit]] was an early issue of discussion about space colonization, with equatorial countries argueing for special rights to the orbit (see [[Bogota Declaration]]).<ref name="Durrani 2019"/>

[[Space debris]], particularly in low Earth orbit, has been characterized as a product of colonization by occupying space and hindering access to space through excessive pollution with debris, with drastic increases in the course of military activity and without a lack of management.<ref name="Durrani 2019"/>

[[File:Axiom modules connected to ISS.jpg|thumb|Through the [[Commercial LEO Destinations program]], the [[Axiom Station]] can gradually establish commercial uses and become economically sustainable.]]

Most of the [[Delta-v budget|delta-''v'' budget]], and thus propellant, of a launch is used bringing a spacecraft to low Earth orbit.<ref name=":3">{{Cite book |last=Wanjek |first=Christopher |title=Spacefarers: how humans will settle the Moon, Mars, and beyond |date=2020 |publisher=Harvard University Press |isbn=978-0-674-98448-6 |location=Cambridge, Massachusetts}}</ref>{{Rp|page=100}} This is the main reason why [[Jerry Pournelle]] said "If you can get your ship into orbit, you're halfway to anywhere".<ref>{{Cite magazine |last=Pournelle |first=Jerry P. |date=April 1974 |title=A Step Farther Out, Halfway to Anywhere |url=https://archive.org/details/Galaxy_v34n07_1974-04/page/n95/mode/2up |magazine=[[Galaxy Magazine]] |pages=94}}</ref> Therefore, the main advantages to constructing a [[space settlement]] in Earth orbit are accessibility to the Earth and already-existing economic motives such as [[space hotels]] and [[space manufacturing]]. However, a big disadvantage is that orbit does not host any materials that is available for exploitation. Space colonization altogether might eventually demand lifting vast amounts of payload into orbit, making thousands of daily launches potentially unsustainable. Various theoretical concepts, such as [[orbital ring]]s and [[Skyhook (structure)|skyhooks]], have been proposed to reduce the cost of accessing space.<ref name=":3" />{{Rp|page=|pages=142–147}}

==== Moon ====
{{Main|Colonization of the Moon}}

[[File:Lunar base concept drawing s99 04195.jpg|thumb|Artist's rendering of an envisioned [[Lunar resources#Mining|lunar mining facility]]]]

The [[Moon]] is discussed as a target for colonization, due to its proximity to Earth and lower [[escape velocity]]. The Moon is reachable from Earth in three days, has a near-instant communication to Earth, with minable minerals, no atmosphere, and low gravity, making it extremely easy to ship materials and products to orbit.<ref name=":3" />{{Rp|page=|pages=175}} Abundant [[ice]] is trapped in [[permanently shadowed crater]]s near the poles, which could provide support for the water needs of a lunar colony,<ref>{{cite news | url =http://www.hindu.com/2009/09/23/stories/2009092357770100.htm | archive-url =https://web.archive.org/web/20090926073133/http://www.hindu.com/2009/09/23/stories/2009092357770100.htm | url-status =dead | archive-date =26 September 2009 | title =Water discovered on Moon?: "A lot of it actually" | date =23 September 2009 | newspaper =[[The Hindu]] | access-date =26 September 2009 |first=Divya |last=Gandhi }}</ref> though indications that [[mercury (element)|mercury]] is also similarly trapped there may pose health concerns.<ref>{{cite journal |last1=Reed Jr. |first1=George W. |date=1999 |title=Don't drink the water |url= |journal=Meteoritics & Planetary Science |volume=34 |issue= 5|pages=809–811 |doi=10.1111/j.1945-5100.1999.tb01394.x |bibcode=1999M&PS...34..809R |s2cid=129733422 |access-date=|doi-access=free }}</ref><ref name=prospecting>{{cite journal |last1=Platts |first1=Warren J. |last2=Boucher |first2=Dale |last3=Gladstone |first3=G. Randall |date=12 December 2013 |title=Prospecting for Native Metals in Lunar Polar Craters |url= |journal=7th Symposium on Space Resource Utilization |volume= |issue= |pages= |doi=10.2514/6.2014-0338 |isbn=978-1-62410-315-5 |access-date=}}</ref> Native [[precious metal]]s, such as [[gold]], [[silver]], and probably [[platinum]], are also concentrated at the lunar poles by electrostatic dust transport.<ref name=prospecting/> There are only a few materials on the Moon which have been identified to make economic sense to ship directly back to the Earth, which are [[helium-3]] (for [[fusion power]]) and rare-earth minerals (for [[electronics]]). Instead, it makes more sense for these materials to be used in-space or being turned into valuable products for export. However, the Moon's lack of atmosphere provides no protection from space radiation or meteoroids, so [[lunar lava tube]]s have been proposed sites to gain protection.<ref>{{cite news|title=Moon hole might be suitable for colony|url=http://www.cnn.com/2010/TECH/space/01/01/moon.lava.hole/ | work=CNN | date=1 January 2010}}</ref> The Moon's low surface gravity is also a concern, as it is unknown whether 1/6[[g-force|g]] is enough to maintain human health for long periods.<ref>{{Cite journal|last=Taylor|first=R. L.|date=March 1993|title=The effects of prolonged weightlessness and reduced gravity environments on human survival.|journal=Journal of the British Interplanetary Society|volume=46|issue=3|pages=97–106|pmid=11539500}}</ref>

Since the Moon has extreme temperature swings and toxic [[lunar regolith]], it is argued by some that the Moon will not become a place of habitation, but instead attract polluting [[Primary sector of the economy|extraction]] and [[Secondary sector of the economy|manufacturing industries]]. Furthermore, it has been argued that moving these industries to the Moon could help protect the Earth's environment and allow poorer countries to be released from the shackles of [[neocolonialism]] by wealthier countries. In the space colonization framework, the Moon will be transformed into an industrial hub of the Solar System.<ref name=":3" />{{Rp|page=|pages=161–172}}

Interest in establishing a [[moonbase]] has increased in the 21st century as an intermediate to Mars colonization.

