Hubble Space Telescope observations of the nucleus and inner coma of comet 67P/Churyumov-Gerasimenko

¹ Laboratoire d'Astrophysique de Marseille, BP 8, 13376 Marseille Cedex 12, France e-mail: philippe.lamy@oamp.fr
² Konkoly Observatory, Budapest, Hungary
³ Applied Physics Laboratory, The Johns Hopkins University, Laurel, MD, USA
⁴ Department of Mathematics and Statistics, University of Helsinki, Finland
⁵ Instituto de Astrofisica de Andalucia, Granada, Spain

Received: 22 March 2006
Accepted: 16 June 2006

Abstract

Context.Following the postponement of the launch of the Rosetta spacecraft scheduled in January 2003, comet 67P/Churyumov-Gerasimenko emerged as the most suitable new target. However a critical issue was the size, that is, the mass of its nucleus, as the surface module Philae was designed to land on a nucleus with a radius no larger than approximately 1.5 km.

Aims. It was therefore crucial to the success of the mission to achieve a timely characterization of the nucleus of 67P/C-G so as to take any proper action on the design needed before the new launch.

Methods. We used the Wide Field Planetary Camera 2 (WFPC2) of the Hubble Space Telescope (HST) whose superior spatial resolution enabled us to accurately separate the signal of the nucleus from that of the coma. The observations were performed on 12 and 13 March 2003, when the comet was 2.50 AU from the Sun, 1.52 AU from the Earth, and at a phase angle of 4.8°.

Results. The nucleus of comet 67P/Churyumov-Gerasimenko was easily detected, and we measured its V and R magnitudes in the Johnson-Kron-Cousins photometric system. Assuming that the nucleus is spherical, with a geometric albedo of 0.04 and a phase coefficient of 0.04 mag/deg for the R band, we derived a radius of  km. The spheroidal solution has semiaxes  km and  km and a rotational period of  h. We inverted the light curve and solved it for a full three-dimensional nucleus having an overall size of  km viewed at an aspect angle of (i.e., near equatorial view) at the time of our observations. When combined with other constraints, the direction of the pole is found at either RA = and Dec = (prograde rotation) or RA = and Dec = (retrograde rotation). The color of the nucleus is moderately red with . From an analysis of the dust coma, we derived  cm, a dust production rate  kg s^-1, and we characterized its color.