THE EVOLUTION OF EARTH-APPROACHING BINARY ASTEROIDS - A MONTE-CARLO DYNAMICAL MODEL

We have developed a Monte Carlo model of the dynamical evolution of a binary asteroid crossing the Earth's orbit. At each close encounter with the Earth, the binary system undergoes impulsive variations of its orbital energy and angular momentum. Thus, after a great number of encounters, the system is dynamically evolved, and the two components may have either collided or escaped ''to infinity.'' Tidal interactions between any two successive encounters also contribute to this dynamical evolution and change the original rotation rates of the components. We have carried out simulations of the evolution process under a variety of initial conditions and confirm that in general the characteristic evolution time is shorter than the typical lifetime of Earth-crossing asteroids vs collisions with the inner planets (approximate to 100 Myr). The radar images obtained recently for several Earth-crossers reveal ''bifurcated'' structures which have been interpreted as contact binaries resulting from low-velocity collisions between two formerly separated components. In our scenario, such structures can be understood as resulting from dynamically evolved binary asteroids-although in the case of (4179) Toutatis the current rotation is so slow that it would require an unlikely cancellation of rotational and orbital angular momentum at the impact between the components. On the other hand, some other near-Earth objects with long rotation periods may be explained by tidal despinning within binary systems, followed by escape of the components. Moreover, collisions with the Earth of relatively young, well-separated binaries may explain the existence of doublet craters. (C) 1995 Academic Press, Inc.