View source for Astronomy:Subsatellite

{{Short description|A satellite that orbits a natural satellite}}
[[File:Exomoon Kepler-1625b-i orbiting its planet (artist’s impression).tif|thumb|Artist's concept of exomoon Kepler-1625b I orbiting exoplanet Kepler-1625b. Kepler-1625b I could theoretically have a subsatellite itself.<ref name="ARX-20181004">{{cite arXiv|last=Forgan|first=Duncan|title=The habitable zone for Earthlike exomoons orbiting Kepler-1625b|date=4 October 2018|eprint=1810.02712v1|class=astro-ph.EP}}</ref><ref name="NASA-20181003" />|alt=]]
A '''subsatellite''', also known as a '''submoon''', is a "moon of a moon" or a hypothetical [[Astronomy:Natural satellite|natural satellite]] that orbits the moon of a planet.<ref>{{Cite web|title=Where is Earth's submoon?|url=https://phys.org/news/2019-01-earth-submoon.html|access-date=2020-10-13|website=phys.org|language=en}}</ref>

It is [[Philosophy:Inference|inferred]] from the empirical study of [[Astronomy:Natural satellite|natural satellite]]s in the [[Astronomy:Solar System|Solar System]] that subsatellites may be rare, albeit possible, elements of [[Astronomy:Planetary system|planetary system]]s. In the Solar System, the giant planets have large collections of natural satellites. The majority of detected [[Astronomy:Exoplanet|exoplanet]]s are [[Astronomy:Giant planet|giant planet]]s; at least one, [[Astronomy:Kepler-1625b|Kepler-1625b]], may have a very large [[Astronomy:Exomoon|exomoon]], named [[Astronomy:Kepler-1625b I|Kepler-1625b I]], which could theoretically host a subsatellite.<ref name="ARX-20181004" /><ref name="NASA-20181003">{{cite press release |last1=Chou |first1=Felcia |last2=Villard |first2=Ray |last3=Hawkes |first3=Alison |editor-last=Brown |editor-first=Katherine |title=Astronomers Find First Evidence of Possible Moon Outside Our Solar System |url= https://www.nasa.gov/press-release/astronomers-find-first-evidence-of-possible-moon-outside-our-solar-system/ |date=3 October 2018 |work=Solar System and Beyond |publisher=[[Organization:NASA|NASA]] |access-date=11 October 2018}}</ref><ref name="NGS-20181003">{{cite news |last=Drake |first=Nadia |title=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. |url=https://www.nationalgeographic.com/science/2018/10/news-first-exomoon-nasa-kepler-planets-facts-space/ |archive-url=https://web.archive.org/web/20181003234439/https://www.nationalgeographic.com/science/2018/10/news-first-exomoon-nasa-kepler-planets-facts-space/ |url-status=dead |archive-date=October 3, 2018 |date=3 October 2018 |work=[[Organization:National Geographic Society|National Geographic Society]] |access-date=11 October 2018 }}</ref><ref>{{cite web |title=Hubble finds compelling evidence for a moon outside the Solar System |date=3 October 2018 |url=https://www.spacetelescope.org/news/heic1817/ |work=Hubble Space Telescope |access-date=11 October 2018}}</ref> Nonetheless, aside from human-launched satellites in temporary [[Astronomy:Lunar orbit|lunar orbit]], no notable subsatellite is known in the Solar System or beyond. In most cases, the [[Physics:Tidal force|tidal effects]] of the planet would make such a system unstable.

==Terminology==
Terms used in scientific literature for subsatellites include "submoons" and "moon-moons". Colloquial terms that have been suggested include moonitos, moonettes, and moooons.<ref name="SM-20181011">{{cite news |last=Daley |first=Jason |title=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 |url=https://www.smithsonianmag.com/smart-news/earths-moon-could-host-moonmoon-if-moonmoons-are-thing-180970520/ |date=11 October 2018 |work=Smithsonian |access-date=10 September 2020 }}</ref>

