Outburst of Comet 17p/Holmes Observed With Thesolar Mass Ejection Imager

We present time-resolved photometric observations of the Jupiter family comet 17P/Holmes during its dramatic 2007 outburst. The observations, from the orbiting Solar Mass Ejection Imager ( SMEI ), provide the most complete measure of the whole-coma brightness, free from the effects of instrumental saturation and with a time resolution well matched to the rapid brightening of the comet. The light curve is divided into two distinct parts. A rapid rise between the first SMEI observation on UT 2007 October 24 06h 37m (mid-integration) and UT 2007 October 25 is followed by a slow decline until the last SMEI observation on UT 2008 April 6 22h 16m (mid-integration). We find that the rate of change of the brightness is reasonably well described by a Gaussian function having a central time of UT 2007 October 24.54 ± 0.01 and a full width at half-maximum of 0.44 ± 0.02 days. The maximum rate of brightening occurs some 1.2 days after the onset of activity. At the peak, the scattering cross-section grows at 1070 ± 40 km 2 s −1 while the (model-dependent) mass loss rates inferred from the light curve reach a maximum at 3 × 10 5 kg s −1 . The integrated mass in the coma lies in the range (2–90) × 10 10 kg, corresponding to 0.2%–10% of the nucleus mass, while the kinetic energy of the ejecta is (0.7–30) megatonnes TNT. The particulate coma mass could be contained within a shell on the nucleus of thickness 1–60 m. This is also the approximate distance traveled by conducted heat in the century since the previous outburst of 17P/Holmes. This coincidence is consistent with, but does not prove, the idea that the outburst was triggered by the action of conducted heat, possibly through the crystallization of buried amorphous ice.