View source for Phobos (moon)

{{Short description|Larger of the two moons of Mars}}
{{redirect|Mars I|the Soviet Mars probe|Mars 1|other uses|Mars 1 (disambiguation)}}
{{Use dmy dates|date=August 2024}}
{{Use British English|date=October 2024}}
{{Infobox planet
| name = Phobos
| symbol = [[File:Phobos symbol (bold).svg|24px]] (rare)
| mpc_name = Mars I
| image = Phobos colour 2008.jpg
| caption = A false color image of Phobos, as captured by the ''[[Mars Reconnaissance Orbiter]]'' in 2008 
| bgcolour =
| discoverer = [[Asaph Hall]]
| discovered = 18 August 1877
| pronounced = {{IPAc-en|ˈ|f|oʊ|b|ɒ|s}}<ref>{{Cite encyclopedia |url=http://www.lexico.com/definition/Phobos |archive-url=https://web.archive.org/web/20200302163117/https://www.lexico.com/definition/phobos |archive-date=2 March 2020 |title=Phobos |dictionary=[[Lexico]] UK English Dictionary |publisher=[[Oxford University Press]]}}</ref> or {{IPAc-en|ˈ|f|oʊ|b|ə|s}}<ref>{{Cite web | url=http://planetary-science.org/mars-research/moons-of-mars/ | title=Moons of Mars – the Center for Planetary Science |archive-url=https://web.archive.org/web/20230425153640/http://planetary-science.org/mars-research/moons-of-mars/ |archive-date=25 April 2023}}</ref>
| adjectives = Phobian<ref>Harry Shipman (2013) ''Humans in Space: 21st Century Frontiers'', p. 317</ref> {{IPAc-en|ˈ|f|oʊ|b|i|ə|n}}<ref>''The Century Dictionary and Cyclopedia'' (1914)</ref>
| named_after = [[Phobos (mythology)|Φόβος]]
| epoch = [[J2000]]
| semimajor = {{val|9,376|u=km}}<ref name="NASA">{{cite web |date=30 September 2003 |title=Mars: Moons: Phobos |publisher=NASA Solar System Exploration |url=http://solarsystem.nasa.gov/planets/profile.cfm?Object=Mar_Phobos&Display=Facts |access-date=2 December 2013 |archive-url=https://web.archive.org/web/20131019162634/http://solarsystem.nasa.gov/planets/profile.cfm?Object=Mar_Phobos&Display=Facts |archive-date=19 October 2013 }}</ref> (2.76 Mars radii/1.472 Earth radii)
| eccentricity = {{val|0.0151}}<ref name="NASA" />
| periapsis = {{val|9,234.42|u=km}}<ref name="NASA" />
| apoapsis = {{val|9,517.58|u=km}}<ref name="NASA" />
| period = {{val|0.31891023|ul=d}}<br/>(7 h 39 m 12 s)<ref>{{cite web |url=https://sci.esa.int/web/mars-express/-/31031-phobos |title=ESA Science and Technology - Martian moons: Phobos |author=<!--Not stated--> |website=ESA |access-date=5 July 2023 }}</ref>
| avg_speed = 2.138 km/s<ref name="NASA" />
| inclination = 1.093° (to Mars's equator)<br/>0.046° (to local [[Laplace plane]])<br/>26.04° (to the [[ecliptic]])
| satellite_of = [[Mars]]
| mean_radius = {{val|11.08|0.04|u=km}}<ref name="Ernst2023"/>
| dimensions = {{val|25.90|x|22.60|x|18.32|u=km}}<br/>(±&nbsp;{{val|0.08|x|0.08|x|0.06|u=km}})<ref name="Ernst2023"/>
| surface_area = {{val|1640|8|u=km2}}<ref name="Ernst2023"/>
| volume = {{val|5695|32|u=km3}}<ref name="Ernst2023"/>
| mass = {{val|1.060|e=16|u=kg}}{{refn|Pätzold et al. (2014), as cited in Ernst et al. (2023).<ref name="Ernst2023"/>}}
| density = {{val|1.861|0.011|u=g/cm3}}<ref name="Ernst2023"/>
| surface_grav = {{val|0.0057|u=m/s2}}<ref name="NASA" /><br/>(581.4 [[Micro-|μ]] [[G-force|g]])
| escape_velocity = 11.39 [[Metre per second|m/s]]<br/>(41 km/h)<ref name="NASA" />
| rotation = [[synchronous rotation|Synchronous]]
| rot_velocity = {{Convert|11.0|km/h|abbr=on}} (at longest axis)
| axial_tilt = 0°
| albedo = 0.071 ± 0.012 at 0.54&nbsp;μm<ref name="LPS">{{cite conference |url=https://www.lpi.usra.edu/meetings/LPSC98/pdf/1768.pdf |first=Beth Ellen |last=Clark |conference=29th Lunar and Planetary Science Conference |conference-url=https://www.lpi.usra.edu/meetings/LPSC98/ |publisher=[[Lunar and Planetary Institute]] |title=Near Photometry of C-Type Asteroid 253 Mathilde |date=March 1998 |access-date=27 April 2023 }}</ref>
| magnitude = 11.8<ref>{{Cite web | url=http://jtg.sjrdesign.net/advanced/mars_moons.html | title=Mars' Moons}}</ref>
| single_temperature = ≈ 233 [[kelvin|K]]
}}

'''Phobos''' ({{IPAc-en|ˈ|f|oʊ|b|ə|s}}) is the innermost and larger of the two [[moons of Mars|natural satellites of Mars]], the other being [[Deimos (moon)|Deimos]]. The two moons were discovered in 1877 by American astronomer [[Asaph Hall]]. Phobos is named after the [[Phobos (mythology)|Greek god of fear and panic]], who is the son of [[Ares]] (Mars) and twin brother of [[Deimos (deity)|Deimos]].

Phobos is a small, irregularly shaped object with a [[mean radius]] of {{convert|11|km|sigfig=1|abbr=on}}. It orbits {{convert|6000|km|mi|abbr=on}} from the Martian surface, closer to its [[Primary (astronomy)|primary body]] than any other known [[natural satellite]] to a planet. It orbits [[Mars]] much faster than Mars rotates and completes an orbit in just 7 hours and 39 minutes. As a result, from the surface of Mars it appears to rise in the west, move across the sky in 4 hours and 15 minutes or less, and set in the east, twice each [[Mars sol|Martian day]]. Phobos is one of the least reflective bodies in the [[Solar System]], with an [[albedo]] of 0.071. Surface temperatures range from about {{convert|−4|C|F}} on the sunlit side to {{convert|−112|C|F}} on the shadowed side. The notable surface feature is the large [[impact crater]] [[Stickney (crater)|Stickney]], which takes up a substantial proportion of the moon's surface. The surface is also marked by many grooves, and there are numerous theories as to how these grooves were formed.

Images and models indicate that Phobos may be a [[rubble pile]] held together by a thin [[Crust (geology)|crust]] that is being torn apart by [[Tidal acceleration|tidal]] interactions. Phobos gets closer to Mars by about {{convert|2|cm}} per year.

== Discovery and etymology ==
{{Main|Moons of Mars#History|l1=History of the moons of Mars}}
Phobos was discovered by the American astronomer [[Asaph Hall]] on 18 August 1877 at the [[United States Naval Observatory]] in [[Washington, D.C.]], at about 09:14 [[Greenwich Mean Time]]. (Contemporary sources, using the pre-1925 [[astronomical day|astronomical convention]] that began the day at noon,<ref name='Campbell "Beginning"'>{{cite journal|last=Campbell|first=W.W.|title=The Beginning of the Astronomical Day |journal=Publications of the Astronomical Society of the Pacific |volume=30 |issue=178 |page=358 |bibcode=1918PASP...30..358C |year=1918 |doi=10.1086/122784|doi-access=free}}</ref> give the time of discovery as 17 August at 16:06 [[Washington Mean Time]], meaning 18 August 04:06 in the modern convention.)<ref>{{Cite journal |url=http://adsabs.harvard.edu//full/seri/Obs../0001//0000181.000.html |title=Notes: The Satellites of Mars |journal=The Observatory |volume=1 |issue=6 |date=20 September 1877 |pages=181–185 |access-date=4 February 2009 |bibcode = 1877Obs.....1..181. }}</ref><ref>{{Cite journal |url=http://adsabs.harvard.edu//full/seri/AN.../0091//0000013.000.html |title=Observations of the Satellites of Mars |last=Hall |first=Asaph |journal=Astronomische Nachrichten |volume=91 |issue=2161 |pages=11/12–13/14 |date=17 October 1877 |type=Signed 21 September 1877 |bibcode = 1877AN.....91...11H |doi=10.1002/asna.18780910103 }}</ref><ref name="Morley1989">{{Cite journal |last=Morley |first=Trevor A. |url=http://adsabs.harvard.edu//full/seri/A+AS./0077//0000220.000.html |title=A Catalogue of Ground-Based Astrometric Observations of the Martian Satellites, 1877–1982 |journal=Astronomy and Astrophysics Supplement Series |volume=77 |issue=2 |date=February 1989 |pages=209–226 |bibcode=1989A&AS...77..209M }} (Table II, p. 220: first observation of Phobos on 18 August 1877.38498)</ref> Hall had discovered [[Deimos (moon)|Deimos]], Mars's other moon, a few days earlier.<ref>{{Citation| title    =Mars Moons: Facts| date =15 November 2017| publisher =NASA| url =https://science.nasa.gov/mars/moons/facts/| archive-url  =https://web.archive.org/web/20231128210959/https://science.nasa.gov/mars/moons/facts/| archive-date =28 November 2023}}</ref>
The discoveries were made using the world's largest [[refracting telescope]], the 26-inch "Great Equatorial".<ref name="Usn">{{cite web |title=Descendants of Asaph Hall Present Memorabilia to USNO: Asaph Hall and the Moons of Mars |url=http://www.usno.navy.mil/hallmedal.html |publisher=USNO Public Affairs Office |access-date=15 October 2024 |archive-url=https://web.archive.org/web/20081204114657/http://www.usno.navy.mil/hallmedal.html |archive-date=4 December 2008 |date=2000}}</ref>

