Hubble Space Telescope observations of the nucleus of comet 45P/Honda-Mrkos-Pajdusakova and its inner coma

The nucleus of Comet 45P/Honda-Mrkos-Pajdusakova was defected 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 hig h spatial resolution (1 pixel projected to 5.6 km at the distance of the co met) 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 t he UBVRI magnitudes of the nucleus in the Johnson-Kron-Cousins photometric system (sometime known as the Landolt system). Assuming a spherical body wh ose 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 domi nated by the unknown phase law; the radius may be as small as 0.22 km if th e phase coefficient is 0.03 mag/deg. The correlated variations of the magni tudes 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 de creases, and its color tends to neutral in the near infrared. We also deriv e a fractional active area of 11% and a dust production rate of 1.0 kg s(-1 ) (86 metric tons day(-1)). (C) 1999 Academic Press.