THE LIFETIME OF BINARY ASTEROIDS VS GRAVITATIONAL ENCOUNTERS AND COLLISIONS

We quantitatively assess the long-term stability of binary asteroids with respect to (1) gravitational encounters with passing asteroids and (2) physical collisions. The effects of gravitational encounters are estimated by a simple physical model, which is based on the distinction between penetrating, close, and far encounters, and on the assumption of a random-walk accumulation of the corresponding energy changes. Explicit computations of lifetimes vs disruption by the above-mentioned processes are then made as a function of the main parameters of the systems (separation, size of the components), and estimates are given for a few specific asteroids for which a binary structure has been inferred from different types of observations. The main conclusion is that, especially when bodies of comparatively small sizes are concerned, the lifetimes of asteroidal binaries are limited by the occurrence of disruptive collisions, while gravitational encounters are generally less important. The frequency of catastrophic impacts, however, is quite uncertain, owing to our poor knowledge of asteroid impact strengths and of the projectile flux in the 1-km size range. In general, for systems whose separation does not exceed a few tens of the primary's radius-a limit also set by tidal evolution and Jovian perturbations-typical lifetimes are comparable to or larger than the age of the Solar System. © 1991.