Minor-planet moon

A minor-planet moon is an astronomical object that orbits a minor planet as its natural satellite. As of November 2021, there are 447 minor planets known or suspected to have moons.[1] Discoveries of minor-planet moons (and binary objects, in general) are important because the determination of their orbits provides estimates on the mass and density of the primary, allowing insights of their physical properties that is generally not otherwise possible.[2]

Several of the moons are quite large compared to their primaries: 90 Antiope, Mors–Somnus and Sila–Nunam (95%), Patroclus–Menoetius, Altjira and Lempo–Hiisi (90%, with LempoPaha at 50%). The largest known minor-planet moon in absolute size is Pluto's largest moon Charon, which itself has about half the diameter of Pluto.

The first modern era mention of the possibility of an asteroid satellite was in connection with an occultation of the bright star Gamma Ceti by the asteroid 6 Hebe in 1977. The observer, amateur astronomer Paul D. Maley, detected an unmistakable 0.5 second disappearance of this naked eye star from a site near Victoria, Texas. Many hours later, several observations were reported in Mexico attributed to the occultation by 6 Hebe itself. Although not confirmed, this documents the first formally documented case of a suspected companion of an asteroid.[3]

There are also several known ring systems around distant objects (see: rings of Chariklo and Chiron).

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