==Possible natural instances==

=== Rhea ===
[[File:Rhean rings PIA10246 Full res.jpg|thumb|Artist's concept of rings around Rhea, a moon of Saturn|alt=]]
The possible detection<ref>{{cite journal
|url=http://www.sciencemag.org/cgi/content/abstract/319/5868/1380
|title=The Dust Halo of Saturn's Largest Icy Moon, Rhea |volume=319 |issue=5868 |pages=1380–1384 |journal=Science
|display-authors=3 |vauthors=Jones GH, Roussos E, Krupp N, Beckmann U, Coates AJ, Crary F, Dandouras I, Dikarev V, Dougherty MK, Garnier P, Hansen CJ, Hendrix AR, Hospodarsky GB, Johnson RE, Kempf S, Khurana KK, Krimigis SM, Krüger H, Kurth WS, Lagg A, McAndrews HJ, Mitchell DG, Paranicas C, Postberg F, Russell CT, Saur J, Seiß M, Spahn F, Srama R, Strobel DF, Tokar R, Wahlund JE, Wilson RJ, Woch J, Young D
|date=7 March 2008
|doi=10.1126/science.1151524
|pmid=18323452 |bibcode=2008Sci...319.1380J |s2cid=206509814 |access-date=12 October 2018
}}</ref> of a [[Astronomy:Rings of Rhea|ring system]] around [[Astronomy:Saturn|Saturn]]'s [[Astronomy:Natural satellite|natural satellite]] [[Astronomy:Rhea (moon)|Rhea]] led to calculations that indicated that satellites orbiting Rhea would have stable orbits. The rings suspected were thought to be narrow,<ref>{{cite web
|url=https://www.newscientist.com/article/dn13421-saturn-satellite-reveals-first-moon-rings.html
|title=Saturn satellite reveals first moon rings |date=6 March 2008 |last=Hecht |first=Jeff
|access-date=12 October 2018
|work=New Scientist
}}</ref> a phenomenon normally associated with [[Astronomy:Shepherd moon|shepherd moon]]s; 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.<ref>{{Cite journal |last1=Tiscareno |first1=Matthew S. |last2=Burns |first2=Joseph A. |last3=Cuzzi |first3=Jeffrey N. |last4=Hedman |first4=Matthew M. |date=July 2010 |title=Cassini imaging search rules out rings around Rhea |arxiv=1008.1764 |journal=Geophysical Research Letters |volume=37 |issue=14 |page=L14205 |doi=10.1029/2010GL043663 |bibcode=2010GeoRL..3714205T |s2cid=59458559 |url-status=dead |archive-url=https://web.archive.org/web/20100810121201/http://www.agu.org/pubs/crossref/2010/2010GL043663.shtml |archive-date=2010-08-10 |url=http://www.agu.org/pubs/crossref/2010/2010GL043663.shtml }}</ref>

===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 [[Astronomy:Equatorial ridge on Iapetus|unusual equatorial ridge]].<ref>{{cite web |last=Fitzpatrick |first=Tony |date=13 December 2010 |title=How Iapetus, Saturn's outermost moon, got its ridge |url=http://news.wustl.edu/news/Pages/21600.aspx |access-date=12 October 2018 |website= |publisher=[[Organization:Washington University in St. Louis|Washington University in St. Louis]]}}</ref> An ancient giant impact on Iapetus could have produced a subsatellite; as Saturn [[Astronomy:Tidal locking|despun]] Iapetus, the subsatellite's orbit would then decay until it crossed Iapetus' [[Physics:Roche limit|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 [[Astronomy:Oberon (moon)|Oberon]] would the resulting ridge have formed after the [[Astronomy:Late Heavy Bombardment|Late Heavy Bombardment]] and thus survived to the present day.<ref>{{cite journal |last1=Dombard |first1=Andrew J. |last2=Cheng |first2=Andrew F. |first3=William B. |last3=McKinnon |first4=Jonathan P. |last4=Kay |date=2012 |title=Delayed formation of the equatorial ridge on Iapetus from a subsatellite created in a giant impact |journal=Journal of Geophysical Research: Planets |volume=117 |issue=E3 |pages= |doi=10.1029/2011JE004010 |bibcode=2012JGRE..117.3002D |doi-access=free }}</ref>

===Irregular moons of Saturn===
Light-curve analysis suggests that Saturn's irregular satellite [[Astronomy:Kiviuq (moon)|Kiviuq]] is extremely prolate, and is likely a [[Astronomy:Contact binary (small Solar System body)|contact binary]] or even a binary moon.<ref name="LPSC2654">{{cite conference
  |title=Cassini Observations of Saturn's Irregular Moons
  |url=https://www.hou.usra.edu/meetings/lpsc2019/pdf/2654.pdf
  |first1=T. |last1=Denk
  |first2=S. |last2=Mottola
  |conference=50th Lunar and Planetary Science Conference
  |publisher=Lunar and Planetary Institute
  |number=2132
  |year=2019}}</ref> Other candidates among the Saturnian irregulars include [[Astronomy:Bestla (moon)|Bestla]], [[Astronomy:Erriapus|Erriapus]], and [[Astronomy:Bebhionn (moon)|Bebhionn]].<ref name="Denk2018">{{cite book |last1=Denk |first1=T. |url=https://tilmanndenk.de/wp-content/uploads/DenkEtAl2018_IrregularMoons.pdf |title=Enceladus and the Icy Moons of Saturn |last2=Mottola |first2=S. |last3=Bottke |first3=W. F. |last4=Hamilton |first4=D. P. |date=2018 |publisher=University of Arizona Press |isbn=9780816537488 |volume=322 |pages=409–434 |chapter=The Irregular Satellites of Saturn |bibcode=2018eims.book..409D |doi=10.2458/azu_uapress_9780816537075-ch020}}</ref>

