  We examine the development of the active asteroid 311P/PANSTARRS (formerly,
2013 P5) in the period from 2013 September to 2014 February using high
resolution images from the Hubble Space Telescope. This multi-tailed object is
characterized by a single, reddish nucleus of absolute magnitude $H \ge$
18.98$\pm$0.10, corresponding to an equal-area sphere of radius $\le$200$\pm$20
m (for assumed geometric albedo 0.29$\pm$0.09). We set an upper limit to the
radii of possible companion nuclei at $\sim$10 m. The nucleus ejected debris in
nine discrete episodes, spread irregularly over a nine month interval, each
time forming a distinct tail. Particles in the tails range from about 10 $\mu$m
to at least 80 mm in radius, and were ejected at speeds $<$1 m s$^{-1}$. The
ratio of the total ejected dust mass to the nucleus mass is
$\sim$3$\times$10$^{-5}$, corresponding to a global surface layer $\sim$2 mm
thick, or to a deeper layer covering a smaller fraction of the surface. The
observations are incompatible with an origin of the activity by impact or by
the sublimation of entrapped ice. This object appears to be shedding its
regolith by rotational (presumably YORP-driven) instability. Long-term fading
of the photometry (months) is attributed to gradual dissipation of near-nucleus
dust. Photometric variations on short timescales ($<$0.7 hr) are probably
caused by fast rotation of the nucleus. However, because of limited time
coverage and dilution of the nucleus signal by near-nucleus dust, we have not
been able to determine the rotation period.
