Comet 162P/Siding Spring: A Surprisingly Large Nucleus

We present an analysis of thermal emission from comet 162P/Siding Spring(P/2004 TU12) measured during its discovery apparition in 2004 December. Thecomet showed no dust coma at this time, so we have sampled emission from thecomet's nucleus. Observations using the Mid-Infrared Spectrometer and Imager(MIRSI) were performed at NASA's Infrared Telescope Facility, where the peak ofthe comet's spectral energy distribution was observed between 8 and 25 microns.In combination with the three near-IR spectra presented by Campins et al.(2006, Astron. J. 132, 1346) that show the Wien-law tail of the thermalemission, the data provide powerful constraints on surface properties of thenucleus. We find that the nucleus's effective radius is 6.0+/-0.8 km. This isone of the largest radii known among Jupiter-family comets, which is unusualconsidering that the comet was discovered only recently. Its geometric albedois 0.059+/-0.023 in the H band, 0.037+/-0.014 in the R band, and 0.034+/-0.013in the V band. We also find that the nucleus of 162P has little IR beaming, andthis implies that the nucleus has low thermal inertia. Including all near-IRspectra yields a beaming parameter of 1.01+/-0.20. This result is in agreementwith others showing that cometary nuclei have low thermal inertia and little IRbeaming. If confirmed for many nuclei, the interpretation of radiometry may notbe as problematic as feared.Comment: 17 pages, 4 figures, appearing in The Astronomical Journal, September 2006 issu