APPROXIMATELY ten per cent of the impact structures on the Earth and Venus are doublets(1,2)-pairs of craters formed by the near-simultaneous impact of asteroids of comparable size, It has been suggested that these doublet craters form from asteroid fragments dispersed by aerodynamic forces during atmospheric entry(1,3), or from asteroids that were tidally disrupted by gravitational forces shortly before impact(4-6), But to form a doublet, the progenitors of the craters must have been well separated before final impact(1), which poses problems for both mechanisms, Here we argue that a hitherto undetected population of well separated binary asteroids can explain the occurrence of doublet craters, By modelling asteroids as weak, gravitationally bound aggregates (’rubble piles’), we show that the tidal forces experienced during close encounters with the Earth can generate binarg asteroids, in a process similar to that which fragmented the comet Shoemaker-Levy 9 (ref, 7) as it passed by Jupiter, Although the resulting binary asteroids may eventually separate or coalesce before colliding with a planet, repeated close encounters with the Earth maintain a steady-state population that is sufficiently large to explain the observed number of doubler craters.