  We present time-resolved photometric observations of Jupiter family comet
17P/Holmes during its dramatic outburst of 2007. 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 lightcurve 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
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 while the (model-dependent) mass loss rates inferred from the
lightcurve reach a maximum at 3+/-10^5 kg/s. The integrated mass in the coma
lies in the range (2 to 90)x10^10 kg, corresponding to 0.2% to 10% of the
nucleus mass, while the kinetic energy of the ejecta is (0.6 to 30) MTonnes
TNT. The particulate coma mass could be contained within a shell on the nucleus
of thickness ~1.5 to 60 m. This is comparable to the 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.
