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Abstract

Main-belt asteroid (6478) Gault was observed to show cometary features in early 2019. To investigate the cause, we conducted BVR observations at Xingming Observatory, China, from 2019 January to April. The two tails were formed around 2018 October 26–November 8, and 2018 December 29–2019 January 8, respectively, and consisted of dust grains of ≳20 μm to 3 mm in radius ejected at a speed of 0.15 ± 0.05 m s−1 and following a broken power-law size distribution bending at grain radius ∼70 μm (bulk density 1 g cm−3 assumed). The total mass of dust within a 104 km-radius aperture around Gault declined from ∼9 × 106 kg since 2019 January at a rate of 2.28 ± 0.07 kg s−1, but temporarily surged around 2019 March 25, because Earth then crossed the orbital plane of Gault, near which the ejected dust was mainly distributed. No statistically significant colour or short-term light-curve variation was seen. Nonetheless we argue that Gault is currently subjected to rotational instability. Using the available astrometry, we did not detect any non-gravitational acceleration in the orbital motion of Gault.