  We report the detection of comet 67P/Churyumov-Gerasimenko's dust trail and
nucleus in 24 micron Spitzer Space Telescope images taken February 2004. The
dust trail is not found in optical Palomar images taken June 2003. Both the
optical and infrared images show a distinct neck-line tail structure, offset
from the projected orbit of the comet. We compare our observations to simulated
images using a Monte Carlo approach and a dynamical model for comet dust. We
estimate the trail to be at least one orbit old (6.6 years) and consist of
particles of size >~100 micron. The neck-line is composed of similar sized
particles, particles of size but younger in age. Together, our observations and
simulations suggest grains 100 micron and larger in size dominate the total
mass ejected from the comet. The radiometric effective radius of the nucleus is
1.87 +/- 0.08 km, derived from the Spitzer observation. The Rosetta spacecraft
is expected to arrive at and orbit this comet in 2014. Assuming the trail is
comprised solely of 1 mm radius grains, we compute a low probability (~10^-3)
of a trail grain impacting with Rosetta during approach and orbit insertion.
