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We present results from SEPPCoN, an on-going Survey of the Ensemble PhysicalProperties of Cometary Nuclei. In this report we discuss mid-infraredmeasurements of the thermal emission from 89 nuclei of Jupiter-family comets(JFCs). All data were obtained in 2006 and 2007 with the Spitzer SpaceTelescope. For all 89 comets, we present new effective radii, and for 57 cometswe present beaming parameters. Thus our survey provides the largest compilationof radiometrically-derived physical properties of nuclei to date. We concludethe following. (a) The average beaming parameter of the JFC population is1.03+/-0.11, consistent with unity, and indicating low thermal inertia. (b) Theknown JFC population is not complete even at 3 km radius, and even for cometswith perihelia near ~2 AU. (c) We find that the JFC nuclear cumulative sizedistribution (CSD) has a power-law slope of around -1.9. (d) This power-law isclose to that derived from visible-wavelength observations, suggesting thatthere is no strong dependence of geometric albedo with radius. (e) The observedCSD shows a hint of structure with an excess of comets with radii 3 to 6 km.(f) Our CSD is consistent with an intrinsic distribution that lacks manysub-kilometer objects.

Thermal Properties, Sizes, and Size Distribution of Jupiter-Family Cometary Nuclei