The [[European Space Agency]] (ESA) head [[Johann-Dietrich Wörner|Jan Woerner]] at the International Astronautical Congress in Bremen, Germany, in October, 2018 proposed cooperation among countries and companies on lunar capabilities, a concept referred to as ''[[Moonbase#Moon Village|Moon Village]]''.<ref name=sn20180529>
{{cite news |last=Foust|first=Jeff |url=https://spacenews.com/bezos-outlines-vision-of-blue-origins-lunar-future/ |title=Bezos outlines vision of Blue Origin's lunar future |work=[[SpaceNews]] |date=29 May 2018 |access-date=21 August 2018 }}</ref>

In a December 2017 [[Space Policy Directive 1|directive]], the [[First presidency of Donald Trump|first Trump administration]] steered [[NASA]] to include a lunar mission on the pathway to other [[beyond Earth orbit]] (BEO) destinations.<ref name=spo20171217>{{cite web|url=https://spacepolicyonline.com/news/text-of-remarks-at-signing-of-trump-space-policy-directive-1-and-list-of-attendees/ |title=Text of Remarks at Signing of Trump Space Policy Directive 1 and List of Attendees |first=Marcia |last=Smith |website=Space Policy Online |date=11 December 2017 |access-date=21 August 2018}}</ref><ref name=sn20180529/>

In 2023, the U.S. Defense Department started a [[DARPA lunar programs|study]] of the necessary infrastructure and capabilities required to develop a moon-based economy over the following ten years.<ref>{{Cite web |last=Easley |first=Mikayla |date=5 December 2023 |title=DARPA taps 14 companies to study infrastructure needs for future lunar economy |url=https://defensescoop.com/2023/12/05/luna-10-darpa-award/ |access-date=22 March 2024 |website=defensescoop.com}}</ref>

As of 2024, on one side, [[China]], along with other partner countries, has announced its intention to establish the [[International Lunar Research Station]]. On the other side, the [[United States]], in collaboration with international partners, is advancing its [[Artemis program]], which includes plans to build [[Moonbase]]s near the lunar poles, close to [[permanently shadowed crater]]s, in the 2030s. The [[Chinese Lunar Exploration Program]] is seen as a means to bolster China's political influence and support its aspirations for [[superpower]] status, while the United States aims to maintain its position as the leading space power.

====Lagrange points====
{{Main|Lagrange point colonization}}
[[File:Lagrange points Earth vs Moon.jpg|thumb|A contour plot of the [[gravitational potential]] of the Moon and Earth, showing the five Earth–Moon Lagrange points]]

Another near-Earth possibility are the stable Earth–Moon [[Lagrangian point|Lagrange points]] {{L4}} and {{L5}}, at which point a space colony can float indefinitely. The [[L5 Society]] was founded to promote settlement by building space stations at these points. [[Gerard K. O'Neill]] suggested in 1974 that the stable region around L<sub>5</sub> could fit several thousand floating colonies, and would allow easy travel to and from the colonies due to the shallow [[effective potential]] at this point.<ref name="o'neill">{{cite journal |last1=O'Neill |first1=Gerard K. |author1-link=Gerard K. O'Neill |title=The colonization of space |journal=Physics Today |date=September 1974 |volume=27 |issue=9 |pages=32–40 |doi=10.1063/1.3128863 |bibcode=1974PhT....27i..32O |doi-access=free }}</ref>

=== Mars ===
{{Main|Colonization of Mars}}
[[File:Artist’s rendering of the approach to Mars.jpg|thumb|SpaceX has long considered [[SpaceX Mars colonization program|settling and colonizing Mars]] as its prime objective.]]

The hypothetical colonization of Mars has received interest from public space agencies and private corporations and has received extensive treatment in science fiction writing, film, and art.

While there have been many plans for a [[Human mission to Mars|human Mars mission]], including affordable ones such as [[Mars Direct]], none has been realized as of 2025. Both the United States and China have plans to send humans to Mars sometime in the 2040s, but these plans are not backed with hardware and funding.<ref name=":3" />{{Rp|page=|pages=219–223}} However, [[SpaceX]] is currently developing [[SpaceX Starship|Starship]], a [[Super heavy-lift launch vehicle|super-heavy-lift reusable launch vehicle]], with a vision of sending humans to Mars. As of November 2024, the company plans to send five uncrewed Starships to Mars in either 2026 or 2028–2029 [[launch window]]s<ref>{{Cite web |title=SpaceX Plans To Send Five Uncrewed Starships to Mars by 2026 |first=Joe |last=Hindy| date=10 October 2024 |url=https://www.cnet.com/science/space/spacex-plans-5-missions-to-mars-by-2026-elon-musk-says/ |access-date=27 November 2024 |website=CNET |language=en}}</ref> and SpaceX's CEO [[Elon Musk]] has repeatingly stated his support for the Mars efforts, both financially and politically.<ref>{{Cite web |last=Koren |first=Marina |date=5 November 2024 |title=MAGA Goes to Mars |url=https://www.theatlantic.com/science/archive/2024/11/musk-trump-mars-spacex/680529/ |access-date=27 November 2024 |website=The Atlantic |language=en}}</ref>

Mars is more suitable for habitation than the Moon, with a stronger gravity, rich amount of materials needed for life, day/night cycle nearly identical to Earth, and a thin atmosphere to protect from [[Micrometeoroid|micrometeroids]]. The main disadvantage of Mars compared to the Moon is the six-to-nine-month transit time and the lengthy launch window, which occurs approximately every two years.<ref name=":3" />{{Rp|page=|pages=175}} Without [[in situ resource utilization]], Mars colonization would be nearly impossible as it would require bringing thousands of tons of payload to sustain a handful of astronauts. If Martian materials can be used to make propellant (such as [[methane]] with the [[Sabatier reaction|Sabatier process]]) and supplies (such as [[oxygen]] for crews), the amount of supplies needed to bring to Mars can be greatly reduced.<ref>{{Cite journal|title = From local resources to in situ propellant and chemical production on Mars. A possible pathway|journal = Chemical Engineering Journal|date = June 2025|volume = 513|doi = 10.1016/j.cej.2025.162490|first1 = Arturo|last1 = Pajares|first2 =Paulina|last2= Govea-Alvarez|first3=Zhiyuan |last3=Chen|first4= Melchiorre|last4= Conti|first5= Bart |last5= Michielsen|doi-access = free}}</ref><ref name=":3" />{{Rp|page=|pages=228–230}} Even then, Mars colonies will not be economically viable in the near term, thus reasons for colonizing Mars will be mostly ideological and prestige-based, such as a desire for [[freedom]].<ref name=":3" />{{Rp|page=|pages=267–270, 280}}

