INFRARED OBSERVATIONS OF COMETS BY COBE

Comets C/Okazaki-Levy-Rudenko 1989 XX (C/OLR), C/Austin 1990 V, (C/Aus tin) P/Schwassmann-Wachmann 3 1990 VIII (P/SW3), and C/Levy 1990 XX (C /Levy) were detected by the COBE/Diffuse Infrared Background Experimen t (DIRBE) with broadband photometry at 1-240 mu m between 1989 Novembe r and 1990 September. Extended dust tails were found at 12 and 25 mu m , with detailed structure due to variations in particle properties and mass-loss rate. Spectrophotometry of the central 42' x 42' was found to agree with that of a graybody of temperature 1.1 times the local bl ackbody temperature for C/OLR and C/Austin, while a nongraybody distri bution with a spectral index of emissivity 0.26 +/- 0.15 and temperatu re 1.25 times the local temperature was found for C/Levy. A model usin g 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 lar ge dark particles of radius greater than 20 mu m predominate by surfac e area for C/OLR and C/Austin, but 1-10 mu m particles predominate for C/Levy. The detection of P/SW3, an optically faint comet, was surpris ing, 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 i n a perihelion passage is similar to 10 times larger than that expecte d from optical observations, and the loss rate is similar to that need ed to supply the interplanetary dust cloud. No comet trails were detec ted to a limiting surface brightness of 1 MJy sr(-1), although large, beta < 5 x 10(4) particles, which could evolve into a dust trail, were detected in C/Austin.