They are also called Minor Planet, or Planetoid, they are small rocky bodies, about 1,000km or less in diameter. Some call them minor planets because of their small size and large numbers relative to the nine major planets. Dynamicists, astronomers who study individual objects with dynamically interesting orbits or groups of objects with similar orbital characteristics, generally use the term minor planet, whereas those who study the physical properties of such objects usually refer to them as asteroids. At first he assumed that this was just another comet, but the orbital characteristic of the body suggested that it was more like a small planet. Scientists believe that originally there were only a few dozen or so asteroids in existence, and these asteroids were fragmented through multiple collisions to lead to the present amount.
Asteroids can vary greatly in size and shape.
The largest known asteroid is Ceres and it has a diameter of about 1030 km. About 200 asteroids have diameters between 200 km and 97 km, and the majority of asteroids, over thousands, are smaller than 97 km in diameter. Asteroids are classified into a few groups. Other less common groups exist and some may be very rare such as M-type objects. Asteroids are also categorized by their position in the solar system.