  Main-belt asteroid (6478) Gault was observed to show cometary features in
early 2019. To investigate the cause, we conducted {\it BVR} observations at
Xingming Observatory, China, from 2019 January to April. The two tails were
formed around 2018 October 26--November 08, and 2018 December 29--2019 January
08, respectively, and consisted of dust grains of $\gtrsim$20 $\mu$m to 3 mm in
radius ejected at a speed of $0.15 \pm 0.05$ m s$^{-1}$ and following a broken
power-law size distribution bending at grain radius $\sim$70 $\mu$m (bulk
density 1 g cm$^{-3}$ assumed). The total mass of dust within a $10^4$
km-radius aperture around Gault declined from $\sim$$9 \times 10^6$ kg since
2019 January at a rate of $2.28 \pm 0.07$ kg s$^{-1}$, but temporarily surged
around 2019 March 25, because Earth thence crossed the orbital plane of Gault,
within which the ejected dust was mainly distributed. No statistically
significant colour or short-term lightcurve variation was seen. Nonetheless we
argue that Gault is currently subjected to rotational instability. Using the
available astrometry, we did not detect any nongravitational acceleration in
the orbital motion of Gault.
