Moons_of_Mars - Wikiparency

# Moons of Mars  **Microsim — three.js (Wikitube framework):** *Phobos in the Solar System explorer* <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/solar/Solar_System.html?obj=Phobos&embed=1" data-title="Phobos in the Solar System explorer"></div> *The Solar System explorer locked on this article's state (`?obj=Phobos`); every object and population of the [[PORTAL_Solar_System|Solar System portal]] has its own state in the same scene.*  *Try: set the speed to 1 day/s and watch Phobos lap Mars about three times every second, 9,377 km from the planet's centre, while Deimos farther out needs about 1.3 seconds; press space to pause and drag to view the two nearly circular, equatorial orbits edge-on; scroll to zoom out until Mars and both moons fit in the frame together.* The **moons of Mars** are Phobos and Deimos, two small, dark, irregularly shaped bodies discovered by Asaph Hall in August 1877.[^R1][^R37] Phobos measures about 26 × 23 × 18 km and orbits only 9,378 km from the centre of [[Mars]]; Deimos, about 16 × 12 × 10 km, orbits at 23,459 km.[^nasa-fs] Both keep one face towards the planet, and both move in almost circular [[Orbit|orbits]] close to Mars's equatorial plane.[^nasa-fs] Phobos orbits faster than Mars rotates and is slowly spiralling inward, while Deimos drifts slowly outward.[^R47][^R49] Their origin is unresolved. Their colour and low reflectivity resemble those of carbon-rich [[Asteroid|asteroids]], which suggests capture, but their tidy orbits and the high porosity and possible Martian minerals of Phobos point instead to formation from debris thrown up by a giant impact on Mars.[^R26][^R11][^R12] Japan's Martian Moons eXploration (MMX) mission is scheduled to launch in October 2026 to return samples from Phobos to [[Earth]], which may settle the question.[^R18] The explorer at the top of this page is locked on Phobos and opens Mars's local frame at true scale, where Phobos orbits 9,377 km from Mars's centre in 7.66 hours, faster than Mars turns; the moons' starting phases are ILLUSTRATIVE, while the orbital sizes and periods follow the NASA fact sheet.[^nasa-fs] ## History ### Early speculation Two Martian moons were imagined long before they were seen. [[Johannes_Kepler|Johannes Kepler]] misread one of Galileo's anagrams, which actually concerned [[Saturn]], as announcing twin companions of Mars.[^R2][^R27] Jonathan Swift's *Gulliver's Travels* (1726) has the astronomers of Laputa find two Martian satellites at 3 and 5 Martian diameters with periods of 10 and 21.5 hours.[^R2] The real moons orbit at about 1.4 and 3.5 Martian diameters from the planet's centre (derived), with periods of 7.65 and 30.3 hours.[^nasa-fs] Most historians see no mystery in the guess: with [[Earth]] known to have one moon and [[Jupiter]] four, analogy suggested two for Mars, and small, close moons were required by their having escaped detection.[^R29] Voltaire's 1752 story *Micromégas* also gave Mars two moons.[^R30] The Deimos craters Swift and Voltaire and the Phobos features Laputa Regio and Lagado Planitia commemorate these stories.[^usgs-gazetteer] ### Discovery Asaph Hall found the moons with the 26-inch (66 cm) refractor of the US Naval Observatory in Washington during the favourable opposition of 1877, deliberately searching close to the planet's glare: Deimos on 12 August and Phobos on 18 August (UTC).[^R37][^R39][^R4] The names, taken from the attendants of Ares, Fear and Terror, in Book XV of the *Iliad*, were suggested by Henry Madan of Eton.[^R41] The same refractor, remounted in 1893, is still at the observatory.[^R40] ### Mars moon hoax In 1959 an April Fool's article claimed that the moons were artificial satellites. The joke spread after the Soviet astrophysicist Iosif Shklovsky suggested, from a density estimate based on a supposed acceleration of its orbit that was later disproved, that Phobos might be a hollow shell.