Artist's concept of exomoon Kepler-1625b I orbiting exoplanet Kepler-1625b. Kepler-1625b I could theoretically have a subsatellite itself.[1][2]

A subsatellite, also known as a submoon or a moonmoon, is a "moon of a moon" or a hypothetical natural satellite that orbits the moon of a planet.[3]

It is inferred from the empirical study of natural satellites in the Solar System that subsatellites may be rare, albeit possible, elements of planetary systems. In the Solar System, the giant planets have large collections of natural satellites. The majority of detected exoplanets are giant planets; at least one, Kepler-1625b, may have a very large exomoon, named Kepler-1625b I, which could theoretically host a subsatellite.[1][2][4][5] Nonetheless, aside from human-launched satellites in temporary lunar orbit, no notable subsatellite is known in the Solar System or beyond. In most cases, the tidal effects of the planet would make such a system unstable.


Terms used in scientific literature for subsatellites include "submoons" and "moon-moons". Colloquial terms that have been suggested include moonitos, moonettes, and moooons.[6]

Possible natural instances


Artist's concept of rings around Rhea, a moon of Saturn

The possible detection[7] of a ring system around Saturn's natural satellite Rhea led to calculations that indicated that satellites orbiting Rhea would have stable orbits. The rings suspected were thought to be narrow,[8] a phenomenon normally associated with shepherd moons; however, targeted images taken by the Cassini spacecraft failed to detect any subsatellites or rings associated with Rhea, at least no particles larger than a few millimeters.[9]


It has also been proposed that Saturn's satellite Iapetus possessed a subsatellite in the past; this is one of several hypotheses that have been put forward to account for its unusual equatorial ridge.[10] An ancient giant impact on Iapetus could have produced a subsatellite; as Saturn despun Iapetus, the subsatellite's orbit would then decay until it crossed Iapetus' Roche limit, forming a transient ring which then impacted Iapetus to form a ridge. Such a scenario could have happened on the other giant-planet satellites as well, but only for Iapetus and perhaps Oberon would the resulting ridge have formed after the Late Heavy Bombardment and thus survived to the present day.[11]

Irregular moons of Saturn

Light-curve analysis suggests that Saturn's irregular satellite Kiviuq is extremely prolate, and is likely a contact binary or even a binary moon.[12] Other candidates among the Saturnian irregulars include Bestla, Erriapus, and Bebhionn.[13]

Artificial subsatellites


Many spacecraft have orbited the Moon, including crewed craft of the Apollo program. As of 2022, none have orbited any other moons. In 1988, the Soviet Union unsuccessfully attempted to put two robotic probes on quasi-orbits around the Martian moon Phobos.[14]

Launched on 11 December 2022, the Lunar Flashlight was a low-cost CubeSat lunar orbiter mission to explore, locate, and estimate size and composition of water ice deposits on the Moon for future exploitation by robots or humans from a polar orbit.[15] The mission was terminated on 12 May 2023, due to failures in the craft's propulsion system.[16]


Launched June 18, 2009, the Lunar Reconnaissance Orbiter (LRO) is a NASA robotic spacecraft currently orbiting the Moon in an eccentric polar mapping orbit. Data collected by LRO have been described as essential for planning NASA's future human and robotic missions to the Moon. Its detailed mapping program is identifying safe landing sites, locating potential resources on the Moon, characterizing the radiation environment, and demonstrating new technologies.

CAPSTONE is a project launched in June 2022. Composed of a 12-unit collection of CubeSats which spent a few months in transit to the Moon to arrive in November 2022. It will spend about 9 months in the Moon's near rectilinear halo orbit. CAPSTONE is intended to test and verify the viability of the planned NRHO of planned future Lunar Gateway and its communication efficiency.[17]

Future planned artificial moon satellites

The interplanetary spacecraft JUICE launched in 2023 will enter an orbit around Ganymede in 2032, becoming the first spacecraft to orbit a moon other than Earth's.

Additionally, the multi-agency supported Lunar Gateway human-rated space station is due to begin construction in 2024 in a near-rectilinear halo orbit (NRHO), primarily in support of the later stage NASA Artemis program missions to the Moon. Lunar Gateway will also potentially support future missions to Mars and outlying asteroids.

