Editing Space colonization (section)

===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>