The sizes, shapes, albedos, and colors of cometary nuclei
Abstract
We critically review the data on the sizes, shapes, albedos, and colors of cometary nuclei. Reliable sizes have been determined for 65 ecliptic comets (ECs) and 13 nearly isotropic comets (NICs). The effective radii fall in the range 0.215 km for the ECs and 1.637 km for the NICs. We note that several nuclei recently measured by the Hubble Space Telescope are subkilometer in radius, and that only 5 of the 65 wellmeasured EC nuclei have effective radii larger than 5 km. We estimate that the cumulative size distribution (CSD) of the ECs obeys a single power law with an exponent qS = 1.9 ± 0.3 down to a radius of ~1.6 km. Below this value there is an apparent deficiency of nuclei, possibly owing to observational bias and/or mass loss. When augmented by 21 nearEarth objects (NEOs) that are thought to be extinct ECs, the CSD flattens to qS = 1.6 ± 0.2. The cumulative size distribution of NICs remains illdefined because of the limited statistical basis compared to ECs. The axial ratios a/b of the measured nuclei of ECs have a median value of ~1.5 and rarely exceed a value of 2, although it must be noted that the observed a/b values are often lower limits because of uncertainties in the aspect angle. The range of rotational periods extends from 5 to 70 h. The lower limit is significantly larger than that of mainbelt asteroids and NEOs (~2.2 h, excluding the monolithic fast rotators), and this has implications for the bulk density of cometary nuclei. By combining rotation and shape data when available, we find a lower limit of 0.6 g cm3 for the nucleus bulk density to ensure stability against centrifugal disruption. Cometary nuclei are very dark objects with globally averaged albedos falling within a very restricted range: 0.020.06, and possibly even narrower. (BV), (VR), and (RI) color indices indicate that, on average, the color of cometary nuclei is redder than the color of the Sun. There is, however, a large diversity of colors, ranging from slightly blue to very red. While two comets have wellcharacterized phase functions with a slope of 0.04 mag deg1, there is evidence for steeper (2P/Encke, 48P/Johnson) and shallower (28P/Neujmin 1) functions, so that the observed range is 0.0250.06 mag deg1. The study of the physical properties of cometary nuclei is still in its infancy, with many unresolved issues, but significant progress is expected in the near future from current and new facilities, both groundbased and spaceborne.
 Publication:

Comets II
 Pub Date:
 2004
 Bibcode:
 2004come.book..223L