  We describe active asteroid 331P/Gibbs (2012 F5) using archival Hubble Space
Telescope data taken between 2015 and 2018. 331P is an outer main-belt active
asteroid with a long-lived debris trail that formed in 2011. Embedded in the
debris trail we identify 19 fragments with radii between 0.04 and 0.11 km
(albedo 0.05 assumed) containing about 1 percent of the mass of the primary
nucleus. The largest shows a photometric range (1.5 magnitudes), a V-shaped
minimum and a two-peaked lightcurve period near 9 hours, consistent with a
symmetric contact binary (Drahus and Waniak 2016). Less convincing explanations
are that 331P-A is a monolithic, elongated splinter or that its surface shows
hemispheric 4:1 albedo variations. The debris trail is composed of centimeter
sized and larger particles ejected with characteristic 10 cm s$^{-1}$ speeds
following a size distribution with index q = 3.7+/-0.1 to 4.1+/-0.2. The HST
data show that earlier, ground-based measurements of the nucleus were
contaminated by near-nucleus debris, which cleared by 2015. We find that the
primary nucleus has effective radius 0.8+/-0.1 km and is in rapid rotation
(3.26+/-0.01 hour, range 0.25 magnitudes, minimum density 1600 kg/m3 if
strengthless. The properties of 331P are consistent with a) formation about 1.5
Myr ago by impact shattering of a precursor body (Novakovic et al. 2014) b)
spin-up by radiation torques to critical rotation c) ejection of about 1
percent of the nucleus mass in mid-2011 by rotational instability and d)
subsequent evolution of the fragments and dispersal of the debris by radiation
pressure.
