Asteroids and Meteorites
 

[B][CENTER]Asteroids[/CENTER][/B]

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Asteroids are rocky and metallic objects that orbit the Sun but are too small to be considered planets. They are known as minor planets. Asteroids range in size from Ceres (The largest and the 1st one to be found) which has a diameter of about 1000 km, down to the size of pebbles. They have been found inside Earth's orbit to beyond Saturn's orbit. Most, however, are contained within a main belt that exists between the orbits of Mars and Jupiter. Some have orbits that cross Earth's path and some have even hit the Earth in times past.
Asteroids are material left over from the formation of the solar system. One of the theory yet presented suggests that they are the remains of a planet that was destroyed in a massive collision long ago. More likely, asteroids are material that never merged into a planet. In fact, if the estimated total mass of all asteroids was gathered into a single object, the object would be less than 1,500 kilometers (932 miles) across i.e. less than half the diameter of Moon.

[B][CENTER]Meteorites[/CENTER][/B]

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The definition of a meteoroid is a solid object moving in interplanetary space, of a size considerably smaller than an asteroid and considerably larger than an atom.
Meteorites are solid extraterrestrial material that survived passage through the Earth's atmosphere and landed on Earth. It is believed that most meteorites are probably fragments from the asteroid belt that lies between Mars and Jupiter. A small number of rare meteorites are fragments of the Moon and Mars.

[B]Types of Meteorites:[/B]
There are three major types of meteorites: stoney, stoney iron, and iron. More that 90 percent of all meteorites are the stoney type, which are made up mostly of silicate minerals.
Furthermore Stone meteorites consist of two groups i.e. [U]chondrites[/U] and [U]achondrites[/U].
Most chondrites have remained unchanged since their formation 4.56 billion years ago, shortly after the formation of the Sun. Almost all chondrites contain chondrules - small, gaseous, spherical inclusions that formed during the solar nebula.
Achondrites appear to have been chondritic before being altered by a heating or impact event. These meteorites are much rarer than chondrites.

asteroids
 

II.ASTEROIDS COMPARED TO OTHER SOLAR SYSTEM OBJECTS


Solar System Tour

Asteroids are sometimes called planetoids or minor planets. Most rotate on their axes every 5 to 20 hours. Some smaller asteroids may orbit larger asteroids as satellites, forming binary pairs. Asteroids differ from true planets mainly by their much smaller size—only about 200 asteroids have diameters of more than 97 km (60 mi). Planets are defined as bodies that have settled into a rounded shape because of the inward pull of their own gravitation. Asteroids are not thought to have enough mass to settle into rounded shapes in the same way as planets. With their low masses, asteroids also do not have atmospheres.In 2006 the International Astronomical Union (IAU) designated a new category of solar system objects called dwarf planets. The dividing line between asteroids and dwarf planets is still being clarified. Dwarf planets have rounded shapes but are not massive enough to clear other bodies from around their orbits. Major or “classical” planets had enough mass to clear their neighborhoods of small bodies, either by pulling such objects in as the planets formed or by throwing the small bodies into distant orbits or out of the solar system. Dwarf planets orbit the Sun in regions of the solar system that contain swarms of small bodies such as the asteroid belt or the Kuiper Belt. The largest object in the asteroid belt, 1 Ceres, is now classified as a dwarf planet because of its rounded shape. Future research may show that additional large asteroids also qualify as dwarf planets because their shape was rounded by their own gravitation.Because of their small size and primitive composition, asteroids share many basic properties with comets. Comets typically have much more elliptical orbits than asteroids and actively shed gas and dust. Although asteroids are mainly rocky, some may also contain water-ice material and so are not clearly distinct from objects that can become comets if heated by enough sunlight. It is also possible that some objects that are considered asteroids are remains of dead comets that have lost their gas and dust. In 2006 astronomers announced finding a number of icy comet-like objects orbiting in the main asteroid belt, suggesting that asteroids and comets can occur together. The term asteroid is sometimes extended to the small icy bodies found in the outer system beyond Jupiter. Unlike rocky asteroids, these more distant bodies are mainly made of ice, along with dust and rock. Closely related to comets, these solar system bodies are now generally called Kuiper Belt Objects (KBOs) or Trans-Neptunian Objects (TNOs). Most of these objects orbit the Sun beyond Neptune in a disk called the Kuiper Belt. A special population of these icy objects called centaurs have left the Kuiper Belt and have unstable orbits that lie between Neptune and Jupiter. Chiron, the first centaur discovered, was originally classified as an asteroid. Like asteroids and dwarf planets, KBOs and centaurs are listed in the official catalog of minor planets.