Konkoly Observatory

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 0.2–15 km for the ECs and 1.6–37 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 well-measured 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 near-Earth 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 ill-defined 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 main-belt asteroids and NEOs (~2.2 h, excluding the monolithic fast rotators), and thi