=== Other inner Solar System bodies ===
====Mercury====
Mercury is rich in metals and volatiles, as well as solar energy. However, Mercury is the [[Delta-v budget|most energy-consuming body on the Solar System]] to land for spacecraft launching from Earth, and astronauts there must contend with the extreme temperature differential and radiation.<ref name=":3" />{{Rp|page=|pages=311–314}}

[[File:TerraformedMercuryGlobe.jpg|thumb|upright=1.2|An artist's conception of a terraformed [[Mercury (planet)|Mercury]]]]
Once thought to be a volatile-depleted body like the Moon, Mercury is now known to be volatile-rich, surprisingly richer in volatiles than any other terrestrial body in the inner Solar System.<ref>{{Cite journal|last1=McCubbin|first1=Francis M.|last2=Riner|first2=Miriam A.|last3=Kaaden|first3=Kathleen E. Vander|last4=Burkemper|first4=Laura K.|date=2012|title=Is Mercury a volatile-rich planet?|journal=Geophysical Research Letters|language=en|volume=39|issue=9|pages=n/a|doi=10.1029/2012GL051711|bibcode=2012GeoRL..39.9202M|issn=1944-8007|doi-access=free}}</ref> The planet also receives six and a half times the solar flux as the Earth/Moon system,<ref name=Bolonkin>{{cite book |last=Bolonkin |first=Alexander A. |date=2015 |editor-last1=Badescu |editor-first1=Viorel |editor-last2=Zacny |editor-first2=Kris |title=Inner Solar System: Prospective Energy and Material Resources |publisher=Springer-Verlag |pages=407–419 |chapter=Chapter 19: Economic Development of Mercury: A Comparison with Mars Colonization |isbn=978-3-319-19568-1}}</ref> making solar energy an effective energy source; it could be harnessed through orbital solar arrays and beamed to the surface or exported to other planets.<ref name=UTMercury/>

Geologist Stephen Gillett suggested in 1996, that this could make Mercury an ideal place to build and launch [[solar sail]] spacecraft, which could launch as folded "chunks" by a [[mass driver]] from Mercury's surface. Once in space, the solar sails would deploy. Solar energy for the mass driver should be easy to produce, and solar sails near Mercury would have 6.5 times the thrust they do near Earth. This could make Mercury an ideal place to acquire materials useful in building hardware to send to (and terraform) Venus. Vast solar collectors could also be built on or near Mercury to produce power for large-scale engineering activities such as laser-pushed light sails to nearby star systems.<ref>{{cite book|first1=Stanley |last1=Schmidt |first2=Robert |last2=Zubrin |title=Islands in the Sky: Bold New Ideas for Colonizing Space |publisher=Wiley |date=1996 |pages=71–84 |isbn=978-0-471-13561-6 |url=https://books.google.com/books?id=YCV0QgAACAAJ|access-date=18 April 2025}}</ref>

As Mercury has essentially no axial tilt, crater floors near its poles lie in [[crater of eternal darkness|eternal darkness]], never seeing the Sun. They function as [[cold trap (astronomy)|cold traps]], trapping volatiles for geological periods. It is estimated that the poles of Mercury contain 10<sup>14</sup>–10<sup>15</sup>&nbsp;kg of water, likely covered by about 5.65×10<sup>9</sup> m<sup>3</sup> of hydrocarbons. This would make agriculture possible. It has been suggested that plant varieties could be developed to take advantage of the high light intensity and the long day of Mercury. The poles do not experience the significant day-night variations the rest of Mercury do, making them the best place on the planet to begin a colony.<ref name=Bolonkin/>

Another option is to live underground, where day-night variations would be damped enough that temperatures would stay roughly constant. There are indications that Mercury contains [[lava tube]]s, like the Moon and Mars, which would be suitable for this purpose.<ref name=UTMercury>{{cite web |url=https://www.universetoday.com/130109/how-do-we-colonize-mercury/ |title=How do we Colonize Mercury? |last=Williams |first=Matt |date=3 August 2016 |website=[[Universe Today]] |access-date=22 August 2021}}</ref> Underground temperatures in a ring around Mercury's poles can reach room temperature on Earth, 22±1&nbsp;°C; and this is achieved at depths starting from about 0.7&nbsp;m. This presence of volatiles and abundance of energy has led [[Alexander Bolonkin]] and James Shifflett to consider Mercury preferable to Mars for colonization.<ref name=Bolonkin/><ref>{{cite web |url=https://einstein-schrodinger.com/mercury_colony.html |title=A Mercury Colony? |last=Shifflett |first=James |date=n.d. |website=einstein-schrodinger.com |access-date=31 July 2021}}</ref>

Yet a third option could be to continually move to stay on the night side, as Mercury's 176-day-long day-night cycle means that the [[terminator (solar)|terminator]] travels very slowly.<ref name=UTMercury/>

Because Mercury is very dense, its surface gravity is 0.38g like Mars, even though it is a smaller planet.<ref name=Bolonkin/> This would be easier to adjust to than lunar gravity (0.16g), but presents advantages regarding lower escape velocity from Mercury than from Earth.<ref name=UTMercury/> Mercury's proximity gives it advantages over the asteroids and outer planets, and its low [[synodic period]] means that launch windows from Earth to Mercury are more frequent than those from Earth to Venus or Mars.<ref name=UTMercury/>

On the downside, a Mercury colony would require significant shielding from radiation and solar flares, and since Mercury is airless, decompression and temperature extremes would be constant risks.<ref name=UTMercury/>

====Venus====
{{Main|Colonization of Venus}}
Though the surface of Venus is extremely hostile, habitats high above the atmosphere of Venus are fairly habitable, with temperatures ranging from 30&nbsp;°C to 70&nbsp;°C (86 to 158&nbsp;°F) and a pressure similar to the Earth's sea level at an altitude of 50 kilometers (30 miles).<ref>{{Cite web |date=9 November 2017 |title=Venus: Facts - NASA Science |url=https://science.nasa.gov/venus/venus-facts/ |access-date=29 January 2025 |language=en-US}}</ref> However, beside tourism opportunities, the economic benefit of a Venusian colony is minimal.<ref name=":3" />{{Rp|page=|pages=308–310}}

