  In the past decade, the number of known binary near-Earth asteroids has more
than quadrupled and the number of known large main belt asteroids with
satellites has doubled. Half a dozen triple asteroids have been discovered, and
the previously unrecognized populations of asteroid pairs and small main belt
binaries have been identified. The current observational evidence confirms that
small (<20 km) binaries form by rotational fission and establishes that the
YORP effect powers the spin-up process. A unifying paradigm based on rotational
fission and post-fission dynamics can explain the formation of small binaries,
triples, and pairs. Large (>20 km) binaries with small satellites are most
likely created during large collisions.
