Radar observations of Asteroid 2100 Ra-Shalom

We report Doppler-only (cw) radar observations of near-Earth Asteroid 2100 Ra-Shalom obtained at the Arecibo Observatory using a transmitter frequency of 2380 MHz (12.6 cm) on 1984 Aug. 18-22. Weighted and filtered sums of cw echoes achieve a maximum signal-to-noise ratio of 74 and cover the asteroi d in rotation phase. A weighted sum of all cw spectra gives an opposite cir cular (OC) radar cross section of 1.13 +/- 0.40 km(2) and a circular polari zation ratio of 0.31 +/- 0.02. Inversion of echo edge frequencies yields a convex hull with an elongation (maximum breadth/minimum breadth) of 1.15 +/ - 0.03 and places a lower bound on the maximum pole-on dimension of 2.4 km/ cos delta, where delta is the angle between the radar line-of-sight and the asteroid's apparent equator. Ra-Shalom has one of the least elongated pole -on silhouettes of the near-Earth asteroids for which similar shape informa tion from radar observations is available. Ra-Shalom's effective diameter ( diameter of a sphere with equal cross-sectional area) is constrained to a r ange of 2.4-3.6 km. We use a two-component radar scattering model to remove the "diffuse" contributions from Ra-Shalom's radar cross section and obtai n a surface bulk density estimate of 1.1-3.3 g cm(-3). When compared with r eported bulk densities and porosities of meteorites, our results are consis tent with either: (1) a C-class asteroid with carbonaceous-chondritic compo sition, effective diameter 2.6-3.6 km, and surface porosity <70%; or (2) an S-class asteroid with ordinary-chondritic or stony-iron composition, effec tive diameter 2.4-2.6 km, and little or no surface regolith. Ra-Shalom's ne ar-surface roughness appears to be globally heterogeneous. (C) 2000 Academi c Press.