The names, originally spelled ''Phobus'' and ''Deimus'' respectively, were suggested by the British academic [[Henry George Madan|Henry Madan]], a science master at [[Eton College]], who based them on [[Greek mythology]], in which [[Phobos (mythology)|Phobos]] is a companion to the god, [[Ares]].<ref>{{Cite journal |last=Madan |first=Henry George |url=https://books.google.com/books?id=fC4CAAAAYAAJ&pg=RA4-PA475 |journal=Nature |volume=16 |issue=414 |title=Letters to the Editor: The Satellites of Mars |date=4 October 1877 |type=Signed 29 September 1877 |page=475 |doi=10.1038/016475b0 |bibcode=1877Natur..16R.475M |s2cid=3998209 |doi-access=free }}</ref><ref>{{Cite journal |last=Hall |first=Asaph |url=http://adsabs.harvard.edu//full/seri/AN.../0092//0000031.000.html |journal=Astronomische Nachrichten |volume=92 |issue=2187 |title=Names of the Satellites of Mars |date=14 March 1878 |type=Signed 7 February 1878 |pages=47–48 |doi=10.1002/asna.18780920304 |bibcode=1878AN.....92...47H }}</ref>

Planetary moons other than Earth's were never given symbols in the astronomical literature. Denis Moskowitz, a software engineer who designed most of the [[dwarf planet]] symbols, proposed a Greek [[phi]] (the initial of Phobos) combined with Mars's spear as the symbol of Phobos ([[File:Phobos symbol (fixed width).svg|16px]]). This symbol is not widely used.<ref name=moons>{{cite web |url=https://www.unicode.org/L2/L2025/25079-phobos-and-deimos.pdf |title=Phobos and Deimos symbols |last1=Bala |first1=Gavin Jared |last2=Miller |first2=Kirk |date=7 March 2025 |website=unicode.org |publisher=The Unicode Consortium |access-date=14 March 2025 |quote=}}</ref>

== Physical characteristics ==
[[File:PIA17351-ApparentSizes-MarsDeimosPhobos-EarthMoon.jpg|thumb|Deimos and Phobos as seen from Mars, compared in [[angular size|apparent size]]  to the [[Moon]] as seen from Earth.]]
Phobos has dimensions of {{convert|26 × 23 × 18|km}},<ref name="Ernst2023"/> and retains too little mass to be rounded under its own gravity. Phobos does not have an [[atmosphere]] due to its low mass and low gravity.<ref>{{cite web |url=http://solarsystem.nasa.gov/planets/profile.cfm?Object=Mar_Phobos |title=Solar System Exploration: Planets: Mars: Moons: Phobos: Overview |publisher=Solarsystem.nasa.gov |access-date=19 August 2013 |archive-url=https://web.archive.org/web/20140624191709/https://solarsystem.nasa.gov/planets/profile.cfm?Object=Mar_Phobos |archive-date=24 June 2014 }}</ref> It is one of the least reflective bodies in the Solar System, with an albedo of about 0.071.<ref name="jplssd">{{cite web |url=http://ssd.jpl.nasa.gov/?sat_phys_par |publisher=[[JPL]] (Solar System Dynamics) |title=Planetary Satellite Physical Parameters |date=13 July 2006 |access-date=29 January 2008 }}</ref> Infrared spectra show that it has carbon-rich material such as that found in [[carbonaceous chondrites]], and its composition shows similarities to that of Mars's surface.<ref>{{Cite journal |last1=Citron |first1=R. I. |last2=Genda |first2=H. |last3=Ida |first3=S. |date=2015 |title=Formation of Phobos and Deimos via a giant impact |url= |journal=Icarus |language=en |volume=252 |pages=334–338 |arxiv=1503.05623 |doi=10.1016/j.icarus.2015.02.011|bibcode=2015Icar..252..334C }}</ref> Phobos's density is too low to be solid rock, and it is known to have significant [[porosity]].<ref>{{cite web |title=Porosity of Small Bodies and a {{sic|nolink=y|reason=error in source|Reassesment}} of Ida's Density |url=http://www.aas.org/publications/baas/v31n4/dps99/65.htm |quote=When the error bars are taken into account, only one of these, Phobos, has a porosity below 0.2... |archive-url=https://web.archive.org/web/20070926224539/http://www.aas.org/publications/baas/v31n4/dps99/65.htm |archive-date=26 September 2007 }}</ref><ref name="Close Inspection for Phobos">{{cite web |title=Close Inspection for Phobos |url=http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=31031 |quote=It is light, with a density less than twice that of water, and orbits just {{Convert|5989|km|sp=us}} above the Martian surface. }}</ref><ref>{{Cite journal |last1=Busch |first1=Michael W. |display-authors=4 |last2=Ostro |first2=Steven J. |last3=Benner |first3=Lance A. M. |last4=Giorgini |first4=Jon D. |last5=Magri |first5=Christopher |last6=Howell |first6=Ellen S. |last7=Nolan |first7=Michael C. |last8=Hine |first8=Alice A. |last9=Campbell |first9=Donald B. |last10=Shapiro |first10=Irwin I. |last11=Chandler |first11=John F. |date=2007 |title=Arecibo Radar Observations of Phobos and Deimos |journal=Icarus |volume=186 |issue=2 |pages=581–584 |doi=10.1016/j.icarus.2006.11.003 |bibcode=2007Icar..186..581B }}</ref> These results led to the suggestion that Phobos might contain a substantial reservoir of ice. Spectral observations indicate that the surface [[regolith]] layer lacks hydration,<ref>{{Cite journal |last1=Murchie |first1=Scott L. |display-authors=4 |last2=Erard |first2=Stephane |last3=Langevin |first3=Yves |last4=Britt |first4=Daniel T. |last5=Bibring |first5=Jean-Pierre |last6=Mustard |first6=John F. |last7=Head |first7=James W. |last8=Pieters |first8=Carle M. |title=Disk-resolved Spectral Reflectance Properties of Phobos from 0.3–3.2 microns: Preliminary Integrated Results from PhobosH 2 |journal=Abstracts of the Lunar and Planetary Science Conference |volume=22 |page=943 |date=1991 |bibcode=1991pggp.rept..249M }}</ref><ref>{{Cite journal |last1=Rivkin |first1=Andrew S. |display-authors=4 |last2=Brown |first2=Robert H. |last3=Trilling |first3=David E. |last4=Bell III |first4=James F. |last5=Plassmann |first5=Joseph (Joe) H. |date=March 2002 |title=Near-Infrared Spectrophotometry of Phobos and Deimos |journal=Icarus |volume=156 |issue=1 |pages=64–75 |bibcode=2002Icar..156...64R |doi=10.1006/icar.2001.6767 }}</ref> but ice below the regolith is not ruled out.<ref>{{cite journal |last1=Fanale |first1=Fraser P. |first2=James R. |last2=Salvail |date=1989 |title=Loss of water from Phobos |journal=Geophys. Res. Lett. |volume=16 |number=4 |pages=287–290 |doi=10.1029/GL016i004p00287 |bibcode=1989GeoRL..16..287F }}</ref><ref>{{cite journal |title=Evolution of the water regime of Phobos |first1=Fraser P. |last1=Fanale |first2=James R. |last2=Salvail |doi = 10.1016/0019-1035(90)90089-R |date=Dec 1990 |journal=Icarus |volume=88 |issue=2 |pages=380–395 |bibcode=1990Icar...88..380F }}</ref> Surface temperatures range from about {{convert|−4|C|F}} on the sunlit side to {{convert|−112|C|F}} on the shadowed side.<ref name="phobos">{{cite web |title=NASA – Phobos |url=http://solarsystem.nasa.gov/planets/profile.cfm?Object=Mar_Phobos |archive-url=https://web.archive.org/web/20140624191709/https://solarsystem.nasa.gov/planets/profile.cfm?Object=Mar_Phobos |archive-date=24 June 2014 |access-date=4 August 2014 |publisher=Solarsystem.nasa.gov}}</ref>

Unlike Deimos, Phobos is heavily cratered,<ref>{{cite web |url=http://www.bbc.co.uk/science/space/solarsystem/mars/phobos.shtml |title=Phobos |work=[[BBC Online]] |archive-url=https://web.archive.org/web/20090422160500/http://www.bbc.co.uk/science/space/solarsystem/mars/phobos.shtml |archive-date=22 April 2009 |date=12 January 2004 |access-date=19 July 2021 }}</ref> with one of the craters near the equator having a central peak despite the moon's small size.<ref>{{cite magazine |url=https://books.google.com/books?id=Q6tkzO06q4oC&pg=PA158 |title=Viking looks at Phobos in detail |magazine=[[New Scientist]] |volume=72 |issue=1023 |page=158 |publisher=[[Reed Business Information]] |date=21 October 1976 |access-date=19 July 2021 |id={{ISSN|0262-4079}} |via=[[Google Books]]}}</ref> The most prominent of these is the crater [[Stickney (crater)|Stickney]], an impact crater {{Convert|9|km|mi|abbr=on}} in diameter, which takes up a substantial proportion of the moon's surface area. As with the [[Saturn]]ian moon [[Mimas (moon)|Mimas]]'s crater [[Herschel (Mimantean crater)|Herschel]], the impact that created Stickney probably almost shattered Phobos.<ref>{{cite web |title=Stickney Crater-Phobos |url=http://www.solarviews.com/cap/mars/phobos2.htm |quote=One of the most striking features of Phobos, aside from its irregular shape, is its giant crater Stickney. Because Phobos is only {{Convert|28|by|20|km||sp=us}}, it must have been nearly shattered from the force of the impact that caused the giant crater. Grooves that extend across the surface from Stickney appear to be surface fractures caused by the impact. }}</ref>

