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Abstract

Observations of active asteroid P/2017 S5 when near perihelion reveal the ejection of large (102–104 μm) particles at 0.2–2 m s−1 speeds, with estimated mass-loss rates of a few kg s−1. The protracted nature of the mass loss (continuous over ~150 days) is compatible with a sublimation origin, meaning that this object is likely an ice-bearing main-belt comet. Equilibrium sublimation of exposed water ice covering as little as 0.1 km2 can match the data. Observations a year after perihelion show the object in an inactive state from which we deduce a nucleus effective radius ${450}_{-60}^{+100}\,{\rm{m}}$ (albedo 0.06 ± 0.02 assumed). The gravitational escape speed from a body of this size is just ~0.3 m s−1, comparable to the inferred ejection speed of the dust. Time-series photometry provides tentative evidence for rapid rotation (lightcurve period 1.4 hr) that may also play a role in the loss of mass and which, if real, is a likely consequence of spin-up by sublimation torques. P/2017 S5 shares both physical and orbital similarities with the split active asteroid pair P/2016 J1-A and J1-B, and all three objects are likely members of the ~7 Myr old, collisionally produced, Theobalda family.