Minor-planet moon

243 Ida and its moon Dactyl
Three images of (66391) and its orbiting moon
Animation of (357439)Animation of (136617), a trinary asteroid
  • Top: 243 Ida and its moon Dactyl as imaged by Galileo in 1993.
  • Middle: Three radar images of 4. The 'streaks' on the image are the moon's trail as it moved while the images were created.
  • Bottom: Animation of 86 and its moon (left). Animation of (136617) 1994 CC, a trinary asteroid with two moons (right).

A minor-planet moon is an astronomical object that orbits a minor planet as its natural satellite. As of February 2019, there are 352 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]

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]


In addition to the terms satellite and moon, the term "binary" (binary minor planet) is sometimes used for minor planets with moons, and "triple" for minor planets with two moons. If one object is much bigger it can be referred to as the primary and its companion as secondary. The term double asteroid is sometimes used for systems in which the asteroid and its moon are roughly the same size, while binary tends to be used independently from the relative sizes of the components. When binary minor planets are similar in size, the Minor Planet Center (MPC) refers to them as "binary companions" instead of referring to the smaller body as a satellite.[4] A good example of a true binary is the 90 Antiope system, identified in August 2000.[5] Small satellites are often referred to as moonlets.[2][6]

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