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M.S. Kelley, C.E. Woodward (U. Minnesota), D.E. Harker (UCSD), W.T. Reach (SSC/Caltech), R.D. Gehrz (U. Minnesota), Spitzer GO Comet Team
The \textit{Spitzer} Space Telescope/Infrared Spectrograph (IRS) enables efficient and detailed investigation of dust properties in a wide range of comets (i.e. from diverse families) through observation and analysis of their mid-infrared spectra. We have observed 24 Jupiter-family, 7 Oort cloud, and 2 Halley family comets. The end products of our survey will consist of nuclei sizes, particle size distributions, and dust productions for all program targets and dust mineralogy for the best observed cases. Here we discuss our data reduction method and present preliminary results for three comets. A combination of imagery and spectroscopy appears to be the best strategy to separate the nucleus (a point source) and coma (extended emission), a requirement to spectrally calibrate the narrow slit spectra. Model fits to the spectra of the Jupiter-family comet 2P/Encke at 2.6\,AU from the Sun reveals a peak grain size of 0.4\,\mum and no evidence for silicates. The Oort cloud comet C/2001~HT50 (LINEAR-NEAT) has a radiometric nuclear radius of 6.1\,km (assuming a geometric albedo of 0.04 and an IR beaming parameter of 1.0) and, again, no evidence for silicates at 3.2 and 4.6\,AU from the Sun. 67P/Churyumov-Gerasimenko, the target of the European Space Agency's Rosetta spacecraft, is a point source in our peak up image at 5.0\,AU and has a broadband flux comparable to a \approx4.5\,km nucleus.
This work is supported in part by a NASA/JPL contract 1263741 to the University of Minnesota, NSF grant AST037-446, and the University of Minnesota Doctoral Dissertation Fellowship.
Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.