The Sporadic Activity of (6478) Gault: A YORP-driven Event?

On 2019 January 5 a streamer associated with the 4-10 km main belt asteroid (6478) Gault was detected by the ATLAS sky survey, a rare discovery of activity around a main belt asteroid. Archival data from ATLAS and Pan-STARRS1 show the trail in early 2018 December, but not between 2010 and 2018 January. The feature has significantly changed over one month, perfectly matching predictions of pure dust dynamical evolution and changes in the observing geometry for a short release of dust around 2018 October 28. Follow-up observations with the Hubble Space Telescope (HST) show a second narrow trail corresponding to a brief release of dust on 2018 December 30. Both releases occurred with negligible velocity. We find the dust grains to be fairly large, with power-law size distributions in the 10^-5-10^-3 m range and power-law indices of ∼-1.5. Three runs of ground-based data find a signature of ∼2 hr rotation, close to the rotational limit, suggesting that the activity is the result of landslides or reconfigurations after Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) spin-up.