Telescope observations of the nucleus and inner coma of comet 19 P / 1904 Y 2 ( Borrelly ) ?

The nucleus of comet 19P/Borrelly was detected using the Planetary Camera (WFPC2) of the Hubble Space Telescope (HST). During the time of our observations, the comet was 0.62 AU from the Earth, 1.40 AU from the Sun, and had a solar phase angle of 38◦. The high spatial resolution of the HST images allowed us to discriminate clearly between the signal from the nucleus and that from the coma. The lightcurve of the 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 geometric albedo of 4% and is a prolate spheroid with a rotational axis pointing in the direction determined by Sekanina (1979), we derive that its semi-axes are 4.4±0.3 km and 1.8±0.15 km. The corresponding fractional active area of ∼8 % suggests a moderately active comet. The highly anisotropic coma is dominated by a strong sunward fan, and the dust production rate exhibited signs of temporal variability throughout our observations.