A meteorite is a relatively small extraterrestrial material body that reaches the Earth's surface. While in space these bodies are called meteoroids ("small" asteroids, approximately boulder-sized or less, which are generally fragments resulting from the collision of two, or more, asteroids). Upon entering the atmosphere air drag and friction will cause the body to heat up, emitting light, thus forming a meteor, fireball, or shooting star.
Most meteors disintegrate in the air, making impact events (Earth impacts) on the surface of Earth uncommon. About 500 baseball sized rocks a year reach the surface. 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 with Earth's surface 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 result 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 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. Carbonaceous chondrites are thought to be unaltered solar nebula material.
Achondrites are similar to terrestrial mafic igneous rocks and sometimes are brecciated. Achondrites are 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. For example, Lunar meteorites or Martian meteorites.
One theory stipulates that a large meteorite impact caused the mass extinction of the dinosaurs, as well as other mass extinction events throughout the history of the earth.
See Siljan and other notable meteorite impacts. See comet, asteroid, Leonids, Geminids and solar system for more information on extraterrestrial bodies.
The only recorded fatality from meteorite impacts is an Egyptian dog, who was killed in 1911. The cause of death was identified in the 1980s as being a Martian meterorite. The first, and as of 2003, only known modern case of a human being hit by a space rock occurred on November 30, 1954 in Sylacauga, Alabama. There an 8.5 pound sulfide meteorite crashed through a roof and hit a Mrs. Elizabeth Hodges in her living room after it bounced off her radio. Other than a nasty bruise she was OK.
See also: Impact event, Tunguska event, asteroid