Ground-based Optical and Spitzer Infrared Imaging Observations of Comet 21p/giacobini-zinner

We present ground-based optical and Spitzer Space Telescope infrared (IR) imaging observations of the ecliptic (Jupiter-family) comet 21P/Giacobini-Zinner, the parent body of the Draconid meteor stream, during its 2005 apparition. The onset of nucleus activity occurred at a pre-perihelion heliocentric distance, rh 3.80 AU, while post-perihelion 21P was dusty (peak Afρ = 131 cm−1) and active out to heliocentric distances 3.3 AU following a logarithmic slope with rh of −2.04. Coma colors, V − R = 0.524 ± 0.003, R − I = 0.487 ± 0.004, are redder than solar, yet comparable to colors derived for other Jupiter-family comets. A nucleus radius of 1.82 ± 0.05 km is derived from photometry at quiescence. Spitzer images post-perihelion exhibit an extensive coma with a prominent dust tail, where excess emission (over the dust continuum) in the 4.5 μm Infrared Array Camera (IRAC) image arises from volatile gaseous CO and/or CO2. No dust trail was detected (3σ surface brightness upper limit of 0.3 MJy sr−1 pixel−1) along the projected velocity vector of comet 21P in the MIPS 24 μm image suggesting that the number density of trail particles is 7 × 10−11 m−3. The bolometric albedo of 21P derived from the contemporaneous optical and Spitzer observations is A(θ = 22◦) = 0.11, slightly lower than values derived for other comets at the same phase angle.