  Active asteroids show (typically transient) cometary activity, driven by a
range of processes. A sub-set, sometimes called main-belt comets, may be driven
by sublimation and so could be useful for tracing the present-day distribution
of asteroid ice. Object P/2018 P3 has a Tisserand parameter 3.096 but a high
eccentricity 0.415, placing it within the dynamical boundary between asteroids
and comets. We aim to determine the cause of activity (sublimation or something
else) and assess the dynamical stability of P3, in order to better constrain
the intrinsic ice content in the main belt. We obtained Hubble Space Telescope
images of P3 at the highest angular resolution. We compared the observations
with a Monte Carlo model of dust dynamics. We identified and analyzed archival
CFHT (2013) and NEOWISE (2018) data. In addition, we numerically integrated the
orbits of P3 clones for 100 Myr. P3 has been recurrently active near two
successive perihelia (at 1.76 AU), indicative of a sublimation origin. The
absence of 4.6 um band excess indicates zero or negligible CO or CO2 gas
production from P3. The properties of the ejected dust are remarkably
consistent with those found in other main-belt comets (continuous emission of
~0.05-5 mm particles at 0.3-3 m/s speeds), with mass-loss rates of >~2 kg/s.
The orbit of P3 is unstable on timescales ~10 Myr. We speculate that P3 has
recently arrived from a more stable source (either the Kuiper Belt or elsewhere
in the main belt) and has been physically aged at its current location, finally
becoming indistinguishable from a weakly sublimating asteroid in terms of its
dust properties. Whatever the source of P3, given the dynamical instability of
its current orbit, P3 should not be used to trace the native distribution of
asteroid ice.
