Before the flyby of Halley's Comet in 1986, little was known about the masses of comets because they produce no detectable gravitational perturbations from which a gravitational mass could be estimated. The best values were found by guessing the composition of the comet and getting a density, usually in the range from 0.2 to 1.0 grams/cc typical of mixtures dominated by ices but with traces of dust grains in a 'Dirty Snowball'. The physical sizes of some comets were then estimated from angular sizes and their distances to be in the range from 1 - 3 kilometers in radius. Taking the average density and the volume, you then derive masses from 10^13 to 10^21 grams, with most falling between 10^14 to 10^17 grams.
The flyby of Halley's Comet let us refine these numbers for one specific comet. It was found to have a mean density of 0.25 grams/cc, and a size of 7.5 x 8.2 x 16 kilometers, making it a large comet. Its mass was then estimated as 1.5 x 10^14 kilograms, with an uncertainty of about 40 percent or so.
Copyright 1997 Dr. Sten Odenwald
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