====Asteroid belt====
{{Main|Colonization of the asteroid belt}} 
Asteroids can provide enough material in the form of water, air, fuel, metal, soil, and nutrients to support ten to a hundred trillion humans in space. Many asteroids contain minerals that are inheriently valuable, such as rare earths and precious metals. However, low gravity, distance from Earth and disperse nature of their orbits make it difficult to settle on small asteroids.<ref name=":3" />{{Rp|page=|pages=203,204,218}}

===Giant planets===
There have also been proposals to place robotic [[aerostat]]s in the upper atmospheres of the Solar System's [[giant planet]]s for exploration and possibly mining of [[helium-3]], which could have a very high value per unit mass as a thermonuclear fuel.<ref name="zubrin1999"/>{{rp|158–160}}<ref name="He3U"/>

Robert Zubrin identified [[Saturn]], [[Uranus]] and [[Neptune]] as "the [[Persian Gulf]] of the Solar System", as the largest sources of [[deuterium]] and helium-3 to drive a [[nuclear fusion|fusion]] economy, with Saturn the most important and most valuable of the three, because of its relative proximity, low radiation, and large system of moons.<ref name="zubrin1999"/>{{rp|161–163}} On the other hand, planetary scientist [[John S. Lewis|John Lewis]] in his 1997 book ''[[Mining the Sky]]'', insists that Uranus is the likeliest place to mine helium-3 because of its significantly shallower gravity well, which makes it easier for a laden tanker spacecraft to thrust itself away. Furthermore, Uranus is an [[ice giant]], which would likely make it easier to separate the helium from the atmosphere.

Because [[Uranus]] has the lowest [[escape velocity]] of the four giant planets, it has been proposed as a mining site for [[helium-3]].<ref name="He3U"/> As Uranus is a gas giant without a viable surface, one of [[Uranus's natural satellites]] might serve as a base.<ref>{{cite web|first=Joseph |last=Castro |date=17 March 2015 |title=What It Would Be Like to Live on Uranus' Moons Titania and Miranda |url=https://www.space.com/28827-living-on-uranus-moons-titania-miranda.html |website=space.com|access-date=19 April 2025}}</ref>

It is hypothesized that one of [[Neptune]]'s satellites could be used for colonization. [[Triton (moon)|Triton]]'s surface shows signs of extensive geological activity that implies a subsurface ocean, perhaps composed of ammonia/water.<ref>{{cite journal| last=Ruiz| first=Javier| year=2003| title=Heat flow and depth to a possible internal ocean on Triton| journal=Icarus| volume=166| doi=10.1016/j.icarus.2003.09.009| page=436| bibcode=2003Icar..166..436R| issue=2| url=http://eprints.ucm.es/10454/1/11-Trit%C3%B3n_1.pdf| access-date=10 April 2023| archive-date=12 December 2019 | archive-url=https://web.archive.org/web/20191212145428/http://eprints.ucm.es/10454/1/11-Trit%C3%B3n_1.pdf| url-status=dead}}</ref> If technology advanced to the point that tapping such geothermal energy was possible, it could make colonizing a cryogenic world like Triton feasible, supplemented by [[nuclear fusion]] power.<ref>{{cite journal|title=Case Study on Human Colonization of Triton |first1=T.A. |last1=Aadithya |first2=Aman |last2=Srivastava |first3=Prinan |last3=Banerjee |first4=P. |last4=Partheban |url=https://www.worldresearchlibrary.org/up_proc/pdf/99-14483036648-10.pdf |journal=Proceedings of 3rd IASTEM International Conference |location=Singapore |date=7 November 2015 |isbn=978-93-85832-33-8}}</ref>

====Moons of outer planets====
[[File:Cryobot.jpg|thumb|Artist's impression of a hypothetical ocean [[cryobot]] in [[Europa (moon)|Europa]]]]
Human missions to the outer planets would need to arrive quickly due to the effects of space radiation and microgravity along the journey.<ref name=Palaszewski>{{cite conference |url=https://arc.aiaa.org/doi/10.2514/6.2015-1654 |title=Solar System Exploration Augmented by In-Situ Resource Utilization: Human Mercury and Saturn Exploration |last1=Palaszewski |first1=Bryan |date=2015 |publisher= |book-title= |pages= |doi=10.2514/6.2015-1654 |location=Kissimmee, Florida |conference=8th Symposium on Space Resource Utilization |id=|hdl=2060/20150004114 |hdl-access=free }}</ref> In 2012, Thomas B. Kerwick wrote that the distance to the outer planets made their human exploration impractical for now, noting that travel times for round trips to Mars were estimated at two years, and that the closest approach of Jupiter to Earth is over ten times farther than the closest approach of Mars to Earth. However, he noted that this could change with "significant advancement on spacecraft design".<ref name=Kerwick/> [[Nuclear thermal rocket|Nuclear-thermal]] or nuclear-electric engines have been suggested as a way to make the journey to Jupiter in a reasonable amount of time.<ref name=UTJupiter/> Another possibility would be plasma [[Magnetic sail|magnet sails]], a technology already suggested for rapidly sending a probe to Jupiter.<ref>{{Cite journal |last1=Freeze |first1=Brent |last2=Greason |first2=Jeff |last3=Nader |first3=Ronnie |last4=Febres |first4=Jaime Jaramillo |last5=Chaves-Jiminez |first5=Adolfo |last6=Lamontagne |first6=Michel |last7=Thomas |first7=Stephanie |last8=Cassibry |first8=Jason |last9=Fuller |first9=John |last10=Davis |first10=Eric |last11=Conway |first11=Darrel |date=1 February 2022 |title=Jupiter Observing Velocity Experiment (JOVE): Introduction to Wind Rider Solar Electric Propulsion Demonstrator and Science Objectives |journal=Publications of the Astronomical Society of the Pacific |volume=134 |issue=1032 |pages=023001 |doi=10.1088/1538-3873/ac4812 |issn=0004-6280|doi-access=free |bibcode=2022PASP..134b3001F }}</ref> The cold would also be a factor, necessitating a robust source of heat energy for spacesuits and bases.<ref name=Kerwick/> Most of the larger moons of the outer planets contain [[Ice|water ice]], [[liquid water]], and organic compounds that might be useful for sustaining human life.<ref name="icemoons">{{cite journal|first=G. J. |last=Consalmagno |title=Ice-rich moons and the physical properties of ice |journal=Journal of Physical Chemistry |volume=87 |number=21 |date=1 October 1983 |pages=4204–4208 |doi=10.1021/j100244a045 |url=https://pubs.acs.org/doi/10.1021/j100244a045 }}</ref><ref name="liftveil">{{cite book|first1=Ralph |last1=Lorenz |first2=Jacqueline |last2=Mitton |title=Lifting Titan's veil: exploring the giant moon of Saturn |publisher=Cambridge University Press |date=2002 |isbn=978-0-521-79348-3 |url=https://books.google.com/books?id=VLDG5awUsPoC}}</ref>

