  We present Hubble Space Telescope and Keck 10 meter telescope observations of
active asteroid 288P/300163 (2006 VW139) taken to examine ejected dust. The
nucleus is a C-type object with absolute magnitude $H_V$ = 17.0$\pm$0.1 and
estimated diameter $\sim$2.6 km (for assumed visual geometric albedo $p_V$ =
0.04). Variations in the brightness of the nucleus at the 10% to 15% level are
significant in both 2011 December and 2012 October but we possess too few data
to distinguish variations caused by activity from those caused by rotation. The
dust scattering cross-section in 2011 December is $\sim$40 km$^2$,
corresponding to a dust mass $\sim$9$\times$10$^6$ kg (88 $\mu$m mean particle
radius assumed). The full width at half maximum of the debris sheet varies from
$\sim$100 km near the nucleus to $\sim$1000 km 30arcsec (40,000 km) east of it.
Dust dynamical models indicate ejection speeds between 0.06 and 0.3 m s$^{-1}$,
particle sizes between 10 and 300 $\mu$m and an inverse square-root relation
between particle size and velocity. Overall, the data are most simply explained
by prolonged, low velocity ejection of dust, starting in or before 2011 July
and continuing until at least 2011 October. These properties are consistent
with the sublimation of near-surface ice aided by centrifugal forces. The high
spatial resolution of our HST images (52 km per pixel) reveals details that
remained hidden in previous ground-based observations, such as the
extraordinarily small vertical extent of the dust sheet, ejection speeds well
below the nucleus escape speed, and the possibility of a binary nucleus.
