Near-aphelion CCD photometry of Comet P/S Schwassmann-Wachmann 2

We present CCD photometry of Comet P/Schwassmann-Wachmann 2 obtained at 4.58 AU. The observations reveal cyclic variations in the comet brightness about a mean apparent red magnitude of 21.05 +/- 0.06 mag and a range of approximately 0.5 mag. We find a best-fit light curve period of 5.58 +/- 0.03 h, which we interpret as the rotation period of the nucleus. No coma was seen in the individual exposures, but a faint coma extension toward the west was observed in a summed image. The derived upper limit to the radius of the nucleus is 3.1 km (assuming a value 0.04 for the geometric albedo). This renders Schwassmann-Wachmann 2 one of the smallest comet nuclei yet studied. The presence of cometary activity in Schwassmann-Wachmann 2 at a large heliocentric distance is consistent with the hypothesis of Rickman et al. (1991) that comets new to the inner solar system possess relatively large free-sublimating areas on their surfaces. (Author)