The reactivation of main-belt comet 259P/Garradd (P/2008 R1)

We present observations of main-belt comet (MBC) 259P/Garradd from 4 months prior to its 2017 perihelion passage to 5 months after perihelion using the Gemini North and South telescopes. The object was confirmed to be active during this period, placing it among seven MBCs confirmed to have recurrent activity. We find an average net pre-perihelion dust production rate for 259P in 2017 of M_d = (4.6 ± 0.2) kg s^-1 (assuming grain densities of ρ = 2500 kg m^-3 and a mean effective particle size of a¯_d = 2 mm) and a best-fit start date of detectable activity of 2017 April 22 ± 1, when the object was at a heliocentric distance of r_h = 1.96 ∓ 0.03 au and a true anomaly of ν = 313°.9 ± 0°.4. We estimate the effective active fraction of 259P’s surface area to be from f_act ∼ 7 × 10^-3 to f_act ∼ 6 × 10^-2 (corresponding to effective active areas of A_act ∼ 8 × 10³ m² to A_act ∼ 7 × 10⁴ m²) at the start of its 2017 active period. A comparison of estimated total dust masses measured for 259P in 2008 and 2017 shows no evidence of changes in activity strength between the two active apparitions. The heliocentric distance of 259P’s activity onset point is much smaller than those of other MBCs, suggesting that its ice reservoirs may be located at greater depths than on MBCs farther from the Sun, increasing the time needed for a solar-irradiation-driven thermal wave to reach subsurface ice. We suggest that deeper ice on 259P could be a result of more rapid ice depletion caused by the object’s closer proximity to the Sun compared to other MBCs.