Comet 67P/Churyumov-Gerasimenko at a large heliocentric distance ^*

¹ Max Planck Institute fuer Sonnensystemforschung, Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany e-mail: [tubiana;boehnhardt;drahus]@mps.mpg.de
² Universidad Metropolitana de Ciencias de la Educación, Santiago, Chile e-mail: lbarrera@umce.cl

Received: 4 October 2007
Accepted: 15 July 2008

Abstract

Aims. The Jupiter family comet 67P/Churyumov-Gerasimenko (67P/C-G) is the target comet of ESA's ROSETTA mission. A detailed portrait of this comet has been drawn from observations around perihelion, but what needs to be enhanced is the description of the comet's behaviour at large heliocentric distance. It is not only important for planning the rendezvous of the ROSETTA spacecraft with the comet, but will also provide valuable information on basic physical properties of the nucleus and the possible distant activity of JFCs.

Methods. We performed broad-band imaging and longslit spectroscopy of 67P/C-G in the visible wavelength range in June 2004, May 2006, and August 2006, when the comet was at the heliocentric distance of AU. The observations were performed at the ESO VLT with the FORS2 instrument.

Results. The comet appears point-like at all observing epochs, indicating that no significant coma is present around the nucleus. From our 67P/C-G images, we determined a synodic rotational period of the nucleus of  h and a linear phase function coefficient  mag/°. We estimated the large-to-small axis ratio and, assuming an albedo of 0.04, an effective nucleus radius of  km, which corresponds to the mean absolute magnitude  mag. No colour variation with the rotational phase is found, based on broadband colour indices and reflectance spectra of the nucleus. A trail-like structure of heavy grains is clearly detected in June 2004, displaying a surface brightness of ~28 mag/ in R the filter. For the first time we also determined visible colours of the trail-like structure:  mag/´´ ² and  mag/´´ ². A trail-like structure is also barely visible in the May 2006 images, but a quantitative study was not possible.