Asteroids are relatively smaller solar system bodies. They have fascinated common man and experts in astronomy from centuries. Asteroids orbit around the sun like other plants in the solar system. They are too small to be considered as planets, though they are referred to as minor planets. There are millions of asteroids. Most of them orbit around the sun in the main asteroid belt between the orbits of Mars and Jupiter. Asteroids nearer to the Earth are called as NEAs (Nar Earth Asteroids). Trojans are located near Jupiter’s Lagrange points. They are categorized under three main groups: C type, S type and M type. C, S and M stand for carbon, stony and metallic respectively. Any newly discovered asteroid is given a provisional designation.
The main asteroid belt is also known as asteroid belt or main belt. It contains more than 1 million asteroids larger than 1 km in diameter. There are millions of smaller asteroids in this asteroid belt. Their size varies from nearly 1000 km in diameter to the size of a dust particle. Most of them are irregularly shaped. Collisions between asteroids are common in the main asteroid belt. Collisions between large asteroids form an asteroid family whose members have similar compositions and orbital behavior. Some of the debris formed by collisions can form meteoroids which some times even enter the Earth’s atmosphere. They provide valuable material for research scientists. In fact more than 90 per cent of the meteorites found on Earth originated in the asteroid belt. Collisions also produce fine dust. Asteroid belt appears to be crowded. But is is not the case. Most of the asteroid belt is empty.
Ceres is the largest object in the asteroid belt (975 km) in diameter. Its mass is around 32 per cent of the mass of all the objects of the main asteroid belt. It is also called as a dwarf planet and it was the first asteroid to be discovered in 1801. Vesta and Pallas are the other 2 large asteroids in this belt, with diameters of approximately 500 km. Ceres, Vesta, Pallas and Hygiea contribute to more than half of the mass of all the objects of the main asteroid belt. Vesta is visible to the naked eye when the sky is very dark, as it has a reflective surface.
Scientists believe that planets were formed by a cloud of inter-stellar dust and gas collapsing under the influence of gravity. They further condensed to form planets. During the first few millions in the history of the solar system there were many collisions which caused the clumping of small particles which increased in size. Some of them became planetesimals in the process by drawing other bodies through gravitational attractions. The planetesimals in the region (of the current asteroid belt) were strongly perturbed by Jupiter’s gravity. They continued to orbit the sun as they were. That is how experts believe asteroids were created in the main asteroid belt. Another theory says that asteroids are the remains of a planet which was destroyed in a massive collision. Asteroids in the belt have been evolving from the time they were created.
Pioneer 10 was the first spacecraft to traverse the asteroid belt. Galileo and NEAR are the other spacecrafts associated with asteroids. Scientists have been conducting a lot of research on these fascinating asteroids.