An asteroid is any of numerous small planetary bodies that revolve around the sun. Asteroids are also called minor planets or planetoids. Most of them are in the asteroid belt between the orbits of Mars and Jupiter, and most asteroids are found in this belt because of the gravitational interaction between the solar system and the celestial bodies in it. Over 6,000 asteroids have been named and 70,000 have been identified. The belt contains more than 200 asteroids larger than 60 miles (100 kilometers) in diameter. Scientists estimate that there could be as many as 800,000 or more asteroids in the belt larger than 3/5 mile (1 kilometer).
Many asteroids follow orbits outside the belt. For example, a number of asteroids called Trojans follow the same orbit as does Jupiter. Three groups of asteroids -- Atens, Amors, and Apollos -- orbit in the inner solar system and are known as near-Earth asteroids. They also vary in their size. Ceres is the largest known asteroid and is 940 km in diameter. Some of the smallest asteroids are just 6 meters in diameter. The mass of all the asteroids added together, is believed to less than the mass of our Moon. Only 200,000 asteroids have been discovered so far, but a billion more undiscovered asteroids likely exist in our solar system-
They are considered to be remnants of an early broken planet of the solar system. Asteroids have no fixed shape and are too small to be spherical in shape. They are ellipsoids (two dimensional figure), dumbbell or irregularly shaped. These objects were left over from the time the planets formed. Elsewhere in the solar system, other such objects gathered together to form the planets and satellites.
Studies of an asteroid's reflected light as well as analyses of meteorites have provided information about the composition of asteroids. Astronomers classify asteroids into two broad groups based on their composition. One group of asteroids dominates the outer part of the belt. These asteroids are rich in carbon. Their composition has not changed much since the solar system formed. Asteroids in the second group, which are located in the inner part of the belt, are rich in minerals. These asteroids formed from melted materials. The average temperature of the surface of a typical asteroid is -100 degrees F (-73 degrees C).
Most asteroids follow elliptical (oval-shaped) orbits in the asteroid belt. Groups of asteroids that follow the same orbit are called Hirayama families, named after Kiyotsugu Hirayama, the Japanese astronomer who first discovered them.
Asteroids are made of different minerals and substances. This depends on the planet they broke away from in a collision, as well as the chemical reactions they might have experienced while orbiting in the solar system. The asteroids closest to the Sun are mostly carbonaceous and the ones further away are composed of silicate rock. The metallic asteroids are made of 70-80% iron and the remaining is nickel with many other metals such as iridium mixed in. Some are also made of half silicate and half metallic. For example the asteroid Ceres (the mass of Ceres is 25% of the combined mass of all the asteroids) is composed of a rocky core covered by an icy mantle, whereas Vesta has a nickel-iron core, basaltic crust and a covering of magnesium iron silicate (olivine mantle). Asteroids composition has been classified as the following: C class asteroids: They are found in the Earth's outer belt and are darker and more carbonaceous than the ones found in the S class. D class asteroids: They are also known as Trojan asteroids of Jupiter and are dark and carbonaceous in composition. S class asteroids: They are found in the Earth's inner belt, closer to Mars and are composed of mostly stone and iron. V class asteroids: They are a far-out group of asteroids that follow a path between the orbits of Jupiter and Uranus and are made of igneous, eruptive materials.