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We present imaging and spectroscopic observations of 6478 Gault, a 6 kmdiameter inner main-belt asteroid currently exhibiting strong, comet-likecharacteristics. Three distinct tails indicate that ultra-slow dust (ejectionspeed 0.15+/-0.05 m/s) was emitted from Gault in separate episodes beginning UT2018 October 28+/-5 (Tail A), UT 2018 December 31+/-5 (Tail B), and UT 2019February 10+/-7, with durations of 10 to 20 days. With a mean particle radius100 micron, the estimated masses of the tails are M_A = 2e7 kg, M_B = 3e6 kgand M_C = 3e5 kg, respectively, and the mass loss rates from the nucleus are 10to 20 kg/s for Tail A, 2 to 3 kg/s for Tail B and about 0.2 kg/s for Tail C. Inits optical colors Gault is more similar to C-type asteroids than to S-types,even though the latter are numerically dominant in the inner asteroid belt. Aspectroscopic upper limit to the production of gas is set at 1 kg/s. Discreteemission in three protracted episodes effectively rules out an impact originfor the observed activity. Sublimation driven activity is unlikely given theinner belt orbit and the absence of detectable gas. In any case, sublimationwould not easily account for the observed multiple ejections. The closestsimilarity is between Gault and active asteroid 311P/(2013 P5), an objectshowing repeated but aperiodic ejections of dust over a 9 month period. WhileGault is 10 times larger than 311P/(2013 P5), and the spin-up time to radiationtorques is 100 times longer, its properties are likewise most consistent withepisodic emission from a body rotating near breakup.

Episodically Active Asteroid 6478 Gault