Pre-perihelion characterization of Comet C/2012 S1 (ISON)

Comet C/2012 S1 (ISON) is a dynamically new comet on a sungrazing orbit. As such, C/ISON represents a unique opportunity to study both the cosmic-ray-irradiated surface, produced during the comet's long residence in the Oort cloud, and much deeper layers in the nucleus, exposed when the comet passes within 2 solar radii of the Sun at perihelion. During the first phase of our investigation, we collected broadband images of C/ISON on April 10, 2012 at a heliocentric distance of 4.15 AU, using the Hubble Space Telescope WFC3/UVIS. We used the F606W and F438W filters in three HST orbits covering a total span of ~19 hrs. The comet shows a well delineated coma in the sunward direction extending about 2" from the nucleus, and a dust tail at least 25" long. The coma has an average red color of 5%/0.1 micron within 1.6" from the nucleus, becoming redder towards the tail. Both the color and the size of the coma in the sunward direction are consistent with outflow of micron sized dust. Broadband photometry yielded Afρ of 1376 cm at 589 nm, and 1281 cm at 433 nm, measured with a 1.6" radius aperture. The total brightness of the comet within a 0.12" radius aperture remained unchanged within 0.03 mag for the entire duration of the observations. A well defined sunward jet is visible after removing the 1/ρ brightness distribution. The jet is centered at position angle 290 deg (E of Celestial N), with a cone angle of 45 deg, a projected length of 1.6", and a slight curvature towards the north near the end. No temporal change in the morphology is observed, suggesting the jet is circumpolar. Under this assumption, the jet's apparent position constrains the rotational pole to lie within 30 deg of (RA, Dec) = (330, 0), and an obliquity of 50-80 deg. Preliminary analysis using a coma-nucleus separation technique suggests a nuclear radius less than 2 km. The survival of such a small nucleus during its perihelion at 2.7 solar radii is certainly questionable.