Subsatellite


A subsatellite is a natural or artificial satellite that orbits a natural satellite, i.e. a "moon of a moon".
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. Therefore, it is reasonable to assume that subsatellites may exist in the Solar System and in other planetary systems.
Nonetheless, no "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.

Possible natural instances

Rhea

The possible detection 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, 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 of a size smaller than a few millimeters.

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.

Artificial subsatellites

Many spacecraft have orbited the Moon, including crewed craft of the Apollo program., 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. Interplanetary spacecraft in development JUICE will enter orbit around Ganymede in 2032, becoming the first spacecraft to orbit a moon other than Earth's.

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.