It has been estimated that a planetary disaster would be created if the earth were struck by an object in excess of one kilometer diameter.
In the case of asteroids, it has been estimated that only 5% of near misses are ever detected. Asteroids and parts of comets have been striking the earth since it began. The geological record of earth shows evidence of more than 100 large objects hitting the planet. Some evidence supports the theory that 65 million years ago a giant object struck the earth, pulverizing a hugh area and spewing so much debris into the atmosphere that the skies darkened for months, temperatures dipped, and much of the life on the earth, most notably the dinosaurs, perished. In 1908, an object exploded in the atmosphere above Siberia, causing a tremendous blast and fireball, felling trees in a 200 square mile area. Objects of one half mile in diameter hit the earth once every 40 million years or so. One theory says that one comet ten meters across strikes the atmosphere every three seconds. A group of large cometary fragments will collide with Jupiter in mid-1994 releasing an amount of energy equivalent to the total destructive power of the world's nuclear weapons at the peak of the Cold War. The consequences cannot at present be predicted.
In 1993 a passing comet left an unusually heavy shower of debris which delayed the launching of a USA satellite.