Introduction

The spectacular collision of the Shoemaker-Levy 9 comet with Jupiter in July 1994 was a dramatic reminder of the fact that the Earth has and will continue to experience such catastrophic events. While the frequency of such massive collisions is very low, smaller objects collide with the Earth regularly and do damage that would be intolerable in any populated region. As an example, the Tunguska (Siberia) event of 1908 is estimated to have involved a 60-m object exploding at a height of 8 km and produced devastation over an area almost the same as that devastated by the eruption of Mt. St. Helens (Morrison et al. 1994). The famous 1-km Meteor Crater in Arizona was made by the impact of an even smaller body only 30 m in diameter (Adushkin and Nemchinov 1994). Human casualties due to direct meteorite strikes are rare but known (Yau 1994). The greater danger is due to the fact that the time between large impacts, such as the Tunguska impact which released tens of megatons of TNT equivalent energy, is significant compared to a human lifetime and there is a small chance that any impact will be in a populated area. The relative scarcity of such areas on the Earth may not offer the protection one might think as recent calculations suggest that larger bodies might do more damage if they didn't hit land; predicting that an impact anywhere in the Atlantic Ocean by a 400-m asteroid would devastate the (well-populated) coasts on both sides of the ocean with tsunamis over 60 m high (Hills et al. 1994).