Rotational Mass Shedding from Asteroid (6478) Gault

The ∼4 km diameter main belt asteroid 6478 Gault has ejected dust intermittently since at least 2013. The character of the emission, including its episodic nature and the low speed of the ejected particles (V ∼ 0.15 m s−1), is most consistent with mass loss from a body rotating near rotational breakup. Owing to dust contamination of the nucleus signal, this conclusion had not yet been confirmed. To test this idea, we have obtained new images of Gault in 2020 August in the absence of dust. Our photometry shows a lightcurve with a very small amplitude (maximum ∼0.05 mag) and a periodicity of 2.55 ± 0.10 hr. The new observations are consistent with a model in which Gault is rotating near breakup, with centrifugal forces responsible for its episodic mass loss. Approximated as a strengthless (fluid) spherical body, the implied density is ρ = 1700 kg m−3. We use the Froude number Fr, defined here as the ratio between centrifugal force and gravitational force, as a way to investigate mass-loss regimes in fast-spinning asteroids and find that mass shedding starts at Fr ∼ 0.5.