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We present time-resolved photometric observations of Jupiter family comet17P/Holmes during its dramatic outburst of 2007. The observations, from theorbiting Solar Mass Ejection Imager (SMEI), provide the most complete measureof the whole-coma brightness, free from the effects of instrumental saturationand with a time-resolution well-matched to the rapid brightening of the comet.The lightcurve is divided into two distinct parts. A rapid rise between thefirst SMEI observation on UT 2007 October 24 06h 37m (mid-integration) and UT2007 October 25, is followed by a slow decline until the last SMEI observationon UT 2008 April 6 22h 16m (mid-integration). We find that the rate of changeof the brightness is reasonably well-described by a Gaussian function having acentral time of UT 2007 October 24.54+/-0.01 and a full-width-at-half-maximum0.44+/-0.02 days. The maximum rate of brightening occurs some 1.2 days afterthe onset of activity. At the peak the scattering cross-section grows at1070+/-40 km^2/s while the (model-dependent) mass loss rates inferred from thelightcurve reach a maximum at 3+/-10^5 kg/s. The integrated mass in the comalies in the range (2 to 90)x10^10 kg, corresponding to 0.2% to 10% of thenucleus mass, while the kinetic energy of the ejecta is (0.6 to 30) MTonnesTNT. The particulate coma mass could be contained within a shell on the nucleusof thickness ~1.5 to 60 m. This is comparable to the distance traveled byconducted heat in the century since the previous outburst of 17P/Holmes. Thiscoincidence is consistent with, but does not prove, the idea that the outburstwas triggered by the action of conducted heat, possibly through thecrystallization of buried amorphous ice.

Outburst of Comet 17P/Holmes Observed With The Solar Mass Ejection Imager