Resurrection of (3200) Phaethon in 2016

Abstract: We present a study of the active asteroid (3200) Phaethon in the 2016 apparition using the STEREO spacecraft and compare the results with data from the previous two perihelia in 2009 and 2012. Once again, Phaethon brightened by $\sim$2 mag soon after its perihelion passage, contradicting expectations from the phase function of a macroscopic monolithic body. Subsequently, a short antisolar tail of $\sim$0{\deg}.1 in length was formed within $\sim$1 day and quickly disappeared. No trail was seen. Our syndyne-synchrone analysis indicates that the tail was comprised of submicron to micron particles and can be approximated by a synchrone coinciding with the outburst. We estimate that the outburst has released a mass of $\sim$10$^{4}$--10$^{5}$ kg, comparable to the two mass ejections in 2009 and 2012, and that the average mass-loss rate is $\sim$0.1--1 kg s$^{-1}$. The forward-scattering effect hinted at low level activity of Phaethon prior to the outburst, which increased the effective cross section by merely $\lesssim$1 km$^2$. Without the forward-scattering enhancement, detecting such activity at side-scattering phase angles is very difficult. The forward-scattering effect also reinforces that the ejected dust grains rather than gas emissions were responsible for the activity of Phaethon. Despite Phaethon's reactivation, it is highly unlikely that the Geminid meteoroid stream can be sustained by similar perihelion mass-loss events.