Hubble Space Telescope Observations of Main Belt Comet (596) Scheila

We present Hubble Space Telescope Observations of (596) Scheila during its recent dust outburst. The nucleus remained point-like with absolute magnitude V(1,1,0) = 8.85+/-0.02 in our data, equal to the pre-outburst value, with no secondary fragments of diameter =>100 m (for assumed albedos 0.04). We find a coma having a peak scattering cross-section ~2.2x10^4 km^2 corresponding to a mass in micron-sized particles of ~4x10^7 kg. The particles are deflected by solar radiation pressure on projected spatial scales ~2x10^4 km, in the sunward direction, and swept from the vicinity of the nucleus on timescales of weeks. The coma fades by ~30% between observations on UT 2010 December 27 and 2011 January 04. The observed mass loss is inconsistent with an origin either by rotational instability of the nucleus or by electrostatic ejection of regolith charged by sunlight. Dust ejection could be caused by the sudden but unexplained exposure of buried ice. However, the data are most simply explained by the impact, at ~5 km/s of a previously unknown asteroid ~35 m in diameter.