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The unique inner-belt asteroid 311P/PANSTARRS (formerly P/2013 P5) is notablefor its sporadic, comet-like ejection of dust in nine distinct epochs spreadover $\sim$250 days in 2013. This curious behavior has been interpreted as theproduct of localized, equator-ward landsliding from the surface of an asteroidrotating at the brink of instability. We obtained new Hubble Space Telescopeobservations to directly measure the nucleus and to search for evidence of itsrapid rotation. However, instead of providing photometric evidence for rapidnucleus 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. Underthis interpretation, the time-series data are consistent with asecondary/primary mass ratio, $m_s/m_p \sim$ 1:6, a ratio of separation/primaryradius, $r/r_p \sim$ 4 and an orbit period $\sim$0.8 days. These properties liewithin the range of other asteroid binaries that are thought to be formed byrotational breakup. While the lightcurve period is long, centripetal dustejection is still possible if one or both components rotates rapidly($\lesssim$ 2 hour) and has a small lightcurve variation because of azimuthalsymmetry. Indeed, radar observations of asteroids in critical rotation reveal"muffin-shaped" morphologies which are closely azimuthally symmetric and whichshow minimal lightcurves. Our data are consistent with 311P being a closebinary in which one or both components rotates near the centripetal limit. Themass loss in 2013 suggests that breakup occurred recently and could even beon-going. A search for fragments that might have been recently ejected beyondthe Hill sphere reveals none larger than effective radius $r_e \sim$ 10 m.

The Nucleus of Active Asteroid 311P/(2013 P5) PANSTARRS