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We present Hubble Space Telescope observations of the active asteroid (andGeminid stream parent) 3200 Phaethon when at its closest approach to Earth(separation 0.07 AU) in 2017 December. Images were recorded within$\sim$1\degr~of the orbital plane, providing extra sensitivity to low surfacebrightness caused by scattering from a large-particle trail. We placed an upperlimit to the apparent surface brightness of such a trail at 27.2 magnitudesarcsecond$^{-2}$, corresponding to an in-plane optical depth $\le3\times10^{-9}$. No co-moving sources brighter than absolute magnitude 26.3,corresponding to circular equivalent radius $\sim$12 m (albedo 0.12 assumed),were detected. Phaethon is too hot for near-surface ice to survive. We brieflyconsider the thermodynamic stability of deeply-buried ice, finding that itssurvival would require either a very small (regolith-like) thermal diffusivity($< 10^{-8}$ m$^2$ s$^{-1}$), or the unexpectedly recent injection of Phaethon(timescale $\lesssim$ 10$^6$ yr) into its present orbit, or both.

Hubble Space Telescope Observations of 3200 Phaethon At Closest Approach