Electronic Telegram No. 1018 Central Bureau for Astronomical Telegrams INTERNATIONAL ASTRONOMICAL UNION M.S. 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A. IAUSUBS@CFA.HARVARD.EDU or FAX 617-495-7231 (subscriptions) CBAT@CFA.HARVARD.EDU (science) URL http://www.cfa.harvard.edu/iau/cbat.html COMET C/2006 P1 (McNAUGHT) C. M. Lisse and N. Dello Russo, Applied Physics Laboratory, Johns Hopkins University; Y. Fernandez, University of Central Florida; G. H. Jones, Mullard Space Science Laboratory, University College, London; and M. Sitko, Space Science Institute, report that the Spitzer Space Telescope observed comet C/2006 P1 on May 4-5 using the IRS instrument when the comet was at r = 2.4 AU and Delta = 2.2 AU. A rounded, central source of emission due to the coma was found in the peak-up imager at 16 microns. The 5- to 35-micron spectrum of the outflowing dust showed only a mild excess (about 10 percent) due to silicate emission at 8-13 microns. The flux density at 10 microns was about 0.1 Jy, and at 20 microns was 0.6 Jy. The effective temperature of the dust was 190 +/- 10 K. The local equilibrium temperature at 2.4 AU was 182 K. Lisse et al. estimate a production rate of dust to be about 6 x 10^3 kg/s. The spectrum is remarkably featureless and dominated by infrared emission from large particles, and is similar to that derived from comet-surface mantles. This is unexpected for a comet that, in Dec. 2006-Jan. 2007, had demonstrated large outflows of material, a highly structured dust tail due to the presence of 0.1- to 10-micron dust particles, and was still emitting dust at the time of Spitzer observations at a rate comparable to the strongly-mid-infrared-featured comets C/1996 B2 (Hyakutake) and 1P/1982 U1 (Halley) when passing closest to the earth. Lisse et al. further surmise that either the comet has a very thick surface mantle that was only temporarily breached during the perihelion passage by a jet or the material being emitted in May 2007 is from a surface mantle that has regrown since the comet's perihelion passage four months earlier. NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2007 CBAT 2007 August 2 (CBET 1018) Daniel W. E. Green