{{multiple image
| direction         = vertical
| align             = 
| width1            = 
| width2            = 
| image1            = Stickney mro.jpg
| image2            = USGS-Phobos-MarsMoon-Map.png
| footer            = (''top'') False color image of the impact crater [[Stickney (crater)|Stickney]] imaged by the ''[[Mars Reconnaissance Orbiter]]'' in March 2008; (''bottom'') Labeled Map of Phobos – Moon of Mars (U.S. Geological Survey)
| total_width       = 220
}}
Many grooves and streaks cover the oddly shaped surface. The grooves are typically less than {{Convert|30|m|sp=us}} deep, {{Convert|100|to|200|m|sp=us}} wide, and up to {{Convert|20|km|sp=us}} in length, and were originally assumed to have been the result of the same impact that created Stickney. Analysis of results from the ''[[Mars Express]]'' spacecraft revealed that the grooves are not radial to Stickney, but are centered on the leading apex of Phobos in its orbit (which is not far from Stickney). Researchers suspected that they had been excavated by material ejected into space by impacts on the surface of Mars. The grooves thus formed as [[crater chain]]s, and all of them fade away as the trailing apex of Phobos is approached. They have been grouped into 12 or more families of varying age, presumably representing at least 12 Martian impact events.<ref name="grooves origin">{{cite web |title=New Evidence on the Origin of Phobos' Parallel Grooves from HRSC Mars Express |last1=Murray |first1=John B. |display-authors=4 |first2=John B. |last2=Murray |first3=Jonathan C. |last3=Iliffe |first4=Jan-Peter A. L. |last4=Muller |first5=Gerhard |last5=Neukum |first6=Stephanie |last6=Werner |first7=Matt |last7=Balme |last8=HRSC Co-Investigator Team |publisher=37th Annual Lunar and Planetary Science Conference, March 2006 |url=http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2195.pdf }}</ref> In November 2018, based on computational probability analysis, astronomers concluded that the many grooves on Phobos were caused by boulders ejected from the asteroid impact that created Stickney crater. These boulders rolled in a predictable pattern on the surface of the moon.<ref name="UT-20181120">{{cite web |last=Gough |first=Evan |title=Strange Grooves on Phobos Were Caused by Boulders Rolling Around on its Surface |url=https://www.universetoday.com/140593/strange-grooves-on-phobos-were-caused-by-boulders-rolling-around-on-its-surface/ |date=20 November 2018 |work=[[Universe Today]] |access-date=21 November 2018 }}</ref><ref name="PSS-20181116">{{cite journal |last1=Ramsley |first1=Kenneth R. |last2=Head |first2=James W. |title=Origin of Phobos grooves: Testing the Stickney Crater ejecta model |journal=[[Planetary and Space Science]] |volume=165 |pages=137–147 |doi=10.1016/j.pss.2018.11.004 |bibcode=2019P&SS..165..137R |year=2019 |s2cid=86859432 }}</ref>

Faint dust rings produced by Phobos and Deimos have long been predicted but attempts to observe these rings have, to date, failed.<ref>{{Cite journal |doi=10.1016/j.pss.2006.05.009 |last1=Showalter |first1=Mark R. |last2=Hamilton |first2=Douglas P. |last3=Nicholson |first3=Philip D. |title=A Deep Search for Martian Dust Rings and Inner Moons Using the Hubble Space Telescope |journal=Planetary and Space Science |volume=54 |date=2006 |issue=9–10 |pages=844–854 |url=http://www.astro.umd.edu/~hamilton/research/reprints/ShoHamNic06.pdf |bibcode=2006P&SS...54..844S }}</ref> Images from ''[[Mars Global Surveyor]]'' indicate that Phobos is covered with a layer of fine-grained regolith at least 100 meters thick; it is hypothesized to have been created by impacts from other bodies, but it is not known how the material stuck to an object with almost no gravity.<ref>{{cite web |last=Britt |first=Robert Roy |title=Forgotten Moons: Phobos and Deimos Eat Mars' Dust |url=http://www.space.com/scienceastronomy/forgotten_moons_010313-3.html |publisher=space.com |date=13 March 2001 |access-date=12 May 2010 }}</ref>

The unique [[Kaidun meteorite]] that fell on a [[Soviet Union|Soviet]] military base in [[Yemen]] in 1980 has been hypothesized to be a piece of Phobos, but this could not be verified because little is known about the exact composition of Phobos.<ref>{{Cite journal |last=Ivanov |first=Andrei V. |bibcode=2004SoSyR..38...97I |title=Is the Kaidun Meteorite a Sample from Phobos? |date=March 2004 |journal=Solar System Research |volume=38 |issue=2 |pages=97–107 |doi=10.1023/B:SOLS.0000022821.22821.84 |s2cid=123669722 }}</ref><ref>{{Cite journal |last1=Ivanov |first1=Andrei |first2=Michael |last2=Zolensky |title=The Kaidun Meteorite: Where Did It Come From? |url=http://www.lpi.usra.edu/meetings/lpsc2003/pdf/1236.pdf |journal=Lunar and Planetary Science |date=2003 |volume=34 |quote=The currently available data on the lithologic composition of the Kaidun meteorite– primarily the composition of the main portion of the meteorite, corresponding to CR2 carbonaceous chondrites and the presence of clasts of deeply differentiated rock&nbsp;– provide weighty support for considering the meteorite's parent body to be a carbonaceous chondrite satellite of a large differentiated planet. The only possible candidates in the modern Solar System are Phobos and Deimos, the moons of Mars. }}</ref>

=== Shklovsky's "Hollow Phobos" hypothesis ===
In the late 1950s and 1960s, the unusual orbital characteristics of Phobos led to speculations that it might be hollow.<ref>{{cite web |date=12 July 2017 |title=A Convenient Truth - One Universe at a Time |url=https://archive.briankoberlein.com/2017/07/12/a-convenient-truth/index.html |access-date=14 July 2020}}</ref> Around 1958, Russian astrophysicist [[Iosif Samuilovich Shklovsky]], studying the [[Secular phenomena|secular]] [[acceleration]] of Phobos's orbital motion, suggested a "thin sheet metal" structure for Phobos, a suggestion which led to speculations that Phobos was of artificial origin.<ref name="Shklovsky">Shklovsky, Iosif Samuilovich; ''The Universe, Life, and Mind'', Academy of Sciences USSR, Moscow, 1962</ref> Shklovsky based his analysis on estimates of the upper Martian atmosphere's density, and deduced that for the weak braking effect to be able to account for the secular acceleration, Phobos had to be very light—one calculation yielded a hollow iron sphere {{Convert|16|km|sp=us}} across but less than {{convert|6|cm}} thick.<ref name="Shklovsky" /><ref>{{Cite journal |last=Öpik |first=Ernst Julius |author-link=Ernst Julius Öpik |date=September 1964 |title=Is Phobos Artificial? |journal=Irish Astronomical Journal |volume=6 |pages=281–283 |bibcode=1964IrAJ....6..281.}}</ref> In a February 1960 letter to the journal ''Astronautics'',<ref name="Singer">[[Fred Singer|Singer, S. Fred]]; ''Astronautics'', February 1960</ref> [[Fred Singer]], then science advisor to U.S. President [[Dwight D. Eisenhower]], said of Shklovsky's theory:
<blockquote>If the satellite is indeed spiraling inward as deduced from astronomical observation, then there is little alternative to the hypothesis that it is hollow and therefore Martian made. The big 'if' lies in the astronomical observations; they may well be in error. Since they are based on several independent sets of measurements taken decades apart by different observers with different instruments, systematic errors may have influenced them.<ref name="Singer" /></blockquote>

Subsequently, the systematic data errors that Singer predicted were found to exist, the claim was called into doubt,<ref name="Öpik1963">{{Cite journal |last=Öpik |first=Ernst Julius |date=March 1963 |title=News and Comments: Phobos, Nature of Acceleration |journal=Irish Astronomical Journal |volume=6 |page=40 |bibcode=1963IrAJ....6R..40.}}</ref> and accurate measurements of the orbit available by 1969 showed that the discrepancy did not exist.<ref>{{Citation |last=Singer |first=S. Fred |title=On the Origin of the Martian Satellites Phobos and Deimos |date=1967 |journal=The Moon and the Planets |page=317 |bibcode=1967mopl.conf..317S}}</ref> Singer's critique was justified when earlier studies were discovered to have used an overestimated value of {{convert|5|cm}} per year for the rate of altitude loss, which was later revised to {{convert|1.8|cm}} per year.<ref>Singer, S. Fred; "More on the Moons of Mars", ''Astronautics'', February 1960. [[American Astronautical Society]], page 16</ref> The secular acceleration is now attributed to tidal effects, which create drag on the moon and therefore cause it to spiral inward.<ref name="JGR-Planets">{{cite news |last1=Efroimsky |first1=Michael |last2=Lainey |first2=Valéry |date=29 December 2007 |title=Physics of bodily tides in terrestrial planets and the appropriate scales of dynamical evolution |work=Journal of Geophysical Research—Planets|volume=112|page=E12003 |doi=10.1029/2007JE002908}}</ref>

The density of Phobos has now been directly measured by spacecraft to be {{convert|1.887| g/cm3||abbr=on}}.<ref name="dlr2008">{{cite web |date=16 October 2008 |title=Mars Express closes in on the origin of Mars' larger moon |url=http://www.dlr.de/mars/en/desktopdefault.aspx/tabid-207/422_read-13776/ |access-date=16 October 2008 |publisher=[[German Aerospace Center|DLR]] |archive-date=4 June 2011 |archive-url=https://web.archive.org/web/20110604233019/http://www.dlr.de/mars/en/desktopdefault.aspx/tabid-207/422_read-13776/ }}</ref> Current observations are consistent with Phobos being a [[rubble pile]].<ref name="dlr2008" /> Images obtained by the [[Viking probes|''Viking'' probes]] in the 1970s showed a natural object, not an artificial one. Nevertheless, mapping by the ''Mars Express'' probe and subsequent volume calculations do suggest the presence of voids and indicate that it is not a solid chunk of rock but a porous body.<ref>Clark, Stuart; "Cheap Flights to Phobos" in ''New Scientist'' magazine, 30 January 2010</ref> The porosity of Phobos was calculated to be 30% ± 5%, or a quarter to a third being empty.<ref name="Andert" />

=== Named geological features ===

Geological features on Phobos are named after [[astronomer]]s who studied Phobos and people and places from [[Jonathan Swift]]'s ''[[Gulliver's Travels]]''.<ref>[https://planetarynames.wr.usgs.gov/append6.html Gazetteer of Planetary Nomenclature] [[USGS]] Astrogeology Research Program, Categories</ref>
{{clear right}}

