Radar Observations of 1620 Geographos

Ostro, S. J., R. F. Jurgens, K. D. Rosema, R. S. Hudson, J. D. Giorgini, R. Winkler, D.K. Yeomans, D. Choate, R. Rose, M. A. Slade, S. D. Howard, D. J. Scheeres, and D. L. Mitchell (1996). Icarus 121, 46-66

Goldstone radar observations of Geographos from Aug. 28 through Sep. 2, 1994, yield over 400 delay-Doppler images whose linear spatial resolutions range from 75 m to 151 m, and 138 pairs of dual-polarization (OC, SC) spectra with one-dimensional resolution of 103 m. Each data type provides thorough rotational coverage. The images contain an intrinsic north/south ambiguity, but the equatorial view allows accurate determination of the shape of the radar-facing part of the asteroid's pole-on silhouette at any rotation phase. Sums of co-registered images that cover nearly a full rotation have defined the extremely elongated shape of that silhouette (Ostro et al. 1995, Nature 375, 474-477). Here we present individual images and co-registered sums over 30 degrees of rotation phase that show the silhouette's structural characteristics in finer detail and also reveal numerous contrast features "inside" the silhouette. Those features include several candidate craters as well as indications of other sorts of large-scale topographic relief, including a prominent central indentation. Protuberances at the asteroid's ends may be related to the pattern of ejecta removal and deposition caused by the asteroid's gravity field. The asteroid's surface is homogeneous and displays only modest roughness at centimeter-to-meter scales. Our estimates of radar cross section and the currently available constraints on the asteroid's dimensions are consistent with a near-surface bulk density between 2 and 3 g/cc. The delay-Doppler trajectory of Geographos' center of mass has been determined to about 200 m on Aug. 28 and to about 100 m on Aug. 31, an improvement of two orders of magnitude over pre-observation ephemerides.

Asteroid Radar Astronomy Publications