Artist's impression of the double asteroid 90 Antiope
A binary asteroid is a system of two asteroids orbiting their common barycenter. 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.
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, most likely form by spin-up and mass shedding, likely as a result of the YORP effect. 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 work confirms 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.