Coherent catastrophism

We review the theoretical and observational evidence that, on timescales relevant to mankind, the prime collision hazard is posed by temporally correlated impacts (coherent catastrophism, Δt ∼ 10 ²-10 ⁴ yr) rather than random ones (stochastic catastrophism, Δt ∼ 10 ⁵-10 ⁸ yr). The mechanism whereby coherent incursions into and through the terrestrial atmosphere occur is described as being the result of giant cometary bodies arriving in orbits with perihelia in the inner solar system. Hierarchical fragmentation of such large (100 km-plus) bodies — due to thermal stresses near perihelion, collisions in the asteroid belt, or passages through the Jovian Roche radius — results in numerous ∼kilometre-sized objects being left in short-period orbits, and appearing in telescopic searches as Apollo-type asteroids. Many more smaller objects, in the 10-100 metre size range and only recently observed, by the Spacewatch team, are expected to be in replenished clusters in particular orbits as a result of continuing disintegrations of large, differentiated, cometary objects. Gravitational perturbations by Jupiter bring these clusters around to have a node at 1 AU in a cyclic fashion, leading to impacts at certain times of year every few years during active periods lasting a few centuries, such periods being separated by intervals of a few millennia. Furthermore, fragmentations within the hierarchy result in significant bombardment commensurabilities ( Δt ∼ 10-10 ² yr) during active periods occurring at random intervals ( Δt ∼ 10 ²-10 ³ yr). It appears that the Earth has been subject to such impacts since the break-up of such a comet ∼2×10 ⁴ years ago; currently we are not passing through a high-risk epoch, although some phenomena originating in the products of this break-up have been observed in the 20th century. This most recent hierarchical disintegration, associated with four well-known meteor showers and termed the Taurid Complex, is now recognized as resulting in a dozen apparently asteroidal objects — almost 10% of the discovered Apollo population — as well as Comet P/Encke. A substantial asteroidal remnant of the Taurid progenitor may still be present in resonance with Jupiter. The implications of the existence of any asteroidal or meteoroidal cluster due to the progenitor are briefly discussed. We emphasize finally the relative importance of active and inactive comets in the cratering record, pointing out the potentially significant contribution by disintegrating large comets to the population of inactive Earth-crossers.