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Subsatellite is a satellite (artificial or natural) that orbits a natural satellite, i.e. a "moon of a moon". There are no known natural subsatellites in the Solar System. In most cases, the tidal effects of the primary would make such a system unstable.
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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 rings associated with Rhea.
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 equatorial ridge.
Artificial subsatellites
Many spacecraft have orbited the Moon, including manned craft of the Apollo program. As of 2016 none have orbited other moons, though several unmanned attempts have been made for Phobos.