Meteorites are lumps of solid matter of extraterrestrial origin which are large enough not to be destroyed by their passage through the atmosphere, and therefore strike the earth's surface. Meteorites frequently occur in large showers. The number of bodies in a shower can be very great - 1868, Pultusk (Poland): 100,000; 1912, Holbrook, Arizona (USA): 14,000. With the increase in the spread of built up areas, the probability that meteorite showers or large meteorites will cause damage and loss of life also increases; in addition, an isolated iron meteorite of the appropriate size could give rise to a radar alert which could lead to a general nuclear counterattack.
It is estimated that 10,000 tonnes of micrometeorites accumulate (harmlessly) on the surface of the earth each day. By December 1965 the total number of well-authenticated distinct meteorites was 7,791. The larger known meteorites range in size from 500 lbs to 60 tonnes. Even larger meteorites have struck the earth, as is evidenced by the presence of craters which range in size up to 37 miles in diameter; it has even been suggested that the Hudson Bay (284 miles diameter) may have resulted from such an impact. Estimates show that at any one time about 1,000 asteroids with diameters greater than 1 km are crossing the earth's orbit and that, on average, one of these bodies would hit the earth every 300,000 years, producing a crater of 20 km in diameter. Impacts of objects with diameters larger than 10 km, producing craters in excess of 200 km, are to be expected every 40 million years. Additional impacts by comets could increase the incidence. It has been suggested that the mass extinction of living species at the Cretaceous-Tertiary boundary was due to climatic effects caused by the impact of an object of about 10 km diameter. It has also been argued that the mass extinction of species at that time was due to a dust storm resulting from resulting from micro-meteorites from outer space, possibly on the same occasion as the impact of the more massive object.