Conclusions

Current evidence implies that km-sized asteroids and comets are rubble piles. When these objects, in the form of ECOs, encounter a planet like the Earth, S-, B-, and M- class tidal disruptions frequently produce elongated objects ( $\varepsilon_{{\rm rem}} > 0.6$) with fast spin rates ($P \sim 5$ hours). These values are consistent with at least two objects in near-Earth space, 1620 Geographos and 433 Eros, which may have made a close slow encounter with Earth or Venus in the past. In addition, the shapes of our model asteroids that have been heavily distorted (and disrupted) by Earth or Venus's tidal forces resemble the radar-derived shapes of Geographos and Eros. Estimates of the frequency of tidal disruption events indicate that a small but detectable fraction of the ECO population should have Geographos-like spins and shapes. For these reasons, we believe that planetary tidal forces should be added to collisional processes as recognized important geological process capable of modifying small bodies.

We thank L. Benner, J. A. Burns, P. Farinella, S. Hudson, J. Jenkins, S. Ostro, and C. Thornton for useful discussions and critiques of this work. We also thank E. Asphaug and C. Chapman for his constructive reviews of this manuscript. PM worked on this paper while holding the External Fellowship of the European Space Agency. DCR was supported by grants from the NASA Innovative Research Program and NASA HPCC/ESS. Preparation of the paper was partly supported by NASA Grant NAGW-310 to J. A. Burns.