Photometric Observations and Modeling of Asteroid 1620 Geographos

Photometric observations of 1620 Geographos in 1993 and 1994 are presented and, in combination with previously published data, are used to derive models of Geographos. We estimate that the sidereal period of rotation is 0.21763860 ± 0.00000003 days (5h13m23.975s± 0.003s). The sense of rotation is retrograde. The ecliptic coordinates of the spin angular velocity vector are estimated to λp= 56° ± 6° and βp= −47° ± 4° (equinox J2000.0). The lightcurve amplitudes are well-explained by an ellipsoidal model with axis ratiosa/b= 2.58 ± 0.16 andb/c= 1.00 ± 0.15. Models that have one or both ends more sharply pointed than the ellipsoid improve the fit to the observations. There are no significant indications of albedo variegation, but non-geometric scattering effects are tentatively suggested based on significant rotational color variation.