Constraining the Dust Coma Properties of Comet C/Siding Spring (2013 A1) at Large Heliocentric Distances

The close encounter of comet C/2013 A1 (Siding Spring) with Mars on 2014 October 19 presented an extremely rare opportunity to obtain the first flyby quality data of the nucleus and inner coma of a dynamically new comet. However, the comet's dust tail potentially posed an impact hazard to those spacecraft orbiting Mars. To characterize the comet at large heliocentric distances, study its long-term evolution, and provide critical inputs to hazard modeling, we imaged C/Siding Spring with the Hubble Space Telescope when the comet was at 4.58, 3.77, and 3.28AU from the Sun. The dust production rate, parameterized by the quantity Af rho, was 2500, 2100, and 1700 cm (5000 km radius aperture) for the three epochs, respectively. The color of the dust coma is (5.0 +/- 0.3)%/100 nm for the first two epochs, and (9.0 +/- 0.3)%/100 nm for the last epoch, and reddens with increasing cometocentric distance out to similar to 3000 km from the nucleus. The spatial distribution and the temporal evolution of the dust color are most consistent with the existence of icy grains in the coma. Two jet-like dust features appear in the northwest and south-southeast directions projected in the sky plane. Within each epoch of 1-2 hr, no temporal variations were observed for either feature, but the position angle of the south-southeastern feature varied between the three epochs by similar to 30 degrees. The dust feature morphology suggests two possible orientations for the rotational pole of the nucleus, (R.A., decl.) = (295 degrees +/- 5 degrees, +43 degrees +/- 2 degrees) and (190 degrees +/- 10 degrees, + 50 degrees +/- 5 degrees), or their diametrically opposite orientations.