The nucleus of comet P/Schwassmann-Wachmann 1

The thermal flux in the 20-μm Q filter band of comet P/Schwassmann-Wachmann 1 was measured some 77 days after the most recent previous eruption, and when the visual magnitude was about 17.6. Considerations of the eruptive history of the comet, and in particular its tendency to fade to a minimum threshold brightness level between eruptions, suggest that the measurement refers to an essentially bare nucleus. Using the standard photometric/radiometric technique for calculating the diameters and geometric albedos of asteroids and planetary satellites, and the assumption that the bare nucleus is measured, we find that the diameter of the nucleus is 40±5 km, and the geometric albedo is 0.13±0.04.