  We have used the Spitzer 22-um peakup array to observe thermal emission from
the nucleus and trail of comet 103P/Hartley 2, the target of NASA's Deep Impact
Extended mission. The comet was observed on UT 2008 August 12 and 13, while the
comet was 5.5 AU from the Sun. We obtained two 200-frame sets of photometric
imaging over a 2.7-hour period. To within the errors of the measurement, we
find no detection of any temporal variation between the two images. The comet
showed extended emission beyond a point source in the form of a faint trail
directed along the comet's anti-velocity vector. After modeling and removing
the trail emission, a NEATM model for the nuclear emission with beaming
parameter of 0.95 +/- 0.20 indicates a small effective radius for the nucleus
of 0.57 +/- 0.08 km and low geometric albedo 0.028 +/- 0.009 (1 sigma). With
this nucleus size and a water production rate of 3 x 10^28 molecules s-1 at
perihelion (A'Hearn et al. 1995) we estimate that ~100% of the surface area is
actively emitting volatile material at perihelion. Reports of emission activity
out to ~5 AU (Lowry et al. 2001, Snodgrass et al. 2008) support our finding of
a highly active nuclear surface. Compared to Deep Impact's first target, comet
9P/Tempel 1, Hartley 2's nucleus is one-fifth as wide (and about one-hundredth
the mass) while producing a similar amount of outgassing at perihelion with
about 13 times the active surface fraction. Unlike Tempel 1, it should be
highly susceptible to jet driven spin-up torques, and so could be rotating at a
much higher frequency. Barring a catastrophic breakup or major fragmentation
event, the comet should be able to survive up to another 100 apparitions (~700
yrs) at its current rate of mass loss.