[[Robert Zubrin]] has suggested Saturn, Uranus, and Neptune as advantageous locations for colonization because their atmospheres are good sources of fusion fuels, such as [[deuterium]] and [[helium-3]]. Zubrin suggested that Saturn would be the most important and valuable as it is the closest and has an extensive satellite system. Jupiter's high gravity makes it difficult to extract gases from its atmosphere, and its strong radiation belt makes developing its system difficult.<ref name=UTSaturn/> On the other hand, fusion power has yet to be achieved, and fusion power from helium-3 is more difficult to achieve than conventional [[deuterium–tritium fusion]].<ref>{{cite news |last1=Day |first1=Dwayne | author-link = Dwayne A. Day |title=The helium-3 incantation |url=http://www.thespacereview.com/article/2834/1 |access-date=11 January 2019 |work=The Space Review |date=28 September 2015}}</ref> Jeffrey Van Cleve, Carl Grillmair, and Mark Hanna instead focus on Uranus, because the [[delta-v]] required to get helium-3 from the atmosphere into orbit is half that needed for Jupiter, and because Uranus' atmosphere is five times richer in helium than Saturn's.<ref name="He3U">{{cite web |first1=Jeffrey |last1=Van Cleve |first2=Carl |last2=Grillmair |first3=Mark |last3=Hanna |url=http://www.mines.edu/research/srr/2001abstracts/vancleve.PDF |title=Helium-3 Mining Aerostats in the Atmosphere of Uranus |archive-url=https://web.archive.org/web/20060630164712/http://www.mines.edu/research/srr/2001abstracts/vancleve.PDF|archive-date=30 June 2006 |access-date=10 May 2006}}</ref>

Jupiter's [[Galilean moons]] (Io, Europa, Ganymede, and Callisto) and Saturn's [[Titan (moon)|Titan]] are the only moons that have gravities comparable to Earth's Moon. The Moon has a 0.17g gravity; Io, 0.18g; Europa, 0.13g; Ganymede, 0.15g; Callisto, 0.13g; and Titan, 0.14g. Neptune's [[Triton (moon)|Triton]] has about half the Moon's gravity (0.08g); other [[planetary-mass moon|round moons]] provide even less (starting from Uranus' [[Titania (moon)|Titania]] and [[Oberon (moon)|Oberon]] at about 0.04g).<ref name=Kerwick/>

====Jovian moons====
[[File:Callisto base.PNG|thumb|upright=1.2|Artist's impression of a base on Callisto<ref name="CallistoBase">{{cite web|title=Vision for Space Exploration|url=http://www.nasa.gov/pdf/55583main_vision_space_exploration2.pdf|publisher=[[NASA]]|year=2004}}</ref>]]
<div style="float:right; margin:2px;">
{| class=wikitable style="text-align:center; font-size:11px"
|+ Jovian radiation
! Moon !! [[Röntgen equivalent man|rem]]/day
|-
| Io || 3600<ref name="ringwald">{{cite web |date=29 February 2000 |title=SPS 1020 (Introduction to Space Sciences) |publisher=California State University, Fresno |last=Ringwald |first=Frederick A. |url=https://zimmer.csufresno.edu/~fringwal/w08a.jup.txt |url-status=dead |access-date=5 January 2014 |archive-url=https://web.archive.org/web/20080725050708/https://zimmer.csufresno.edu/~fringwal/w08a.jup.txt |archive-date=25 July 2008 }}</ref>
|-
| Europa || 540<ref name="ringwald"/>
|-
|Ganymede || 8<ref name="ringwald"/>
|-
| Callisto || 0.01<ref name="ringwald"/>
|-
! Earth (Max) !!  0.07
|-
! Earth (Avg) !! 0.0007

|}</div>

The [[Jupiter|Jovian]] system in general has particular disadvantages for colonization, including a deep [[gravity well]]. The [[magnetosphere of Jupiter]] bombards the [[moons of Jupiter]] with intense [[ionizing radiation]]<ref name="radjup">{{cite journal|first1=R. Walker |last1=Fillius |first2=Carl E. |last2=McIlwain |first3=Antonio |last3=Mogro-Campero |title=Radiation Belts of Jupiter: A Second Look |journal=Science |volume=188 |number=4187 |pages=465–467 |date=2 May 1975 |doi=10.1126/science.188.4187.465 |pmid=17734363 |url=https://www.science.org/doi/10.1126/science.188.4187.465}}</ref> delivering about 36 [[Sievert#Dose examples|Sv]] per day to unshielded colonists on [[Io (moon)|Io]] and about 5.40 Sv per day on [[Europa (moon)|Europa]]. Exposure to about 0.75 Sv over a few days is enough to cause [[Acute radiation syndrome|radiation poisoning]], and about 5&nbsp;Sv over a few days is fatal.<ref name="zubrin1999"/>{{rp|166–170}}

Jupiter itself, like the other gas giants, has further disadvantages. There is no accessible surface on which to land, and the light hydrogen atmosphere would not provide good buoyancy for some kind of aerial habitat as has been proposed for Venus.

