A binary asteroid is a system of two asteroids orbiting their common center of mass, in analogy with binary stars. 243 Ida was the first binary asteroid to be identified when the Galileo spacecraft did a flyby in 1993. Since then numerous binary asteroids have been detected.
When both binary asteroids are similar in size, they are sometimes called "binary companions", "double asteroids" or "doublet asteroids". A good example of true binary companions is the 90 Antiope system. Binary asteroids with a small satellite, called a "moonlet", have been more commonly observed (see 22 Kalliope, 45 Eugenia, 87 Sylvia, 107 Camilla, 121 Hermione, 130 Elektra, 243 Ida, 283 Emma, 379 Huenna, etc.). They are also called high-size-ratio binary-asteroid systems.
Paired impact craters, such as the Clearwater Lakes in Canada, may have been formed by binary asteroids.
Several theories have been posited to explain the formation of binary-asteroid systems. Recent work suggests that most of them have a significant macro-porosity (a "rubble-pile" interior). The satellites orbiting large main-belt asteroids such as 22 Kalliope, 45 Eugenia or 87 Sylvia could have formed by disruption of a parent body after impact or fission after an oblique impact. Trans-Neptunian binaries may have formed during the formation of the Solar System by mutual capture or three-body interaction. Near-Earth asteroids, which orbit in the inner part of the Solar System, may have split by tidal disruption after a close encounter with a terrestrial planet. A possible explanation for the relatively greater occurrence of binary asteroids near or inside of Earth's orbit was described in the journal Nature (10 June 2008): this theory states that when solar energy (see YORP effect) spins a “rubble pile” asteroid to a sufficiently fast rate, material is thrown from the asteroid’s equator. This process also exposes fresh material at the poles of the asteroid.