  We present imaging and spectroscopic observations of 6478 Gault, a 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) 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, with durations of 10 to 20 days. With a mean particle radius
100 micron, the estimated masses of the tails are M_A = 2e7 kg, M_B = 3e6 kg
and M_C = 3e5 kg, respectively, and the mass loss rates from the nucleus are 10
to 20 kg/s for Tail A, 2 to 3 kg/s for Tail B and about 0.2 kg/s 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. 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 spin-up time to radiation
torques is 100 times longer, its properties are likewise most consistent with
episodic emission from a body rotating near breakup.
