2008A&A...490..377T

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. 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 r≥4.9AU. The observations were performed at the ESO VLT with the FORS2 instrument. 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 12.7047±0.0011h and a linear phase function coefficient β=0.076±0.003mag/°. We estimated the large-to-small axis ratio a/b≥1.45±0.09 and, assuming an albedo of 0.04, an effective nucleus radius of 2.38±0.04km, which corresponds to the mean absolute magnitude H=15.35±0.04mag. 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 ∼28mag/arcsec² in R the filter. For the first time we also determined visible colours of the trail-like structure: (V-R)=0.95±0.14mag/arcsec² and (R-I)=0.39±0.14mag/arcsec². A trail-like structure is also barely visible in the May 2006 images, but a quantitative study was not possible.