[^R42] ### Recent surveys In 2003 a telescopic survey of nearly all of Mars's Hill sphere, the region of about 1 million km radius (derived) in which Mars's gravity dominates the [[Sun|Sun's]], found no further moons larger than about 0.09 km in radius, although glare prevented a search close to the planet.[^R43] The [[Hill_sphere|Hill sphere]] radius follows from 227.96 million km × (m_Mars / 3 m_Sun)^(1/3) ≈ 1.08 million km (derived).[^nasa-fs] ## Characteristics Phobos has a mass of 1.06 × 10¹⁶ kg and a mean [[Density|density]] of 1,900 kg/m³; Deimos has 2.4 × 10¹⁵ kg and 1,750 kg/m³, both far below the density of Mars itself.[^nasa-fs] Both are among the darkest objects in the Solar System, with geometric albedos of 0.07 and 0.08.[^nasa-fs] Phobos is covered by a thick layer of fine dust, as Mars Global Surveyor's thermal measurements showed in 1998.[^R64] From the Martian equator Phobos would appear about 13 arcminutes across when overhead, roughly a third of the full Moon's width seen from Earth, and Deimos about 2 arcminutes (both derived from the moons' sizes and distances above the surface).[^R44] Neither can cover the [[Sun]], whose disc spans about 21 arcminutes from Mars, so Mars has only brief transits and partial eclipses, while Phobos passes through Mars's shadow on most orbits.[^R45] Seen from the surface, Phobos rises in the west and sets in the east, returning about every 11 hours, while Deimos, only a little outside the synchronous distance, rises in the east and takes about 2.7 days to set (both derived from the orbital and rotation periods).[^R44][^nasa-fs] [[Tide|Tides]] decide the moons' fates. The synchronous orbit around Mars, where a moon would keep pace with the planet's 24.62-hour rotation, lies at about 20,400 km from the centre (derived from [[Kepler's_laws_of_planetary_motion|Kepler's third law]] with GM = 42,828 km³/s²).[^nasa-fs] Phobos, inside it, raises a tidal bulge that lags behind and pulls it inward; in some tens of millions of years it will either strike Mars or be torn into a ring when it crosses the Roche limit.[^R46][^R47] Deimos, outside it, is being pushed very slowly outward, like Earth's [[Moon]].[^R49] ### Orbital details | | Phobos | Deimos | |---|---|---| | Semi-major axis | 9,378 km | 23,459 km | | Orbital period | 0.31891 d (7.65 h) | 1.26244 d (30.3 h) | | Eccentricity | 0.0151 | 0.0005 | | Inclination | 1.08° | 1.79° | | Radii (km) | 13.0 × 11.4 × 9.1 | 7.8 × 6.0 × 5.1 | | Mass | 1.06 × 10¹⁶ kg | 2.4 × 10¹⁵ kg | All values are from the NASA NSSDCA Mars fact sheet; inclinations are to Mars's equator.[^nasa-fs] ## Origin Two families of hypotheses compete.[^R26] In the capture scenario, the moons are asteroids that strayed from the [[Asteroid_belt|asteroid belt]]; their spectra and albedos resemble those of dark C- or D-type asteroids.[^R51][^R6] Capture is hard to reconcile with their orbits, however: a captured body would arrive on an eccentric, inclined path, and circularising it into the equatorial plane would need strong atmospheric drag and tides, possibly more than the time available, at least for Deimos.[^R5][^R8] Geoffrey Landis suggested that the break-up of a binary asteroid during a close pass could have left one member bound to Mars.[^R7] The alternative is formation in orbit from debris. Radio tracking of the [[Gravity|gravity]] of Phobos by Mars Express gave it a density of about 1.88 g/cm³, implying that 25–35% of its volume is empty space, and thermal infrared spectra suggest phyllosilicates, minerals common on Mars and unlike any class of chondrite meteorite.[^R11][^R12] Both findings fit an origin from material blasted off Mars by a giant impact, as Earth's [[Moon]] is thought to have formed.[^R13] Simulations have the impact form a disc whose inner part built a large moon that later fell back to Mars, while Phobos and Deimos assembled in the outer disc.[^R52] Other studies have argued that the two moons are fragments of a single parent body broken up between about 1 and 2.7 billion years ago, although N-body simulations suggest such a split would quickly lead to a collision between them.