Comet C/2012 S1 (Ison)

W. A. Delamere, Delamere Space Sciences; A. S. McEwen, Rod Heyd, and Sarah Mattson, University of Arizona; J.-Y. Li, Planetary Science Institute; and C. M. Lisse, Applied Physics Laboratory, Johns Hopkins University, report detection of comet C/2012 S1 using the Mars Reconnaissance Orbiter's High Resolution Imaging Science Experiment (HiRISE) on Sept. 29-Oct. 2 UT. The comet was detected in all five orbits (ten pointings) of MRO, with the signal-to-noise ratio increasing as the comet approached Mars and the phase angle of the observations rose from 47 to 89 deg. The authors find total V-band magnitudes of 16.2, 16.5, 16.7, and 16.5 in the 0".2 aperture at phase angles of 47, 47, 51, and 51 degrees for the first four observations. At closest approach, the HiRISE pixel scale at the comet was 13 km. Assuming all the observed flux to be due to scattered sunlight from the surface (noting that dust from the inner coma dominates the higher-phase-angle observations) and assuming a nucleus surface with an albedo of 0.03 with an adopted nucleus phase law of 10**(-0.016*phase angle), the authors find upper limits for the nucleus diameter of 1.25, 1.12, 1.05 and 1.12 km for the first four observations, with phase angles of 47 and 51 degrees for the broadband red filter. Similarly for the narrower blue-green filter, the upper limits are 1.25, 1.15, 0.95 and 1.12 km. B. P. Bonev, Center for Astrobiology (CfAb), Goddard Space Flight Center (GSFC), NASA, and Catholic University of America (CUA); M. A. DiSanti, CfAb, GSFC; E. L. Gibb, CfAb/GSFC and University of Missouri, St. Louis; G. L. Villanueva and L. Paganini, CfAb/GSFC and CUA; and M. J. Mumma, CfAb, GSFC, observed comet C/2012 S1 with CSHELL at the NASA Infrared Telescope Facility on Nov. 15, 16, 17, 18, and 22 UT, and report here their preliminary results. Multiple primary (parent) volatiles were detected, including H_{2}O, C_{2}H_{6}, CH_{3}OH, CO, HCN, NH_{3}, C_{2}H_{2}, and possibly CH_{4}, plus two fragmentation products (NH_{2} and OH). Based on seven spectral lines of H_{2}O, and assuming an atmospheric seeing correction (growth-factor) of 1.8, the global water-production rate varied between about 1.2 and 2.6 x 10**29 molecules/s during Nov. 15-18. The possible role of short-term variability in production rates, such as could be produced by nucleus rotation, is under consideration. On Nov. 17 (with the comet at r = 0.53 AU and Delta = 0.87 AU), the authors oriented the slit along the sun-comet line (position angle 289 degrees, as projected on the sky plane) and measured spatially resolved rotational temperatures (T_rot) and column densities for H_{2}O. They observed a decrease in T_rot with nucleocentric distance, from 115 K near the position of peak emission intensity to around 90 K at projected distances offset by about 1000 km. Distinct differences in the spatial distributions of the dust continuum intensity and H_{2}O column density are being investigated. The following revised orbital elements by G. V. Williams are from 6120 observations (2011 Sept. 30-2013 Nov. 22, mean residual 0".6) with non-gravitational parameters A_1 = +8.93 +/- 0.12 and A_2 assumed zero), with residuals published on MPEC 2013-W16. Williams also writes that the corresponding barycentric "original" and "future" values of 1/a are -0.00000465 and -0.00005417 AU**-1, respectively; the latter hyperbolic value indicates that the comet will be ejected from the solar system, while uncertainties in the non-gravitational forces presumably would explain away the "original" hyperbolic solution. Epoch = 2013 Dec. 14.0 TT T = 2013 Nov. 28.78555 TT Peri. = 345.56521 e = 1.0000013 Node = 295.65272 2000.0 q = 0.0124431 AU Incl. = 62.39824