A meteorite is the solid portion of a meteoroid that survives
its fall to Earth, or some other body. Meteorites are classified as stony
meteorites, iron meteorites,
and stony iron meteorites, and further
categorized according to their mineralogical content as shown in the table
(see individual entries for each of these types). They range in size from
microscopic to many meters across. Of the several tens of tons of cosmic
material entering Earth's atmosphere each day, only about one ton reaches
the ground – an object's survival chances depending on its initial
mass, speed of entry, and composition. Incoming meteoroids with masses between
10-6 g and 1 kg tend to burn up completely as meteors.
Smaller objects are dramatically slowed down without being incinerated and
fall as a continuous, gentle, invisible rain of micrometeorites.
Larger objects, up to 1,000 tons, are decelerated to a lesser extent, fall
through the lower atmosphere at high speed, causing them to glow brightly
as a fireball.
A meteorite fall is a meteorite that was seen to fall and was recovered. Because this type of meteorite is usually collected soon after falling, weathering and other terrestrial processes do not have an opportunity to degrade the sample. A find is a meteorite that was not seen to fall, but was found at some later date. For example, many finds from Antarctica fell 10,000 to 700,000 years ago.
The origin of meteorites
Rumors of fiery objects briefly lighting up the heavens and the impromptu
arrival of rocks that apparently dropped out of the blue (or the black)
had contributed to folklore for thousands of years. But the origin of these
"sky stones" remained a matter of great controversy. One idea, popular among
18th-century scientists, was that they were hurled out of volcanoes.
Many ordinary folk, though, were of a different opinion. They thought that
sky stones really did come from the heavens – an idea considered absurd
by most astronomers and geologists. All the same, it was a tricky problem
to resolve. Sky stones would just turn up unpredictably, so there was only
the occasional eyewitness report by a startled onlooker to offer any kind
|A monument at Wold Newton, Yorkshire, England, to
a meteorite which fell there in 1795. Photo credit: Mike Thornton
Then came the big breakthrough. A football-sized rock blazed out of the
sky on a winter's day in 1795 and landed right in the middle of a field
in Yorkshire, England, narrowly missing an astonished plowman. Contacted
by excited villagers, the playwright, journalist, and soldier Major Edward
Topham, the local squire of Wold Newton, took charge of the strange missile
and conveyed it to London. There he showed it to his friend Sir Joseph Banks,
the president of the Royal Society, who commissioned a chemist and a mineralogist
to examine the rock. Asa result of their studies the remarkable truth began
to dawn that sky stones really do arrive on Earth from the depths of space.
A monument, an isolated brick obelisk on Wold Cottage Farm in Wold Newton,
commemorates the great event: "Here, On this Spot, December 13th, 1795,
fell from the Atmosphere, An Extraordinary Stone." An extraordinary stone
indeed, an alien stone – a meteorite.
AND PLANETARY SCIENCE