Ceres, the largest asteroid known, is 480 miles (772 km) in diameter. Eros is 18 miles (29 km) in diameter. Amor is only about a mile across. They are all too small to hold any air on them. They do not have enough gravity. There is so little gravity on Amor that, if a person could go there and could take a train with him, he could lift the train with one hand and swing it around his head.
About 1,500 asteroids have been discovered. New ones keep being found. Of course, these new ones are not really new. They have simply never been seen before. None of them can be seen without telescopes. Most of them have been discovered in pictures taken through telescopes.
The asteroid Kermes made a name for itself in 1937. It carne very close to the earth on one of its trips around the Sun. It was only 621,000 miles away!
No one knows surely how the asteroids were formed. One guess is that they came from a large planet which was once between Mars and Júpiter. Perhaps the large planet came too close to the giant Júpiter and was pulled to pieces.
Sometimes asteroids are called planetoids. Planetoids is really a better name, for these small bodies do not shine by their own light as stars do.
