Diagram of asteroids

Over tens of millions of years, a rubble-pile asteroid less than 10 kilometers in diameter can spin up to a speed that allows the asteroid to split in two, forming a proto-binary system. If the smaller companion is less than 60 percent the size of the larger asteroid, they will gently separate from one other at a relatively low velocity. The system will eventually become an asteroid pair, or "divorced" asteroid. (Image: Pravec, et al.)