As the solar nebula cooled, small aggregates of material called planetesimals began to accrete mass. The inner planetesimals accrued heavier material through condensation and the fact that the lighter material had been swept away by the solar wind. The planetesimals further out accrued lighter material. As the planetesimals grew larger, they began to act as vacuum cleaners, ridding the solar system of debris and eventually becoming the current planets. Asteroids are small rocky worlds.
Most asteroids orbit the sun in a region between Mars and Jupiter called the Asteroid Belt.
This belt is thought to be the remains of a planet that failed to form at a distance of 2.8 AU from the sun. There are about 200 asteroids larger than 60 miles in diameter, about 200 larger than 6 miles, and 500,000 larger than 0.6 miles. Comets are large dirty snowballs. They consist of three parts. The nucleus is a ball of frozen water and carbon dioxide, and is usually a few dozen Km in diameter.