  The unique inner-belt asteroid 311P/PANSTARRS (formerly P/2013 P5) is notable
for its sporadic, comet-like ejection of dust in nine distinct epochs spread
over $\sim$250 days in 2013. This curious behavior has been interpreted as the
product of localized, equator-ward landsliding from the surface of an asteroid
rotating at the brink of instability. We obtained new Hubble Space Telescope
observations to directly measure the nucleus and to search for evidence of its
rapid rotation. However, instead of providing photometric evidence for rapid
nucleus rotation, our data set a lower limit to the lightcurve period, $P \ge$
5.4 hour. The dominant feature of the lightcurve is a V-shaped minimum,
$\sim$0.3 magnitudes deep, that is suggestive of an eclipsing binary. Under
this interpretation, the time-series data are consistent with a
secondary/primary mass ratio, $m_s/m_p \sim$ 1:6, a ratio of separation/primary
radius, $r/r_p \sim$ 4 and an orbit period $\sim$0.8 days. These properties lie
within the range of other asteroid binaries that are thought to be formed by
rotational breakup. While the lightcurve period is long, centripetal dust
ejection is still possible if one or both components rotates rapidly
($\lesssim$ 2 hour) and has a small lightcurve variation because of azimuthal
symmetry. Indeed, radar observations of asteroids in critical rotation reveal
"muffin-shaped" morphologies which are closely azimuthally symmetric and which
show minimal lightcurves. Our data are consistent with 311P being a close
binary in which one or both components rotates near the centripetal limit. The
mass loss in 2013 suggests that breakup occurred recently and could even be
on-going. A search for fragments that might have been recently ejected beyond
the Hill sphere reveals none larger than effective radius $r_e \sim$ 10 m.
