Radar observations of comets

Seven comets have been detected by Earth-based radars during the period 198 0-1995. All but one of these gave a detectable echo fi om the nucleus, whil e three of the comets also showed a broad-band echo from large (similar to cm-size) grains in the inner coma. Although all observations have been of t he CW (continuous-wave) type, which precludes direct size measurement, the radar cross sections are consistent with nucleus diameters averaging a few kilometers and varying over a range of ten. Comparisons with independent si ze estimates indicate relatively low radar albedos, implying nucleus surfac e densities of 0.5 to 1 g/cm(3), The surfaces of comet nuclei appear to be as rough as typical asteroid surfaces, but are considerably less dense. Ana lysis of coma echoes indicates that some comets emit large grains at rates (similar to ton/s) which are comparable with their gas and dust production rates, There is also some indirect evidence for grain evaporation or fragme ntation within a few hundred to a few thousand kilometers of the nucleus. T he highest priority of future radar observations will be to obtain delay-Do ppler images of a nucleus, which would give direct size and shape estimates as well as a more reliable albedo. Delay-Doppler or interferometric imagin g of the coma echo would also help to better characterize the grain halo. T en short-period comets are potentially detectable during the next two decad es, although the best radar opportunities may well come from comets yet to be discovered. (C) 1999 Elsevier Science Ltd. All rights reserved.