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