Hubble Space Telescope Observations of 3200 Phaethon At Closest Approach

TL;DR

Hubble Space Telescope observations of 3200 Phaethon during its closest approach in 2017 set upper limits on trail brightness and size of detectable co-moving objects, providing insights into its surface and subsurface properties.

Contribution

This study provides the first sensitive upper limits on Phaethon's trail brightness and co-moving object size during close approach, informing models of its surface and subsurface composition.

Findings

No detectable trail brightness above 27.2 mag arcsec$^{-2}$.

No co-moving objects brighter than magnitude 26.3 found.

Phaethon’s surface likely lacks near-surface ice due to high temperature.

Abstract

We present Hubble Space Telescope observations of the active asteroid (and Geminid stream parent) 3200 Phaethon when at its closest approach to Earth (separation 0.07 AU) in 2017 December. Images were recorded within $\sim$1\degr~of the orbital plane, providing extra sensitivity to low surface brightness caused by scattering from a large-particle trail. We placed an upper limit to the apparent surface brightness of such a trail at 27.2 magnitudes arcsecond$^{-2}$, corresponding to an in-plane optical depth $\le 3\times10^{-9}$. No co-moving sources brighter than absolute magnitude 26.3, corresponding to circular equivalent radius $\sim$12 m (albedo 0.12 assumed), were detected. Phaethon is too hot for near-surface ice to survive. We briefly consider the thermodynamic stability of deeply-buried ice, finding that its survival would require either a very small (regolith-like) thermal diffusivity ($< 10^{-8}$ m$^2$ s$^{-1}$), or the unexpectedly recent injection of Phaethon (timescale $\lesssim$ 10$^6$ yr) into its present orbit, or both.