OUR COMETARY ENVIRONMENT

The encounter of a small armada of spacecraft with Halley's Comet in 1 986, the disintegration and multiple impact of Comet Shoemaker-Levy 9 on Jupiter in 1994, and the application of new technologies to the det ection of distant solar system bodies, have led to great revisions in the understanding of comets. Further, rapid improvements in computing power and numerical techniques have permitted the dynamical evolution of comets and asteroids to be followed far into the future and past, a nd the relationships between families of small interplanetary bodies t o be explored. The small body environment is now generally recognized as strongly interacting with the terrestrial one, and may be hazardous on timescales of human as well as geological interest. We review our current understanding of the cometary environment, with particular reg ard to the hazard it presents. It appears that many comets are handed down from the Oort-Opik cloud, which is dynamically sensitive to the g alactic environment, through the planetary system into Earth-crossing orbits. Thus, the terrestrial environment is subject to stresses which vary cyclically on a number of timescales from planetary to galactic.