Has 1620 Geographos Been Reshaped by Planetary Tides?

W. F. Bottke (Cornell U.), D. C. Richardson (U. Washington), S. G. Love (JPL)

Images of Earth-crossing asteroid 1620 Geographos obtained using delay-Doppler radar techniques show it to be highly elongated (aspect ratio 2.76; dimensions tex2html_wrap_inline21 km; Ostro et al., 1995, Nature 375, 474). Moreover, the rotation period of Geographos is so short (5.22 h) that its ends are near the rotational break-up limit. Interestingly, these ends are also swept back slightly against the direction of rotation, giving Geographos the appearance of a ``pinwheel'' when its silhouette is viewed from various orientations (Ostro et al. 1996, Icarus 121, 46).

Since these features seem antithetical with a collisional origin, Solem and Hills (1996, AJ 111, 1382) suggested that Geographos might have been reshaped by planetary tides during a close encounter with a planet. We investigated this idea further using a sophisticated N-body code which models km-sized asteroids and comets as aggregates of spherical particles held together by self-gravity (Bottke et al., 1997, Icarus 126, 470). Testing non-spherical progenitors with different initial rotation states (P= 4-20 h), spin axis orientations, and hyperbolic encounter trajectories with Earth, we found a wide range of disruption endstates. In particular, tidal torques cause some objects to be stretched and spun-up until they suffer mass shedding. Particles near the ends of these distorted bodies were often swept back by Keplerian shear, yielding Geographos-like shapes. We argue that many of Geographos's physical characteristics are diagnostic of tidal distortion.

This hypothesis is strengthened by Geographos's orbital parameters (a = 1.25 AU, e = 0.336, i = 13.3 tex2html_wrap_inline33 ), which cause it to make frequent close encounters with Earth at low enough velocities that tidal distortion/disruption is possible (Milani et al., 1989, Icarus 78, 212). Since many ECAs share these orbital characteristics, and planetary close encounters are more likely than collisions, we find it probable that many ECAs have been reshaped by tidal effects or are by-products of tidal disruption (e.g. 4769 Castalia; 4179 Toutatis).