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A meteorite is a natural object originating in Outer Space that survives an impact with the Earth 's surface without being destroyed. While in space it is called a Meteoroid . When it enters the atmosphere, Air Resistance causes the body to heat up and emit light, thus forming a Fireball , also known as a Meteor or '''shooting star'''. The term '''bolide''' refers to either an extraterrestrial body that collides with the Earth, or to an exceptionally bright, fireball-like meteor regardless of whether it ultimately impacts the surface. The meteorite is the source of the light. More generally, a meteorite on the surface of any celestial body is an object that has come from elsewhere in space. Meteorites have been found on the .1 Meteorites that are recovered after being observed as they transited the atmosphere or impacted the Earth are called falls. All other meteorites are known as '''finds'''. As of mid-2006, there are approximately 1,050 Witnessed Falls having specimens in the world's collections. In contrast, there are over 31,000 well-documented meteorite finds Meteoritical Bulletin Database . Meteorites are always named for the place where they were found, Meteoritical Society Guidelines for Meteorite Nomenclature usually a nearby town or geographic feature. In cases where many meteorites were found in one place, the name may be followed by a number or letter (e.g., Allan Hills 84001 or Dimmitt (b)). Meteorites have traditionally been divided into three broad categories: stony meteorites are rocks, mainly composed of silicate minerals; Iron Meteorite s are largely composed of metallic iron-nickel; and, stony-iron meteorites contain large amounts of both metallic and rocky material. Modern classification schemes divide meteorites into groups according to their structure, chemical and isotopic composition and mineralogy. See Meteorites Classification . FALL PHENOMENA in 1868 ('''')]] Most meteoroids disintegrate when entering the Earth's atmosphere. However an estimated 500 meteorites ranging in size from Marble s to Basketball s or larger do reach the surface each year; only 5 or 6 of these are typically recovered and made known to scientists. Few meteorites are large enough to create Impact Crater s. Instead, they typically arrive at the surface at their Terminal Velocity (free-fall) and, at most, create a small pit. Even so, falling meteorites have reportedly caused damage to property, livestock, and people. Very large meteoroids may strike the ground with a significant fraction of their cosmic velocity, leaving behind a , Odessa Meteor Crater , Wabar Craters , and Wolfe Creek Crater ; iron meteorites are found in association with all of these craters. In contrast, even relatively large stony or icy bodies like small Comet s or Asteroid s, up to millions of tons, are disrupted in the atmosphere, and do not make impact craters.Bland P.A. and Artemieva, N A. (2006) The rate of small impacts on Earth. Meteoritics and Planetary Science 41, 607-631. Although such disruption events are uncommon, they can cause a considerable concussion to occur; the famed Tunguska Event probably resulted from such an incident. Very large stony objects, hundreds of meters in diameter or more, weighing tens-of-millions of Ton s or more, can reach the surface and cause large craters, but are very rare. Such events are generally so energetic that the impactor is completely destroyed, leaving no meteorites. (The very first example of a stony meteorite found in association with a large impact crater, the Morokweng Crater in South Africa, was reported in May 2006.Maier, W.D. et al. (2006) Discovery of a 25-cm asteroid clast in the giant Morokweng impact crater, South Africa. Nature 441, 203-206) Several phenomena are well-documented during witnessed meteorite falls too small to produce hypervelocity craters. Sears, D. W. (1978) The Nature and Origin of Meteorites, Oxford Univ. Press, New York The fireball that occurs as the meteoroid passes through the atmosphere can appear to be very bright, rivaling the sun in intensity, although most are far dimmer and may not even be noticed during daytime. Various colors have been reported, including yellow, green and red. Flashes and bursts of light can occur as the object breaks up. Explosions, detonations, and rumblings are often heard during meteorite falls, which can be caused by Sonic Boom s as well as shock waves resulting from major fragmentation events. These sounds can be heard over wide areas, up to many thousands of square Km . Whistling and hissing sounds are also sometimes heard, but are poorly understood. Following passage of the fireball, it is not unusual for a dust trail to linger in the atmosphere for some time. As meteoroids are heated