Until recently, most asteroids were thought to be solid bodies whose shapes
were determined largely by collisions with other asteroids. Recent work by
Burns and others has shown that many asteroids may be little more than rub
ble piles, held together by self-gravity; this means that their shapes may
be strongly distorted by tides during close encounters with planets. Here w
e report on numerical simulations of encounters between an ellipsoid-shaped
rubble-pile asteroid and Earth. After an encounter, many of the simulated
asteroids develop the same rotation rate and distinctive shape as 1620 Geog
raphos (i.e., highly elongated with a single convex side, tapered ends, and
small protuberances swept back against the rotation direction). Since our
numerical studies show that these events occur with some frequency, we sugg
est that Geographos may be a tidally distorted object. In addition, our wor
k shows that 433 Eros, which will be visited by the NEAR spacecraft in 1999
, is much like Geographos, suggesting that it too may have been molded by t
ides in the past.