Mid-infrared 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 November 1992. The images were taken through a 1 μm wide filter centered at 11.65 μm. During the observing run, the heliocentric distance decreased 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 pixel. In addition, photometric data at wavelengths between 3.6 and 18.5 μm were obtained on one night. The infrared images cover the cometary activity on time scales from hours to weeks and reveal large changes 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% higher 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 μm. The dust mass loss rate varied with the heliocentric distance as R^-6.3 and displayed ∼40% variation with the rotational phase. Two major jets were present in the images obtained 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 relative positions of the three active areas on the surface of the nucleus are consistent with the positions of the most stable active zones identified by Sekanina [AJ, 86, 1741 (1981)] from the data on the P/Swift-Tuttle apparition in 1862. The radial brightness profiles suggest the radius of the nucleus is 15±3 km. This implies that the nucleus of P/Swift-Tuttle is ∼34 times more massive than the nucleus of P/Halley.