Issue A: Evidence that ECOs are ``rubble piles''

Planetary tidal forces are, in general, too weak to modify the shapes of solid asteroids or comets unless the bodies are composed of very weak material ([Jeffreys 1947,Öpik 1950]). Recent evidence, however, supports the view that most km-sized asteroids (and comets) are weak ``rubble-piles'', aggregates of smaller fragments held together by self-gravity rather than material strength ([Chapman 1978], [Love and Ahrens 1996]). We list a few salient points; additional information can be found in Richardson et al. (1998). (i) Comet Shoemaker-Levy-9 (SL9) tidally disrupted when it passed within 1.6 planetary radii of Jupiter in 1992; numerical modelling suggests this could only have happened if SL9 were virtually strengthless ([Asphaug and Benz 1996]). (ii) C-class asteroid 253 Mathilde has such a low density (1.3 g ${\rm cm}^{-3}$; [Veverka et al., 1998]) compared to the inferred composition of its surface material (i.e., if carbonaceous chondrite-like, it would have a density $\sim 2$ g ${\rm cm}^{-3}$; Wasson 1985) that its interior must contain large void spaces, small fragments with substantial interparticle porosity, or a combination of the two. (iii) A set of 107 near-Earth and main belt asteroids smaller than 10 km shows no object with a rotation period shorter than 2.27 h; this spin rate matches where rubble-pile bodies would begin to fly apart from centrifugal forces ([Harris 1996]). (iv) Most collisionally evolved bodies larger than $\sim 1$ km are highly fractured, according to numerical simulations of asteroid impacts ([Asphaug and Melosh 1993,Greenberg et al., 1996,Love and Ahrens 1996]). (v) All of the small asteroids (or asteroid-like bodies) imaged so far by spacecraft (e.g., 253 Mathilde, 243 Ida, 951 Gaspra, and Phobos) have large craters on their surface, implying their internal structures are so broken up that they damp the propagation of shock waves and thereby limit the effects of an impact to a localized region ([Asphaug 1998]). If this were not the case, many of these impacts would instead cause a catastrophic disruption.

If the above lines of evidence have been properly interpreted, we can conclude that Geographos (and other ECOs) are probably rubble piles, since ECOs are generally thought to be collisionally evolved fragments derived from catastrophic collisions in the main belt. Thus, we predict that Geographos is weak enough to be susceptible to tidal distortion during a close pass with a planet.