Hubble Space Telescope observations of the nucleus fragment 73P/Schwassmann–Wachmann 3-C

The investigation of fragmented comets provides information on the physical properties and internal structure of cometary nuclei, as well as insights into the mechanisms responsible for cometary breakups. The Jupiter-family Comet 73P/Schwassmann–Wachmann 3 (73P/SW3) fragmented non-tidally into at least four components, and probably more, in the autumn of 1995. Fragment C was detected with the Wide Field Planetary Camera 2 (WFPC2) of the Hubble Space Telescope (HST) on 26 November 2001 when it was 3.26 AU from the Sun and 2.34 AU from the Earth. The high spatial resolution of the HST allowed us to separate the signal of the fragment from that of its coma, and to determine its R magnitude in the Johnson–Kron–Cousins photometric system from four images taken with the F675W filter. Assuming a spherical body with a geometric albedo of 0.04 and a linear phase coefficient of 0.04 mag deg−1 for the R band, we derived an effective radius of 0.68 ± 0.04 km . The pre-breakup radius of the original nucleus was estimated to be 1.1 km, which implies that the volume of fragment C is ∼ 25 % of the total volume of the pre-breakup nucleus. The limited temporal coverage of our observations preclude deriving an accurate shape or rotational period; our measurements are consistent with a rather spherical body but an elongated shape cannot be excluded. Fragment C was very active despite its rather large heliocentric distance, with an estimated dust production rate of ∼ 1.5 kg s −1 (∼130 metric tons day−1). A very large fraction of the surface area of fragment C must have been sublimating to sustain such a high level of activity. Fragment C may be recovered at its next return in 2006, if it does not experience further fragmentation.