Subsatellite

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

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

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.

Terminology

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

Possible natural instances

Rhea
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]

Iapetus

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

Historical

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.[11]

Current

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.

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 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 Earth's moon. Lunar Gateway will also potentially support future missions to Mars and outlying asteroids.

CAPSTONE is a projected May 2022 launch of a 12-unit collection of CubeSats intended to test the viability of the planned NRHO of Lunar Gateway.[12]

With a planned launch no earlier than 2022, the Lunar Flashlight is 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. Originally scheduled to launch alongside Artemis 1, Lunar Flashlight missed the integration window, and as of 2022 has to find an alternative ride to the Moon.[13]

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.
  • In Planet 51, the titular planet is orbited by a moon with a Saturn-like ring system.
  • Dumb Martian, a short story by John Wyndham, mostly takes place on Jupiter IV/II, a sub-moon of Callisto (see The Seeds of Time).
  • In Star Wars, the moon Ajan Kloss has two moons of its own.[14]

See also

References
  1. Forgan, Duncan (4 October 2018). "The habitable zone for Earthlike exomoons orbiting Kepler-1625b". arXiv:1810.02712v1 [astro-ph.EP].
  2. 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?". phys.org. 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. 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. Archived from the original on 2010-08-10.
  10. Fitzpatrick, Tony (13 December 2010). "How Iapetus, Saturn's outermost moon, got its ridge". the Source. Washington University in St. Louis. Retrieved 12 October 2018.
  11. Edwin V. Bell II (11 April 2016). "Phobos Project Information". NASA Space Science Data Coordinated Archive. NASA. Retrieved 2018-10-15.
  12. NASA Ames [@NASAAmes] (2 March 2022). "LAUNCH UPDATE: Our CAPSTONE spacecraft has a new launch window between May 3 – May 15!" (Tweet) – via Twitter.
  13. Ohana, Lavie (3 October 2021). "Four Artemis I CubeSats miss their ride". Space Scout. Retrieved 6 October 2021.
  14. Hidalgo, Pablo (20 December 2019). Star Wars: The Rise of Skywalker: The Visual Dictionary. DK Publishing. ISBN 9781465479037.
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