HUBBLE-SPACE-TELESCOPE OBSERVATIONS OF THE NUCLEUS AND INNER COMA OF COMET 19P 1904 Y2 (BORRELLY)

The nucleus of comet 19P/Borrelly was detected using the Planetary Cam era (WFPC2) of the Hubble Space Telescope (HST). During the time of ou r observations, the comet was 0.62 AU from the Earth, 1.40 AU from the Sun, and had a solar phase angle of 38 degrees. The high spatial reso lution of the HST images allowed us to discriminate clearly between th e signal from the nucleus and that from the coma. The lightcurve of th e nucleus indicates that it is a highly elongated body rotating with a synodic period of 25.0 +/- 0.5 hr. Assuming that the nucleus has a ge ometric albedo of 4% and is a prolate spheroid with a rotational axis pointing in the direction determined by Sekanina (1979), we derive tha t its semi-axes are 4.4 +/- 0.3 km and 1.8 +/- 0.15 km. The correspond ing fractional active area of similar to 8% suggests a moderately acti ve comet. The highly anisotropic coma is dominated by a strong sunward fan, and the dust production rate exhibited signs of temporal variabi lity throughout our observations.