The Nucleus of Comet 45P/Honda-Mrkos-Pajdusakova

P.P.L. Lamy (LAS/CNRS), I. Toth (Konkoly Obs.), M.F. A'Hearn (UMD), H.A. Weaver (JHU)

We present the detailed analysis of observations of comet 45P/Honda- Mrkos-Pajdusakova performed with the Planetary Camera (WFPC2) of the Hubble Space Telescope approximately 40 days after its past perihelion passage when the comet was at 0.17 AU from the Earth. A sequence of images taken with five broad-band filters was repeated on 4 and 5 Feb 1996. The high spatial resolution (a PC pixel projects to 5.6 km at the distance of the comet) allows to separate the nucleus signal from that of the coma and to determine the UBVRI magnitudes of the nucleus. Assuming a spherical body of geometric albedo equals to 0.04 for the V band and a phase coefficient of 0.04 mag/deg, we derive a radius of 0.35 km. The correlated variations of the magnitudes over 1 day suggest an elongated body with an axis ratio of at least 1.3. The normalized reflectivity spectrum is characterized by a strong gradient in the ultra-violet which progressively decreases and levels off in the near infra-red. We also derive a fractional active area of 10 percents and a dust production rate of 1.8 kg/sec.