A subsatellite, also known as a submoon, or moonmoon, is a "moon of a moon" or a hypothetical

natural satellite A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body (or sometimes another natural satellite). Natural satellites are often colloquially referred to as ''moons'' ...

that orbits the moon of a planet. It is

inferred Inferences are steps in reasoning, moving from premises to logical consequences; etymologically, the word ''infer'' means to "carry forward". Inference is theoretically traditionally divided into deduction and induction, a distinction that in ...

from the

empirical Empirical evidence for a proposition is evidence, i.e. what supports or counters this proposition, that is constituted by or accessible to sense experience or experimental procedure. Empirical evidence is of central importance to the sciences and ...

study of

s in the

Solar System The Solar System Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...

that subsatellites may be rare, albeit possible, elements of

planetary system A planetary system is a set of gravity, gravitationally bound non-Star, stellar objects in or out of orbit around a star or star system. Generally speaking, systems with one or more planets constitute a planetary system, although such systems m ...

s. In the Solar System, the giant planets have large collections of natural satellites. The majority of detected

exoplanet An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not recognized as such. The first confirmation of detection occurred in 1992. A different planet, init ...

s are

giant planet The giant planets constitute a diverse type of planet much larger than Earth. They are usually primarily composed of low-boiling-point materials (volatiles), rather than rock or other solid matter, but massive solid planets can also exist. Ther ...

s; at least one,

Kepler-1625b Kepler-1625b is a super-Jupiter exoplanet orbiting the Sun-like star Kepler-1625 about away. The large gas giant is approximately the same radius as Jupiter and orbits its star every 287.4 days. In 2017, hints of a Neptune-sized exomoon in orbit ...

, may have a very large

exomoon An exomoon or extrasolar moon is a natural satellite that orbits an exoplanet or other non-stellar extrasolar body. Exomoons are difficult to detect and confirm using current techniques, and to date there have been no confirmed exomoon detecti ...

, named

Kepler-1625b I Kepler 1625b I, a possible moon of exoplanet Kepler-1625b, may be the first exomoon ever discovered (pending confirmation), and was first indicated after preliminary observations by the Kepler Space Telescope. A more thorough observing campaign b ...

, which could theoretically host a subsatellite. Nonetheless, aside from human-launched satellites in temporary

lunar orbit In astronomy, lunar orbit (also known as a selenocentric orbit) is the orbit of an object around the Moon. As used in the space program, this refers not to the orbit of the Moon about the Earth, but to orbits by spacecraft around the Moon. The ...

, 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".

Colloquial Colloquialism (), also called colloquial language, everyday language or general parlance, is the style (sociolinguistics), linguistic style used for casual (informal) communication. It is the most common functional style of speech, the idiom norm ...

terms that have been suggested include moonitos, moonettes, and moooons.

Possible natural instances

Rhea

The possible detection of a

ring system A ring system is a disc or ring, orbiting an astronomical object, that is composed of solid material such as dust and moonlets, and is a common component of satellite systems around giant planets. A ring system around a planet is also known as ...

around

Saturn Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It has only one-eighth the average density of Earth; h ...

Rhea led to calculations that indicated that satellites orbiting Rhea would have stable orbits. The rings suspected were thought to be narrow, 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.

Iapetus

It has also been proposed that Saturn's satellite

Iapetus In Greek mythology, Iapetus (; ; grc, Ἰαπετός, Iapetós), also Japetus, is a Titan, the son of Uranus and Gaia and father of Atlas, Prometheus, Epimetheus, and Menoetius. He was also called the father of Buphagus and Anchiale in oth ...

possessed a subsatellite in the past; this is one of several hypotheses that have been put forward to account for its unusual equatorial ridge. 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 In celestial mechanics, the Roche limit, also called Roche radius, is the distance from a celestial body within which a second celestial body, held together only by its own force of gravity, will disintegrate because the first body's tidal forc ...

, 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 Oberon () is a king of the fairies in medieval and Renaissance literature. He is best known as a character in William Shakespeare's play ''A Midsummer Night's Dream'', in which he is King of the Fairies and spouse of Titania, Queen of the Fairi ...

would the resulting ridge have formed after the

Late Heavy Bombardment The Late Heavy Bombardment (LHB), or lunar cataclysm, is a hypothesized event thought to have occurred approximately 4.1 to 3.8 billion years (Ga) ago, at a time corresponding to the Neohadean and Eoarchean eras on Earth. According to the hypot ...

and thus survived to the present day.

Kiviuq

Light-curve analysis suggests that Saturn's irregular satellite,

Kiviuq Kiviuq (also spelled "Qiviuq", "Kiviok" and other variants) is a legendary hero of the epic stories of the Inuit of the Arctic regions of northern Canada, Alaska and Greenland. Kiviuq is an eternal Inuit wanderer. Spirits, giants, cannibals, bea ...

