Physical Model of Asteroid 1620 Geographos from Radar and Optical Data

We develop a physical model of asteroid 1620 Geographos using Goldstone delay–Doppler radar images obtained August 1994 (Ostro et al. 1996, Icarus121, 46–66) with resolution as fine as 75 m, and optical lightcurves obtained in 1969, 1983, and 1993–1994 (Magnusson et al. 1996, Icarus123, 227–244). The data set admits a geometric ambiguity that precludes a unique model. Within this constraint, our model has maximum dimensions of (5.0, 2.0, 2.1)±0.15 km and a volume of ≤8.8 km3, equivalent to a sphere of ≤2.56 km diameter. The radar equivalent spherical albedo is ≥0.12. The photometric solution provides Hapke parameters w≥0.22, g=−0.34±0.10, and θ=25±10° with assumed values h=0.02 and B0=1.32. The spin state solution does not differ significantly from that of Magnusson et al. having λ=55±6°, β=−46±4°, and P=5.2233270±0.00000072 h. We identify seven main features in the delay–Doppler images and their corresponding locations on the model.