The Contemporary Hazard of Comet Impacts

Cosmic impacts by asteroids and comets pose a continuing hazard of loss of human life and property. Significant contemporary risk is associated with projectiles in the energy range from about 5 megatons of TNT up to the size of the KT impactor (100 million megatons). The lower threshold for damage is defined by the atmosphere of the Earth, which effectively shields us from smaller projectiles. Up to energies of about a gigaton of TNT, the effects are local or regional for impacts on the land, or coastal for ocean impacts, which can generate large tsunamis. A greater risk is associated with still larger impacts, which are capable of causing global ecological catastrophe, possibly leading to mass mortality from starvation and epidemics. The primary objective of any program to deal with this hazard is to determine whether or not such a near-term impact is likely, which is the objective of the Spaceguard Survey of near-Earth asteroids and short-period comets. Longperiod comets, however, pose a different challenge, since they cannot be discovered long in advance of a possible impact, their orbits are harder to predict, and they are significantly more difficult to deflect or destroy.