meteorite

A meteorite is a solid piece of extraterrestrial matter that survives its passage through Earth's atmosphere and lands on the surface. Most meteorites originate in the asteroid belt, though some come from the Moon or Mars after impact ejections and subsequent delivery to Earth.

Meteorites are commonly grouped into three broad types: irons, stones, and stony-irons. Irons are composed mainly of nickel-iron alloy. Stones are silicate rocks and include chondrites, which contain small rounded grains called chondrules and are remnants of primitive solar-system material, and achondrites, which are differentiated rocks that have undergone melting. Stony-iron meteorites contain substantial amounts of both metal and silicate. During atmospheric entry, a meteorite creates a bright meteor or fireball. Most of its outer layers ablate, forming a fusion crust as it slows down. Larger fragments may reach the ground as meteorites, while many others are found as micrometeorites in soils or sediments.

Meteorites provide direct samples of other bodies and help scientists study the early solar system, planetary differentiation, and the formation of planets. Radiometric dating and isotopic analyses place many meteorites at about 4.5 billion years old.

Notable meteorites include the Allende chondrite, a carbonaceous chondrite that fell in Mexico in 1969; the Murchison meteorite, another carbonaceous chondrite that fell in Australia in 1969 and is rich in organic compounds; the Hoba iron meteorite in Namibia, the largest known intact meteorite; and the Canyon Diablo iron meteorite associated with the Barringer Crater in the United States. The 2013 Chelyabinsk event highlighted how an atmospheric airburst can occur without a surface impact, while leaving recovered fragments.