Disintegration of Long-period Comet C/2021 A1 (Leonard)

Abstract

Abstract We present imaging observations of the disintegrating long-period comet C/2021 A1 (Leonard). High-resolution observations with the Hubble Space Telescope show no evidence for surviving fragments, and place a 3σ upper limit on their possible radius of ∼60 m (albedo 0.1 assumed). In contrast, wide-field observations from the Swan Hill Observatory, Australia, show an extensive debris cloud, the cross section and estimated mass of which are consistent with complete disintegration of the nucleus near 2021 mid-December (at about 0.8 au). Two methods give the pre-disruption radius of the nucleus, r n = 0.6 ± 0.2 km. Tidal, collisional, sublimation, and pressure-confined explosion models provide implausible explanations of the disintegration. However, rotational instability driven by outgassing torques has a very short timescale (∼0.1 yr) given the orbit and size of the nucleus of C/2021 A1, and offers the most plausible mechanism for the disruption. Initial rotational breakup is accelerated by the exposure and strong sublimation of previously buried volatiles, leading to catastrophic destruction of the nucleus.