Astronomers may also measure an asteroid during an occultation, when the asteroid passes in front of a star and is silhouetted against it. In addition, astronomers use radio telescopes to produce images of asteroids. The asteroids Gaspra, left, and Ida. Orbitting Ida is a tiny satellite object called Dactyl, which is seen at the far right. In 1991, the United States space probe Galileo took the first detailed photograph of an asteroid.
Astronomers classify asteroids into two broad groups based on their composition.
One group of asteroids dominates the outer part of the belt. These asteroids are rich in carbon. Their composition has not changed much since the solar system formed. Asteroids in the second group, which are located in the inner part of the belt, are rich in minerals. These asteroids formed from melted materials.