Such objects have collided with the Earth since its formation, and brought the carbon and water which made life possible. They have also caused widespread changes in the Earth s surface, and occasional extinctions of such living organisms as the dinosaurs. The threat has only recently been recognised and accepted. This has come about through advances in telescope technology allowing the study of these usually faint objects, the identification of craters on the moon, other planets and the Earth as a result of impacts, and the dramatic collision of pieces of the comet Shoemaker-Levy 9 with Jupiter in 1994. Impacts represent a significant risk to human and other forms of life.
Means now exist to mitigate the consequences of such impacts for the human species.
The largest uncertainty in risk analysis arises from our incomplete knowledge of asteroids whose orbits bring them near to the Earth. With greater information about them, fairly accurate predictions can be made. The risk from comets is between 10 and 30 per cent of that from asteroids. The advance warning period for a potential impact from a long period comet may be as short as a year compared to decades or centuries for asteroids. Short period comets can be considered along with asteroids.