On UT 2000 August 21 we obtained simultaneous visible and mid-infrared observations
of Comet 9P/Tempel 1, the target of the upcoming NASA Discovery Program mission
Deep Impact. The comet was still quite active while 2.55 AU from the Sun (post-perihelion).
Two independent analyses of our data, one parameterizing the coma morphology and
the other modeling infrared spectrophotometry, show that the nucleus's cross section
at the time the data were taken corresponds to an effective radius of /3.0+/-0.2 km.
Based on visible-wavelength photometry of the comet taken during this observing run
and others in the summer of 2000, all of which show the rotational modulation of
the nucleus's brightness, we find that the infrared data were obtained near the
maximum of the light curve. If we assume that the nucleus's light curve had a
peak-to-valley range of /0.6+/-0.2 mag, then the mean effective radius is /2.6+/-0.2 km.
Visible-wavelength photometry of the nucleus, including data published by other groups,
lets us constrain the nucleus's R-band geometric albedo: /0.072+/-0.016. The nucleus's
flux contributed about 85% of the light in the mid-infrared images.