  We present imaging observations of the disintegrating long-period comet
C/2021 A1 (Leonard). High resolution observations with Hubble Space Telescope
show no evidence for surviving fragments, and place a 3 sigma upper limit to
their possible radius about 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 mid- December 2021 (at about 0.8
au). Two methods give the pre-disruption nucleus radius, r = 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 (of order 0.1 year)
given the orbit and size of the C/2021 A1 nucleus, 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.
