Asteroid and comet impacts: the ultimate environmental catastrophe.

Cosmic impacts represent the most extreme class of natural hazards, combining a very low probability of occurrence with a capability of killing hundreds of millions and destabilizing global civilization. Fortunately, these disasters are amenable to precise prediction and even (in principle) can be avoided entirely by appropriate application of space technology. However, impacts take place so rarely that they have only recently been recognized as a significant natural hazard. Concerted international action to deal with impacts depends on increased public awareness.