2008A&A...489..777L

Comet 67P/Churyumov-Gerasimenko is the target of the Rosetta mission, and an early characterization of its nucleus is required to assist in preparing the orbital strategy of the spacecraft, the delivery of the Philae surface module and the science operations. We detected the nucleus using the Hubble Space Telescope in March 2003, but had to assume an albedo to derive its size from its observed magnitudes. It is important to derive an additional constraint for independently determining both the comet size and albedo. We implemented the well-known ``radiometric method'', which combines visible and infrared photometry. Sixteen thermal images of 67P/C-G were obtained by the Multiband Imaging Photometer (MIPS) 24 µm channel of the Spitzer Space Telescope (SST) on 25 February 2004: the observations spanned a time interval of ∼12.5h, which sampled the rotational light curve of its nucleus. The comet was then outbound at a heliocentric distance of 4.48AU, at a distance of 4.04AU from SST, and at a solar phase angle of 12.1°. The nucleus conspicuously appeared as a bright point source superimposed on a dust trail; it was necessary to apply the point-spread function fitting technique using an adequate model of the trail to correctly determine the thermal flux from the nucleus. The data were analyzed using a standard thermal model that incorporated the thermal inertia. Our preferred solution with a low thermal inertia has overall dimensions measured along the principal axis of 4.40-5.20km, 4.16-4.30km, and 3.40-3.50km, corresponding to an effective radius of a sphere with the same volume in the range of 1.93-2.03km. Larger values of thermal inertia produce larger sizes but the effective radius cannot exceed ∼2.3km. The albedo is in the range 0.039-0.043, remarkably consistent with the canonical value of 0.04 for cometary nuclei. The success of the landing of the Philae surface module remains critically dependent upon the bulk density of the nucleus: it would be safe if close to 0.35g/cm³, but a larger value, for instance 0.5g/cm³, would present some risks.