Hubble Space Telescope Observations of Active Asteroid P/2020 O1 (Lemmon-PANSTARRS)

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 at ≲1 kg s⁻¹ 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 m² 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⁻¹ 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 an icy asteroid with the smallest semimajor axis (highest temperature), setting new bounds on the distribution of ice in the asteroid belt.