==== {{anchor|Craters on Phobos}} Craters on Phobos ====
Some craters have been named, and are listed in the following map and table.<ref>[https://planetarynames.wr.usgs.gov/jsp/FeatureTypes2.jsp?system=Mars&body=Phobos&systemID=4&bodyID=22&sort=AName&show=Fname&show=Lat&show=Long&show=Diam&show=Stat&show=Orig Gazetteer of Planetary Nomenclature] [[USGS]] Astrogeology Research Program, Craters</ref>
{{PhobosCraterNames}}
{| class="wikitable" 
! [[impact crater|Crater]] !! width=90pt| Coordinates !! [[Diameter]]<br />(km) !! Approval<br />Year !! class="unsortable" | [[Eponym]] !! Ref 
|-id="Clustril"
| Clustril || {{Coord|60|N|91|W|globe:phobos_type:landmark|name=Clustril}} || align=right | 3.4 || align=center | 2006 || Character in [[Lilliput and Blefuscu|Lilliput]] who informed [[Flimnap, the Lilliputian Premier|Flimnap]] that his wife had visited Gulliver privately in Jonathan Swift's novel ''[[Gulliver's Travels]]'' || {{WGPSN|14257}}
|-id="D'Arrest"
| D'Arrest || {{Coord|39|S|179|W|globe:phobos_type:landmark|name=D'Arrest}} || align=right | 2.1 || align=center | 1973 || [[Heinrich Louis d'Arrest]]; German/Danish astronomer (1822–1875) || {{WGPSN|1696}}
|-id="Drunlo"
| Drunlo || {{Coord|36.5|N|92|W|globe:phobos_type:landmark|name=Drunlo}} || align=right | 4.2 || align=center | 2006 || Character in Lilliput who informed Flimnap that his wife had visited Gulliver privately in ''Gulliver's Travels'' || {{WGPSN|14258}}
|-id="Flimnap"
| Flimnap || {{Coord|60|N|10|E|globe:phobos_type:landmark|name=Flimnap}} || align=right | 1.5 || align=center | 2006 || Treasurer of Lilliput in ''Gulliver's Travels'' || {{WGPSN|14259}}
|-id="Grildrig"
| Grildrig || {{Coord|81|N|165|E|globe:phobos_type:landmark|name=Grildrig}} || align=right | 2.6 || align=center | 2006 || Name given to Gulliver by the farmer's daughter [[Glumdalclitch]] in the giants' country [[Brobdingnag]] in ''Gulliver's Travels'' || {{WGPSN|14260}}
|-id="Gulliver"
| Gulliver || {{Coord|62|N|163|W|globe:phobos_type:landmark|name=Gulliver}} || align=right | 5.5 || align=center | 2006 || [[Lemuel Gulliver]]; surgeon captain and voyager in ''Gulliver's Travels'' || {{WGPSN|14261}}
|-id="Hall"
| Hall || {{Coord|80|S|150|E|globe:phobos_type:landmark|name=Hall}} || align=right | 5.4 || align=center | 1973 || [[Asaph Hall]]; American astronomer discoverer of Phobos and Deimos (1829–1907) || {{WGPSN|2328}}
|-id="Limtoc"
| [[Limtoc]] || {{Coord|11|S|54|W|globe:phobos_type:landmark|name=Limtoc}} || align=right | 2 || align=center | 2006 || General in Lilliput who prepared articles of impeachment against Gulliver in ''Gulliver's Travels'' || {{WGPSN|14262}}
|-id="Öpik"
| Öpik || {{Coord|7|S|63|E|globe:phobos_type:landmark|name=Öpik}} || align=right | 2 || align=center | 2011 || [[Ernst J. Öpik]], Estonian astronomer (1893–1985) || {{WGPSN|14865}}
|-id="Reldresal"
| Reldresal || {{Coord|41|N|39|W|globe:phobos_type:landmark|name=Reldresal}} || align=right | 2.9 || align=center | 2006 || Secretary for Private Affairs in Lilliput; Gulliver's friend in ''Gulliver's Travels'' || {{WGPSN|14263}}
|-id="Roche"
| Roche || {{Coord|53|N|177|E|globe:phobos_type:landmark|name=Roche}} || align=right | 2.3 || align=center | 1973 || [[Édouard Roche]]; French astronomer (1820–1883) || {{WGPSN|5167}}
|-id="Sharpless"
| Sharpless || {{Coord|27.5|S|154|W|globe:phobos_type:landmark|name=Sharpless}} || align=right | 1.8 || align=center | 1973 || [[Bevan Sharpless]]; American astronomer (1904–1950) || {{WGPSN|5474}}
|-id="Shklovsky"
| Shklovsky || {{Coord|24|N|112|E|globe:phobos_type:landmark|name=Shklovsky}} || align=right | 2 || align=center | 2011 || [[Iosif Shklovsky]], Soviet astronomer (1916–1985) || {{WGPSN|14866}}
|-id="Skyresh"
| Skyresh || {{Coord|52.5|N|40|E|globe:phobos_type:landmark|name=Skyresh}} || align=right | 1.5 || align=center | 2006 || Skyresh Bolgolam; High Admiral of the Lilliput council who opposed Gulliver's plea for freedom and accused him of being a traitor in ''Gulliver's Travels'' || {{WGPSN|14264}}
|-id="Stickney"
| [[Stickney (crater)|Stickney]] || {{Coord|1|N|49|W|globe:phobos_type:landmark|name=Stickney}} || align=right | 9 || align=center | 1973 || [[Angeline Stickney]] (1830–1892); wife of American astronomer Asaph Hall (above) || {{WGPSN|5707}}
|-id="Todd"
| Todd || {{Coord|9|S|153|W|globe:phobos_type:landmark|name=Todd}} || align=right | 2.6 || align=center | 1973 || [[David Peck Todd]]; American astronomer (1855–1939) || {{WGPSN|6042}}
|-id="Wendell"
| Wendell || {{Coord|1|S|132|W|globe:phobos_type:landmark|name=Wendell}} || align=right | 1.7 || align=center | 1973 || [[Oliver Wendell]]; American astronomer (1845–1912) || {{WGPSN|6518}}
|}{{clear}}

==== Other named features ====
There is one named [[Planetary geology#Regio (astronomy)|regio]], ''Laputa Regio'', and one named [[planitia]], ''Lagado Planitia''; both are named after places in ''Gulliver's Travels'' (the fictional [[Gulliver's Travels#Part III: A Voyage to Laputa, Balnibarbi, Luggnagg, Glubbdubdrib and Japan|Laputa]], a flying island, and [[Lagado]], imaginary capital of the fictional nation [[Balnibarbi]]).<ref>[https://planetarynames.wr.usgs.gov/Page/PHOBOS/target Gazetteer of Planetary Nomenclature] [[USGS]] Astrogeology Research Program, Phobos</ref> The only named ridge on Phobos is ''Kepler Dorsum'', named after the astronomer [[Johannes Kepler]].<ref>{{cite journal |title=Working Group for Planetary System Nomenclature (Groupe de Travail Pour la Nomenclature du Systeme Planetaire) |journal=Transactions of the International Astronomical Union |date=1988 |volume=20 |issue=2 |page=372 |doi=10.1017/S0251107X0002767X|doi-access=free }}</ref>

== Orbital characteristics ==
[[File:Phobos fly-by animation ESA223006.gif|thumb|Orbits of Phobos and [[Deimos (moon)|Deimos]]]]
The [[orbital motion]] of Phobos has been intensively studied, making it "the best studied [[natural satellite]] in the Solar System" in terms of orbits completed.<ref name="Bills">{{cite journal |last1=Bills |first1=Bruce G. |first2=Gregory A. |last2=Neumann |first3=David E. |last3=Smith |first4=Maria T. |last4=Zuber |date=2005 |title=Improved estimate of tidal dissipation within Mars from MOLA observations of the shadow of Phobos |journal=Journal of Geophysical Research |volume=110 |issue=E07004 |pages=E07004 |doi=10.1029/2004je002376 |bibcode=2005JGRE..110.7004B |doi-access=free }}</ref> Its close orbit around Mars produces some distinct effects. With an altitude of {{convert|5989|km|abbr=on}}, Phobos orbits Mars below the [[synchronous orbit]] radius, meaning that it moves around Mars faster than Mars itself rotates.<ref name="Close Inspection for Phobos" /> Therefore, from the point of view of an observer on the surface of Mars, it rises in the west, moves comparatively rapidly across the sky (in 4 h 15 min or less) and sets in the east, approximately twice each Martian day (every 11 h 6 min). Because it is close to the surface and in an [[equator]]ial orbit, it cannot be seen above the horizon from [[latitude]]s greater than 70.4°. Its orbit is so low that its [[angular diameter]], as seen by an observer on Mars, varies visibly with its position in the sky. Seen at the horizon, Phobos is about 0.14° wide; at [[zenith]], it is 0.20°, one-third as wide as the full [[Moon]] as seen from Earth. By comparison, the Sun has an apparent size of about 0.35° in the Martian sky. Phobos's phases, inasmuch as they can be observed from Mars, take 0.3191 days (Phobos's [[Synodic period|synodic]] period) to run their course, a mere 13 seconds longer than Phobos's [[sidereal period]].
{{clear left}}

=== Solar transits ===
{{Main|Transit of Phobos from Mars}}
[[File:Phobos_transit_in_real_color.webm|thumb|Phobos [[Astronomical transit|transits]] the [[Sun]], as viewed by the [[Perseverance (rover)|''Perseverance'' rover]] on 2 April 2022]]
An observer situated on the Martian surface, in a position to observe Phobos, would see regular [[astronomical transit|transits]] of Phobos across the Sun. Several of these transits have been photographed by the Mars Rover ''[[MER-B|Opportunity]]''. During the transits, Phobos casts a shadow on the surface of Mars; this event has been photographed by several spacecraft. Phobos is not large enough to cover the Sun's disk, and so cannot cause a [[total eclipse]].<ref>{{cite web |url= https://www.theverge.com/2022/4/21/23035514/nasa-perseverance-rover-mars-eclipse-phobos|title= Check out NASA's latest footage of a solar eclipse on Mars|author= Mary Beth Griggs|date= 21 April 2022|publisher= The Verge|access-date=19 April 2022}}</ref>

