Nonetheless, no "moon of a moon" or subsatellite is known as of 2018 in the Solar System or beyond the Solar System. In most cases, the tidal effects of the planet would make such a system unstable.
Artist's concept of rings around Rhea, a moon of Saturn
The possible detection of a ring system around Saturn's natural satelliteRhea 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.
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.