[^R55][^R56] Samples returned by MMX are expected to discriminate between these ideas.[^R26] ## Exploration ### Past attempts and proposals Many spacecraft sent to Mars have imaged its moons, but few have targeted them. The Soviet Phobos 1 and Phobos 2 probes launched in 1988; Phobos 1 was lost on the way, and Phobos 2 reached Mars orbit and gathered data on Mars and Phobos before contact was lost on 27 March 1989, shortly before it was to release landers near the moon.[^nssdc-phobos2] Russia's Fobos-Grunt sample-return mission was stranded in Earth orbit after its 2011 launch and re-entered over the Pacific on 15 January 2012.[^R58][^R14] The Aladdin sample-return concept was a [[NASA]] Discovery finalist in 1997–1998 but lost to MESSENGER, and NASA's PADME multi-flyby proposal was also not selected.[^R15][^R60] Concepts for Phobos rovers and a Deimos sample return called Gulliver were studied but not flown.[^R16][^R62] ### Current proposals JAXA's MMX is scheduled to launch on 20 October 2026, reach Mars about a year later, land on Phobos to collect at least 10 g of [[Regolith|regolith]], fly past Deimos, and return its samples to Earth, with re-entry planned for fiscal year 2031.[^R18][^R21] It would be the first sample return from the Martian system.[^R26] ## Gallery The best views of the moons come from spacecraft around Mars. The Mars Reconnaissance Orbiter's HiRISE camera photographed Phobos and Deimos in colour in 2008, showing Phobos's large crater Stickney and the grooves that run across its surface.[^R51] Mars Global Surveyor imaged Phobos at close range in 1998, and its thermal data showed a surface covered in fine powder about a metre deep.[^R64] In March 2010 Mars Express flew within 67 km of Phobos, its closest pass, to measure the moon's gravity precisely.[^R9] From the surface, Perseverance filmed Phobos crossing the face of the Sun on 2 April 2022, an eclipse lasting a little over 40 seconds; timing such events over two decades has refined measurements of the moon's inward drift.[^jpl-eclipse] The explorer above offers a different view: the whole system at once, to true scale. At that scale Phobos orbits only about 1.8 Mars radii above the surface (derived: 9,378 − 3,390 ≈ 5,990 km of altitude), and the two moons are tiny points compared with the planet. Their positions along their orbits are illustrative, but the orbital sizes and periods, and therefore the contrast between speedy Phobos and slow Deimos, follow the NASA values.[^nasa-fs] ## See also - [[Mars]] - [[Moon]] - [[Galilean_moons]] - [[Hill_sphere]] - [[PORTAL_Solar_System|Solar System portal]] ## Notes Derived numbers use the NASA fact-sheet values: orbital distances in Martian diameters are 9,378 / 6,779 ≈ 1.38 and 23,459 / 6,779 ≈ 3.46; the synchronous radius is (GM T² / 4π²)^(1/3) with GM = 42,828 km³/s² and T = 88,642 s, about 20,400 km; the Hill radius is 227.96 × 10⁶ km × (0.6417 × 10²⁴ / (3 × 1.989 × 10³⁰))^(1/3) ≈ 1.08 × 10⁶ km; the angular size of Phobos overhead from the equator is about 22 km / (9,378 − 3,396) km ≈ 0.0037 rad ≈ 13′, and of Deimos about 12.6 km / 20,060 km ≈ 2.2′. Phobos returns to the same place in the Martian sky after 1 / (1/7.654 − 1/24.623) ≈ 11.1 hours, and Deimos after 1 / (1/24.623 − 1/30.30) ≈ 131 hours, so it is above the horizon for about half that, roughly 2.7 days. At 1 day/s in the explorer, Phobos's 0.319-day orbit takes about a third of a second. ## References [^nasa-fs]: Williams, D. R. "Mars Fact Sheet" (section "Satellites of Mars"). NASA NSSDCA, last updated 19 May 2025. https://nssdc.gsfc.nasa.gov/planetary/factsheet/marsfact.html (fetched 2026-09-18). [^usgs-gazetteer]: USGS Astrogeology / IAU WGPSN. *Gazetteer of Planetary Nomenclature*: Phobos and Deimos. https://planetarynames.wr.usgs.gov/ [^jpl-eclipse]: NASA Jet Propulsion Laboratory (20 April 2022). "NASA's Perseverance rover captures video of solar eclipse on