A binary system is an astronomical term referring to two objects in space (usually stars, but also planets, galaxies, or asteroids) which are so close that their gravitational interaction causes them to orbit about a common center of mass. Some definitions (e.g. that of double planet, but not that of binary star) require that this center of mass is not located within the interior of either object. A multiple system is like a binary system but consists of three or more objects.
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When binary asteroids are similar in size, they may be called "binary companions" instead of referring to the smaller body as a satellite.[1] A good example of a true binary companion is the 90 Antiope system.