Asteroids and comets are two more heavenly bodies that are quite close to the earth. They are mostly in form or rocks, ice, or the two. And scientists consider them as remnants of the planet, and they near-earth objects that have the likeness of having a collision with the earth. These two objects actually orbit the sun between the Jupiter and Mars planets, but the asteroids move more in a circular manner, while the comets move more in an elongated manner.
The scientists find it easier to study the asteroids than the comets, simply because the asteroids are nearer, while the orbits are farther. Only 3,572 comets could be accounted for as of 2008, whereas I read that there are typically millions of asteroids. The asteroids are found concentrated around the asteroid belt. However, the comets are being said to originate from a belt of orbiting material referred to as the "Oort Cloud."