Introduction

It is a demonstrable fact that asteroids of all sizes and less frequently cometary nuclei suffer collisions with the Earth's surface. The impact hazard, which is defined in Morrison et al. (2002) as “… the probability for an individual of premature death as a consequence of impact,” has undergone considerable analysis with the conclusion that the greatest risk is from the very rare collisions of relatively large asteroids that can create a global scale catastrophe in the biosphere (Chapman and Morrison 1994). In the last decade, the question of how to deal with the hazard has led to considerable activity and advocacy on the part of the interested scientific community, and activity at government level has been stimulated in the United States, Europe, and Japan (a detailed overview is given by Morrison et al. 2002): there are now survey programs to search for objects that could be potentially hazardous; there are high-level calls for increased observational efforts to characterize the physical and compositional nature of near-Earth objects (NEOs) (e.g., The UK NEO Task Force report: Atkinson et al. 2000); an impact hazard scale has been invented to provide the public with an assessment of the magnitude of the hazard from a particular object; there have been considerable advances in the accuracy of orbit determination and impact probability.

Nevertheless, it seems that the question of how governments should go about preparing to mitigate the hazard needs some further attention.