The term binary asteroid refers to a system in which two asteroids orbit their common centre of gravity, in analogy with binary stars. An asteroid is a small, solid object in our Solar System, orbiting the Sun. ... The barycenter (from the Greek βαρύκεντρον) is the center of mass of two or more bodies which are orbiting each other, and is the point around which both of them orbit. ... A binary star system consists of two stars both orbiting around their barycenter. ...
Asteroids that have moons of roughly the same size are sometimes called "double asteroids". An example is the 90 Antiope system. 90 Antiope (an-tye-a-pee) is an asteroid discovered on October 1, 1866 by Robert Luther. ...
For more detail, see the entry on asteroid moons. An asteroid moon is an asteroid that orbits another asteroid. ...
For single asteroids, that kind of information can only be obtained by sending a spacecraft close to the body, and so most asteroids' densities, compositions and meteorite associations are not well known.
The first binaryasteroid was found in August 1993 when NASA's Galileo spacecraft took pictures of asteroid Ida and revealed its tiny moon Dactyl.
Ostro said that the existence of binaryasteroids on potentially hazardous orbits means that we have to start figuring out how to maneuver spacecraft close to such objects.
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