Center for Radiophysics and Space Research, Cornell University, Ithaca, NY 14853-6801
Department of Astronomy, University of Washington, Box 351580, Seattle, WA 98195
Osservatorio Astronomico di Torino, Strada Osservatorio 20, 10025 Pino Torinese (TO), Italy
Jet Propulsion Laboratory, California Institute of Technology, M/S 306-438, 4800 Oak Grove Drive, Pasadena, CA 91109-8099
Until recently, most asteroids were thought to be solid bodies whose shapes were determined largely by collisions with other asteroids ([Davis et al., 1989]). It now seems that many asteroids are little more than rubble piles, held together by self-gravity ([Burns 1998]); this means that their shapes may be strongly distorted by tides during close encounters with planets. Here we report on numerical simulations of encounters between a ellipsoid-shaped rubble-pile asteroid and the Earth. After an encounter, many of the simulated asteroids develop the same rotation rate and distinctive shape (i.e., highly elongated with a single convex side, tapered ends, and small protuberances swept back against the rotation direction) as 1620 Geographos. Since our numerical studies show that these events occur with some frequency, we suggest that Geographos may be a tidally distorted object. In addition, our work shows that 433 Eros, which will be visited by the NEAR spacecraft in 1999, is much like Geographos, which suggests that it too may have been molded by tides in the past.
asteroids, dynamics, celestial mechanics