Radar observations of three comets and detection of echoes from one: P/Grigg-Skjellerup

Radar echoes from the nucleus of the short-period comet Grigg-Skjellerup were obtained using the 12.6 cm wavelength radar of the Arecibo Observatory during the apparition of 20 May–2 June 1982. Dual circularly-polarized receiving channels were employed. The receiving mode orthogonally polarized to the transmission yielded an echo equal to nearly 10 times the standard deviation of the accompanying noise; no echo was detected in the polarization of the same sense as transmitted. The observations give a radar cross-section of 0.50±0.13 km2, and an upper limit to the width of the (unresolved) echo spectrum of less than 1 Hz. These results are consistent with specular reflection from a solid nucleus having a radius of less than 0.4 km. The upper limit depends on the unknown rotation rate and scattering law. Two other comets, comets Austin and P/Churyumov-Gerasimenko have also been probed but have not returned any detectable echo, thus allowing one to put only an upper limit on the sizes of their nuclei.