Comets consist of a nucleus (of the order of 1 km in diameter), a diffuse envelope of dust and gases driven out of the nucleus by solar radiation (possibly up to 400,000 km in diameter), and a tail composed of molecules and very finely divided dust particles (possibly over a million km long). There is a vast reservoir of comets in orbit around the sun. Their periods range from a few years to several million years, so that many may exist which have not been visible during recorded history. There is therefore some possibility that at least one future comet may intersect the path of the earth or cause meteors to do this. Disastrous effects include a direct cometary collision; a near collision with gravitational and tidal interactions; dense, heavy showers of meteorites striking the earth's surface; and interferences in space affecting earth missions, terrestrial magnetic or meteorological phenomena, or the moon.
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.
Comets are considered the source of great danger and many disasters. The Chinese believed them to be celestial brooms wielded by the gods to sweep the heavens free of evil, which then fell to earth, bring wars, floods, drought an other disasters. In AD 66 the passing of Halley's comet presaged the fall of Jerusalem. Its return in 451 was a portend to the defeat of Attila the Hun's armies at Châlons. The Saxons may have attributed their defeat in 1066 to its appearance. It heralded the descent of Turkish armies on Belgrade in 1456. When it came into view in 1910, some residents in Chicago prepared themselves for death by cyanogen-gas poisoning. Some contemporary scientists believe comets are the source of life and diseases like AIDS on earth.
There are many thousands of comets in the Earth's neighbourhood; up to 1,700 are estimated to cross the Earth's path and to be of a size large enough to cause global havoc. It has been estimated that the probability of such an occurrence is approximately once in every 300,000 years.
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 huge 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.
In 1996 it was estimated that only 10 percent of the potential Earth-hitting asteroids had been detected and without an adequate early warning system, Earth could be taken by surprise.