  We present observations of main-belt comet 358P/PANSTARRS (P/2012 T1)
obtained using the Gemini South telescope from 2017 July to 2017 December, as
the object approached perihelion for the first time since its discovery. We
find best-fit IAU phase function parameters of H_R=19.5+/-0.2 mag and
G_R=-0.22+/-0.13 for the nucleus, corresponding to an effective radius of
r_N=0.32+/-0.03 km (assuming an albedo of p_R=0.05). The object appears
significantly brighter (by >1 mag) than expected starting in 2017 November,
while a faint dust tail oriented approximately in the antisolar direction is
also observed on 2017 December 18. We conclude that 358P has become active
again for the first time since its previously observed active period in
2012-2013. These observations make 358P the seventh main-belt comet candidate
confirmed to exhibit recurrent activity near perihelion with intervening
inactivity away from perihelion, strongly indicating that its activity is
sublimation-driven. Fitting a linear function to the ejected dust masses
inferred for 358P in 2017 when it is apparently active, we find an average net
dust production rate of 2.0+/-0.6 kg/s (assuming a mean effective particle
radius of 1 mm) and an estimated activity start date of 2017 November 8+/-4
when the object was at a true anomaly of 316+/-1 deg and a heliocentric
distance of R=2.54 AU. Insufficient data is currently available to ascertain
whether activity strength has changed between the object's 2012-2013 and 2017
active periods. Further observations are therefore highly encouraged during the
object's upcoming observing window (2018 August through 2019 May).

Earth and Planetary Astrophysics