Active Asteroid P/2017 S5 (ATLAS)

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 {{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.