MIDINFRARED OBSERVATIONS OF THE NUCLEUS AND DUST OF COMET P SWIFT-TUTTLE/

We report pre-perihelion observations of comet P/Swift-Tuttle obtained with the UCSD mid-infrared astronomical camera on 12 nights in Novemb er 1992. The images were taken through a 1 mu m wide filter centered a t 11.65 mu m. During the observing run, the heliocentric distance decr eased from 1.13 to 0.98 AU, and the geocentric distance increased from 1.16 to 1.33 AU. The spatial scale of the images was 700-800 km per p ixel. In addition, photometric data at wavelengths between 3.6 and 18. 5 mu m were obtained on one night. The infrared images cover the comet ary activity on time scales from hours to weeks and reveal large chang es in the overall morphology of the coma. From periodic changes in the jet patterns we determined the period of nuclear rotation of 67.5+/-0 .4 h. Photometry indicates that the temperature of the coma was 35% hi gher than the blackbody temperature at the same heliocentric distance and, hence, that the coma was dominated by small particles of average radius of 0.7 mu m. The dust mass loss rate varied with the R(-6.3) an d displayed similar to 40% variation with the rotational phase. Two ma jor jets were heliocentric distance as R present in the images obtaine d on November 07-17, and a third area became active on November 24-29, increasing the average dust loss rate by a factor of 1.4. The relativ e positions of the three active areas on the surface of the nucleus ar e consistent with the positions of the most stable active zones identi fied by Sekanina [AJ, 86, 1741 (1981)] from the data on the P/Swift-Tu ttle apparition in 1862. The radial brightness profiles suggest the ra dius of the nucleus is 15+/-3 km. This implies that the nucleus of P/S wift-Tuttle is similar to 34 times more massive than the nucleus of P/ Halley. (C) 1995 American Astronomical Society.