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J.K. Harmon, M.C. Nolan (Arecibo Observatory)
The nucleus of Comet 2P/Encke was detected with the Arecibo S-band 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 0.84 km2 was close to that obtained in 1980, the 2003 Doppler bandwidth (22.9 Hz) was nearly four times larger. This bandwidth increase can be attributed simply to the change in observing aspect relative to a stable spin axis close to that specified by the pole directions of Sekanina (1988) and Festou and Barale (2000). Combining the 2003 Doppler bandwidth with infrared-based size estimates (Fernández et al., 2000; Campins, 1988) supports an 11-h dominant rotation period and precludes a slower (15-h) period that has also been suggested. If one assumes a short-axis-mode rotation with an 11.1-h period (Belton et al., 2004), then the radar data indicate that the nucleus is an oblong object with a long-axis dimension of 9.2 kilometers. The estimated radar albedo of 0.05 is similar to that measured for C/IRAS-Araki-Alcock, providing further evidence that comet nuclei have surface densities in the range 0.5-1.0 g/cm3. No broadband echo component was detected from large coma grains despite predictions, based on optical and infrared models, that such a component might be detectable.
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.