Mainbelt Asteroids: Results of Arecibo and Goldstone Radar Observations of 37 Objects during 1980–1995

We report detailed results of Arecibo and Goldstone radar observations of 30 mainbelt asteroids (MBAs) during 1980–1995. In addition to estimates of radar cross section, radar albedo, and circular polarization ratio, we obtain new constraints on pole direction for several asteroids, with those for 21 Lutetia being particularly restrictive. We carry out statistical analyses of disk-integrated properties (radar albedo and polarization ratio) of all 37 radar-observed MBAs. M asteroids seem to have higher radar albedos and a wider range of albedos than do asteroids from the other taxonomic classes; there is no evidence that C and S MBAs have different albedo distributions; and there is some suggestion, worthy of future study, that primitive B, F, G, and P asteroids are not as radar-bright as C and S objects. There is no statistically significant evidence that different taxonomic classes have different polarization ratio distributions, despite suggestions to the contrary based on visual inspection of these distributions. The similarity between the C and S albedo distributions implies similar near-surface regolith bulk densities. The hypothesis of ordinary chondritic composition for the S-class asteroids is reasonably consistent with the radar data, provided that these asteroids have typical lunar porosities. Nevertheless, it is possible that some of these targets have high-porosity regoliths of stony-iron composition. Our M-class sample presumably contains both metallic objects (such as 216 Kleopatra and, probably, 16 Psyche) and less metallic objects.