The region located roughly between the orbits of planet Mars and Jupiter, is known as asteroid belt. Numerous irregular shaped bodies occupy this region. These bodies are called asteroids or minor planets. The asteroid belt was formed from primeval solar nebula. It was formed as a group of planetisimals which are smaller precursors of planets. Between Mars and Jupiter perturbations due to gravitational pull from the giant planet instilled the protoplanets with a lot of energy (orbital energy) which caused them to assimilate into a planet. The intensity of the collisions increased and they became too violent as a result instead of sticking together the planetisimals shattered. Some of the fragments sometimes find their way into the atmosphere resulting in formation of meteors.
The asteroids are not the exact samples of primeval Solar system. Since their formation they have undergone a lot evolution like internal heating, surface melting caused by impacts, space weathering due to radiation and bombardment by micrometeorites. It is believed that the current asteroid belt has only a small fraction of the mass that was actually there in the original asteroid belt. According to computer simulations the mass of the original asteroid belt would have been equal to that of earth. The size distribution of the asteroid belt has remained constant since the formation. No significant change has been observed in the typical dimensions of the main belt.
The popular descriptions of the asteroid belt say that there is not much empty space in the asteroid belt contrary to this the belt is mostly empty. Currently about hundreds of thousands of asteroids are distinctly known and the total number of asteroids is estimated to be in millions or even more. According to the estimates the total mass of the asteroid belt is 2.8×1021 to 3.2×1021. This mass is just about 4% that of moon.
The current main belt consists of primarily three categories of asteroids:
• C-type or carbonaceous asteroids
• S-type or silicate asteroids
• M-type or metallic asteroids
The C-type asteroids are rich in carbon and they are found in the outer region of the belt. They comprise about 75% of the visible asteroids. S-type asteroids, which are rich in silica, are mostly dominant in the inner region of the belt. M-type asteroids are rich in metal and they form about 10% of the total population of asteroids.
The first spacecraft which traversed the main asteroid belt was called Pioneer 10. It entered the asteroid belt in 16 July, 1972. During that period it was a major concern that the debris which was there in the belt could damage the spacecraft and posed a danger. But since then 9 spacecrafts have successfully traversed the belt. Since, the density of the material of the belt is very low so there are very few chances that a probe would collide with an asteroid. All the images of the asteroid belt that have been obtained till today by all spacecrafts have come from a short flyby opportunity when the spacecraft was heading towards some other target.
Will a big asteroid will ever hit the earth and cause the end of the world ? This is purely teorical but it could happen sooner then we think. With all the latest phenomena happening around the world, we are not strange to recent bizarre events happening suddently. The latest theory about an asteroid hitting eart is the apophis 99942 asteroid that is suppose to hit the earth in 2036.