Radar Observations of Asteroid 2100 Ra-Shalom

Abstract

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 asteroid in rotation phase. A weighted sum of all cw spectra gives an opposite circular (OC) radar cross section of 1.13±0.40 km 2 and a circular polarization 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 δ, where δ 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 information from radar observations is available. Ra-Shalom's effective diameter (diameter of a sphere with equal cross-sectional area) is constrained to a range 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 obtain a surface bulk density estimate of 1.1–3.3 g cm −3. When compared with reported bulk densities and porosities of meteorites, our results are consistent with either: (1) a C-class asteroid with carbonaceous-chondritic composition, effective diameter 2.6–3.6 km, and surface porosity <70%; or (2) an S-class asteroid with ordinary-chondritic or stony-iron composition, effective diameter 2.4–2.6 km, and little or no surface regolith. Ra-Shalom's near-surface roughness appears to be globally heterogeneous.