Infrared Observations of Comets by COBE

Comets C/Okazaki-Levy-Rudenko 1989 XX (C/OLR), C/Austin 1990 V, (C/Austin) P/Schwassmann-Wachmann 3 1990 VIII (P/SW3), and C/Levy 1990 XX (C/Levy) were detected by the COBE/Diffuse Infrared Background Experiment (DIRBE) with broadband photometry at 1-240 μm between 1989 November and 1990 September. Extended dust tails were found at 12 and 25 μm, with detailed structure due to variations in particle properties and mass-loss rate. Spectrophotometry of the central 42' × 42' was found to agree with that of a graybody of temperature 1.1 times the local blackbody temperature for C/OLR and C/Austin, while a nongraybody distribution with a spectral index of emissivity 0.26 +/- 0.15 and temperature 1.25 times the local temperature was found for C/Levy. A model using modified Mie dust particles composed of fractal mixtures of vacuum, silicates, and carbonaceous material was found to fit the observations. Comparison with IUE and ground-based observations indicates that large dark particles of radius greater than 20 μm predominate by surface area for C/OLR and C/Austin, but 1-10 μm particles predominate for C/Levy. The detection of P/SW3, an optically faint comet, was surprising, especially since four optically brighter comets were not detected by DIRBE. This may be related to the nuclear breakup observed during its next apparition. The total estimated mass loss from these comets in a perihelion passage is ~10 times larger than that expected from optical observations, and the loss rate is similar to that needed to supply the interplanetary dust cloud. No comet trails were detected to a limiting surface brightness of 1 MJy sr^-1, although large, β < 5 × 10⁴ particles, which could evolve into a dust trail, were detected in C/Austin.