Deep Impact MRI Observations Of Comet Garradd (C/2009 P1)

The Deep Impact spacecraft observed comet C/2009 P1 Garradd with the Medium Resolution Instrument at ∼hourly intervals from 2012 Feb 20 to Mar 7, with additional observations between Mar 25 and Apr 8. Images were obtained with the broadband CLEAR filter and narrowband CN, OH, C2 and Green Continuum filters. During the time of the observations, the comet ranged from 1.75 to 2.11 AU from the Sun and 1.87 to 1.30 AU from the spacecraft. These images were used to measure the coma lightcurve, which showed a 1% peak-to-peak variation in the CLEAR filter measurements and a 4% variation in the continuum-subtracted CN observations. Periodicity analyses of the two lightcurves reveal a 10.4±0.05 hour period. Because the variation is produced by changes in the comet's activity, it is assumed that this is a single peaked lightcurve, with the 10.4 hour value representing the rotation period of the nucleus. The photometry was also used to compute the CN and OH production rates and Afρ for the times of observation. Average production rates derived from a 50-arcsec aperture for Feb 23, Mar 6 and Apr 6, were 2.3e28, 1.9e28, and 1.4e28 molecules/s for OH and 3.2e26, 2.7e26, and 1.5e26 molecules/s for CN, with relative and systematic uncertainties of 10% and 25%, respectively. Afρ of 3657, 3270, and 2946 cm, (±5%) were computed for the same dates. We will discuss these results in comparison with the DI High Resolution Instrument IR spectra obtained on Mar 26 and Apr 2, and with other ground-based observations.