  Comet disintegration proceeds both through sublimation and discrete splitting
events. The cross-sectional area of material ejected by a comet may, within
days, become many times greater than that of the Earth, making encounters with
such debris much more likely than collisions with the nucleus itself. The
hierarchic fragmentation and sublimation of a large comet in a short period
orbit may yield many hundreds of such short-lived clusters. We model this
evolution with a view to assessing the probability of an encounter which might
have significant terrestrial effects, through atmospheric dusting or multiple
impacts. Such an encounter may have contributed to the large animal extinctions
and sudden climatic cooling of 12,900 years ago, and the near-simultaneous
collapse of civilisations around 2350 BC.