, is extremely prolate, and is likely a

contact binary In astronomy, a contact binary is a binary star system whose component stars are so close that they touch each other or have merged to share their gaseous envelopes. A binary system whose stars share an envelope may also be called an overcontac ...

or even a binary moon.

Artificial subsatellites

Historical

Many spacecraft have orbited the

Moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...

, including crewed craft of the Apollo program. , none have orbited any other moons. In 1988, the

Soviet Union The Soviet Union,. officially the Union of Soviet Socialist Republics. (USSR),. was a transcontinental country that spanned much of Eurasia from 1922 to 1991. A flagship communist state, it was nominally a federal union of fifteen nationa ...

unsuccessfully attempted to put two robotic probes on quasi-orbits around the Martian moon Phobos.

Current

Launched June 18, 2009, the

Lunar Reconnaissance Orbiter 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 ...

(LRO) is a NASA robotic spacecraft currently orbiting the

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 CAPSTONE (Cislunar Autonomous Positioning System Technology Operations and Navigation Experiment) is a lunar orbiter that will test and verify the calculated orbital stability planned for the Lunar Gateway space station. The spacecraft is a 12- ...

is a project launched in June 2022. Composed of a 12-unit collection of

CubeSats A CubeSat is a class of miniaturized satellite based around a form factor consisting of cubes. CubeSats have a mass of no more than per unit, and often use commercial off-the-shelf (COTS) components for their electronics and structure. CubeSats ...

which spent a few months in transit to the moon to arrive in November 2022. It will spend about 9 months in the moons near rectilinear halo orbit. CAPSTONE is intended to test and verify the viability of the planned NRHO of planned future Lunar Gateway and it's communication efficiency.

Future planned artificial moon satellites

Launched on 11 December 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 A polar orbit is one in which a satellite passes above or nearly above both poles of the body being orbited (usually a planet such as the Earth, but possibly another body such as the Moon or Sun) on each revolution. It has an inclination of about ...

. As of December 2022, Lunar Flashlight is en route to the Moon. The interplanetary spacecraft

JUICE Juice is a drink made from the extraction or pressing of the natural liquid contained in fruit and vegetables. It can also refer to liquids that are flavored with concentrate or other biological food sources, such as meat or seafood, such as ...

currently in development will enter an orbit around Ganymede in

2032 The 2030s (pronounced "twenty-thirties"; shortened to the '30s) is the next decade in the Gregorian calendar that will begin on 1 January 2030, and will end on 31 December 2039. Plans and goals * NASA plans to execute a crewed mission to Mars ...

, becoming the first spacecraft to orbit a moon other than

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surface ...

's. Additionally, the multi-agency supported

Lunar Gateway The Lunar Gateway, or simply Gateway, is the first planned extraterrestrial space station in lunar orbit intended to serve as a solar-powered communication hub, science laboratory, and short-term habitation module for government-agency ast ...

human-rated space station is due to begin construction in 2024 in a

near-rectilinear halo orbit A near-rectilinear halo orbit (NRHO) is a halo orbit with slightly curved – or nearly straight – sides between close passes with an orbiting body. The 2022 CAPSTONE mission is the first such orbit in cislunar space, and this Moon-centric orb ...

(NRHO), primarily in support of the later stage NASA

Artemis program The Artemis program is a robotic and human Moon exploration program led by the United States' National Aeronautics and Space Administration (NASA) along with three partner agencies: European Space Agency (ESA), Japan Aerospace Exploration Age ...

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

In fiction

* In ''

Planet 51 ''Planet 51'' is a 2009 computer-animated science fiction comedy film directed by Jorge Blanco and co-directed by Javier Abad and Marcos Martínez, from a script by Joe Stillman, based on an original idea by Javier Abad, Jorge Blanco, Marcos ...

'', 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 Callisto most commonly refers to: * Callisto (mythology), a nymph *Callisto (moon), a moon of Jupiter Callisto may also refer to: Art and entertainment *''Callisto series'', a sequence of novels by Lin Carter *''Callisto'', a novel by Torsten Kr ...

(see ''

The Seeds of Time ''The Seeds of Time'' is a collection of science fiction short stories by British writer John Wyndham, published in 1956 by Michael Joseph. The title is presumably from ''Macbeth'', Act I Scene III. Contents The collection contains: * a fo ...

''). * In '' Star Wars'', the moon Ajan Kloss has two moons of its own.

References

External links

*
Shadow Moons: The Unknown Sub-Worlds that Might Harbor Life

Position statement on the definition of a planet. (IAU)
The Hunt for Exomoons with Kepler (HEK): I. Description of a New Observational Project
{{Portal bar, Astronomy, Stars, Spaceflight, Outer space, Solar System

Moons A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body (or sometimes another natural satellite). Natural satellites are often colloquially referred to as ''moons'' ...

Hypothetical moons