Often called “minor planets,” asteroids are large chunks of rock or metal that orbit the Sun just as the nine major planets do. Unlike the major planets, however, most asteroids are confined to the Asteroid Belt, an area between the orbits of Mars and Jupiter.

Scientists once believed asteroids were fragments of a planet that had been destroyed, but the consensus today is they are remnants left over from the formation of the solar system. While other chunks of rock adhered together to form planets, Jupiter’s gravitational pull prevented these bodies from developing into a larger object. Even if they did unite, there would not be enough material to constitute a planet--the resulting object would measure only 950 miles in diameter, a fraction of the size of Earth’s moon, and about 1/1000 of Earth’s mass. Most asteroids are made of rock, primarily carbon and silicon compounds. Only 5 percent are metallic, consisting mainly of nickel and iron.

The first asteroid to be discovered, Ceres, was identified in 1801. Ceres is the largest known asteroid, with a diameter of about 580 miles, and is thought to constitute about one-third of the mass of all asteroids. Since the discovery of Ceres, thousands more asteroids have been identified, and since 1847, not a year has passed without the discovery of still more. Precise orbits have been calculated for many asteroids, but only preliminary orbits have been worked out for most. There are about 264,000 known asteroids, 12,136 of which have been named. It is believed there are millions of asteroids in the solar system.

Most asteroids follow an elliptical orbit, and though the majority stay in the outer section of the solar system, some do cross Earth’s path. These are called Near Earth Objects, or NEOs, and over 600 of these are classified as Potentially Hazardous Asteroids, or PHAs. The Minor Planet Center of the International Astronomical Union monitors PHAs closely, though researchers don’t yet know how to deflect an Earth-bound asteroid. NEAs are also monitored by The Lincoln Near Earth Asteroid Research project (LINEAR) and NASA’s Near-Earth Asteroid Tracking project (NEAT).

Collisions among asteroids are frequent, creating smaller chunks and resulting in the irregular shape many asteroids possess. In addition, Jupiter’s gravitational pull and encounters with Mars can sometimes knock asteroids out of the Asteroid Belt and into the orbits of their planetary neighbors. It is believed Mars’ moons Phobos and Deimos were asteroids or asteroid fragments captured by the planet. Occasionally, small asteroid fragments reach Earth’s surface, and are called meteorites. Larger asteroids may have collided with Earth in the past, however. It is believed that the extinction of the dinosaurs, some 65 million years ago, resulted from an impact with an asteroid, which set off a chain of environmental changes.