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Abstract

We present imaging and spectroscopic observations of 6478 Gault, a similar to 6 km diameter inner main-belt asteroid currently exhibiting strong, comet-like characteristics. Three distinct tails indicate that ultra-slow dust (ejection speed 0.15 +/- 0.05 m s(-1)) was emitted from Gault in separate episodes beginning UT 2018 October 28 +/- 5 (Tail A), UT 2018 December 31 +/- 5 (Tail B), and UT 2019 February 10 +/- 7 (Tail C), with durations of Delta T similar to 10-20 days. With a mean particle radius (a) over bar similar to 200 mu m, the estimated masses of the tails are M-A;similar to 4 x 10(7) kg, M-B similar to 6 x 10(6) kg, and M-C similar to 6 x 10(5) kg, respectively, and the mass-loss rates from the nucleus are 20-40 kg s(-1) for Tail A, 4-6 kg s(-1) for Tail B, and -0.4 kg s(-1) for Tail C. In its 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. A spectroscopic upper limit to the production of gas is set at 1 kg s(-1). Discrete emission in three protracted episodes effectively rules out an impact origin for the observed activity. Sublimation driven activity is unlikely given the inner-belt orbit and the absence of detectable gas. In any case, sublimation would not easily account for the observed multiple ejections. The closest similarity is between Gault and active asteroid 311P/(2013 P5), an object showing repeated but aperiodic ejections of dust over a 9 month period. While Gault is 10 times larger than 311P/(2013 P5), and the relevant timescale for spin-up by radiation torques is similar to 100 times longer, its properties are likewise most consistent with episodic emission from a body rotating near breakup.