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Every year, thousands of cosmic bodies bombard the Earth's atmosphere, with a few hundred surviving the journey to impact land or sea. Meteorites the size of a basketball strike Earth about once a month, with nearly 75 percent landing in water. Larger asteroids with potential for major global environment changes hit once every 100,000 years on average, with similar-sized comets impacting approximately every 500,000 years.



Asteroids are generally composed of rock, with some made of metals such as iron and nickel. The vast majority of asteroids in our solar system exist in the Asteroid Belt between Mars and Jupiter. They have diameters that range from under one mile to nearly 500 miles across. Upon entry into the atmosphere, these big rocks are made incandescent by friction with the Earth's atmosphere, burning a luminous trail into the sky called a meteor. Exceptionally bright meteors are also sometimes called bolides, or fireballs. The core of the object falling towards Earth is considered a meteoroid. When a meteoroid survives the descent and strikes Earth, it is renamed a meteorite. Meteorites are no more likely to be radioactive than ordinary terrestrial rocks, and no meteorite yet has been found to contain any element not occurring naturally on Earth. Larger meteorites have left a number of craters around the world both on land and on the ocean floor.

Comets are "dusty iceballs" of rock, frozen water and organic compounds that consist of a head, or coma, and a gaseous tail potentially thousands of miles long. Its head consists of a solid nucleus surrounded by a nebulous coma up to 1.5 million miles in diameter. The tail is an elongated curved vapor trail arising from the coma when sufficiently close to the sun, and is thought to consist chiefly of ammonia, methane, carbon dioxide, and water. Comets are observed only in that part of their orbits that is relatively close to the sun.



The Chicxulub crater on the north shore of the Yucatán Peninsula in Mexico is a 65-million-year-old asteroid or comet impact scar the source of which is thought to have brought about the end of the dinosaurs, along with about 70 percent of life on Earth. The recently discovered impact kicked up a global cloud of dust and sulfur gases that blocked sunlight from penetrating through the atmosphere and introduced Earth to a decade of near-freezing temperatures. Small glassy bodies dating 65 million years called tektites, which are believed to be of extraterrestrial origin, have been found all around the Gulf. The crater is estimated to be 120 miles in diameter, and has since filled with ocean sediment. Scientists believe events the size of Chicxulub occur once every 50-100 million years.

The most famous and best-preserved exposed crater, the Barringer Meteor Crater near Winslow, Arizona is almost a mile in diameter and has been dated to approximately 50,000 years old. It was discovered in 1891 and was created by a nickel-iron meteorite about 150 feet across, weighing roughly 300,000 tons and traveling at a speed of 40,000 miles per hour. The impact caused an explosion equivalent to 20 million tons of TNT and has left behind a chasm 570 feet deep.

The 1908 Tunguska Event in Siberia occurred when an asteroid or comet estimated at 200 feet wide and weighing about 100,000 tons exploded five miles above the Earth, releasing approximately 20-40 megatons of energy and leaving tiny grains of cosmic matter embedded in the trees knocked down by the blast. An area the size of Rhode Island was devastated in the explosion, killing wildlife within 20 miles of the impact, and creating fires that burned for weeks. Falls in the class of the Tunguska and Barringer variety occur over land only about once every millennium.

On March 23, 1989 an asteroid named 1989 FC with a diameter about 0.3 miles and a kinetic energy of over 1,000 one-megaton hydrogen bombs passed within 430,000 miles of the Earth. This asteroid was not discovered until it had passed its point of closest approach, and only after calculating backwards its orbital path. Since then several other celestial bodies of similar sizes have been measured as coming within 62,000 miles of Earth.



Although scientists have now identified over 150 impact sites, the history of Earth impacts will continue to grow as new discoveries of hidden craters are made. Nearly all known craters have been recognized as such since 1950 and a number of new structures are found each year. Evidence suggests that there have been many thousands of other impacts over the course of the Earth's history, some of which have yet to be uncovered on the ocean floor or in hidden surface locations.






Updated October 14, 1998
Roger D. Launius, NASA Chief Historian
Steve Garber, NASA History Web Curator
For further information contact the NASA History Office