The Astrophysical Journal 728, 31−39, 2011
© The American Astronomical Society

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

J. Li
Inst. for Geophysics and Planetary Physics, Univ. of California Los Angeles

D. Jewitt
Inst. for Geophysics and Planetary Physics, Univ. of California Los Angeles
Depts. Earth and Space Sciences and Physics and Astronomy, Univ. of California Los Angeles

J.M. Clover and B.V. Jackson
Cntr. for Astrophysics and Space Sciences, Univ. of California San Diego


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 06h37m (mid-integration) and UT 2007 October 25, is followed by a slow decline until the last SMEI observation on UT 2008 April 6 22h16m (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 km2 s−1 while the (model-dependent) mass loss rates inferred from the lightcurve reach a maximum at 3×105 kg s−1. The integrated mass in the coma lies in the range (2 to 90)×1010 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 travelled 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.