==Artificial subsatellites==

===Historical===
Many spacecraft have orbited the [[Astronomy:Moon|Moon]], including crewed craft of the [[Astronomy:Apollo program|Apollo program]]. {{As of|2022|}}, none have orbited any other moons. In 1988, the [[Place:Soviet Union|Soviet Union]] unsuccessfully attempted to put [[Astronomy:Phobos program|two robotic probes]] on [[Astronomy:Quasi-satellite|quasi-orbits]] around the Martian moon [[Astronomy:Phobos (moon)|Phobos]].<ref>{{Cite web |url=https://nssdc.gsfc.nasa.gov/planetary/phobos.html |title=Phobos Project Information |author=Edwin V. Bell II |date=11 April 2016 |website=NASA Space Science Data Coordinated Archive |publisher=[[Organization:NASA|NASA]] |access-date=2018-10-15}}</ref>

Launched on 11 December 2022, the [[Astronomy:Lunar Flashlight|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 [[Astronomy:Polar orbit|polar orbit]].<ref>{{Cite web |last=Greicius |first=Tony |date=2022-11-30 |title=NASA's Lunar Flashlight Has Launched – Follow the Mission in Real Time |url=http://www.nasa.gov/feature/jpl/nasa-s-lunar-flashlight-has-launched-follow-the-mission-in-real-time |access-date=2022-12-11 |website=NASA}}</ref> The mission was terminated on 12 May 2023, due to failures in the craft's propulsion system.<ref>{{Cite web |last=https://www.jpl.nasa.gov |title=NASA Calls End to Lunar Flashlight After Some Tech Successes |url=https://www.jpl.nasa.gov/news/nasa-calls-end-to-lunar-flashlight-after-some-tech-successes |access-date=2023-05-28 |website=NASA Jet Propulsion Laboratory (JPL) |language=en-US}}</ref>

===Current===

Launched June 18, 2009, the [[Astronomy:Lunar Reconnaissance Orbiter|Lunar Reconnaissance Orbiter]] (LRO) is a NASA robotic spacecraft currently orbiting the [[Astronomy:Moon|Moon]] in an eccentric [[Astronomy:Polar orbit|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.

[[Engineering:CAPSTONE (spacecraft)|CAPSTONE]] is a project launched in June 2022. Composed of a 12-unit collection of [[Engineering:CubeSat|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.<ref>{{cite web |last=Figliozzi |first=Gianine |date=May 20, 2022 |title=CAPSTONE Spacecraft Launch Targeted No Earlier Than June 6 |url=https://blogs.nasa.gov/artemis/2022/05/20/capstone-spacecraft-launch-targeted-no-earlier-than-june-6/ |accessdate=May 25, 2022 |work=[[Organization:NASA|NASA]]}}</ref>

===Future planned artificial moon satellites===
The interplanetary spacecraft [[Astronomy:Jupiter Icy Moons Explorer|JUICE]] launched in 2023 will enter an orbit around [[Astronomy:Ganymede (moon)|Ganymede]] in 2032, becoming the first spacecraft to orbit a moon other than [[Astronomy:Earth|Earth]]'s.

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

==In fiction==
* In ''Planet 51'', the titular planet is orbited by 2 moons with Saturn-like ring systems.
* ''Dumb Martian'', a short story by John Wyndham, mostly takes place on Jupiter IV/II, a sub-moon of [[Astronomy:Callisto (moon)|Callisto]] (see ''The Seeds of Time'').
* In ''Star Wars'', the moon Ajan Kloss has two moons of its own.<ref>{{cite book|title=Star Wars: The Rise of Skywalker: The Visual Dictionary|publisher=DK Publishing|first=Pablo|last=Hidalgo|date=20 December 2019|isbn=9781465479037}}</ref>

==See also==
{{div col|colwidth=30em}}
* [[Astronomy:Binary asteroid|Binary asteroid]]
* {{annotated link|Astronomy:Circumplanetary disk}}
* [[Astronomy:Minor-planet moon|Minor-planet moon]]
* [[Astronomy:Moons of Jupiter|Moons of Jupiter]]
* [[Astronomy:Moons of Neptune|Moons of Neptune]]
* [[Astronomy:Moons of Saturn|Moons of Saturn]]
* [[Astronomy:Moons of Uranus|Moons of Uranus]]
* [[Astronomy:Satellite system|Satellite system]]
{{div col end}}

==References==
{{Reflist}}

== External links ==
* [http://www.space.com/scienceastronomy/shadow_moons_021008-1.html Shadow Moons: The Unknown Sub-Worlds that Might Harbor Life]
* [http://www.space.com/scienceastronomy/planet_photo_040910.html Likely First Photo of Planet Beyond the Solar System]
* [https://web.archive.org/web/20060916161707/http://www.dtm.ciw.edu/boss/definition.html Working Group on Extrasolar Planets – Definition of a "Planet"] Position statement on the definition of a planet. (IAU)
* [https://arxiv.org/abs/1201.0752 The Hunt for Exomoons with Kepler (HEK): I. Description of a New Observational Project]

{{Solar System moons}}
{{exoplanet}}

[[Category:Exoplanetology|Moons]]
[[Category:Hypothetical moons]]

{{Sourceattribution|Subsatellite}}