In fiction

See also


  1. ^ a b Forgan, Duncan (4 October 2018). "The habitable zone for Earthlike exomoons orbiting Kepler-1625b". arXiv:1810.02712v1 [astro-ph.EP].
  2. ^ a b Chou, Felcia; Villard, Ray; Hawkes, Alison (3 October 2018). Brown, Katherine (ed.). "Astronomers Find First Evidence of Possible Moon Outside Our Solar System". Solar System and Beyond (Press release). NASA. Retrieved 11 October 2018.
  3. ^ "Where is Earth's submoon?". Retrieved 2020-10-13.
  4. ^ Drake, Nadia (3 October 2018). "Weird giant may be the first known alien moon - Evidence is mounting that a world the size of Neptune could be orbiting a giant planet far, far away". National Geographic Society. Archived from the original on October 3, 2018. Retrieved 11 October 2018.
  5. ^ "Hubble finds compelling evidence for a moon outside the Solar System". Hubble Space Telescope. 3 October 2018. Retrieved 11 October 2018.
  6. ^ Daley, Jason (11 October 2018). "If a Moon Has a Moon, Is Its Moon Called a Moonmoon? - A new study suggests it's possible some moons could have moons and the internet wants to give them a name—but scientists have yet to actually find one". Smithsonian. Retrieved 10 September 2020.
  7. ^ Jones GH, Roussos E, Krupp N, et al. (7 March 2008). "The Dust Halo of Saturn's Largest Icy Moon, Rhea". Science. 319 (5868): 1380–1384. Bibcode:2008Sci...319.1380J. doi:10.1126/science.1151524. PMID 18323452. S2CID 206509814. Retrieved 12 October 2018.
  8. ^ Hecht, Jeff (6 March 2008). "Saturn satellite reveals first moon rings". New Scientist. Retrieved 12 October 2018.
  9. ^ Tiscareno, Matthew S.; Burns, Joseph A.; Cuzzi, Jeffrey N.; Hedman, Matthew M. (July 2010). "Cassini imaging search rules out rings around Rhea". Geophysical Research Letters. 37 (14): L14205. arXiv:1008.1764. Bibcode:2010GeoRL..3714205T. doi:10.1029/2010GL043663. S2CID 59458559. Archived from the original on 2010-08-10.
  10. ^ Fitzpatrick, Tony (13 December 2010). "How Iapetus, Saturn's outermost moon, got its ridge". Washington University in St. Louis. Retrieved 12 October 2018.
  11. ^ Dombard, Andrew J.; Cheng, Andrew F.; McKinnon, William B.; Kay, Jonathan P. (2012). "Delayed formation of the equatorial ridge on Iapetus from a subsatellite created in a giant impact". Journal of Geophysical Research: Planets. 117 (E3). Bibcode:2012JGRE..117.3002D. doi:10.1029/2011JE004010.
  12. ^ Denk, T.; Mottola, S. (2019). Cassini Observations of Saturn's Irregular Moons (PDF). 50th Lunar and Planetary Science Conference. Lunar and Planetary Institute.
  13. ^ Denk, T.; Mottola, S.; Bottke, W. F.; Hamilton, D. P. (2018). "The Irregular Satellites of Saturn". Enceladus and the Icy Moons of Saturn (PDF). Vol. 322. University of Arizona Press. pp. 409–434. doi:10.2458/azu_uapress_9780816537075-ch020. ISBN 9780816537488.
  14. ^ Edwin V. Bell II (11 April 2016). "Phobos Project Information". NASA Space Science Data Coordinated Archive. NASA. Retrieved 2018-10-15.
  15. ^ Greicius, Tony (2022-11-30). "NASA's Lunar Flashlight Has Launched – Follow the Mission in Real Time". NASA. Retrieved 2022-12-11.
  16. ^ "NASA Calls End to Lunar Flashlight After Some Tech Successes". NASA Jet Propulsion Laboratory (JPL). Retrieved 2023-05-28. ((cite web)): External link in |last= (help)
  17. ^ Figliozzi, Gianine (May 20, 2022). "CAPSTONE Spacecraft Launch Targeted No Earlier Than June 6". NASA. Retrieved May 25, 2022.
  18. ^ Hidalgo, Pablo (20 December 2019). Star Wars: The Rise of Skywalker: The Visual Dictionary. DK Publishing. ISBN 9781465479037.