Mars". https://www.jpl.nasa.gov/news/nasas-perseverance-rover-captures-video-of-solar-eclipse-on-mars/ [^nssdc-phobos2]: NASA NSSDCA. "Phobos 2". NSSDCA Master Catalog, spacecraft 1988-059A. https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1988-059A [^R1]: Andrews, Robin George (25 July 2020). "Why the 'Super Weird' Moons of Mars Fascinate Scientists - What's the big deal about little Phobos and tinier Deimos?". *The New York Times*. https://www.nytimes.com/2020/07/25/science/mars-moons-phobos-deimos.html [^R37]: Hall, Asaph (1877). "Observations of the Satellites of Mars". *Astronomische Nachrichten* 91, 11–14. https://doi.org/10.1002/asna.18780910103 [^R47]: December 2017, Nola Taylor Redd 08 (8 December 2017). "Phobos: Facts About the Doomed Martian Moon". *Space.com*. https://www.space.com/20346-phobos-moon.html [^R49]: "Mission To Mars - Project-based Learning". *www.edb.utexas.edu*. http://www.edb.utexas.edu/missiontomars/pdf/MarsSSBody.pdf [^R26]: Kuramoto, Kiyoshi (2024-07-23). "Origin of Phobos and Deimos Awaiting Direct Exploration". *Annual Review of Earth and Planetary Sciences* 52, 495–519. https://doi.org/10.1146/annurev-earth-040522-110615 [^R11]: Andert, T. P.; Rosenblatt, P.; Pätzold, M.; Häusler, B.; Dehant, V.; Tyler, G. L. et al. (7 May 2010). "Precise mass determination and the nature of Phobos". *Geophysical Research Letters* 37, n/a. https://doi.org/10.1029/2009GL041829 [^R12]: Giuranna, M.; Roush, T. L.; Duxbury, T.; Hogan, R. C.; Geminale, A.; Formisano (2010). "Compositional Interpretation of PFS/MEx and TES/MGS Thermal Infrared Spectra of Phobos". *European Planetary Science Congress Abstracts, Vol. 5*. http://meetingorganizer.copernicus.org/EPSC2010/EPSC2010-211.pdf [^R18]: "MMX - Martian Moons eXploration". *MMX at JAXA*. https://www.mmx.jaxa.jp/en/ [^R2]: "Galileo's Anagrams and the Moons of Mars". *www.mathpages.com*. http://www.mathpages.com/home/kmath151/kmath151.htm [^R27]: "Galileo, Kepler, & Two Anagrams: Two Wrong Solutions Turn Into Two Correct Solutions". *Tumblr*. https://judgestarling.tumblr.com/post/62652246148/galileo-kepler-two-anagrams-two-wrong-solutions [^R29]: Lamont, Roscoe (1925). "The moons of Mars". *Popular Astronomy* 33, 496. bibcode 1925PA.....33..496L [^R30]: Sheehan, William (1 September 1996). "The Planet Mars: A History of Observation and Discovery". *University of Arizona Press*. https://archive.org/details/planetmarshistor00shee ISBN 9780816516414 [^R39]: "Telescope: Naval Observatory 26-inch Refractor". *amazing-space.stsci.edu*. http://amazing-space.stsci.edu/resources/explorations/groundup/lesson/scopes/naval/index.php [^R4]: Morley, T. A. (1989). "A catalogue of ground-based astrometric observations of the Martian satellites, 1877-1982". *Astronomy and Astrophysics Supplement Series* 77, 209. bibcode 1989A&AS...77..209M [^R41]: Knorre, V. (1878). "Entdeckung zweier Planeten". *Astronomische Nachrichten* 92, 47–48. https://doi.org/10.1002/asna.18780920305 [^R40]: "The 26-inch "Great Equatorial" Refractor". *United States Naval Observatory*. https://www.usno.navy.mil/USNO/about-us/usnos-telescopes/the-26-inch-refractor [^R42]: *Jefferson City Post-Tribune* (4 May 1959), newspaper report of Shklovsky's artificial-satellite claim. [^R43]: Sheppard, Scott S.; Jewitt, David; Kleyna, Jan (November 2004). "A Survey for Outer Satellites of Mars: Limits to Completeness". *The Astronomical Journal* 128, 2542–2546. https://doi.org/10.1086/424541 [^R64]: Ainsworth, Diane (September 11, 1998). "Martian moon Phobos hip-deep in powder". *JPL*. http://www.jpl.nasa.gov/releases/98/mgsphobos.html [^R44]: "Astrobiology Exclusive News You Can't Afford to Miss - Astrobio.net". *Astrobiology Magazine*. http://www.astrobio.net/news/index.php?name=News&file=article&sid=775 [^R45]: "Astrobiology Exclusive News You Can't Afford to Miss - Astrobio.net". *Astrobiology Magazine*. http://www.astrobio.net/news/index.php?name=News&file=article&sid=775 [^R46]: (14 April 2008). "Phobos: Doomed Moon of Mars". [^R51]: "New Views