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abstract
The size distribution of comets, although crucial for assessing theories of their origin, remains largely uncertain. Measurements of cometary nuclear magnitudes have been made for a few comets when their activity level was low. Estimates for their corresponding nuclear diameters can then be made if their albedos are known. While albedos have been determined for only about 5 short period comets, those albedos have been consistently very low averaging about 3--4\%. Attempts to directly measure nuclear magnitudes have been made for only a small handful of objects. An alternate method which uses total magnitude measurements of comets, assumming that the measured total magnitudes approach their nuclear magnitudes when the object is far from the Sun has yielded measurements for about 60 short period comets.
The 0.91-m Spacewatch Telescope on Kitt Peak has been used since 1985 to make astrometric observations of a large number of comets. A Tektronix 2048$\times$2048 pixel CCD is used unfiltered at the f/5 Newtonian focus. The data include a large and relatively consistent set of measurements of faint, nearly quiescent comets. We have extracted nuclear magnitudes from these data by estimating and removing the contribution of any coma which may be present. Given the geometry at the moment of observation, the nuclear absolute magnitudes can then be calculated, and then, given an assummed albedo of 3\%, the nuclear sizes can be estimated. We find that the sizes of most short period comet nuclei are in the 2-10 kilometer range. While a trend of larger comets at larger perihelion distance is noted, this trend is most likely due to detection biases in the discovery of the comets. We find no short period comets with nuclear magnitudes fainter than 18.0 (diameter 2 kilometers assumming an albedo of 3\%) in our sample of 56 well observed short period comets. This may also be due to detection biases in the discoveries of comets, or it may be due to intrinsic properties of smaller short period comet nuclei. Detailed analysis of the nuclear magnitude measurements for P/Encke has resulted in a determination of the phase coefficient for that comet of 0.08 magnitudes/degree.
