Asteroids are rocky-metallic objects which range in size from about the size of pebbles to around 600 miles (~1,000 km) across. Although they orbit the Sun, they are too small to be considered planets. Asteroids are thought to be leftover material from the formation of our solar system. Most are found in the Asteroid Belt, a doughnut-shaped ring which lies between the orbits of Mars and Jupiter. Astronomers have also identified a group of asteroids whose orbits cross Earth's orbit. Several hundred thousand asteroids are known to exist in our solar system, and many are yet to be discovered. Most of the undiscovered asteroids are the smaller ones (less than 100 km across) which are more difficult to detect. It is estimated that there are over a million of these smaller asteroids.
Although asteroids are visually dark objects, they can glow in the infrared. Asteroids usually absorb more sunlight than they reflect. This causes them to warm up and emit infrared (or heat) radiation. By studying the infrared radiation from an asteroid, we can learn about its location, composition, rotation and its shape and size. Many asteroids are too dim to be detected in visible light and can only be detected in the infrared. For example, the first infrared satellite mission, IRAS, discovered over 400 new asteroids and also provided infrared data on about 1800 previously known asteroids. Because asteroids were formed when the solar system was formed, scientists are very interested in their composition (what they are made of). The composition of most asteroids is determined using infrared spectroscopy. This information can tell us about the conditions which existed during solar system formation.

Near infrared map of the asteroid
Eros from the NEAR sattellite

The study of asteroids is one important key to the understanding of the early solar system. The Spitzer Space Telescope will measure the infrared emission from asteroids, to gather additional information about their composition and structure. In particular, Spitzer will study smaller asteroids - those with diameters of less than 10 km.