MRO HiRISE Observations of the Nucleus of Comet C/2013 A1 (Siding Spring)

Comet C/2013 A1 (Siding Spring) will pass Mars at a distance of 131,000 ± 3,000 km on Oct 19, 2014. Mars Reconnaissance Orbiter (MRO) will observe the comet on multiple orbits within 2.5 days before and after closest approach. The HiRISE telescope has a 50 cm aperture and an instantaneous field of view of one micro-radian per pixel, so the best image will have a scale near 131 m/pixel. If imaging is successful (i.e., not too smeared) and if the nucleus is larger than ~200 m diameter, this will provide the first resolved images of the nucleus of a long-period comet. Previous resolved images cover the nuclei of short-period comets, whose surfaces have been processed by many close passes by the sun. The encounter velocity is 56 km/s, so the best observations of the nucleus are only possible a few hours near closest approach, as seen from portions of 3 MRO orbits. A total of 20 HiRISE observations are planned for these 3 orbits, with exposure times varying from 0.13 to 2.5 s to cover a range of potential brightness. The HiRISE CCDs are operated at much higher temperatures, 20-30C, than those used for astronomical observations, so the maximum exposure time is limited by dark current signal. HiRISE objectives are to measure the shape and dimensions of the nucleus, determine its albedo (currently unknown for long-period comets), measure the rotation period and axis, and observe the inner coma for bright jets or outbursts. The phase angles range from 90 to 107 degrees, which will make imaging the nucleus challenging. The fast relative motions and changing geometries of the comet and MRO also make it challenging to acquire unsmeared images, since the image must pass directly down the 128 lines used for time-delay integration (TDI). Although the comet's trajectory is probably well known (as nongravitational effects have not been detected), the exact timing is uncertain. Small timing errors could result in the image passing down the TDI rows at an angle, producing smeared images. Larger timing errors could result in the comet falling outside the HiRISE image boundaries.