Asubsatellite, also known as asubmoon or informally amoonmoon, is a "moon of a moon" or a hypotheticalnatural satellite that orbits the moon of a planet.[3]
It isinferred from theempirical study ofnatural satellites in theSolar System that subsatellites may be rare, albeit possible, elements ofplanetary systems. In the Solar System, the giant planets have large collections of natural satellites. The majority of detectedexoplanets aregiant planets; at least one,Kepler-1625b, may have a very largeexomoon, namedKepler-1625b I, which could theoretically host a subsatellite.[1][2][4][5] Nonetheless, aside from human-launched satellites in temporarylunar orbit, no subsatellite is known in the Solar System or beyond. In most cases, thetidal effects of the planet would make such a system unstable on an astronomical timescale.
Terms used in scientific literature for subsatellites include "submoons" and "moon-moons".Colloquial terms that have been suggested include moonitos (compareplutino), moonettes, and moooons.[6]
There is a possible detection[7] ofa ring system aroundSaturn'snatural satelliteRhea that led to calculations that indicated that satellites orbiting Rhea would have stable orbits. The rings suspected were thought to be narrow,[8] a phenomenon normally associated withshepherd moons; however, targeted images taken by theCassini 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.[9]
It has also been proposed that Saturn's satelliteIapetus possessed a subsatellite in the past; this is one of several hypotheses that have been put forward to account for itsunusual equatorial ridge.[10] An ancient giant impact on Iapetus could have produced a subsatellite; as Saturndespun 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 perhapsOberon would the resulting ridge have formed after theLate Heavy Bombardment and thus survived to the present day.[11]
Light-curve analysis suggests that Saturn's irregular satelliteKiviuq is extremely prolate, and is likely acontact binary or even a binary moon.[12] Other candidates among the Saturnian irregulars includeBestla,Erriapus, andBebhionn.[13]
Many spacecraft have orbited theMoon since the first one in 1966 (Luna 10).
As of 2024[update], no spacecraft has successfully orbited any natural satellite other than the Moon. In 1988, theSoviet Union unsuccessfully attempted to puttwo robotic probes onquasi-orbits around the Martian moonPhobos.[14]
Launched June 18, 2009, theLunar Reconnaissance Orbiter (LRO) is a NASA robotic spacecraft currently orbiting theMoon in an eccentricpolar 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 is a project that successfully launched on June 28, 2022. Composed of a 12-unit collection ofCubeSats 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'sNear-rectilinear halo orbit. CAPSTONE is testing and verifying the viability of the planned NRHO of planned future Lunar Gateway and its communication efficiency.[15]
The interplanetary spacecraftJUICE launched in 2023 will enter an orbit aroundGanymede in 2034, becoming the first spacecraft to orbit a moon other thanEarth's.
Additionally, the multi-agency supportedLunar Gateway human-rated space station began construction in April 2024 in anear-rectilinear halo orbit (NRHO), primarily in support of the later stage NASAArtemis program missions to the Moon. Lunar Gateway will also potentially support future missions to Mars and outlying asteroids.
Media related toExtrasolar moons at Wikimedia Commons
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