=== Predicted destruction ===
[[Tidal deceleration]] is gradually decreasing the orbital radius of Phobos by approximately {{convert|2|m|abbr=on}} every 100 years,<ref name="Falling apart">{{cite web |last=Zubritsky |first=Elizabeth |url=https://www.nasa.gov/feature/goddard/phobos-is-falling-apart |title=Mars' Moon Phobos is Slowly Falling Apart |work=[[NASA]] |date=10 November 2015 |access-date=19 July 2021}}</ref> and with decreasing orbital radius the likelihood of breakup due to [[tidal force]]s increases, estimated in approximately 30–50 million years,<ref name="Falling apart" /><ref name="Bills" /> or about 43 million years in one study's estimate.<ref name="Efroimsky">{{Cite journal |last1=Efroimsky |first1=Michael |last2=Lainey |first2=Valéry |title=Physics of bodily tides in terrestrial planets and the appropriate scales of dynamical evolution. |date=2007 |journal=Journal of Geophysical Research |volume=112 |number=E12 |page=E12003 |doi=10.1029/2007JE002908 |arxiv=0709.1995 |bibcode=2007JGRE..11212003E |s2cid=9480498 }}</ref>

Phobos's grooves were long thought to be fractures caused by the impact that formed the Stickney crater. Other modelling suggested since the 1970s support the idea that the grooves are more like "stretch marks" that occur when Phobos gets deformed by tidal forces, but in 2015 when the tidal forces were calculated and used in a new model, the stresses were too weak to fracture a solid moon of that size, unless Phobos is a rubble pile surrounded by a layer of powdery regolith about {{convert|100|m|abbr=on}} thick. Stress fractures calculated for this model line up with the grooves on Phobos. The model is supported with the discovery that some of the grooves are younger than others, implying that the process that produces the grooves is ongoing.<ref name="Falling apart" /><ref>Hurford, Terry A.; Asphaug, Erik; Spitale, Joseph; Hemingway, Douglas; et al.<!-- Rhoden, Alyssa; Henning, Wade; Bills, Bruce; Kattenhorn, Simon; Walker, Matthew -->; "Surface Evolution from Orbital Decay on Phobos", Division of Planetary Sciences of the American Astronomical Society meeting #47, National Harbor, MD, November 2015</ref>{{Inconsistent|date=June 2019|reason=Chapter 'Physical characteristics' says the grooves were caused by rolling boulders}}

Given Phobos's irregular shape and assuming that it is a pile of rubble (specifically a [[Mohr–Coulomb theory|Mohr–Coulomb body]]), it will eventually break up due to tidal forces when it reaches approximately 2.1 Mars radii.<ref name="death">
{{Cite journal |first=Keith A. |last=Holsapple |date=December 2001 |title=Equilibrium Configurations of Solid Cohesionless Bodies |journal=Icarus |volume=154 |issue=2 |pages=432–448 |doi=10.1006/icar.2001.6683 |bibcode=2001Icar..154..432H |s2cid=10781522 |url=http://pdfs.semanticscholar.org/131a/1b59e2f721e98b95653327f691ef171a4e63.pdf |archive-url=https://web.archive.org/web/20200412142548/http://pdfs.semanticscholar.org/131a/1b59e2f721e98b95653327f691ef171a4e63.pdf |archive-date=12 April 2020 }}</ref> When Phobos is broken up, it will form a [[Ring system|planetary ring]] around Mars.<ref>{{Cite web |last=Sample |first=Ian |url=https://www.theguardian.com/science/2015/nov/23/gravity-will-rip-mars-moon-apart-dust-rubble-ring |title=Gravity will rip Martian moon apart to form dust and rubble ring |work=[[The Guardian]] |date=23 November 2015 |access-date=17 July 2016}}</ref> This predicted ring may last from 1 million to 100 million years. The fraction of the mass of Phobos that will form the ring depends on the unknown internal structure of Phobos. Loose, weakly bound material will form the ring. Components of Phobos with strong cohesion will escape tidal breakup and will enter the Martian atmosphere.<ref>Black, Benjamin A.; and Mittal, Tushar; (2015), "The demise of Phobos and development of a Martian ring system", ''Nature Geosci'', advance online publication, doi:10.1038/ngeo2583</ref>

== Origin ==
[[File:MoonsOfMarsImproved2.gif|thumb|An illustration of main-belt asteroid capture hypothesis]]
The origin of the Martian moons has been disputed.<ref name="burns">Burns, Joseph A.; "Contradictory Clues as to the Origin of the Martian Moons" in ''Mars'', H. H. Kieffer et al., eds., University of Arizona Press, Tucson, AZ, 1992</ref> Phobos and Deimos both have much in common with carbonaceous [[C-type asteroid]]s, with [[electromagnetic spectrum|spectra]], [[albedo]], and [[density]] very similar to those of C- or D-type asteroids.<ref name="c-type">{{cite web |url=https://www.nasa.gov/mission_pages/MRO/multimedia/20071127-caption.html |title=Views of Phobos and Deimos |work=[[NASA]] |date=27 November 2007 |access-date=19 July 2021 |archive-date=4 May 2016 |archive-url=https://web.archive.org/web/20160504234224/http://www.nasa.gov/mission_pages/MRO/multimedia/20071127-caption.html }}</ref> Based on their similarity, one hypothesis is that both moons may be captured [[main-belt asteroids]].<ref>{{cite web |title=Close Inspection for Phobos |url=http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=31031 |quote=One idea is that Phobos and Deimos, Mars's other moon, are captured asteroids. }}</ref><ref name="landis">Landis, Geoffrey A.; "Origin of Martian Moons from Binary Asteroid Dissociation", ''American Association for the Advancement of Science Annual Meeting; Boston, MA, 2001'', [https://ntrs.nasa.gov/search.jsp?R=946501&id=8&qs=No%3D70&N%3D4294808501 abstract]</ref> Since both moons have nearly circular orbits that lie almost exactly in Mars's [[equatorial plane]], a capture origin for them requires a mechanism for circularizing their initially highly eccentric orbits and adjusting their inclinations into the equatorial plane, most probably by a combination of atmospheric drag and tidal forces.<ref name="cazenave">{{Cite journal |last1=Cazenave |first1=Anny |author-link=Anny Cazenave |last2=Dobrovolskis |first2=Anthony R. |last3=Lago |first3=Bernard |date=1980 |title=Orbital history of the Martian satellites with inferences on their origin |journal=Icarus |volume=44 |issue=3 |pages=730–744 |doi=10.1016/0019-1035(80)90140-2 |bibcode=1980Icar...44..730C }}</ref> However, it is not clear that sufficient time is available for this to occur for Deimos.<ref name="burns" /> Capture also requires dissipation of energy. The current Martian atmosphere is too thin to capture a Phobos-sized object by atmospheric braking.<ref name="burns" /> [[Geoffrey A. Landis]] has pointed out that the capture could have occurred if the original body was a [[binary asteroid]] that separated under tidal forces.<ref name="landis" /><ref>{{cite journal | last = Canup | first = Robin | author-link = Robin Canup | title = Origin of Phobos and Deimos by the impact of a Vesta-to-Ceres sized body with Mars | date = 18 April 2018 | journal = Science Advances | volume = 4 | issue = 4 | article-number = eaar6887 | doi = 10.1126/sciadv.aar6887| pmid = 29675470 | pmc = 5906076 | bibcode = 2018SciA....4.6887C | doi-access = free }}</ref>

Phobos could be a second-generation Solar System object that [[Accretion (astrophysics)|coalesced]] in orbit after Mars formed, rather than forming concurrently out of the same birth cloud as Mars.<ref name="ESA2010">{{cite web |first1=Martin |last1=Pätzold |first2=Olivier |last2=Witasse |name-list-style=amp |url=https://www.esa.int/Science_Exploration/Space_Science/Mars_Express/Phobos_flyby_success |title=Phobos Flyby Success |publisher=[[ESA]] |date=4 March 2010 |access-date=4 March 2010}}</ref>

Another hypothesis is that Mars was once surrounded by many Phobos- and Deimos-sized bodies, perhaps ejected into orbit around it by a collision with a large [[planetesimal]].<ref name="Craddock">Craddock, Robert A.; (1994); "The Origin of Phobos and Deimos", ''Abstracts of the 25th Annual Lunar and Planetary Science Conference, held in Houston, TX, 14–18 March 1994'', p. 293</ref> The high porosity of the interior of Phobos (based on the density of 1.88 g/cm<sup>3</sup>, voids are estimated to comprise 25 to 35 percent of Phobos's volume) is inconsistent with an asteroidal origin.<ref name="Andert">{{Cite journal |last1=Andert |first1=Thomas P. |display-authors=4 |last2=Rosenblatt |first2=Pascal |last3=Pätzold |first3=Martin |last4=Häusler |first4=Bernd |last5=Dehant |first5=Véronique M. |last6=Tyler |first6=George Leonard |last7=Marty |first7=Jean-Charles |title=Precise mass determination and the nature of Phobos |journal=[[Geophysical Research Letters]] |volume=37 |issue=9 |pages=L09202 |date = 7 May 2010 |doi=10.1029/2009GL041829 |bibcode=2010GeoRL..37.9202A |doi-access=free }}</ref> Observations of Phobos in the [[thermal infrared]] suggest a composition containing mainly [[phyllosilicates]], which are well known from the surface of Mars. The spectra are distinct from those of all classes of [[chondrite]] meteorites, again pointing away from an asteroidal origin.<ref name="Giuranna">{{Cite conference |first1=Marco |last1=Giuranna |display-authors=4 |last2=Roush |first2=Ted L. |last3=Duxbury |first3=Thomas |last4=Hogan |first4=Robert C. |last5=Geminale |first5=Anna |last6=Formisano |first6=Vittorio |title=Compositional Interpretation of PFS/MEx and TES/MGS Thermal Infrared Spectra of Phobos |book-title=European Planetary Science Congress Abstracts, Vol. 5 |date=2010 |url=http://meetingorganizer.copernicus.org/EPSC2010/EPSC2010-211.pdf |access-date=1 October 2010 }}</ref> Both sets of findings support an origin of Phobos from material ejected by an impact on Mars that reaccreted in Martian orbit,<ref name="Blast">{{cite web |url=https://www.space.com/9201-mars-moon-phobos-forged-catastrophic-blast.html |title=Mars Moon Phobos Likely Forged by Catastrophic Blast |work=[[Space.com]] |date=27 September 2010 |access-date=1 October 2010}}</ref> similar to the [[Giant-impact hypothesis|prevailing theory]] for the origin of Earth's moon.

