Kuiper binaries Observations from the Galileo spacecraft in 1993 revealed a satellite, Dactyl, orbiting the main-belt asteroid Ida. Other asteroid binaries have since been discovered, including seven transneptunian bodies in the Kuiper belt. This was unexpected, because physical collisions there are too infrequent to explain binary formation. Now Goldreich et al. report a possible mechanism that would generate perhaps 5% incidence of ‘wide’ binaries among Kuiper belt objects, via gravitational effects during the period of runaway accretion in the early Solar System. See the letter to Nature in the December 12th edition