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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. Exomoon Kepler-1625b-i orbiting its planet (artist's impression).tif
Artist's concept of exomoon Kepler-1625b I orbiting exoplanet Kepler-1625b. Kepler-1625b I is thought to possibly have a subsatellite itself.

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 Rhean rings PIA10246 Full res.jpg
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, at least no particles larger 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 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 planned late-2021 launch of a 12-unit collection of CubeSats 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.

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 ).

See also

Related Research Articles

Phobos (moon) Larger, inner, moon of Mars

Phobos is the innermost and larger of the two natural satellites of Mars, the other being Deimos. Both moons were discovered in 1877 by American astronomer Asaph Hall. Phobos is named after the Greek god Phobos, a son of Ares (Mars) and Aphrodite (Venus) and twin brother of Deimos. Phobos was the god and personification of fear and panic.

Natural satellite Astronomical body that orbits a planet

A natural satellite is in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small solar system body. While natural satellites are often colloquially referred to as moons, there is only the Moon of Earth.

Timeline of Solar System astronomy Timeline of the history of Solar System astronomy

The following is a timeline of Solar System astronomy.

Rhea (moon) Moon of Saturn

Rhea is the second-largest moon of Saturn and the ninth-largest moon in the Solar System. It is the smallest body in the Solar System for which precise measurements have confirmed a shape consistent with hydrostatic equilibrium. It was discovered in 1672 by Giovanni Domenico Cassini.

Iapetus (moon) Moon of Saturn

Iapetus is the third-largest natural satellite of Saturn and the eleventh-largest in the Solar System. Discoveries by the Cassini mission in 2007 revealed several unusual features, such as a massive equatorial ridge running three-quarters of the way around the moon.

Lander (spacecraft) Type of spacecraft

A lander is a spacecraft that descends towards, comes to rest on, the surface of an astronomical body. In contrast to an impact probe, which makes a hard landing that damages or destroys the probe upon reaching the surface, a lander makes a soft landing after which the probe remains functional.

Phoebe (moon) Moon of Saturn

Phoebe is an irregular satellite of Saturn with a mean diameter of 213 km (132 mi). It was discovered by William Henry Pickering on March 18, 1899 from photographic plates that had been taken starting on 16 August 1898 at the Boyden Station of the Carmen Alto Observatory near Arequipa, Peru, by DeLisle Stewart. It was the first satellite to be discovered photographically.

Discovery Program Ongoing solar system exploration program by NASA

The Discovery Program is a series of solar system exploration missions funded by the US National Aeronautics and Space Administration (NASA) through its Planetary Missions Program Office. The cost of each mission is capped at a lower level than missions from NASA's New Frontiers or Flagship Programs. As a result, Discovery missions tend to be more focused on a specific scientific goal rather than serving a general purpose.

Moons of Saturn Natural satellites of the planet Saturn

The moons of Saturn are numerous and diverse, ranging from tiny moonlets only tens of meters across to enormous Titan, which is larger than the planet Mercury. Saturn has 82 moons with confirmed orbits that are not embedded in its rings – of which only 13 have diameters greater than 50 kilometers – as well as dense rings that contain millions of embedded moonlets and innumerable smaller ring particles. Seven Saturnian moons are large enough to have collapsed into a relaxed, ellipsoidal shape, though only one or two of those, Titan and possibly Rhea, are currently in hydrostatic equilibrium. Particularly notable among Saturn's moons are Titan, the second-largest moon in the Solar System, with a nitrogen-rich Earth-like atmosphere and a landscape featuring dry river networks and hydrocarbon lakes, Enceladus, which emits jets of gas and dust from its south-polar region, and Iapetus, with its contrasting black and white hemispheres.

Exomoon Moon beyond the Solar System

An exomoon or extrasolar moon is a natural satellite that orbits an exoplanet or other non-stellar extrasolar body.

