An asteroid moon is an asteroid that orbits another asteroid. It is thought that many asteroids may possess moons, in some cases quite substantial in size. 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.
The origin of asteroid moons is not currently known with certainty, and a variety of possibilities 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 asteroid is captured by the gravity of a larger one.
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. 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 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 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.