during passage through the atmosphere, their surfaces melt and experience Ablation . They can be sculpted into various shapes during this process, sometimes resulting in deep "thumb-print" like indentations on their surfaces called regmaglypts. If the meteoroid maintains a fixed orientation for some time, without tumbling, it may develop a conical "nose cone" or "heat shield" shape. As it decelerates, eventually the molten surface layer solidifies into a thin fusion crust, which on most meteorites is black (on some achondrites, the fusion crust may be very light colored). On stony meteorites, the heat-affected zone is at most a few mm deep; in iron meteorites, which are more thermally conductive, the structure of the metal may be affected by heat up to 1 cm below the surface. Meteorites are sometimes reported to be warm to the touch when they land, but they are never hot. Reports, however, vary greatly, with some meteorites being reported as "burning hot to the touch" upon landing, and others forming a frost upon their surface. Meteoroids that experience disruption in the atmosphere may fall as meteorite showers, which can range from only a few up to thousands of separate individuals. The area over which a meteorite shower falls is known as its strewn field. Strewn fields are commonly Elliptical in shape, with the major axis parallel to the direction of flight. In most cases, the largest meteorites in a shower are found farthest down-range in the strewn field. METEORITE TYPES About 86% of the meteorites that fall on Earth are s, are composed mostly of silicate minerals that appear to have been melted while they were free-floating objects in space. Chondrites also contain small amounts of organic matter, including Amino Acid s, and Presolar Grains . Chondrites are typically about 4.55 billion years old and are thought to represent material from the Asteroid Belt that never formed into large bodies. Like Comet s, chondritic asteroids are some of the oldest and most primitive materials in the solar system. Chondrites are often considered to be "the building blocks of the planets". , New York ]] About 8% of the meteorites that fall on Earth are Achondrite s, some of which appear to be similar to terrestrial mafic Igneous Rock s. Most achondrites are also ancient rocks, and are thought to represent crustal material of asteroids. One large family of achondrites (the HED Meteorite s) may have originated on the asteroid 4 Vesta . Others derive from different asteroids. Two small groups of achondrites are special, as they are younger and do not appear to come from the asteroid belt. One of these groups comes from the Moon, and includes rocks similar to those brought back to Earth by Apollo and Luna programs. The other group is almost certainly from Mars and are the only materials from other planets ever recovered by man. About 5% of meteorites that fall are Iron meteorites with intergrowths of iron- Nickel Alloy s, such as Kamacite and Taenite . Most iron meteorites are thought to come from the core of a number of asteroids that were once molten. As on Earth, the denser metal separated from silicate material and sank toward the center of the asteroid, forming a core. After the asteroid solidified, it broke up in a collision with another asteroid. Due to the near absence of irons from finds in collection areas such as Antarctica, where little, if any meteoric material that has fallen is not found, it is thought that, although irons constitute approximately 5% of recovered falls, they might actually be considerably less common than previously supposed. Stony-iron meteorites constitute the remaining 1%. They are a mixture of iron-nickel metal and Silicate minerals. One type, called Pallasite s, is thought to have originated in the boundary zone above the core regions where iron meteorites originated. The other major type of stony-iron meteorites is the Mesosiderite s. Tektites (from Greek ''tektos'', molten) are not themselves meteorites, but are rather natural glass objects up to a few centimeters in size which were formed--according to most scientists--by the impacts of large meteorites on Earth's surface. A few researchers have favored Tektites originating from the Moon as volcanic ejecta, but this theory has lost much of its support over the last few decades. METEORITE RECOVERY Falls Most Meteorite Falls are recovered on the basis of eye-witness accounts of the fireball or the actual impact of the object on the ground, or both. Therefore, despite the fact that meteorites actually fall with virtually equal probability everywhere on Earth, verified meteorite falls tend to be concentrated in areas with high human population densities such as Europe, Japan, and northern India. A small number of meteorite falls have been observed with automated