of Martian Moons". http://www.physorg.com/news115483748.html [^R6]: "Close Inspection for Phobos". http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=31031 [^R5]: Burns, J. A. "Contradictory Clues as to the Origin of the Martian Moons," in Mars, H. H. Kieffer et al., eds., U. Arizona Press, Tucson, 1992 [^R8]: Cazenave, A.; Dobrovolskis, A.; Lago, B. (1980). "Orbital history of the Martian satellites with inferences on their origin". *Icarus* 44, 730–744. https://doi.org/10.1016/0019-1035(80)90140-2 [^R7]: Landis, G. A. (2002). "Origin of Martian Moons from Binary Asteroid Dissociation". arXiv:0903.3434 [^R13]: (27 September 2010). "Mars Moon Phobos Likely Forged by Catastrophic Blast". *www.space.com*. http://www.space.com/scienceastronomy/martian-moon-forged-by-catastrophic-blast-100927.html [^R52]: Rosenblatt, Pascal; Charnoz, Sébastien; Dunseath, Kevin M.; Terao-Dunseath, Mariko; Trinh, Antony; Hyodo, Ryuki et al. (2016). "Accretion of Phobos and Deimos in an extended debris disc stirred by transient moons". *Nature Geoscience* 9, 581–583. https://doi.org/10.1038/ngeo2742 [^R55]: Bagheri, Amirhossein; Khan, Amir; Efroimsky, Michael; Kruglyakov, Mikhail; Giardini, Domenico (22 February 2021). "Dynamical evidence for Phobos and Deimos as remnants of a disrupted common progenitor". *Nature Astronomy* 5, 539–543. https://doi.org/10.1038/s41550-021-01306-2 [^R56]: Hyodo, Ryuki; Genda, Hidenori; Sekiguchi, Ryosuke; Madeira, Gustavo; Charnoz, Sébastien (1 August 2022). "Challenges in Forming Phobos and Deimos Directly from a Splitting of an Ancestral Single Moon". *The Planetary Science Journal* 3, 204. https://doi.org/10.3847/psj/ac88d2 [^R58]: (14 January 2012). "Russia's failed Phobos-Grunt space probe heads to Earth". *BBC News*. https://www.bbc.co.uk/news/science-environment-16491457 [^R14]: (15 January 2012). "Russian space probe crashes into Pacific Ocean". *Fox News Channel*. http://www.foxnews.com/scitech/2012/01/15/russian-space-probe-to-crash-on-earth-within-hours/ [^R15]: Mueller, J. T.; Guo, Y.; von Mehlem, U. I.; Cheng, A. F. (January 2003). "Aladdin mission concept". *Acta Astronautica* 52, 211–218. https://doi.org/10.1016/S0094-5765(02)00159-5 [^R60]: (4 January 2017). "NASA Selects Two Missions to Explore the Early Solar System". https://www.nasa.gov/press-release/nasa-selects-two-missions-to-explore-the-early-solar-system/ [^R16]: Pandika, Melissa (28 December 2012). "Stanford researchers develop acrobatic space rovers to explore moons and asteroids". *Stanford Report*. http://news.stanford.edu/news/2012/december/rover-mars-phobos-122812.html [^R62]: Britt, D. T.; Robinson, M.; Gulliver Team (2004). "The Gulliver sample return mission to Deimos". *35th COSPAR Scientific Assembly* 35, 3897. https://ui.adsabs.harvard.edu/abs/2004cosp...35.3897B/abstract [^R21]: (31 August 2017). "Gravity both too strong and too weak: landing on the Martian moons". *JAXA News*. https://mmx-news.isas.jaxa.jp/?p=331&lang=en [^R9]: Martin Pätzold; Olivier Witasse (4 March 2010). "Phobos Flyby Success". *ESA*. http://www.esa.int/esaSC/SEMIPX6K56G_index_0.html ## Further reading - Kuramoto, K. (2024). "Origin of Phobos and Deimos awaiting direct exploration". *Annual Review of Earth and Planetary Sciences* 52: 495–519. https://doi.org/10.1146/annurev-earth-040522-110615 ## External links - NASA Science: Mars moons. https://science.nasa.gov/mars/moons/ - JAXA Martian Moons eXploration (MMX). https://www.mmx.jaxa.jp/en/ - USGS/IAU Gazetteer of Planetary Nomenclature: Mars system. https://planetarynames.wr.usgs.gov/Page/MARS/target ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Moons_of_Mars) : [Wikitube](https://en.wikitube.io/wiki/Moons_of_Mars) · pinned revision [1369832505](https://en.wikipedia.org/w/index.php?oldid=1369832505) · 2026-09-18 ## Previous hub tags Hubs: `Life_Physics`. Portals: [[PORTAL_Solar_System]]. --- *Solar System portal child articles, wave 1 · 2026-09-18 · drafted · row SOL-047 · explorer state `?obj=Phobos`.*