Active asteroid P/2020 O1 has an orbit in the middle asteroid belt (a = 2.647 AU) that may define the innermost extent of the asteroid belt where objects retaining water ice can be found, or the "ice line". Beyond the ice line, many asteroids may contain subsurface ice, including main-belt comets, which orbit in the asteroid belt but exhibit comet-like sublimation-driven dust emission.
We present Hubble Space Telescope observations of P/2020 O1 taken to examine its development for a year after perihelion. We find that the mass loss peaks at ~0.5 kg s-1 in 2020 August and then declines to nearly zero over four months. The protracted nature of the mass loss (continuous over 180 days), its onset near perihelion, its termination at true anomaly ~60°, and the dust velocity proportional to the inverse square root of the particle size are compatible with a sublimation origin. Time-series photometry provides tentative evidence for extremely rapid rotation 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 of the nucleus, while larger particles are released at speeds less than the gravitational escape velocity. These properties are consistent with the sublimation of near-surface ice aided by centrifugal forces.
While sublimation provides the most plausible explanation for the activity, we need additional observations to demonstrate the expected recurrence of activity at subsequent perihelia. P/2020 O1 will next reach perihelion in 2024 August. If the activity is repetitive near perihelion and water ice sublimation is thus confirmed, P/2020 O1 would be the icy asteroid with the smallest known semimajor axis (highest temperature), setting new bounds on the distribution of ice in the asteroid belt. This would allow us to extend the ice line inward by ~0.12 AU, increasing the number of main-belt asteroids with potentially surviving ice content by a factor of 1.4.