Asteroids are (relatively) small rocky entities that orbit the sun. Asteroids are distinguished from comets and meteors by their size, composition and their orbit. They are created under the same conditions as the planets and the moon, through the violent collision of clouds, gas and dust in a nebula.
Heavy and dense in nature, these compress together and condense to create celestial bodies in space. In recent astrophysical history, asteroids have seldom interacted violently with one another because of a relatively stable orbit and weak gravitational field. Thus they maintain an irregular shape. The asteroid belt is situated between Mars and Jupiter and contains the majority of the solar system's asteroid population. The largest asteroids are found within the asteroid belt and measure over 500 miles in diameter.
An asteroid large enough could collide with Earth and threaten human civilisation. However such a cataclysmic event occurs only once every few million years. More frequently, Earth is struck by smaller asteroids which leave craters but endangers no species. In February 2013, a small asteroid passed by Earth and fragmented as the atmosphere began to break it up. A large portion and several smaller meteorites crashed in Chelyabinsk, Russia.
The principle part of this former asteroid weighed 1440LB and measured 65ft in diameter. There were no deaths. But around 1,500 people had been injured, principally by shattering glass.