Some areas of the surface are reddish in color, while others are bluish. The hypothesis is that gravity pull from Mars makes the reddish regolith move over the surface, exposing relatively fresh, unweathered and bluish material from the moon, while the regolith covering it over time has been weathered due to exposure of solar radiation. Because the blue rock differs from known Martian rock, it could contradict the theory that the moon is formed from leftover planetary material after the impact of a large object.<ref>{{cite web |last=Choi |first=Charles Q. |url=https://www.space.com/mars-moon-phobos-orbit-surface-powder.html |title=A Weird Powder Puzzle on the Martian Moon Phobos May Be Solved |work=[[Space.com]] |date=18 March 2019 |access-date=19 July 2021}}</ref>

In February 2021, Amirhossein Bagheri ([[ETH Zurich]]), Amir Khan (ETH Zurich), [[Michael Efroimsky]] (US Naval Observatory) and their colleagues proposed a new hypothesis on the origin of the moons. By analyzing the seismic and orbital data from [[InSight|Mars InSight Mission]] and other missions, they proposed that the moons are born from disruption of a common parent body around 1 to 2.7 billion years ago. The common progenitor of Phobos and Deimos was most probably hit by another object and shattered to form both moons.<ref>{{Cite journal|last1=Bagheri|first1=Amirhossein|last2=Khan|first2=Amir|last3=Efroimsky|first3=Michael|last4=Kruglyakov|first4=Mikhail|last5=Giardini|first5=Domenico|date=22 February 2021|title=Dynamical evidence for Phobos and Deimos as remnants of a disrupted common progenitor|url=https://www.nature.com/articles/s41550-021-01306-2|journal=Nature Astronomy|volume=5|issue=6|language=en|pages=539–543|doi=10.1038/s41550-021-01306-2|bibcode=2021NatAs...5..539B|s2cid=233924981|issn=2397-3366|url-access=subscription}}</ref>

== Exploration ==

=== Launched missions ===
[[File:Phobos over Mars - Peridier Crater, Syrtis Major quadrangle - ESA Mars Express (52568167027).png|thumb|Phobos over Mars (ESA ''Mars Express'')]]
Phobos has been photographed in close-up by several spacecraft whose primary mission has been to photograph Mars. The first was ''[[Mariner 7]]'' in 1969, followed by ''[[Mariner 9]]'' in 1971, ''[[Viking 1]]'' in 1977, [[Phobos 2]] in 1989<ref name=Harvey253>{{cite book |last=Harvey |first=Brian |date=2007 |title=Russian Planetary Exploration History, Development, Legacy and Prospects |publisher=Springer-Praxis |pages=253–254 |isbn=978-0-387-46343-8}}</ref> ''[[Mars Global Surveyor]]'' in 1998 and 2003, ''[[Mars Express]]'' in 2004, 2008, 2010<ref>{{Cite news |url=https://news.bbc.co.uk/1/hi/sci/tech/8550362.stm |access-date=7 March 2010 |title=Closest Phobos flyby gathers data |date=4 March 2010 |location=London |work=BBC News }}</ref> and 2019, and ''[[Mars Reconnaissance Orbiter]]'' in 2007 and 2008. On 25 August 2005, the [[Spirit Rover|''Spirit'' rover]], with an excess of energy due to wind blowing dust off of its solar panels, took several short-exposure photographs of the night sky from the surface of Mars, and was able to successfully photograph both Phobos and Deimos.<ref>{{cite web |title=Two Moons Passing in the Night |url=http://marsrovers.jpl.nasa.gov/gallery/press/spirit/20050909a.html |publisher=NASA |access-date=27 June 2011 }}</ref>

The [[Soviet Union]] undertook the [[Phobos program]] with two probes, both launched successfully in July 1988. ''[[Phobos 1]]'' was shut down by an erroneous command from ground control issued in September 1988 and lost while still en route. ''[[Phobos 2]]'' arrived at the Mars system in January 1989 and, after transmitting a small amount of data and imagery shortly before beginning its detailed examination of Phobos's surface, abruptly ceased transmission due either to failure of the onboard computer or of the radio transmitter, already operating on backup power. Other Mars missions collected more data, but no dedicated [[sample return mission]] has been successfully performed.

The [[Russian Space Agency]] launched a sample return mission to Phobos in November 2011, called ''[[Fobos-Grunt]]''. The return capsule also included a life science experiment of [[The Planetary Society]], called [[Living Interplanetary Flight Experiment]], or LIFE.<ref>{{cite web |url=http://www.planetary.org/programs/projects/life/ |title=Projects LIFE Experiment: Phobos |publisher=[[The Planetary Society]] |access-date=12 May 2010 |archive-date=17 February 2010 |archive-url=https://web.archive.org/web/20100217221528/http://planetary.org/programs/projects/life/ }}</ref> A second contributor to this mission was the [[China National Space Administration]], which supplied a surveying satellite called "[[Yinghuo-1]]", which would have been released in the orbit of Mars, and a soil-grinding and sieving system for the scientific payload of the Phobos lander.<ref>{{cite web |url=http://www.hktrader.net/200705/lead/lead-SpaceMission200705.htm |title=HK triumphs with out of this world invention |publisher=Hong Kong Trader |date=1 May 2007 |access-date=12 May 2010 |archive-url=https://web.archive.org/web/20120213201112/http://www.hktrader.net/200705/lead/lead-SpaceMission200705.htm |archive-date=13 February 2012 }}</ref><ref>{{cite web |url=https://www.polyu.edu.hk/web/en/media/media_releases/index_id_815.html |title=PolyU-made space tool sets for Mars again |publisher=[[Hong Kong Polytechnic University]] |date=2 April 2007 |access-date=23 January 2018 }}</ref> After achieving [[Geocentric orbit|Earth orbit]], the ''Fobos-Grunt'' probe failed to initiate subsequent burns that would have sent it to Mars. Attempts to recover the probe were unsuccessful and it crashed back to Earth in January 2012.<ref>{{cite news|url=https://www.bbc.co.uk/news/science-environment-16491457|title=Russia's Failed Phobos-Grunt Space Probe Heads to Earth|work=BBC News|date=14 January 2012}}</ref>

On 1 July 2020, the [[Mars Orbiter Mission|Mars orbiter]] of the [[Indian Space Research Organisation]] was able to capture photos of the body from 4,200&nbsp;km away.<ref>{{Cite news|title=Phobos imaged by MOM on 1st July|url=https://www.isro.gov.in/pslv-c25-mars-orbiter-mission/phobos-imaged-mom-1st-july|work=Indian Space Research Organisation|date=5 July 2020|archive-url=https://web.archive.org/web/20200705020859/https://www.isro.gov.in/pslv-c25-mars-orbiter-mission/phobos-imaged-mom-1st-july|archive-date=5 July 2020|access-date=6 July 2020}}</ref>

During the end of its 12 March 2025 [[gravity assist]] from Mars, en route to [[65803 Didymos]], the [[European Space Agency|ESA]]'s [[Hera (space mission)|''Hera'']] was able to observe Phobos retreating from the planet in its orbit at distances less than 13,000 km away.<ref>{{Cite AV media |url=https://www.youtube.com/watch?v=V6swIjI-7RI |title=Timelapse of Hera's Mars flyby |date=2025-04-15 |last=European Space Agency, ESA |access-date=2025-07-27 |via=YouTube}}</ref>

=== Planned missions ===
[[File:MMX-CG02.jpg|thumb|An artist's concept of Mars Moons eXploration spacecraft]]
The [[Japanese Aerospace Exploration Agency]] (JAXA) unveiled on 9 June 2015 the [[Martian Moons Exploration]] (MMX), a sample return mission targeting Phobos.<ref>{{cite news |url=http://www.japantimes.co.jp/news/2015/06/10/national/science-health/jaxa-plans-probe-bring-back-samples-martian-moons/ |title=JAXA plans probe to bring back samples from moons of Mars |date=10 June 2015 |newspaper=The Japan Times Online }}</ref> MMX will land and collect samples from Phobos multiple times, along with conducting Deimos flyby observations and monitoring Mars's climate. By using a [[core drill|corer]] sampling mechanism, the spacecraft aims to retrieve a minimum 10&nbsp;g amount of samples.<ref name="fujimoto20170111">{{cite web |url=http://www.lpi.usra.edu/sbag/meetings/jan2017/presentations/Fujimoto.pdf |title=JAXA's exploration of the two moons of Mars, with sample return from Phobos |last1=Fujimoto |first1=Masaki |publisher=[[Lunar and Planetary Institute]] |date=11 January 2017 |access-date=23 March 2017 }}</ref> NASA, DLR, and [[CNES]]<ref>{{cite press release |title=Coopération spatiale entre la France et le Japon Rencontre à Paris entre le CNES et la JAXA-ISAS |url=https://presse.cnes.fr/sites/default/files/drupal/201702/default/cp025-2017_-_japon.pdf |publisher=[[CNES]] |date=10 February 2017 |access-date=23 March 2017 |language=fr }}</ref> are also participating in the project, and will provide scientific instruments<ref>{{cite web |url=http://www.isas.jaxa.jp/outreach/isas_news/files/ISASnews430.pdf |title=ISASニュース 2017.1 No. 430 |publisher=[[Institute of Space and Astronautical Science]] |date=22 January 2017 |access-date=23 March 2016 |language=ja }}</ref><ref>{{cite web |url=http://www.lpi.usra.edu/pss/jun2016/presentations/Green.pdf#page=22 |title=Planetary Science Division Status Report |last1=Green |first1=James |publisher=[[Lunar and Planetary Institute]] |date=7 June 2016 |access-date=23 March 2017 }}</ref> and a rover for the mission, named [[Idefix (rover)|Idefix]]. MMX is scheduled for launch in 2026, and will return samples to Earth in 2031.<ref name="fujimoto20170111" />

