Hubble Space Telescope Observations of the Nucleus of Comet 45P/Honda–Mrkos–Pajdusakova and Its Inner Coma

Abstract

The nucleus of Comet 45P/Honda–Mrkos–Pajdusakova was detected during Hubble Space Telescope observations taken with the Wide-Field Planetary Camera 2 (WFPC2) in February 1996 when the comet was 0.17 AU from the Earth. The high spatial resolution (1 pixel projected to 5.6 km at the distance of the comet) allowed us to separate the signal of the nucleus from that of the coma, and images taken through five broadband filters allowed us to determine the UBVRI magnitudes of the nucleus in the Johnson–Kron–Cousins photometric system (sometime known as the Landolt system). Assuming a spherical body whose geometric albedo is 0.04 for the V band and a phase coefficient of 0.04 mag/deg, we derive an effective radius of 0.34 km. The uncertainty is dominated by the unknown phase law; the radius may be as small as 0.22 km if the phase coefficient is 0.03 mag/deg. The correlated variations of the magnitudes over 1 day suggest an elongated body with an axis ratio of at least 1.3. The strong reddening of the nucleus in the ultraviolet progressively decreases, and its color tends to neutral in the near infrared. We also derive a fractional active area of 11% and a dust production rate of 1.0 kg s−1 (86 metric tons day−1).