# New Active Asteroid (6478) Gault

## TL;DR

This study investigates the dust ejection and activity of asteroid (6478) Gault in 2019, revealing dust properties, ejection speeds, and suggesting rotational instability as the cause of its cometary features.

## Contribution

It provides detailed observations of Gault's dust ejection, analyzes dust size distribution and ejection dynamics, and proposes rotational instability as the activity mechanism.

## Key findings

- Dust tails formed in late 2018 and early 2019 with grains 20 μm to 3 mm.
- Dust mass declined at 2.28 kg/s, with a temporary surge in March 2019.
- No significant non-gravitational acceleration detected in Gault's orbit.

## Abstract

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.