Radiation levels on [[Io (moon)|Io]] and [[Europa (moon)|Europa]] are extreme, enough to kill unshielded humans within an Earth day.<ref name="zubrin1999">{{cite book |first=Robert |last=Zubrin |title=Entering Space: Creating a Spacefaring Civilization |publisher=Tarcher/Putnam |date=1999 |isbn=978-1-58542-036-0|url=https://books.google.com/books?id=67XuAAAAMAAJ |access-date=18 April 2025}}</ref>{{rp|163–170}} Therefore, only [[Callisto (moon)|Callisto]] and perhaps [[Ganymede (moon)|Ganymede]] could reasonably support a human colony. Callisto orbits outside Jupiter's radiation belt.<ref name=Kerwick/> Ganymede's low latitudes are partially shielded by the moon's magnetic field, though not enough to completely remove the need for radiation shielding. Both of them have available water, silicate rock, and metals that could be mined and used for construction.<ref name=Kerwick/>

Although Io's volcanism and tidal heating constitute valuable resources, exploiting them is probably impractical.<ref name=Kerwick>{{cite journal |last1=Kerwick |first1=Thomas B. |date=2012 |title=Colonizing Jupiter's Moons: An Assessment of Our Options and Alternatives |url=https://www.jstor.org/stable/24536505 |journal=Journal of the Washington Academy of Sciences |volume=98 |issue=4 |pages=15–26 |jstor=24536505 |access-date=1 August 2021}}</ref> Europa is rich in water (its subsurface ocean is expected to contain over twice as much water as all Earth's oceans together)<ref name=UTJupiter/> and likely oxygen, but metals and minerals would have to be imported. If alien microbial life exists on Europa, human immune systems may not protect against it. Sufficient radiation shielding might, however, make Europa an interesting location for a research base.<ref name=Kerwick/> The private ''[[Artemis Project]]'' drafted a plan in 1997 to colonize Europa, involving surface igloos as bases to drill down into the ice and explore the ocean underneath, and suggesting that humans could live in "air pockets" in the ice layer.<ref>[http://www.asi.org/ Artemis Society International], {{Webarchive|url=https://web.archive.org/web/20110820180833/http://asi.org/|date=20 August 2011}} official website.</ref><ref>{{cite journal|first1=Peter |last1=Kokh |first2=Mark |last2=Kaehny |first3=Doug |last3=Armstrong |first4=Ken |last4=Burnside |url=http://asi.org/adb/06/09/03/02/110/europa2-wkshp.html |title=Europa II Workshop Report |archive-url=https://web.archive.org/web/20190607102248/http://asi.org/adb/06/09/03/02/110/europa2-wkshp.html|archive-date=7 June 2019 |journal=Moon Miner's Manifesto |volume=110 |date=November 1997}}</ref><ref name=UTJupiter>{{cite web |url=https://www.universetoday.com/130637/colonize-jupiters-moons/ |title=How do we Colonize Jupiter's Moons? |last=Williams |first=Matt |date=23 November 2016 |website=[[Universe Today]] |access-date=10 January 2022}}</ref> Ganymede<ref name=UTJupiter/> and Callisto are also expected to have internal oceans.<ref name='OW Roadmap 2019'>{{cite journal | last1 = Hendrix | first1 = Amanda R. | last2 = Hurford | first2 = Terry A. | last3 = Barge | first3 = Laura M. | last4 = Bland | first4 = Michael T. | last5 = Bowman | first5 = Jeff S. | last6 = Brinckerhoff | first6 = William | last7 = Buratti | first7 = Bonnie J. | last8 = Cable | first8 = Morgan L. | last9 = Castillo-Rogez | first9 = Julie | last10 = Collins | first10 = Geoffrey C. | display-authors = etal | year = 2019| title = The NASA Roadmap to Ocean Worlds | journal = Astrobiology | volume = 19| issue = 1 | pages = 1–27| doi = 10.1089/ast.2018.1955 | pmid = 30346215 | pmc = 6338575 | bibcode = 2019AsBio..19....1H | doi-access = free }}</ref> It might be possible to build a surface base that would produce fuel for further exploration of the Solar System.

In 2003, NASA performed a study called ''HOPE'' (Revolutionary Concepts for Human Outer Planet Exploration) regarding the future exploration of the Solar System.<ref>{{cite report|first1=Patrick A. |last1=Troutman |first2=Kristen |last2=Bethke |first3=Frederic H. |last3=Stillwagen |first4=Darrell L. |last4=Caldwell, Jr |first5=Ram |last5=Manvi |first6=Chris |last6=Strickland |first7=Shawn A. |last7=Krizan |url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20030063128.pdf |title=Revolutionary Concepts for Human Outer Planet Exploration (HOPE) |archive-url=https://web.archive.org/web/20170815051016/https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20030063128.pdf|archive-date=15 August 2017 |date=28 January 2003 |access-date= 19 April 2025|publisher=NASA Langley Research Center}}</ref> The target chosen was [[Callisto (moon)|Callisto]] due to its distance from Jupiter, and thus the planet's harmful radiation. It could be possible to build a surface base that would produce fuel for further exploration of the Solar System.<ref>{{cite book|last=Seedhouse|first=Erik|title=Interplanetary Outpost: The Human and Technological Challenges of Exploring the Outer Planets|year=2012 |url=https://books.google.com/books?id=Cf3N1ejDNO4C|publisher=Springer |location=Berlin |isbn=978-1-4419-9747-0|access-date=19 April 2025}}</ref>{{rp|21}} HOPE estimated a round trip time for a crewed mission of about 2–5 years, assuming significant progress in propulsion technologies.<ref name=Kerwick/>

[[Io (moon)|Io]] is not ideal for colonization, due to its hostile environment. The moon is under influence of high tidal forces, causing high volcanic activity. Jupiter's strong radiation belt overshadows Io, delivering 36 Sv a day to the moon. The moon is also extremely dry. Io is the least ideal place for the colonization of the four Galilean moons.
Despite this, its volcanoes could be energy resources for the other moons, which are better suited to colonization.

[[File:Currents in Jovian Magnetosphere.png|thumb|upright=2|The magnetic field of Jupiter and co-rotation rotation enforcing currents]]

[[Ganymede (moon)|Ganymede]] is the largest moon in the Solar System. Ganymede is the only moon with a [[magnetosphere]], albeit overshadowed by [[Magnetosphere of Jupiter|Jupiter's magnetic field]]. Because of this magnetic field, Ganymede is one of only two Jovian moons where surface settlements would be feasible because it receives about 0.08 [[Sievert|Sv]] of radiation per day. Ganymede could be terraformed.<ref name="ringwald"/>

The [[Keck Observatory]] announced in 2006 that the binary [[Jupiter trojan]] [[617 Patroclus]], and possibly many other Jupiter trojans, are likely composed of water ice, with a layer of dust. This suggests that mining water and other volatiles in this region and transporting them elsewhere in the Solar System, perhaps via the proposed [[Interplanetary Transport Network]], may be feasible in the not-so-distant future. This could make [[colonization of the Moon]], [[Colonization of Mercury|Mercury]] and main-belt [[Colonization of the asteroids|asteroids]] more practical.

