Comet C/2012 S1 (ISON): Final observations from the Deep Impact spacecraft

The Deep Impact spacecraft observed comet C/2012 S1 (ISON) between 17 January and 10 March 2013 when the comet was ∼5 AU from the Sun. Continuous, high-cadence, images spanning as much as 6 days at a time, and high-cadence IR spectral scans spanning 2 days, represent the most intensive set of observations available from the early part of ISON's apparition. These observations were used to investigate the comet's detailed behavior, including variability in the lightcurve and changes in the coma morphology. ISON experienced a gradual brightening throughout this time period, with A(0)fρ increasing from 1150 cm in January to 1430 cm in March. Although no periodic variability was detected to a level <3%, DI did record several events showing the comet spontaneously brightening by 10-15% for several hours, indicating that the comet was experiencing spontaneous bursts of enhanced activity. These small outbursts may be the result of residual pockets of the volatiles that drove the rapid brightening seen between 8 and 5 AU. No changes were detected in the coma morphology over the course of the observations, and no gas emission was detected in either the narrowband comet gas filters or the IR spectra.