Evolution of Earth-crossing binary asteroids due to gravitational encounters with the Earth

During close encounters with the Earth, binary Earth-approaching asteroids have their orbital energy changed by Earth tidal forces. The total amount of this change, over the binary's lifetime, is of the order of the energy itself. As a consequence, an initial population of binaries with separations of a few times the sum of the radii of the components evolves to a broader orbital distribution, which includes both increased separations, leading to formation of "loose binaries" and possibly to escape of the components, and smaller orbital distances, eventually creating contact binary systems. The existence of well-separated systems is suggested by the terrestrial cratering record, while contact systems have been recently observed by radar.