====Saturn====
Saturn's radiation belt is much weaker than Jupiter's, so radiation is less of an issue here. Dione, Rhea, Titan, and Iapetus all orbit outside the radiation belt, and Titan's thick atmosphere would adequately shield against cosmic radiation.<ref name=UTSaturn/>

Saturn has seven moons [[planetary-mass moon|large enough to be round]]: in order of increasing distance from Saturn, they are [[Mimas (moon)|Mimas]], [[Enceladus]], [[Tethys (moon)|Tethys]], [[Dione (moon)|Dione]], [[Rhea (moon)|Rhea]], [[Titan (moon)|Titan]], and [[Iapetus (moon)|Iapetus]].

=====Enceladus=====
The small moon Enceladus is also of interest, having a subsurface ocean that is separated from the surface by only tens of meters of ice at the south pole, compared to kilometers of ice separating the ocean from the surface on Europa. Volatile and organic compounds are present there, and the moon's high density for an ice world (1.6&nbsp;g/cm<sup>3</sup>) indicates that its core is rich in silicates.<ref name=UTSaturn>{{cite web |url=https://www.universetoday.com/132413/colonize-saturns-moons/ |title=How do we Colonize Saturns' Moons |last=Williams |first=Matt |date=22 December 2016 |website=[[Universe Today]] |access-date=22 August 2021}}</ref>

On 9 March 2006, [[NASA]]'s ''[[Cassini–Huygens|Cassini]]'' space probe found possible evidence of liquid water on [[Enceladus]].<ref>{{cite news|url=https://spacenews.com/nasas-cassini-discovers-potential-liquid-water-on-enceladus/ |title=NASA's Cassini Discovers Potential Liquid Water on Enceladus |publisher=Space News|date=9 March 2006 |access-date=16 April 2025}}</ref> According to that article, "pockets of liquid water may be no more than tens of meters below the surface." These findings were confirmed in 2014 by NASA. This means liquid water could be collected much more easily and safely on Enceladus than, for instance, on Europa (see above). Discovery of water, especially liquid water, generally makes a celestial body a much more likely candidate for colonization. An alternative model of Enceladus's activity is the decomposition of methane/water [[clathrate]]s – a process requiring lower temperatures than liquid water eruptions. The higher density of Enceladus indicates a larger than Saturnian average silicate core that could provide materials for base operations.

=====Titan=====
{{Main|Colonization of Titan}}
Authors like [[Robert Zubrin]] have offered that Saturn is the most important and valuable of the four [[gas giant]]s in the [[Solar System]], because of its relative proximity, low radiation, and excellent system of moons. He named Titan as the best candidate on which to establish a base to exploit the resources of the Saturn system.<ref name="zubrin1999"/>{{rp|161-163}} He pointed out that Titan possesses an abundance of all the elements necessary to support life, saying "In certain ways, Titan is the most hospitable extraterrestrial world within our solar system for human colonization."<ref name="zubrin1999"/>{{rp|163-166}}

To consider a colony on [[Saturn]]'s largest moon [[Titan (moon)|Titan]], protection against the extreme cold must be a primary consideration.<ref>{{cite web|title=Mars vs. Titan: A Showdown of Human Habitability |first=Kasha |last=Patel  |date=1 October 2018 | url=https://www.acs.org/education/chemmatters/past-issues/2018-2019/october2018/mars-vs-titan.html |website=acs.org |access-date=20 April 2025}}</ref> Titan offers a gravity of approximately 1/7 of Earth gravity, in the same range as Earth's Moon. Atmospheric pressure at the surface of the planet is about 1.5x that of the surface of the Earth; there is however, no oxygen present in the environment. The atmosphere is about 95% nitrogen and 5% methane.<ref>{{cite web|title=Titan: Facts|url=https://science.nasa.gov/saturn/moons/titan/facts/ |website=NASA|date=2 May 2018 |access-date=21 April 2025}}</ref> Some estimates suggest that abundant energy resources on Titan could power a colony with a population size of the United States.<ref>{{cite web |title=Titan Has Enough Energy to Power a Colony The Size of The US |url=https://www.sciencealert.com/titan-has-enough-energy-to-power-a-colony-the-size-of-the-us |date=10 July 2017 |first=David |last=Nield |website=sciencealert.com|access-date=20 April 2025}}</ref>

The dense atmosphere of Titan shields the surface from radiation and would make any structural failures problematic, rather than catastrophic. With an oxygen mask and thermal clothing protection, humans could roam Titan's surface in the dim sunlight. Or, given the low gravity and dense atmosphere, they could float above it in a balloon or on personal wings.<ref>{{cite web|title=Forget Mars—let's go colonize Titan! |first=John |last=Timmer – |date= 13 May 2017 |url=https://arstechnica.com/science/2017/05/forget-mars-lets-go-colonize-titan/ |website=arstechnica.com|access-date=26 April 2025}}</ref><ref>{{cite web|title=The tech we need to build a colony on Titan |first=Jamie |last=Carter |date=28 August 2017 |url=https://www.techradar.com/news/the-tech-we-need-to-build-a-colony-on-titan |website=techradar.com|access-date=21 April 2025}}</ref>

===Trans-Neptunian region===
{{main|Colonization of trans-Neptunian objects}}

{{Excerpt|Colonization of trans-Neptunian objects|paragraph=1}}

===Beyond the Solar System===
{{Further|Interstellar travel}}
Beyond the Solar System colonization targets might be identified in the [[List of nearest stars|surrounding stars]]. The main difficulty is the vast distances to other stars.

To reach such targets travel times of millennia would be necessary, with current technology. At average speeds of even 0.1% of the speed of light (''c'') interstellar expansion across the entire [[Milky Way galaxy]] would take up to one-half of the Sun's galactic orbital period of ~240,000,000 years, which is comparable to the timescale of other galactic processes.<ref>{{cite web|url=https://hypertextbook.com/facts/2002/StacyLeong.shtml|work=The Physics Factbook|last=Leong|first=Stacy|year=2002|title=Period of the Sun's Orbit around the Galaxy (Cosmic Year)|access-date=19 April 2025}}</ref> Due to fundamental energy and reaction mass consideration such speeds would be with current technology limited to small spaceships. If humanity would gain access to a large amount of energy, on the order of the mass-energy of entire planets, it may become possible to construct spaceships with [[Alcubierre drive]]s.<ref>{{Cite news |last=Williams |first=Matt |date=1 March 2020 |title=Scientists Are Starting to Take Warp Drives Seriously, Especially This One Concept |work=ScienceAlert.com |url=https://www.sciencealert.com/scientists-are-starting-to-take-warp-drives-seriously-especially-this-one-concept |access-date=19 April 2025}}</ref>

