We report Spitzer Space Telescope observations of comet
67P/Churyumov-Gerasimenko at 5.5 and 4.3 AU from the Sun, post-aphelion. Comet
67P is the primary target of the European Space Agency's Rosetta mission. The
Rosetta spacecraft will rendezvous with the nucleus at heliocentric distances
similar to our observations. Rotationally resolved observations at 8 and 24
microns (at a heliocentric distance, rh, of 4.8 AU) that sample the size and
color-temperature of the nucleus are combined with aphelion R-band light curves
observed at the Very Large Telescope (VLT) and yield a mean effective radius of
2.04 +/- 0.11 km, and an R-band geometric albedo of 0.054 +/- 0.006. The
amplitudes of the R-band and mid-infrared light curves agree, which suggests
that the variability is dominated by the shape of the nucleus. We also detect
the dust trail of the comet at 4.8 and 5.5 AU, constrain the grain sizes to be
less than or similar to 6 mm, and estimate the impact hazard to Rosetta. We
find no evidence for recently ejected dust in our images. If the activity of
67P is consistent from orbit to orbit, then we may expect the Rosetta
spacecraft will return images of an inactive or weakly active nucleus as it
rendezvous with the comet at rh = 4 AU in 2014.Comment: 19 pages, 2 tables, 10 figures. Accepted for publication in the
Astronomical Journa
