  Asteroid pairs sharing similar heliocentric orbits were found recently.
Backward integrations of their orbits indicated that they separated gently with
low relative velocities, but did not provide additional insight into their
formation mechanism. A previously hypothesized rotational fission process4 may
explain their formation - critical predictions are that the mass ratios are
less than about 0.2 and, as the mass ratio approaches this upper limit, the
spin period of the larger body becomes long. Here we report photometric
observations of a sample of asteroid pairs revealing that primaries of pairs
with mass ratios much less than 0.2 rotate rapidly, near their critical fission
frequency. As the mass ratio approaches 0.2, the primary period grows long.
This occurs as the total energy of the system approaches zero requiring the
asteroid pair to extract an increasing fraction of energy from the primary's
spin in order to escape. We do not find asteroid pairs with mass ratios larger
than 0.2. Rotationally fissioned systems beyond this limit have insufficient
energy to disrupt. We conclude that asteroid pairs are formed by the rotational
fission of a parent asteroid into a proto-binary system which subsequently
disrupts under its own internal system dynamics soon after formation.
