C © 2014. The American Astronomical Society. All rights reserved. Printed in the U.S.A. DISINTEGRATING ASTEROID P/2013 R3

Splitting of the nuclei of comets into multiple components has been frequently observed but, to date, no main-belt asteroid has been observed to break up. Using the Hubble Space Telescope, we find that main-belt asteroid P/2013 R3 consists of 10 or more distinct components, the largest up to 200 m in radius (assumed geometric albedo of 0.05) each of which produces a coma and comet-like dust tail. A diffuse debris cloud with total mass ∼2 × 108 kg further envelopes the entire system. The velocity dispersion among the components, ΔV ∼ 0.2–0.5 m s−1, is comparable to the gravitational escape speeds of the largest members, while their extrapolated plane-of-sky motions suggest a break up between 2013 February and September. The broadband optical colors are those of a C-type asteroid. We find no spectral evidence for gaseous emission, placing model-dependent upper limits to the water production rate 1 kg s−1. Breakup may be due to a rotationally induced structural failure of the precursor body