Radar observations of Comet 2P/Encke during the 2003 apparition

The nucleus of Comet 2P/Encke was detected with the Arecibo radar during the close approach of November, 2003, making this the first comet to yield radar detections at two different apparitions. Although the measured radar cross section of 1.0 km2 was close to that obtained in 1980, the Doppler bandwidth was nearly four times larger. Most of this bandwidth difference can simply be attributed to a different observing aspect relative to the spin axis proposed by Sekanina [1988, Astron. J. 95, 911] and Festou and Barale [2000, Astron. J. 119, 3119]. Comparison of the 2003 Doppler bandwidth with infrared-based size estimates supports an 11-h dominant rotation period and excludes slower 15- and 22-h periods that have also been suggested. If one assumes a short-axis-mode rotation with an 11-h period, then the Doppler bandwidth indicates that the nucleus is an oblong object with a long-axis dimension of 9 km. The estimated radar albedo of 0.05 is similar to that measured for C/IRAS–Araki–Alcock, providing further evidence that comet nuclei have relatively low surface densities of ∼0.5–1.0 g cm−3. No broadband echo component was detected from large coma grains despite predictions, based on optical/infrared models, that such a component might be detectable.