Meteors and Meteorites

If a meteoroid is to survive its passage through Earth's atmosphere to become a meteorite, it must be both large and dense. If these physical conditions are not met, it is more than likely that the meteoroid, as it ploughs through Earth's atmosphere, will either crumble into many small fragments, or it will be completely vaporized before it hits Earth's surface. Most of the meteoroids that produce meteorites are believed to be asteroidal in origin. In essence they are the small fragmentary chips thrown off when two minor planets (asteroids) collide. Meteorites are very valuable then, for bringing samples of asteroidal material to Earth. A few very rare meteorite samples are believed to have come from the planet Mars and the Moon. It is believed that these rare meteorite specimens characterize material that was ejected from the surfaces of Mars and the Moon during the formation of large impact craters.

Accurate orbits are presently known for a few recovered meteorites, including the the Pibram meteorite, which fell in the Czech Republic in 1959; the Lost City meteorite, which fell in Oklahoma in 1970; the Innisfree meteorite, which fell in Alberta, Canada, in 1977; and the Peekskill meteorite, which fell in New York State in 1992. All four of these meteorites have orbits that extend to the main asteroid belt between the planets Mars and Jupiter.

Meteorites are superficially described as being either falls or finds. A meteorite fall is scientifically more useful than a find because the exact time that it hit Earth's surface is known. Finds, on the other hand, are simply that—meteorites that have been found by chance. The largest meteorite find to date is that of the 66-ton (60-metric ton) Hoba meteorite in South Africa. Meteorites are either named after the specific geographic location in which they fall, or after the nearest postal station to the site of the fall.

An analysis of meteorite fall statistics suggests that about 30,000 meteorites of mass greater than 3.5 oz (100 g) fall to Earth each year. Of these meteorites the majority weigh just a few hundred grams, only a few (about 5,000) weigh more than 2.2 lb (1 kg), and fewer still (about 700) weigh more than 22 lb (10 kg). In general the number of meteoroids hitting the Earth's atmosphere increases with decreasing meteoroid mass: milligram meteoroids, for example, are about a million times more common than meteoroids weighing a kilogram.