=== Proposed and undeveloped missions ===
In 1997 and 1998, the ''Aladdin'' mission was selected as a finalist in the NASA [[Discovery Program]]. The plan was to visit both Phobos and Deimos, and launch projectiles at the satellites. The probe would collect the ejecta as it performed a slow flyby (~1&nbsp;km/s).<ref>{{cite book |last=Barnouin-Jha |first=Olivier S. |doi=10.1109/AERO.1999.794346 |title=1999 IEEE Aerospace Conference. Proceedings (Cat. No. 99TH8403) |volume=1 |pages=403–412 vol.1 |year=1999 |isbn=978-0-7803-5425-8 |chapter=Aladdin: Sample return from the moons of Mars |s2cid=129101577 }}</ref> These samples would be returned to Earth for study three years later.<ref>{{cite web |last=Pieters |first=Carle |title=Aladdin: Phobos -Deimos Sample Return |url=http://www.lpi.usra.edu/meetings/lpsc97/pdf/1113.PDF |work=28th Annual Lunar and Planetary Science Conference |access-date=28 March 2013 }}</ref><ref>{{cite web |title=Messenger and Aladdin Missions Selected as NASA Discovery Program Candidates |url=http://www.jhuapl.edu/newscenter/pressreleases/1998/managed.asp |access-date=28 March 2013 |archive-date=4 April 2017 |archive-url=https://web.archive.org/web/20170404212543/http://www.jhuapl.edu/newscenter/pressreleases/1998/managed.asp }}</ref> The Principal Investigator was Dr. [[Carle Pieters]] of [[Brown University]]. The total mission cost, including launch vehicle and operations was $247.7 million.<ref>{{cite web |title=Five Discovery mission proposals selected for feasibility studies |url=http://nssdc.gsfc.nasa.gov/planetary/text/discovery_pr_19981112.txt |access-date=28 March 2013 }}</ref> Ultimately, the mission chosen to fly was ''[[MESSENGER]]'', a probe to Mercury.<ref>{{cite web |title=NASA Selects Missions to Mercury and a Comet's Interior as Next Discovery Flights |url=http://nssdc.gsfc.nasa.gov/planetary/news/discovery_pr_19990707.html |access-date=28 March 2013 }}</ref>

In 2007, the European aerospace subsidiary [[EADS Astrium]] was reported to have been developing a mission to Phobos as a [[technology demonstrator]]. Astrium was involved in developing a [[European Space Agency]] plan for a sample return mission to Mars, as part of the ESA's [[Aurora programme]], and sending a mission to Phobos with its low gravity was seen as a good opportunity for testing and proving the technologies required for an eventual sample return mission to Mars. The mission was envisioned to start in 2016, was to last for three years. The company planned to use a "mothership", which would be propelled by an [[ion engine]], releasing a lander to the surface of Phobos. The lander would perform some tests and experiments, gather samples in a capsule, then return to the mothership and head back to Earth where the samples would be jettisoned for recovery on the surface.<ref>Amos, Jonathan; [https://news.bbc.co.uk/2/hi/science/nature/6324923.stm ''Martian Moon 'Could be Key Test'<nowiki />''], BBC News (9 February 2007)</ref>

[[File:Monolith55103h-crop.jpg|thumb|left|The [[Phobos monolith]] (right of center) as taken by the [[Mars Global Surveyor]] (MOC Image 55103, 1998)]]

In 2007, the [[Canadian Space Agency]] funded a study by [[Optech]] and the [[Mars Institute]] for an uncrewed mission to Phobos known as ''[[Phobos Reconnaissance and International Mars Exploration]]'' (PRIME). A proposed landing site for the PRIME spacecraft is at the "[[Phobos monolith]]", a prominent object near [[Stickney (crater)|Stickney crater]].<ref>Optech press release, "[http://www.spaceref.com/news/viewpr.html?pid=22554 Canadian Mission Concept to Mysterious Mars moon Phobos to Feature Unique Rock-Dock Maneuver]", 3 May 2007</ref><ref>[http://www.marsinstitute.info/docs/PRIME.Poster.061018.pdf PRIME: Phobos Reconnaissance & International Mars Exploration] {{webarchive|url=https://web.archive.org/web/20070724075929/http://www.marsinstitute.info/docs/PRIME.Poster.061018.pdf |date=24 July 2007 }}, Mars Institute website, accessed 27 July 2009.</ref><ref>Lee, Pascal; Richards, Robert; Hildebrand, Alan; and the PRIME Mission Team 2008, [http://www.lpi.usra.edu/meetings/lpsc2008/pdf/2268.pdf "The PRIME (Phobos Reconnaissance and International Mars Exploration) Mission and Mars sample Return"], in ''39th Lunar Planetary Science Conference'', Houston, TX, March 2008, [#2268]</ref> The PRIME mission would be composed of an orbiter and lander, and each would carry 4 instruments designed to study various aspects of Phobos's geology.<ref>{{Cite news |first=Leslie |last=Mullen |title=New Missions Target Mars Moon Phobos |date=30 April 2009 |publisher=Space.com |url=http://www.space.com/scienceastronomy/090430-mars-phobos-missions.html|work=Astrobiology Magazine |access-date=5 September 2009 }}</ref>

In 2008, [[NASA Glenn Research Center]] began studying a Phobos and Deimos sample return mission that would use [[solar electric propulsion]]. The study gave rise to the "Hall" mission concept, a [[New Frontiers program|New Frontiers]]-class mission under further study as of 2010.<ref>Lee, Pascal; Veverka, Joseph F.; Bellerose, Julie; Boucher, Marc; <!-- Boynton, John; Braham, Stephen; Gellert, Ralf; Hildebrand, Alan; Manzella, David; Mungas, Greg; Oleson, Steven; Richards, Robert; Thomas, Peter C.; West, Michael D. -->et al.; 2010; "Hall: A Phobos and Deimos Sample Return Mission", ''44th Lunar Planetary Science Conference'', The Woodlands, TX. 1–5 Mar 2010. [#1633] {{bibcode|2010LPI....41.1633L }}.</ref>

Another concept of a sample return mission from Phobos and Deimos is [[OSIRIS-REx#OSIRIS-REx II|OSIRIS-REx II]], which would use heritage technology from the first [[OSIRIS-REx]] mission.<ref>Elifritz, Thomas Lee; (2012); [http://adsabs.harvard.edu/abs/2012LPICo1679.4017E OSIRIS-REx II to Mars]. (PDF)</ref>

In 2013, ''[[Phobos Surveyor]]'' mission wa proposed by [[Stanford University]], NASA's [[Jet Propulsion Laboratory]], and the [[Massachusetts Institute of Technology]].<ref name="pandika-2012">{{cite news |url=http://news.stanford.edu/news/2012/december/rover-mars-phobos-122812.html |title=Stanford researchers develop acrobatic space rovers to explore moons and asteroids |work=Stanford Report |date=28 December 2012 |agency=Stanford News Service |access-date=3 January 2013 |last=Pandika |first=Melissa |location=Stanford, CA }}</ref>

In 2014, a Discovery-class mission was proposed to place an orbiter in Mars orbit by 2021 to study Phobos and Deimos through a series of close flybys. The mission is called [[Phobos And Deimos & Mars Environment]] (PADME).<ref name="lpsc 2014">{{cite conference |last1=Lee |first1=Pascal |last2=Bicay |first2=Michael |last3=Colapre |first3=Anthony |last4=Elphic |first4=Richard |title=Phobos And Deimos & Mars Environment (PADME): A LADEE-Derived Mission to Explore Mars's Moons and the Martian Orbital Environment |url=http://www.hou.usra.edu/meetings/lpsc2014/pdf/2288.pdf |conference=45th Lunar and Planetary Science Conference (2014) |conference-url=http://www.hou.usra.edu/meetings/lpsc2014/ |date=17–21 March 2014 }}</ref><ref name="Reyes 2014">{{cite news |last=Reyes |first=Tim |url=http://www.universetoday.com/114871/making-the-case-for-a-mission-to-the-martian-moon-phobos/ |title=Making the Case for a Mission to the Martian Moon Phobos |work=Universe Today |date=1 October 2014 |access-date=5 October 2014 }}</ref><ref name="lpsc 2015">{{cite conference |last1=Lee |first1=Pascal |last2=Benna |first2=Mehdi |last3=Britt |first3=Daniel T. |last4=Colaprete |first4=Anthony |title=PADME (Phobos And Deimos & Mars Environment): A Proposed NASA Discovery Mission to Investigate the Two Moons of Mars |url=http://www.hou.usra.edu/meetings/lpsc2015/pdf/2856.pdf |conference=46th Lunar and Planetary Science Conference (2015) |conference-url=http://www.hou.usra.edu/meetings/lpsc2015/ |date=16–20 March 2015 }}</ref> Two other Phobos missions that were proposed for the Discovery 13 selection included a ''Merlin'', which would flyby Deimos but actually orbit and land on Phobos, and ''Pandora'' which would orbit both Deimos and Phobos.<ref name="tps">[http://www.planetary.org/blogs/guest-blogs/van-kane/20150810-merlin-proposal-to-explore-the-martian-moons.html MERLIN: The Creative Choices Behind a Proposal to Explore the Martian Moons] (Merlin and PADME info also)</ref>

Russia plans to repeat Fobos-Grunt mission in the late 2020s, and the European Space Agency is assessing a sample-return mission for 2024 called [[Phootprint]].<ref name="Airbus 2014">{{cite conference |last1=Barraclough |first1=Simon |last2=Ratcliffe |first2=Andrew |last3=Buchwald |first3=Robert |last4=Scheer |first4=Heloise |last5=Chapuy |first5=Marc |last6=Garland |first6=Martin |title=Phootprint: A European Phobos Sample Return Mission |url=http://solarsystem.nasa.gov/docs/03_Phootprint_A%20European%20Phobos%20Sample%20Return%20Mission_Ratcliffe.pdf |conference=11th International Planetary Probe Workshop |publisher=Airbus Defense and Space |date=16 June 2014 |access-date=22 December 2015 |archive-url=https://web.archive.org/web/20160129204028/http://solarsystem.nasa.gov/docs/03_Phootprint_A%20European%20Phobos%20Sample%20Return%20Mission_Ratcliffe.pdf |archive-date=29 January 2016 }}</ref><ref name="COSPAR 2014">{{cite journal |bibcode=2014cosp...40E1592K |title=Phootprint – A Phobos sample return mission study |last1=Koschny |first1=Detlef |last2=Svedhem |first2=Håkan |first3=Denis |last3=Rebuffat |journal=ESA |volume=40 |pages=B0.4–9–14 |date=2 August 2014 }}</ref>

