Meteors+-+SA

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Classification of Meteorites
Not until the early 19th cent. did scientists fully accept the fact that meteorites came to the earth from outer space. Since then many studies have been made of their composition and crystalline structure; the use of microchemical analysis, X rays, and the mass spectrograph has facilitated such work. The age of meteorites can be determined by measuring their radium and helium contents. Some meteorites might be fragments of comets; others, small asteroids whose orbital paths crossed that of the earth. Geochemical analysis has shown that more than a dozen known meteorites are of lunar origin and more than a dozen others are of Martian origin, one of which--known as ALH84001--is believed by some scientists to show evidence of there once having been primitive bacterial life on Mars. These lunar and Martian meteorites are thought to have been broken away from the moon and Mars by the impact of large asteroids. Three general categories are used to classify meteorites. The siderites, or irons, are composed entirely of metal (chiefly nickel and iron). The aerolites, or stony meteorites, show a diversity of mineral elements including large percentages of silicon and magnesium oxides; the most abundant type of aerolite is the chondrite, so called because the metal embedded in it is in the form of grainlike lumps, or chondrules. The siderolites, which are rarer than the other types, are of both metal and stone in varying proportions. As a meteor speeds through the atmosphere, its outer surface becomes liquefied; the friction of the atmosphere finally reduces its velocity (if the meteor is not large), and the surface cools and solidifies into a dark, smooth crust. Lines of flow in the hardened surface can indicate its motions in flight. Cone-shaped meteorites show that one end was directed forward. Others, which are unevenly shaped, probably spun while falling.

Formation of Craters
Friction with the atmosphere has little effect in slowing down a very large, fast-moving meteorite. When it reaches the earth, it strikes with tremendous force and becomes buried beneath the surface. This sudden impact causes great compression, heating, and partial vaporization of the outer part of the meteorite and of the materials in the ground; expansion of the gases thus formed and of steam produced from groundwater causes an explosion that shatters the meteorite and carves out a crater in the ground.

Many people mix up the words //meteoroid//,//meteorite//, and //meteor//. These words are similar, but they mean different things. A meteoroid is a piece of rock moving through space. A meteoroid that falls to the earth's surface is called a meteorite. A meteor is the bright streak of light a meteoroid makes in the sky as it f

Most meteoroids are the size of a tennis ball. Some are as tiny as a grain of salt and are called interplanetary dust. Really large meteoroids are called asteroids. A meteoroid travels 5,000 times faster than a car zipping down a highway. Because it is moving so fast, the meteoroid compresses the air in front of it. It pushes even harder on the air than a moving boat does on the water in front of it. As a boat moves through water, it leaves a wake behind it. Meteoroids do the same with the air. The force of a meteoroid pushing hard against the air is called ram pressure. This action heats the air around the meteoroid. The heated air makes the meteoroid burn hot. Meteoroids can be as hot as 3,000 degrees Fahrenheit (1,649 degrees Celsius). Because of this great heat, most meteoroids burn up as they fall through the atmosphere. Scientists say that as much as 1,000 to 10,000 tons of meteoritic bits fall to Earth every day. If all these bits were piled in one place, they would weigh as much as 200 to 2,000 elephants. Luckily, most of the pieces that fall are as tiny as bits of dust. And since they are so light, they float gently through the air. alls. Meteors are produced when bits of cometary or asteroidal debris in space, usually between the size of a sand grain and a pebble, enter the Earth's atmosphere and burn up, creating a brief -- usually white -- streak of light. Fireball, very bright meteor leaving a trail in the sky that can remain visible for several minutes; often a distinct sound, perhaps caused by very low frequency radio waves, is associated with it.