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We examine the rise and sudden demise of comet C/2010 X1 (Elenin) on itsapproach to perihelion. Discovered inbound at 4.2 AU, this long-period cometwas predicted to become very bright when near perihelion, at 0.48 AU on 2011September 10. Observations starting 2011 February (heliocentric distance$\sim$3.5 AU) indeed show the comet to brighten by about 11 magnitudes, withmost of the increase occurring inside 1 AU from the Sun. The peak brightnessreached $m_R$ = 6 on UT 2011 August 12.95$\pm$0.50, when at $\sim$0.83 AU fromthe Sun. We find that most of the surge in brightness in mid-August resultedfrom dust particle forward-scattering, not from a sudden increase in theactivity. A much smaller ($\sim$3 magnitudes) brightening reached a maximum onUT 2011 August 30$\pm$1 (at 0.56 AU), and reflects the true break-up of thenucleus. This second peak was matched by a change in the morphology fromcentrally condensed to diffuse. The estimated cross-section of the nucleus whenat 1 AU inbound was $\sim$1 km$^2$, corresponding to an equal-area circle ofradius 0.6 km. No surviving fragments were found to a limiting red magnitude$r'$ = 24.4, corresponding to radii $\lesssim$40 m (red geometric albedo = 0.04assumed). Our observations are consistent with disintegration of the nucleusinto a power law size distribution of fragments with index $q$ = 3.3$\pm$0.2combined with the action of radiation pressure. We speculate about physicalprocesses that might cause nucleus disruption in a comet when still 0.7 AU fromthe Sun. Tidal stresses and devolatilization of the nucleus by sublimation areboth negligible at this distance. However, the torque caused by mass loss, evenat the very low rates measured in comet Elenin, is potentially large enough tobe responsible by driving the nucleus to rotational instability.

Disappearance of Comet C/2010 X1 (Elenin): Gone with a Whimper, not a Bang