World's second largest Meteorite in Culiacan, Mexico

A meteorite is a relatively small extra-terrestrial body that reaches the Earth's surface. While in space these bodies are called meteoroids and are called meteors after entering Earth's atmosphere, but before reaching the surface. These are "small" asteroids, approximately boulder-sized or less. Upon entering the atmosphere, air drag and friction cause the body to heat up and emit light, thus forming a fireball or shooting star.

More generally, a meteorite on a celestial body is a small body that has come from elsewhere in space. One has been found on Mars: Heat Shield Rock.

Overview

Meteorite which fell in Wisconsin in 1868 (Full image)

Most meteors disintegrate when entering the Earth's atmosphere, making impact events (Earth impacts) on the surface uncommon. About 500 baseball-sized rocks reach the surface each year. Large meteorites may strike the ground with considerable force, leaving behind a meteor crater. The kind of crater will depend on the size, composition, degree of fragmentation, and incoming angle of the meteor. The force of collision may cause widespread destruction. Occasional damage to property, livestock, and even people has been recorded in historic times. In the case of comet fragments, which are largely composed of ice, a considerable concussion may occur, even though no fragment of the original meteoroid survives; the famed Tunguska event is thought to have resulted from such an incident.

79% of meteorites are Chondrites - balls of mafic minerals with small grain size indicative of rapid cooling. In most chondrites small spherules, called chondrules, can be found. Chondrites are typically about 4.6 billion years old and are thought to represent material from the asteroid belt. It is unknown how they formed. Carbonaceous Chondrites, thought to be unaltered solar nebula material, constitute about 5% of meteorites and contain small amounts of organic materials, including amino acids. Also, presolar grains are identified in carbonaceous chondrites. The isotope ratios of carbonaceous chondrites are similar to those of the Sun.

Achondrites are similar to terrestrial mafic igneous rocks and sometimes are brecciated. Achondrites constitute about 8% of the incoming material and are thought to represent crustal material of larger asteroids. About 6% of meteorites are iron meteorites with intergrowths of iron-nickel alloys, such as kamacite. Unlike chondrites, the crystals are large and appear to represent slow crystallization. Iron meteorites are thought to be the core material of one or more planets that subsequently broke up. Stony iron meteorites constitute the remaining 2%. They are a mixture of iron-nickel and silicate minerals. They are thought to have originated in the boundary zone above the core regions where iron meteorites originated. A small number of meteorites belong to additional groups or subgroups with unique chemical characteristics relative to other members of the larger groups, such as Lunar meteorites or Martian meteorites.

One theory stipulates that a large meteorite impact caused the mass extinction of the dinosaurs. It is also theorized that meteorites caused other mass extinction events as well throughout the history of the earth.

The Willamette Meteorite, the largest ever to fall on the United States

The only reported fatality from meteorite impacts is an Egyptian dog who was killed in 1911, although this report is disputed. The meteorites that struck this area were identified in the 1980s as Martian in origin. The first and only known modern case of a human hit by a space rock occurred on November 30, 1954 in Sylacauga, Alabama. There a 4 kg sulfide meteorite crashed through a roof and hit Ann Hodges in her living room after it bounced off her radio. She was badly bruised.

Whole and partial meteorites are valuable to collectors, with large fully intact pieces selling for prices above USD$20,000 at auction.

See also

• Sayh al Uhaymir
• Lake Siljan
• Leonids
• Geminids
• Solar System
• Willamette Meteorite (the largest meteorite ever found in the United States)

External links

• Meteorite.fr - All about Meteorites (http://www.meteorite.fr/en/news/)
• Natural History Museum of Vienna (http://www.nhm-wien.ac.at/NHM/Mineral/MetCollecte.htm)
• Meteoritical Society (http://www.meteoriticalsociety.org/)
• The Natural History Museum's Meteorite Catalogue Database (http://flood.nhm.ac.uk/cgi-bin/earth/metcat/)
• Meteorite hits (http://www.branchmeteorites.com/metstruck.html)
• Largest meteorites (http://www.jensenmeteorites.com/largestmeteorites.htm)