Multiple Outbursts of Asteroid (6478) Gault

Main-belt asteroid (6478) Gault unexpectedly sprouted two tails in late 2018 and early 2019, identifying it as a new active asteroid. Here we present observations obtained by the 1.2 m Zwicky Transient Facility survey telescope that provide detailed time-series coverage of the onset and evolution of Gault’s activity. Gault exhibited two brightening events, with the first one starting on 2018 October 18 ± 5 days and a second one starting on 2018 December 24 ± 1 days. The amounts of mass released are 2 × 10⁷ kg and 1 × 10⁶ kg, respectively. Based on photometric measurements, each event persisted for about a month. Gault’s color has not changed appreciably over time, with a pre-outburst color of g _PS1 - r _PS1 = 0.50 ± 0.04 and g _PS1 - r _PS1 = 0.46 ± 0.04 during the two outbursts. Simulations of dust dynamics shows that the ejecta consists of dust grains of up to 10 μm in size that are ejected at low velocities below 1 {{m}} {{{s}}}^-1 regardless of particle sizes. This is consistent with non-sublimation-driven ejection events. The size distribution of the dust exhibits a broken power law, with particles at 10-20 μm following a power law of -2.5 to -3.0, while larger particles follow a steeper slope of -4.0. The derived properties can be explained by either rotational excitation of the nucleus or a merger of a near-contact binary, with the latter scenario to be statistically more likely.

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