MOST asteroids, those lumps of cosmic rock too small to be considered planets, confine themselves to a dense swarm in a belt that lies between the orbits of Jupiter and Mars. A minority, however, follow more unusual paths, and a few in the course of their eccentric wanderings come close enough to Earth from time to time to be considered threats. In this guise, they are "Near Earth Objects", NEOs for short.
According to Sky High 1996, the current edition of the Irish Astronomical Society's annual year book, an NEO will pass within a mere three million miles of Earth today. The asteroid Toutatis is about three miles long and one mile wide, or about half the size of Valentia Island, Co Kerry. It tumbles head overheels on its journey around the solar system in an orbit that sometimes brings it even closer to us than it is today. On September 29th, 2004, for example, Toutatis will skim past only a million miles away from Earth. And some of its fellows have come nearer still our closest known encounter with an NEO in recent years was in May 1993, when one three miles wide passed within 90,000 miles of us, a distance less than half that between the Moon and Earth.
Now when an NEO stops being an NEO and scores a hit on Earth, its energy on impact, to put it mildly, can be quite considerable. One such, for example, estimated to have been about 50 yards in diameter came to ground in June 1908, near the Tunguska river in Siberia. Trees were levelled radially in a zone extending 30 miles or more and tremors were recorded on seismographs as far away as London. Obviously, had it landed on a major city, the potential loss of life would have been very great indeed.
Traces of such impacts over the millennia are rather difficult to find, since nature abhors a crater just as much as she abhors a vacuum, and erodes and covers up the evidence. A bout 130 craters, however, have been identified so far, and based on this information and our knowledge of the tangle of asteroid orbits known to intersect the orbit of the Earth, scientists have calculated the odds of being hit.
It is reckoned that collisions on the scale of the Tunguska event can be expected every 300 years or so. Impact by an object a mile or greater in diameter, an occurrence which would be almost catastrophic on a global scale, is likely to happen every half a million years. And much smaller objects, ranging in size from imperceptible particles of dust to rocks some inches or a foot or two across, are liable to hit us any time at all.