cameras and recovered following calculation of the impact point. The first of these was the s captured images of the fireball. The images were used both to determine the location of the stones on the ground and, more significantly, to calculate for the first time an accurate orbit for a recovered meteorite. Following the Pribram fall, other nations established automated observing programs aimed at studying infalling meteorites. One of these was the ''Prairie Network'', operated by the Smithsonian Astrophysical Observatory from 1963 to 1975 in the midwestern US. This program also observed a meteorite fall, the ''Lost City'' chondrite, allowing its recovery and a calculation of its orbit.McCrosky, R.E. et al. (1971) J. Geophys. Res. 76, 4090-4108 Another program in Canada, the Meteorite Observation and Recovery Project, ran from 1971 to 1985. It too recovered a single meteorite, ''Innisfree'', in 1977.Campbell-Brown, M. D. and Hildebrand, A. (2005) A new analysis of fireball data from the Meteorite Observation and Recovery Project (MORP). Earth, Moon, and Planets 95, 489 - 499 Finally, observations by the European Fireball Network, a descendant of the original Czech program that recovered Pribram, led to the discovery and orbit calculations for the '' Neuschwanstein '' meteorite in 2002.Oberst, J. et al. (2004) The multiple meteorite fall of Neuschwanstein: Circumstances of the event and meteorite search campaigns. Meteoritics & Planetary Science 39, 1627-1641 PS...39.1627O NASA ADS Finds Until the 20th century, only a few hundred meteorite finds had ever been discovered. Over 80% of these were iron and stony-iron meteorites, which are easily distinguished from local rocks. To this day, few stony meteorites are reported each year that can be considered to be "accidental" finds. The reason there are now over 30,000 meteorite finds in the world's collections started with the discovery by Harvey H. Nininger that meteorites are much more common on the surface of the Earth than was previously thought. The Great Plains of the US Nininger's strategy was to search for meteorites in the Great Plains of the United States, where the land was largely cultivated and the soil contained few rocks. Between the late 1920s and the 1950s, he traveled across the region, educating local people about what meteorites looked like and what to do if they thought they had found one, for example, in the course of clearing a field. The result was the discovery of over 200 new meteorites, mostly stony types. Website by A. Mitterling In the late 1960s, Roosevelt County, New Mexico in the Great Plains was found to be a particularly good place to find meteorites. After the discovery of a few meteorites in 1967, a public awareness campaign resulted in the finding of nearly 100 new specimens in the next few years, with many being found by a single person, Mr. Ivan Wilson. In total, nearly 140 meteorites were found in the region since 1967. In the area of the finds, the ground was originally covered by a shallow, loose soil sitting atop a Hardpan layer. During the Dustbowl era, the loose soil was blown off, leaving any rocks and meteorites that were present stranded on the exposed surface.Huss, G.I. and Wilson, I.E. (1973) A census of the meteorites of Roosevelt County, New Mexico. Meteoritics 8, 287-290 NASA ADS Antarctica A few meteorites had been found by field parties in ) program. European teams, starting with a consortium called "EUROMET" in the late 1980s, and continuing with a program by the Italian Programma Nazionale di Ricerche in Antartide have also conducted systematic searches for Antarctic meteorites. Most recently, a Chinese program, the Antarctic Scientific Exploration of China, has conducted highly successful meteorite searches since the year 2000. The combined efforts of all of these expeditions have produced over 23,000 classified meteorite specimens since 1974, with thousands more that have not yet been classified. For more information see the article by Harvey (2003).Harvey, Ralph (2003) The origin and significance of Antarctic meteorites Chemie der Erde 63, 93-147 Australia At about the same time as meteorite concentrations were being discovered in the cold desert of Antarctica, collectors discovered that many meteorites could also be found in the hot desert of Australia. Several dozen meteorites had already been found in the . In the extremely arid climate, there has been relatively little Weathering or Sedimentation on the surface for tens of thousands of years, allowing meteorites to accumulate without being buried or destroyed. The dark colored meteorites can then be recognized among the very different looking limestone pebbles and rocks. The Sahara and rising commercialization |
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