Spitzer Observations of Comet 67P/Churyumov-Gerasimenko at 5.5-4.3 AU From the Sun

In previous perihelion passages, the comet was not characterized over the orbital arc at which the Rosetta spacecraft will rendezvous with the nucleus (Agarwal et al. 2007b). Dynamical models by Fulle et al. (2004) of the dust coma and tail of 67P observed near perihelion in 2002-2003 have indicated that the comet is significantly active and able to eject millimeter and centimeter sized grains at 3.6 AU, post-aphelion (i.e., pre-perihelion). However, this conclusion is disputed by Agarwal et al. (2007b), who point out that the dust tail analyzed by Fulle et al. (2004) contains overlaping contributions from all dust ejected before the comet reached 1.5 AU. Direct imaging of the comet with ground-based observations have shown that the comet is active at 2.0 AU, post-aphelion (Agarwal et al. 2007b). Recent ground-based optical and Spitzer Space Telescope mid-infrared (mid-IR) observations have extended that limit to 2.9-3.0 AU (Kadota et al. 2008;Wooden et al. 2008). Some ecliptic comets are active at more than 4 AU from the Sun, e.g., comets 65P/Gunn, 74P/Smirnova-Chernykh, 152P/Helin-Lawrence, and 103P/Hartley (Lowry & Fitzsimmons 2001;Lowry et al. 1999;Snodgrass et al. 2008). Indeed, even comet 67P was brighter than expected for a bare nucleus and dust trail in Spitzer spectra of the comet at a heliocentric distance (r h ) of 4.98 AU, pre-aphelion (Kelley et al. 2006), and in ground-based CCD observations at 4.9 AU, pre-aphelion (Mueller 1992;Lamy et al. 2006)-equivalent postaphelion observations have only recently been presented (Tubiana et al. 2008). The present study of comet 67P is motivated by the lack of aphelion observations and the importance of the characterization of the nucleus and comet activity of 67P for the Rosetta mission.

In this paper, we present and discuss Spitzer Space Telescope (Gehrz et al. 2007;Werner et al. 2004) observations of comet 67P at heliocentric distances between 5.5 and 4.3 AU, postaphelion-the latter distance is comparable to the r h at which Rosetta will rendezvous with the comet nucleus. In §2, we present imaging observations of the comet obtained with both the Multiband Imaging Photometer for Spitzer (MIPS; Rieke et al. 2004) at 24 µm and the Infrared Array Camera (IRAC; Fazio et al. 2004) at 8 µm, and 14-35 µm spectra of the nucleus with the Infrared Spectrograph (IRS; Houck et al. 2004). In 24 µm MIPS images, we detect the comet’s dust trail (Sykes & Walker 1992) and a tail composed of large grains. We constrain the largest grain size ejected by 67P to be 6 mm, and then estimate the large grain number density and its impact hazard to Rosetta in §3.1. Our observations are consistent with an inactive nucleus between 5.5 and 4.3 AU, post-aphelion. We present upper-limits to emission from dust at 5.5 and 4.8 AU in §3.2. In §3.3, we derive the infrared (IR) light curve at 4.8 AU by sampling the emission from the nucleus at 11 epochs with consecutive MIPS and IRAC observations. The Spitzer derived light curves of comet 67P constrain the shape, color-temperature, and effective radius of the nucleus. We examine those properties and their behavior with rotational phase to show in §3.4 that the visible light curve is likely dominated by nucleus shape rather than albedo. However, there are some discrepancies between the visible and IR light curves, therefore some albedo variation cannot be ruled out. Our results are summarized in §4.

Spitzer observed comet 67P with the MIPS 24 µm instrument in Photometry/Super Resolution mode (pixel scale 2.55 arcsec pixel -1 ). This mode provides 14 dithered images covering an approximate field-of-view of 7 × 8 (each image is approximately 5 × 5 ). The comet nucleus is centered in the dither pattern, and the telescope tracks the comet with its computed non-sidereal rates. Each image has an exposure time of 10 s. The dither pattern was executed 9 times for each of 2 epochs at r h = 5.5 AU (2006 August 16 and 2006 September 01), 3 times for each of 11 epochs at 4.8 AU (2007 May 18), and once for each of 3 epochs at 4.4 AU (2007 September 19). The comet is readily identified as a moving point source, centered in the field-of-view, and bisected by the comet’s dust emission (Fig. 1), except at 4.4 AU where dust is not observed due to crowding by point sources and diffuse emission arisin

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