  Quasi-Hilda asteroid P/2010 H2 (Vales) underwent a spectacular photometric
outburst by 7.5 magnitudes (factor of 1000) in 2010. Here, we present our
optical observations of this event in the four month period from April 20 to
August 10. The outburst, starting UT 2010 April 15.76, released dust particles
of total cross-section 17,600 sq km (albedo 0.1 assumed) and mass 1.2e9 kg,
this being about 1e-4 of the mass of the nucleus, taken as a sphere of radius
1.5 km and density 500 kg/m3. While the rising phase of the outburst was very
steep (brightness doubling time of hours), subsequent fading occurred slowly
(fading timescales increasing from weeks to months), as large, low velocity
particles drifted away from the nucleus. A simple model of the fading
lightcurve indicates that the ejected particles occupied a broad range of
sizes, from microns to centimeters, and followed a differential power-law
distribution with index 3.6+/-0.1 (similar to that in other comets). The
fastest particles had speeds 210 m/s, indicating gas-drag acceleration of small
grains well-coupled to the flow. Low energy processes known to drive mass loss
in active asteroids, including rotational disruption, thermal and desiccation
stress cracking, and electrostatic repulsion, cannot generate the high
particles speeds measured in P/Vales, and are discounted. Impact origin is
unlikely given the short dynamical lifetimes of the quasi-Hildas and the low
collision probabilities of these objects. The specific energy of the ejecta is
estimated at 220 J/kg. The outburst follows a series of encounters with Jupiter
in the previous century, consistent with the delayed activation of buried
supervolatiles (and/or the crystallization of sub-surface amorphous ice) by
conducted heat following an inward displacement of the perihelion. A potential
origin in the debris cloud produced by avalanche is also considered.
