Asteroid moon

An asteroid moon is an asteroid that orbits another asteroid as its natural satellite. Many asteroids possess moons, in some cases quite substantial in size.

Terminology

In addition to the term satellite and the popular term moon, the term binary (triple, quadruple, multiple system) is used for objects with a companion (respectively 2, 3, 4 or more companions). The term binary is often used independently from the relative sizes of the components. If one object is much bigger it is usually referred to as the primary and its companion as secondary. 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.

Origin

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 object is captured by the gravity of a larger one.

Formation by collision is constrained by the angular momentum of components i.e. by the masses and their separation. Close binaries fit this model. Distant binaries however, with components of comparable size, are unlikely to have followed this scenario, unless considerable mass has been lost in the event.

The distances of the components for the known binaries vary from a few hundreds of kilometres to more than 50 000 km.