Albedos of Asteroids in Comet-Like Orbits

We present the results of a mid-infrared survey of 26 asteroids in comet-like orbits, including six Damocloids and six near-Earth asteroids (NEAs). We define a ''comet-like'' orbit as one having a Tisserand invariant TJ under 3 (but only including objects that are NEAs or otherwise unusual). Visible-wavelength data were also obtained, so geo- metric albedos (in theCousins R band)and effective radii are presented for 25 objects (plus one more with 3 ! limits) as derived using the NEA Thermal Model. Nine of our objects were observed at two or more mid-infrared wave- lengths, and in all cases the low-thermal inertia thermal model was found to be applicable, with various values of the beaming parameter. Our work more than quintuples the total number of observationally constrained albedos among TJ < 3asteroidsto32.Definingthe''comet-like''albedosasthosebelow0.075,wefindthat64% ! 5%ofthesample has comet-like albedos. Objects in comet-like orbits with comet-like albedos are candidates for being dormant or extinct comets. Indeed, one of our targets, 2001 OG108,became active again a few months after our observations. We find a very strong correlation between the albedo distribution and TJ, with the percentage of dark TJ < 3 asteroids beingmuch greater thanthat of theTJ > 3NEAs. Thereare 10NEAs among the 32 objects, andof those, 53% ! 9% have comet-like albedos. With the current crop of NEAs, this implies that about 4% of all known NEAs are extinct comets. A comparison of the histogram of TJ < 3 asteroid albedos with that of active cometary nuclei shows that the former has a larger spread.