  We present Hubble Space Telescope observations of active asteroid P/2020 O1
taken to examine its development for a year after perihelion. We find that the
mass loss peaks <~1 kg/s in 2020 August and then declines to nearly zero over
four months. Long-duration mass loss (~180 days) is consistent with a
sublimation origin, indicating that this object is likely an ice-bearing
main-belt comet. Equilibrium sublimation of water ice from an area as small as
1580 m^2 can supply the observed mass loss. Time-series photometry shows
tentative evidence for extremely rapid rotation (double-peaked period < 2 hr)
of the small nucleus (effective radius ~420 m). Ejection velocities of 0.1 mm
particles are comparable to the 0.3 m/s gravitational escape speed from the
nucleus, while larger particles are ejected at speeds less than the escape
velocity. These properties are consistent with the sublimation of near-surface
ice aided by centripetal acceleration. If water ice sublimation is confirmed,
P/2020 O1 would be the icy asteroid with the smallest semimajor axis (highest
temperature), setting new bounds on the distribution of ice in the asteroid
belt.
