A subsatellite, also known as a submoon or informally a 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 colloquially referred to as moons, a deriv ...

that orbits the moon of a planet. It is

inferred Inferences are steps in logical 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 ...

from the

empirical Empirical evidence is evidence obtained through sense experience or experimental procedure. It is of central importance to the sciences and plays a role in various other fields, like epistemology and law. There is no general agreement on how t ...

study of

s in the

Solar System The Solar SystemCapitalization 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 "Sola ...

that subsatellites may be rare, albeit possible, elements of

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

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 confirmed detection of an exoplanet was in 1992 around a pulsar, and the first detection around a main-sequence star was in 1995. A different planet, first det ...

s are

giant planet A giant planet, sometimes referred to as a jovian planet (''Jove'' being another name for the Roman god Jupiter (mythology), Jupiter), is a diverse type of planet much larger than Earth. Giant planets are usually primarily composed of low-boiling ...

s; at least one, Kepler-1625b, may have a very large

exomoon Artist's impression of candidate exomoon Kepler-1625b I orbiting its planet. 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 us ...

, named

Kepler-1625b I Kepler-1625b I is a possible moon of exoplanet Kepler-1625b, which 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 ...

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

lunar orbit In astronomy and spaceflight, a lunar orbit (also known as a selenocentric orbit) is an orbit by an object around Earth's Moon. In general these orbits are not circular. When farthest from the Moon (at apoapsis) a spacecraft is said to be at apo ...

, no subsatellite is known in the Solar System or beyond. In most cases, the tidal effects of the planet would make such a system unstable on an astronomical timescale.

Terminology

Terms used in scientific literature for subsatellites include "submoons" and "moon-moons".

Colloquial Colloquialism (also called ''colloquial language'', ''colloquial speech'', ''everyday language'', or ''general parlance'') is the linguistic style used for casual and informal communication. It is the most common form of speech in conversation amo ...

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

Possible natural instances

Rhea

There is a possible detection of a ring system 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 9 times that of Earth. It has an eighth the average density of Earth, but is over 95 tim ...

Rhea that 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, making the chance of a ring system around Rhea slim.

Iapetus

It has also been proposed that Saturn's satellite

Iapetus In Greek mythology, Iapetus (; ; ), 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 other sources. Iapetus was linked ...

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 force ...

, 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 fairy, fairies in Middle Ages, 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 ...

would the resulting ridge have formed after the

Late Heavy Bombardment The Late Heavy Bombardment (LHB), or lunar cataclysm, is a hypothesized astronomical 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 ...

and thus survived to the present day.

Irregular moons of Saturn

Light-curve analysis suggests that Saturn's irregular satellite Kiviuq 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. Other candidates among the Saturnian irregulars include

Bestla Bestla (Old Norse: ) is a jötunn in Norse mythology, and the mother of the gods Odin, Vili and Vé (by way of Borr). She is also the sister of an unnamed man who assisted Odin, and the daughter (or granddaughter depending on the source) of the j ...

, Erriapus, and Bebhionn.

Artificial subsatellites

Historical

Many spacecraft have orbited the

Moon The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...

since the first one in 1966 (

Luna 10 Luna 10 (or Lunik 10) was a 1966 Soviet lunar robotic spacecraft mission in the Luna program. It was the first artificial satellite of the Moon, and any other body other than Earth and the Sun (in heliocentric orbit). Luna 10 conducted extens ...

). , no spacecraft has successfully orbited any natural satellite other than the Moon. In 1988, the

Soviet Union The Union of Soviet Socialist Republics. (USSR), commonly known as the Soviet Union, was a List of former transcontinental countries#Since 1700, transcontinental country that spanned much of Eurasia from 1922 until Dissolution of the Soviet ...

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 orbit, polar mapping orbit. Data collected by LRO have been described as essential for planning NASA's future human and robotic ...

(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 __NOTOC__ Capstone may refer to: Architecture * Keystone (architecture), also known as a capstone Brands and enterprises * Capstone Investment Advisors, a US investment management firm * Capstone Partners, an investment banking firm * Capstone P ...

is a project that successfully launched on June 28, 2022. Composed of a 12-unit collection of

CubeSats A CubeSat is a class of small satellite with a form factor of cubes. CubeSats have a mass of no more than per unit,, url=https://static1.squarespace.com/static/5418c831e4b0fa4ecac1bacd/t/5f24997b6deea10cc52bb016/1596234122437/CDS+REV14+2020-07-3 ...

which spent a few months in transit to the Moon to arrive on November 14, 2022. It has spend over 2 years in the Moon's

Near-rectilinear halo orbit In orbital mechanics a near-rectilinear halo orbit (NRHO) is a halo orbit that passes close to the smaller of two astronomical object, bodies and has nearly stable behavior. The CAPSTONE mission, launched in 2022, is the first spacecraft to use ...

. CAPSTONE is testing and verifying the viability of the planned NRHO of planned future Lunar Gateway and its communication efficiency.

Future planned artificial moon satellites

The interplanetary spacecraft

JUICE Juice is a drink made from the extraction or Cold-pressed juice, 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 ...

launched in 2023 will enter an orbit around Ganymede in 2032, 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 Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

's. Additionally, the multi-agency supported

Lunar Gateway The Lunar Gateway, or simply Gateway, is a planned space station which is to be assembled in orbit around the Moon. The Gateway is intended to serve as a communication hub, science laboratory, and habitation module for astronauts as part ...

human-rated space station began construction in April 2024 in a

near-rectilinear halo orbit In orbital mechanics a near-rectilinear halo orbit (NRHO) is a halo orbit that passes close to the smaller of two astronomical object, bodies and has nearly stable behavior. The CAPSTONE mission, launched in 2022, is the first spacecraft to use ...

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

Artemis program The Artemis program is a Exploration of the Moon, Moon exploration program led by the United States' National Aeronautics and Space Administration (NASA), formally established in 2017 via Space Policy Directive 1. The program's stated long-ter ...

missions to the Moon. Lunar Gateway will also potentially support future missions to Mars and outlying asteroids.

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 colloquially referred to as moons, a deriva ...

Hypothetical moons Hypothetical astronomical objects