A subsatellite, also known as a submoon or informally a moonmoon, is a "moon of a moon" or a hypothetical natural satellite that orbits the moon of a planet.
It is inference from the empirical study of natural satellites in the Solar System that subsatellites may be rare, albeit possible, elements of . In the Solar System, the giant planets have large collections of natural satellites. The majority of detected are ; at least one, Kepler-1625b, may have a very large exomoon, named Kepler-1625b I, which could theoretically host a subsatellite.
Terminology
Terms used in scientific literature for subsatellites include "submoons" and "moon-moons".
Colloquial 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's natural satellite 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
; 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 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
tidal locking Iapetus, the subsatellite's orbit would then decay until it crossed Iapetus'
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 Oberon would the resulting ridge have formed after the Late Heavy Bombardment 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 or even a binary moon.
Other candidates among the Saturnian irregulars include Bestla,
Erriapus, and Bebhionn.
Artificial subsatellites
Historical
Many spacecraft have orbited the
Moon since the first one in 1966 (Luna 10).
, no spacecraft has successfully orbited any natural satellite other than the Moon. In 1988, the Soviet Union unsuccessfully attempted to put Phobos program on Quasi-satellite around the Martian moon Phobos.
Current
Launched June 18, 2009, the Lunar Reconnaissance Orbiter (LRO) is a NASA robotic spacecraft currently orbiting the
Moon in an eccentric
Polar 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 is a project that successfully launched on June 28, 2022. Composed of a 12-unit collection of CubeSat which spent a few months in transit to the Moon to arrive on November 14, 2022. It has spent over 2 years in the Moon's Near-rectilinear halo orbit. 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 launched in 2023 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 began construction in April 2024 in a near-rectilinear halo orbit (NRHO), primarily in support of the later stage NASA Artemis program missions to the Moon. Lunar Gateway will also potentially support future missions to Mars and outlying asteroids.
See also
-
Binary asteroid
-
Minor-planet moon
-
Moons of Jupiter
-
Moons of Neptune
-
Moons of Saturn
-
Moons of Uranus
-
Satellite system (astronomy)
-
Selam (moon) – a contact-binary satellite of 152830 Dinkinesh
-
Three-body problem
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