=== Human missions ===
[[File:Phobos Base.png|thumb|NASA concept of a human mission to Phobos]]
Phobos has been proposed as an early target for a [[human mission to Mars]]. The [[teleoperation]] of robotic scouts on Mars by humans on Phobos could be conducted without significant time delay, and [[planetary protection]] concerns in early Mars exploration might be addressed by such an approach.<ref>Landis, Geoffrey A.; "Footsteps to Mars: an Incremental Approach to Mars Exploration", in ''Journal of the British Interplanetary Society'', vol. 48, pp. 367–342 (1995); presented at Case for Mars V, Boulder CO, 26–29 May 1993; appears in ''From Imagination to Reality: Mars Exploration Studies'', R. Zubrin, ed., ''AAS Science and Technology Series Volume 91'', pp. 339–350 (1997). (text available as [http://www.sff.net/people/Geoffrey.Landis/Footsteps.pdf Footsteps to Mars])</ref>

A landing on Phobos would be considerably less difficult and expensive than a landing on the surface of Mars itself. A lander bound for Mars would need to be capable of [[atmospheric entry]] and subsequent return to orbit without any support facilities, or would require the creation of [[In situ resource utilization|support facilities in-situ]]. A lander instead bound for Phobos could be based on equipment designed for lunar and [[asteroid]] landings.<ref>Lee, Pascal; Braham, Stephen; Mungas, Greg; Silver, Matt; Thomas, Peter C.; and West, Michael D. (2005), "Phobos: A Critical Link Between Moon and Mars Exploration", ''Report of the Space Resources Roundtable VII: LEAG Conference on Lunar Exploration'', League City, TX 25–28 Oct 2005. ''LPI Contrib. 1318'', p. 72. {{bibcode|2005LPICo1287...56L }}</ref> Furthermore, due to Phobos's very weak gravity, the [[delta-v]] required to land on Phobos and return is only 80% of that required for a trip to and from the surface of the Moon.<ref>{{cite web |last=Oberg |first=Jamie |url=https://www.discovermagazine.com/the-sciences/russias-dark-horse-plan-to-get-to-mars |title=Russia's Dark Horse Plan to Get to Mars |work=[[Discover (magazine)|Discover]] |date=20 May 2009 |access-date=19 July 2021 |url-access=limited |quote=The total delta-v required for a mission to land on Phobos and come back is startlingly low—only about 80 percent that of a round trip to the surface of Earth's moon. (That is in part because of Phobos's feeble gravity; a well-aimed pitch could launch a softball off its surface.) |archive-date=12 August 2022 |archive-url=https://web.archive.org/web/20220812205435/https://www.discovermagazine.com/the-sciences/russias-dark-horse-plan-to-get-to-mars }}</ref>

It has been proposed that the sands of Phobos could serve as a valuable material for [[aerobraking]] during a Mars landing. A relatively small amount of chemical fuel brought from Earth could be used to lift a large amount of sand from the surface of Phobos to a transfer orbit. This sand could be released in front of a spacecraft during the descent maneuver causing a densification of the atmosphere just in front of the spacecraft.<ref name="arias16">{{Cite conference |last=Arias |first=Francisco J. |date=2017 |title=On the Use of the Sands of Phobos and Deimos as a Braking Technique for Landing Large Payloads on Mars |conference=53rd AIAA/SAE/ASEE Joint Propulsion Conference |location=Atlanta, GA |id=AIAA 201–4876|doi=10.2514/6.2017-4876 |isbn=978-1-62410-511-1}}</ref><ref name="arias1">{{cite journal |last1=Arias |first1=Francisco J. |last2=De Las Heras |first2=Salvador A. |title=Sandbraking. A technique for landing large payloads on Mars using the sands of Phobos |journal=Aerospace Science and Technology |volume=85 |pages=409–415 |doi=10.1016/j.ast.2018.11.041 |issn=1270-9638|year=2019 |bibcode=2019AeST...85..409A |hdl=2117/127428 |s2cid=115285339 |hdl-access=free }}</ref>

While human exploration of Phobos could serve as a catalyst for the human exploration of Mars, it could be scientifically valuable in its own right.<ref>{{cite conference |last=Lee |first=Pascal |url=https://www.lpi.usra.edu/meetings/phobosdeimos2007/pdf/7044.pdf |title=Phobos-Deimos ASAP: A Case for the Human Exploration of the Moons of Mars |conference=First Int'l Conf. Explor. Phobos & Deimos |location=NASA Research Park, Moffett Field, CA |page=25 [#7044] |work=[[Lunar and Planetary Institute|LPI]] Contrib. 1377 |publisher=[[Universities Space Research Association|USRA]] |date=5–7 November 2007 |access-date=19 July 2021}}</ref>

=== Space elevator base ===
First discussed in fiction in 1956 by Fontenay,<ref>{{cite web | url=https://www.gutenberg.org/files/32067/32067-h/32067-h.htm | title=The Project Gutenberg eBook of Atom Drive, by Charles Fontenay }}</ref> Phobos has been proposed as a future site for [[space elevator]] construction. This would involve a pair of space elevators: one extending 6,000&nbsp;km from the Mars-facing side to the edge of Mars's atmosphere, the other extending {{convert|6000|km|abbr=on}} from the other side and away from Mars. A spacecraft launching from Mars's surface to the lower space elevator would only need a delta-v of {{convert|0.52|km/s|abbr=on}}, as opposed to the over {{convert|3.6|km/s|abbr=on}} needed to launch to low Mars orbit. The spacecraft could be lifted up using electrical power and then released from the upper space elevator with a hyperbolic velocity of {{convert|2.6|km/s|abbr=on}}, enough to reach Earth and a significant fraction of the velocity needed to reach the [[asteroid belt]]. The space elevators could also work in reverse to help spacecraft enter the Martian system. The great mass of Phobos means that any forces from space elevator operation would have minimal effect on its orbit. Additionally, materials from Phobos could be used for space industry.<ref>{{cite journal |last1=Weinstein |first1=Leonard M. |date=January 2003 |title=Space Colonization Using Space-Elevators from Phobos |url=https://space.nss.org/wp-content/uploads/2003-Space-Colonization-Using-Space-Elevators-From-Phobos.pdf |journal=AIP Conference Proceedings |language=en |volume=654 |pages=1227–1235 |bibcode=2003AIPC..654.1227W |doi=10.1063/1.1541423 |s2cid=1661518 |access-date=23 December 2022 |hdl=2060/20030065879}}</ref>

== See also ==
* [[List of natural satellites]]
* [[List of missions to Mars#Missions to the moons of Mars|List of missions to the moons of Mars]]
* [[Phobos monolith]]
* [[Transit of Phobos from Mars]]

== Further reading ==
* {{cite journal |last1=Pätzold |first1=Martin |last2=Andert |first2=Tom P. |last3=Cardesín-Moinelo |first3=Alejandro |last4=Cicchetti |first4=Andrea |last5=Duxbury |first5=Thomas |last6=Futaana |first6=Yoshifumi |last7=Godfrey |first7=James |last8=Hegler |first8=Sebastian |last9=Hu |first9=Xuanyu |last10=Lainey |first10=Valéry |last11=Matz |first11=Klaus-Dieter |last12=Oberst |first12=Jürgen |last13=Orosei |first13=Roberto |last14=Plettemeier |first14=Dirk |last15=Rosenblatt |first15=Pascal |last16=Stark |first16=Alexander |last17=Willner |first17=Konrad |last18=Witasse |first18=Olivier |last19=Ziese |first19=Ramona |title=Investigations of the Moon Phobos by Mars Express and Implications Towards Its Origin |journal=Space Science Reviews |date=23 April 2025 |volume=221 |issue=4 |pages=41 |doi=10.1007/s11214-025-01165-7 |url=https://link.springer.com/article/10.1007/s11214-025-01165-7 |language=en |issn=1572-9672|doi-access=free }}

== References ==
<references>
<ref name="Ernst2023">{{cite journal
  |display-authors = etal
  |first1     = Carolyn M. |last1 = Ernst
  |first2     = R. Terik |last2 = Daly
  |first3     = Robert W. |last3 = Gaskell
  |first4     = Olivier S. |last4 = Barnouin
  |first5     = Hari |last5 = Nair
  |first6     = Benjamin A. |last6 = Hyat
  |title      = High-resolution shape models of Phobos and Deimos from stereophotoclinometry
  |journal    = Earth, Planets and Space
  |date       = December 2023
  |volume     = 75
  |issue      = 1
  |page = 103 |id         = 103
  |doi-access = free
  |doi        = 10.1186/s40623-023-01814-7
  |pmid = 37378051 |bibcode    = 2023EP&S...75..103E|pmc= 10290967
  }}</ref>
</references>

== External links ==
{{Commons and category|Phobos|Phobos (moon)}}
* [http://hirise.lpl.arizona.edu/phobos.php Phobos Imaged by HiRISE] from the [[University of Arizona]]
* [https://planetarynames.wr.usgs.gov/Page/PHOBOS/target Target: Phobos] in the Gazetteer of Planetary Nomenclature ([[USGS]])
* [http://www.solarviews.com/cap/mars/vphobos3.htm Flight around Phobos] and [http://www.solarviews.com/cap/mars/vphobos2.htm High-Resolution Animation of Phobos] (two animations from ''Views of the Solar System'')
* [https://web.archive.org/web/20160731205625/http://mexlab.miigaik.ru/eng/projects/phobos_cartography/ Phobos cartography] (MIIGAiK Extraterrestrial Laboratory)

{{Navboxes
|title=Articles and topics related to Phobos
|state=collapsed
|list1=
{{Mars}}
{{Solar System moons}}
{{Solar System}}
{{Solar System table}}
{{Portal bar|Astronomy|Stars|Spaceflight|Outer space|Solar system}}
}}
{{Authority control}}

{{DEFAULTSORT:Phobos}}
[[Category:Phobos (moon)| ]]
[[Category:Astronomical objects discovered in 1877|18770812]]
[[Category:Moons of Mars]]
[[Category:Articles containing video clips]]
[[Category:Moons with a prograde orbit]]
[[Category:Solar System]]