meteorite

Piece of rock or metal from space that reaches the surface of the Earth, Moon, or other body. Most meteorites are thought to be fragments from asteroids, although some may be pieces from the heads of comets. Most are stony, although some are made of iron and a few have a mixed rock–iron composition.

Stony meteorites can be divided into two kinds: chondrites and achondrites. Chondrites contain chondrules, small spheres of the silicate minerals olivine and orthopyroxene, and comprise 85% of meteorites. Achondrites do not contain chondrules. Meteorites provide evidence for the nature of the Solar System and may be similar to the Earth's core and mantle, neither of which can be observed directly.

Thousands of meteorites hit the Earth each year, but most fall in the sea or in remote areas and are never recovered. The largest known meteorite is one composed of iron, weighing 66 tons, which lies where it fell in prehistoric times at Grootfontein, Namibia.

Meteorites are slowed down by the Earth's atmosphere, but if they are moving fast enough they can form a crater on impact. Meteor Crater in Arizona, about 1,200 m/4,000 ft in diameter and 200 m/650 ft deep, is the site of a meteorite impact about 50,000 years ago. Mrs Hewlett Hodges of Sylacauga, Alabama, was bruised on the hip by a meteorite that fell through the roof of her house on 30 November 1954 – the only well-authenticated case of human injury by a meteorite.

A meteorite from Mars, found in Antarctica in 1984, was claimed by NASA scientists in August 1996 to contain possible evidence of life. The 15 million-year-old rock, which entered the Earth's atmosphere in the form of a meteorite 12,000 years ago, was probably broken from the surface of Mars when the planet collided with a large object in space. The meteorite, ALH84001, weighs 1.9 kg/4.2 lb, and was found to contain carbonate globules, on the surface of which are minute round or long and thin micro-organisms. These microfossils are a hundred times smaller than the smallest known Earth microfossils. The globules also contain magnetite and iron sulphide particles comparable in shape and quantity to those produced by Earth micro-organisms. It is now thought more likely, however, that the meteorite had been contaminated with terrestrial organisms.

By 2005, 34 known meteorites were believed to have come from Mars.