Space probe unmanned spacecraft that doesnt orbit the Earth, but, instead, explores further into outer space

A space probe, or simply probe, is a robotic spacecraft that doesn't orbit the Earth, but instead explores farther into outer space. A space probe may approach the Moon; travel through interplanetary space; flyby, orbit, or land or fly on other planetary bodies; or enter interstellar space.

Habitability of natural satellites Measure of the potential of natural satellites to have environments hospitable to life

The habitability of natural satellites is a measure of the potential of natural satellites to have environments hospitable to life. Habitable environments do not necessarily harbor life. Natural satellite habitability is an emerging field which is considered important to astrobiology for several reasons, foremost being that natural satellites are predicted to greatly outnumber planets and it is hypothesized that habitability factors are likely to be similar to those of planets. There are, however, key environmental differences which have a bearing on moons as potential sites for extraterrestrial life.

Satellite system (astronomy) Set of gravitationally bound objects in orbit

A satellite system is a set of gravitationally bound objects in orbit around a planetary mass object or minor planet, or its barycenter. Generally speaking, it is a set of natural satellites (moons), although such systems may also consist of bodies such as circumplanetary disks, ring systems, moonlets, minor-planet moons and artificial satellites any of which may themselves have satellite systems of their own. Some bodies also possess quasi-satellites that have orbits gravitationally influenced by their primary, but are generally not considered to be part of a satellite system. Satellite systems can have complex interactions including magnetic, tidal, atmospheric and orbital interactions such as orbital resonances and libration. Individually major satellite objects are designated in Roman numerals. Satellite systems are referred to either by the possessive adjectives of their primary, or less commonly by the name of their primary. Where only one satellite is known, or it is a binary with a common centre of gravity, it may be referred to using the hyphenated names of the primary and major satellite.

The following outline is provided as an overview of and topical guide to Saturn:

Kepler-1625 is a 14th-magnitude solar-mass star located in the constellation of Cygnus approximately 8,000 light years away. Its mass is within 5% of that of the Sun, but its radius is approximately 70% larger reflecting its more evolved state. A candidate gas giant exoplanet was detected by the Kepler Mission around the star in 2015, which was later validated as a likely real planet to >99% confidence in 2016. In 2018, the Hunt for Exomoons with Kepler project reported that this exoplanet has evidence for a Neptune-sized exomoon around it, based on observations from NASA’s Kepler Mission. Subsequent observations by the larger Hubble Space Telescope provided compounding evidence for a Neptune-sized satellite, with an on-going debate about the reality of this exomoon candidate.

Kepler-1625b is an extrasolar planet (exoplanet) orbiting the yellow star Kepler-1625 about 2,500 parsecs away. A large gas giant, it 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 of the planet was found using photometric observations collected by the Kepler Mission. Further evidence for a Neptunian moon was found the following year using the Hubble Space Telescope, where two independent lines of evidence constrained the mass and radius to be Neptune-like. The mass-signature has been independently recovered by two other teams. However, the radius-signature was independently recovered by one of the teams but not the other. The original discovery team later showed that this latter study appears affected by systematic error sources which may be influencing their findings.

Kepler-1625b I Exomoon

Kepler 1625b I, a possible moon of exoplanet Kepler-1625b, may be the first exomoon ever discovered, and was first indicated after preliminary observations by the Kepler Space Telescope. A more thorough observing campaign by the Hubble Space Telescope took place in October 2017, ultimately leading to a discovery paper published in Science Advances in early October 2018. Studies related to the discovery of this moon suggest that the host exoplanet is up to several Jupiter masses in size, and the moon is thought to be approximately the mass of Neptune. There is a possibility that the large exomoon may have a moon itself, called a subsatellite.

CAPSTONE (spacecraft) NASA satellite to test the Lunar Gateway orbit

Cislunar Autonomous Positioning System Technology Operations and Navigation Experiment (CAPSTONE) is a lunar orbiter that will test and verify the calculated orbital stability planned for the Gateway space station. The spacecraft is a 12-unit CubeSat that will also test a navigation system that will measure its position relative to NASA's Lunar Reconnaissance Orbiter (LRO) without relying on ground stations.


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