High-resolution thermal infrared imaging of 3200 Phaethon

We present thermal infrared observations of the active asteroid (and Geminid meteoroid stream parent) 3200 Phaethon using the Very Large Telescope. The images, at 10.7 μm wavelength, were taken with Phaethon at its closest approach to Earth (separation of 0.07 au) in 2017 December, at a linear resolution of about 14 km. We probe the Hill sphere (of radius ~66 km) for trapped dust and macroscopic bodies, finding neither, and we set limits to the presence of unbound dust. The derived limits to the optical depth of dust near Phaethon depend somewhat on the assumed geometry, but are of an order of 10−5. The upper limit to the rate of loss of mass in dust is lesssim14 kg s−1. This is ~50 times smaller than the rate needed to sustain the Geminid meteoroid stream in steady state. The observations thus show that the production of the Geminids does not proceed in a steady state.