1620 Geographos and 433 Eros: Shaped by Planetary Tides?

Abstract

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 rubble piles, held together by self-gravity ; 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 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 Geographos (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 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, suggesting that it too may have been molded by tides in the past.

The Porosity of Dark Meteorites and the Structure of Low-Albedo Asteroids

“…Of particular interest are the bulk density values determined by the NEAR spacecraft as it flew by 253 Mathilde at 1.3 ± 0.2 g/cm 3 (Yeomans et al 1997) and a similar value of 1.2 +0.6 −0.2 determined by Merline and co-workers for 45 Eugenia (Merline et al 1999). These values are extremely low for meteoritic material but are consistent with an emerging picture of the low-density "rubble pile" structure of many asteroids , Richardson et al 1998, Bottke et al 1999. The low bulk density values are similar to determinations of the densities of the small martian moons Phobos and Deimos (Smith et al 1995, Kieffer et al 1992.…”

Petrogenesis of acapulcoites and lodranites: A shock-melting model

“…Many asteroids appear to be high-porosity bodies akin to rubble piles with densities as low as 1.2 g cm À3 (Bottke et al, 1999;Cheng and Barnouin-Jha, 1999;Veverka et al, 1999). In porous asteroids, collisional kinetic energy is distributed through relatively small volumes of material (Stewart and Ahrens, 1999) and efficiently converted into heat in the crater vicinity (Melosh, 1989;Housen and Holsapple, 1999;Britt et al, 2002).…”

YORP-Induced Long-Term Evolution of the Spin State of Small Asteroids and Meteoroids: Rubincam's Approximation

“…Again, though interesting, this number needs to be validated by a more precise model as already outlined. Geographos is thought to have undergone a "recent" close approach to the Earth that modified both its shape and its rotation state (e.g., Bottke et al 1999). Such events are rare enough that the YORP effect may secularly change its rotation state before the next deep encounter, but the ≈10-to 100-Myr dynamical lifetime of the Geographos orbit may prevent a significant effect.…”