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Artist's concept of exomoon Kepler-1625b I orbiting exoplanet Kepler-1625b. Kepler-1625b I is thought to possibly have a subsatellite itself.[1][2]

A subsatellite, also known as a submoon or moonmoon, is a natural or artificial satellite that orbits a natural satellite, i.e. a "moon of a moon".[3]

It is inferred from the empirical study of natural satellites in the Solar System that subsatellites may be 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, with speculations of a subsatellite.[1][2][4][5] Nonetheless, aside from human-launched satellites in temporary lunar orbit, no notable "moon of a moon" or 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 moons of moons include "submoons" and "moon-moons". Other 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. Furthermore, the suspected rings are 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 of a size smaller 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]

Artificial subsatellites


Many spacecraft have orbited the Moon, including crewed craft of the Apollo program. As of 2020, 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.[11]


Launched June 18, 2009, the Lunar Reconnaissance Orbiter (LRO) is a NASA robotic spacecraft currently orbiting Earths moon Luna 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.

Future planned artificial moon satellites

The interplanetary spacecraft JUICE currently in development 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 manned 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 Earth's moon. Lunar Gateway will also potentially support future missions to Mars and outlying asteroids.

CAPSTONE is a planned mid-2021 launch of a 12-unit collection of CubeSat's intended to test the viability of the planned NRHO of Lunar Gateway.

With a projected November 2021 launch, the Lunar Flashlight is planned low-cost CubeSat lunar orbiter mission to explore, locate, and estimate size and composition of water ice deposits on Luna for future exploitation by robots or humans from a polar orbit.

In fiction

The Elder Scrolls series of video games takes place on the planet of Nirn, orbited by the satellite Masser, which in turn is orbited by the subsatellite Secunda.[citation needed]

In Planet 51 the titular planet is orbited by a moon with a Saturn-like ring system.

See also


  1. 1.0 1.1 Forgan, Duncan (4 October 2018). "The habitable zone for Earthlike exomoons orbiting Kepler-1625b". arXiv:1810.02712v1 [astro-ph.EP].
  2. 2.0 2.1 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?" (in en). 
  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. 
  5. "Hubble finds compelling evidence for a moon outside the Solar System". Hubble Space Telescope. 3 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. 
  7. "The Dust Halo of Saturn's Largest Icy Moon, Rhea". Science 319 (5868): 1380–1384. 7 March 2008. doi:10.1126/science.1151524. PMID 18323452. Bibcode2008Sci...319.1380J. Retrieved 12 October 2018. 
  8. Hecht, Jeff (6 March 2008). "Saturn satellite reveals first moon rings". New Scientist. 
  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. doi:10.1029/2010GL043663. Bibcode2010GeoRL..3714205T. 
  10. Fitzpatrick, Tony (13 December 2010). "How Iapetus, Saturn's outermost moon, got its ridge". Washington University in St. Louis. 
  11. Edwin V. Bell II (11 April 2016). "Phobos Project Information". NASA. 

External links