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An asteroid moon is an asteroid orbiting an asteroid.
Asteroid moon is an asteroid that orbits another asteroid as its natural satellite. Many asteroids may possess moons, in some cases quite substantial in size. Discoveries of asteroid moons (and binary objects, in general) are very important because the determination of their orbits provides estimates (or least constraints) on their density and mass allowing an insight into their physical properties, impossible otherwise.
Terminology of asteroid moon.
In addition to the term satellite and the popular term moon, the term binary (triple, quadruple, multiple system) is used for objects with a companion (respectively 2, 3 or more companions). The term binary is often used independently from the relative sizes of the components†. If one object is much bigger it is usually referred to as the primary and its companion as secondary. Asteroids with moons are commonly referred to as binary asteroids. The term double asteroid is sometimes used for systems in which the asteroid and its moon are roughly the same size.
†Typically, masses/albedos are not actually known so the determination of the barycentre would be problematic anyway.
Discovery milestones of asteroid moons.
As early as 1978, following a stellar occultation, 532 Herculina had been suggested to have a moon and there were reports of other asteroids having companions (usually referred to as satellites) in the following years. A letter in Sky & Telescope magazine at this time pointed to pairs of large craters (e.g. the Clearwater Lakes in Quebec) also suggesting asteroids having companions. However, the first asteroid moon to be confirmed was Dactyl which orbits 243 Ida. It was discovered by the Galileo probe in 1993. The second was discovered around 45 Eugenia in 1998. The first TNO binary, 1998 WW31 was resolved optically in 2002.
As of February 2004, nearly 37 more asteroid moons had been discovered by Earth-bound telescopes. Asteroid moons have been discovered orbiting Main belt asteroids, Trojan asteroids, near-Earth objects, and Kuiper Belt objects. In 2005, the asteroid 87 Sylvia was discovered to have two moons, making it the first known triple asteroid. Later the same year, the KBO (136108) 2003 EL61 was also discovered to have two moons, making it the second known KBO to have at least two moons after Pluto.
An example of a double asteroid is 90 Antiope, where two roughly equal-sized components orbit the common centre of gravity. 617 Patroclus and its same-sized companion Menoetius is the only known binary system in the Trojan population.
Asteroid moons: are they common or rare?
The data about the populations of binary objects are still patchy. In addition to the inevitable observational bias (dependence on the distance from Earth, size, albedo and separation of the components) the frequency appears to be different among different categories of objects. Among asteroids, an estimated 2% would have satellites. Among trans-Neptunian objects (TNO), an estimated 11% are believed to be binary or multiple objects, but three of the four known large TNO (75%) have at least one satellite.
More than 20 binaries are known in each of the main groupings: Near Earth asteroids, Main belt asteroids, and Trans-Neptunians, not including numerous claims based solely on the light curve variation. No binaries have been found so far among Centaurs, presumably due to the much smaller number and relative faintness of these objects.
Origin of asteroid moons.
The origin of asteroid moons is not currently known with certainty, and a variety of theories exist. A widely accepted theory is that asteroid moons are formed from debris knocked off of the primary asteroid by an impact. Other pairings may be formed when a small object is captured by the gravity of a larger one.
Formation by collision is constrained by the angular momentum of components i.e. by the masses and their separation. Close binaries fit this model (e.g. Pluto/Charon). Distant binaries however, with components of comparable size, are unlikely to have followed this scenario, unless considerable mass has been lost in the event.
The distances of the components for the known binaries vary from a few hundreds of km (243 Ida, 3749 Balam) to more than 3000 km (379 Huenna) for the asteroids. Among TNOs, the known separations vary from 3,000 to 50,000 km.
Notable asteroids with moons.
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