Recent Radar Modeling Results: 1620 Geographos and 4179 Toutatis

R.S. Hudson (WSU), S.J. Ostro (JPL)

We report on recent radar-based modeling of two near-Earth asteroids. For 1620 Geographos, which was to have been a Clementine I target, radar imaging at Goldstone in August 1994 revealed a very elongated body with peculiar protuberances at its ends (Ostro et al., Icarus 121, 46-66, 1996). Unfortunately the imaging geometry placed the radar close to the asteroid's equatorial plane for the entire experiment, a situation that provides no leverage to resolve the ``north-south ambiguity'' in the delay-Doppler radar images. However, the extensive Geographos lightcurve data set (Magnusson et al., Icarus 123, 227-244, 1996) provides additional geometrical diversity. Using these two data sets, we have produced a comprehensive physical model of Geographos' shape, spin state, and photometric properties. We will present this model, explore its limitations, and discuss the process of integrating radar and optical data in a single physical model.

For the Clementine II target 4179 Toutatis (Nature 387, 110, 1997) nine consecutive days of high-resolution delay-Doppler imaging at Goldstone during December 1996 extended the time base of available radar data from nineteen days to about four years (Ostro et al., BAAS in press). With small refinements to its non-principal-axis spin state, a physical model based on 1992 data (Hudson and Ostro, Science 270, 84-86, 1995) accounts for the orientation of the asteroid observed in the 1996 data.