We present ground-based optical and Spitzer Space Telescope infrared imaging observations of the ecliptic ( Jupiter-family ) comet 21P/Giacobini-Zinner , the parent body of the Draconid meteor stream , during its 2005 apparition .
Onset of nucleus activity occurred at a pre-perihelion heliocentric distance , r _ { h } \simeq 3.80 AU , while post-perihelion 21P was dusty ( peak Af \rho = 131 cm ^ { -1 } ) and active out to heliocentric distances \raisebox { -2.58 pt } { $ \stackrel { \raisebox { -0.86 pt } { $ \textstyle > $ } } { \sim } $ } 3.3 AU following a logarithmic slope with r _ { h } of -2.04 .
Coma colors , V - R = 0.524 \pm 0.003 ,R - I = 0.487 \pm 0.004 are redder than solar , yet comparable to colors derived for other Jupiter-family comets .
A nucleus radius of 1.82 \pm 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 IRAC image arises from volatile gaseous CO and/or CO _ { 2 } .
No dust trail was detected ( 3 \sigma 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 \raisebox { -2.58 pt } { $ \stackrel { \raisebox { -0.86 pt } { $ \textstyle < $ } } { \sim } $ } 7 % \times 10 ^ { -11 } m ^ { -3 } .
The bolometric albedo of 21P derived from the contemporaneous optical and Spitzer observations is A ( \theta = 22 \arcdeg ) = 0.11 , slightly lower than values derived for other comets at the same phase angle .