Hazards from Comets and Asteroids

William Napier

The idea that celestial bodies may on occasion strike the Earth with catastrophic consequences very likely pre-dates Christianity. Its history and current status are reviewed here from an astronomical perspective. The concept essentially died out in the 19th century following the Darwinian revolution, but was revived following the discovery of increasing numbers of small bodies in Earth-crossing orbits throughout the 1970s, and the realisation that high-energy impacts with such bodies are relatively common on geological timescales. The Oort comet cloud appears to yield most of the largest hazards (impact energies 10-100 million megatons equivalent of TNT). The cloud is sensitive to external perturbers, and these yield Galactic signatures in the impact cratering record of the past 250 million years, mainly in the form of discrete bombardment episodes recurring with a periodicity of about 36 Myr. We are in or near the peak of such an episode now.

There is a weak correlation between these impact episodes and mass extinctions, and a stronger one with the creation dates of large igneous provinces on Earth. On timescales of historical rather than geological interest, extrapolation from 30 yr of observation suggests that prompt extinction of humankind is unlikely, but that horrendous damage on a continental or global scale is possible from impacts by stray bodies. However, such extrapolation is based on the assumption of statistical completeness, and this is open to challenge on several fronts. In particular the role of rare, large comets, and their modes of disintegration, remains uncertain, and passage through dust and other debris from a large, disintegrating comet has the potential to crash civilisation. Evidence exists that such debris from an erstwhile large, short-period comet, is trapped in a 7:2 orbital resonance with Jupiter and that the Earth encounters it every 2600-2800 years.