The following are plausible approaches with current technology:
* A [[generation ship]] which would travel much slower than light, with consequent interstellar trip times of many decades or centuries. The crew would go through generations before the journey was complete, so none of the initial crew would be expected to survive to arrive at the destination, assuming current human lifespans.<ref name="hein-pak-putz-revisited">{{cite journal |last1=Hein |first1=Andreas M. |last2=Pak |first2=Mikhail |last3=Pütz |first3=Daniel |last4=Bühler |first4=Christian |last5=Reiss |first5=Philipp |title=World ships—architectures & feasibility revisited |journal=Journal of the British Interplanetary Society |date=2012 |volume=65 |issue=4 |page=119|url=https://www.researchgate.net/publication/236177990 }}</ref>
* A [[sleeper ship]], where most or all of the crew spend the journey in some form of [[hibernation]] or [[suspended animation]], allowing some or all to reach the destination.<ref>{{cite news|title=Sleeper spaceship could carry first humans to Mars in hibernation state
|first=Ben |last=Brumfield |date=7 October 2014 |url=https://www.cnn.com/2014/10/07/tech/innovation/mars-hibernation-flight/index.html |publisher=CNN|access-date=18 April 2025}}</ref>
* An [[embryo space colonization|embryo-carrying interstellar starship]] (EIS), much smaller than a generation ship or sleeper ship, transporting human [[embryo]]s or DNA in a frozen or dormant state to the destination. (Obvious biological and psychological problems in birthing, raising, and educating such voyagers, neglected here, may not be fundamental.)<ref name="Crowl, 2012">{{cite journal|last=Crowl|first=Adam|title=Embryo Space Colonisation to Overcome the Interstellar Time Distance Bottleneck|url=http://www.jbis.org.uk/paper.php?p=2012.65.283|journal=Journal of the British Interplnanetary Society, 65, 283-285, 2012|volume=65 |display-authors=etal}}</ref>
* A [[nuclear fusion]] or [[nuclear fission|fission]] powered ship (e.g. [[ion drive]]) of some kind, achieving velocities of up to perhaps 10% ''c''&nbsp; permitting one-way trips to nearby stars with durations comparable to a human lifetime.<ref>{{cite news|title=Nuclear Pulse Propulsion: Gateway to the Stars|date=27 March 2013 |first=Stan |last=Tackett |url=https://www.ans.org/news/article-1294/nuclear-pulse-propulsion-gateway-to-the-stars/ |publisher=American Nuclear Society|access-date=19 April 2025}}</ref>
* A [[Project Orion (nuclear propulsion)|Project Orion]]-ship, a nuclear-powered concept proposed by [[Freeman Dyson]] which would use [[nuclear bomb|nuclear explosions]] to propel a starship. A special case of the preceding nuclear rocket concepts, with similar potential velocity capability, but possibly easier technology.<ref>{{Cite web |title=The Case For Orion |url=https://www.spacedaily.com/news/nuclearspace-03h.html |access-date=2023-08-03 |website=www.spacedaily.com |archive-date=August 3, 2023 |archive-url=https://web.archive.org/web/20230803065646/https://www.spacedaily.com/news/nuclearspace-03h.html |url-status=live }}</ref>
* [[Solar sail|Laser propulsion]] concepts, using some form of beaming of power from the Solar System might allow a [[solar sail|light-sail]] or other ship to reach high speeds, comparable to those theoretically attainable by the fusion-powered electric rocket, above.<ref>{{cite journal|title=Roundtrip interstellar travel using laser-pushed lightsails |first=Robert L. |last=Forward |date=April 1984 |url=https://doi.org/10.2514/3.8632 |journal=Journal of Spacecraft|volume=21|number=2|pages=187–195 |doi=10.2514/3.8632 |access-date=19 April 2025}}</ref> These methods would need some means, such as supplementary nuclear propulsion, to stop at the destination, but a hybrid (light-sail for acceleration, fusion-electric for deceleration) system might be possible.
* [[Mind uploading|Uploaded human minds]] or [[artificial intelligence]] may be transmitted via radio or laser at light speed to interstellar destinations where [[self-replicating spacecraft]] have traveled subluminally and set up infrastructure and possibly also brought some minds. [[Extraterrestrial intelligence]] might be another viable destination.<ref>{{cite web|title= Uploaded e-crews for interstellar missions |date= 12 December 2012 |url= https://www.thekurzweillibrary.com/uploaded-e-crews-for-interstellar-missions |website= kurzweilai.net| access-date= 19 April 2025}}</ref>

====Intergalactic travel====
{{Main|Intergalactic travel}} 
[[File:Interstellar vessel of Island One size, ion-driven, Don Davis, 1977.gif|thumb|upright=1.2|Proposed interstellar vessel based on [[Gerard K. O'Neill]]'s Island One version of [[Bernal sphere]] space habitat]]
The distances between galaxies are on the order of a million times farther than those between the stars, and thus intergalactic colonization would involve voyages of millions of years via special self-sustaining methods.<ref name="burruss">{{cite journal | first1 = Robert Page | last1 = Burruss | first2= J. | last2= Colwell | title = Intergalactic Travel: The Long Voyage From Home | journal = The Futurist | date=September–October 1987 | volume= 21 | issue= 5 | pages = 29–33}}</ref><ref>{{cite journal | author= Fogg, Martyn | title= The Feasibility of Intergalactic Colonisation and its Relevance to SETI | journal= Journal of the British Interplanetary Society | volume= 41 | number= 11 | date= November 1988 | pages= 491–496 | url= https://www.academia.edu/4166742 | bibcode= 1988JBIS...41..491F }}</ref><ref>{{cite journal | author= Armstrong, Stuart | author2= Sandberg, Anders | url= http://www.fhi.ox.ac.uk/intergalactic-spreading.pdf | title=Eternity in six hours: intergalactic spreading of intelligent life and sharpening the Fermi paradox | journal= Acta Astronautica | year= 2013 | volume= 89 | pages= 1–13 | publisher= Future of Humanity Institute, Philosophy Department, Oxford University| doi= 10.1016/j.actaastro.2013.04.002 | bibcode= 2013AcAau..89....1A }}</ref>