Minor planets and related objects. XV. Asteroid (1620) Geographos

Observations were made in 1969 of lightcurves, brightness, colors, and polarization of asteroid (1620) Geogra- phos. The absolute magnitude of the lightcurve maximum is B (1,0) =16.08, and the colors, which d not change appreciably with phase, are B - V = +0.87 and U - B = +0.50. The lighteurves are relatively smooth with two distinct maxima and minima; there is, however, an unexplained variation in the arrival times of the minima. The largest amplitude is 2.03 mag, which is the largest observed on asteroids to date. One end of geographos may be rougher or of a different shape than the other. The geometric albedo is 0.1 6i0.03 at 0.4 m. The rotation is retrograde with a sidereal period of 5h1 3"23 . 9 1i0 03; the axis of rotation is at ? , = 200'i8', , = 60'i4' ecliptic longitude and latitude, respectively. Precession of the axis of rotation was not required to obtain this solution. An attempt was made to find a laboratory model which would reproduce the Geographos lightcurves. The best fitting model (a cylinder with hemispherical ends) had a length to width ratio of 2.7, which results in Geographos being 1.50i0. 15 km wide and 4.0i0.5 km long.