About 2-4 AU (186-370 million miles) away from the Sun, between the orbits of Mars and Jupiter, is a region called the Asteroid Belt. This region is a ring of tens of thousands of relatively small rocky objects called Asteroids. Asteroids can vary greatly in size. The smallest are the size of small pebbles, while the largest known asteroid, Ceres, is about 600 miles in diameter. Even Ceres is small compared to the planets of our solar system: about 500 Ceres-sized objects could fit inside the earth! It's important to understand that even though there are millions of asteroids, they are spread out over a large area. It's not like the asteroid field in Star Wars! If you were on an asteroid in our solar system, it would be very difficult to find another one without a telescope.
Why does our solar system have an Asteroid Belt? One theory that astronomers have is that 4.6 billion years ago, when our solar system was being formed, a tenth planet tried to form between Mars and Jupiter. However, Jupiter’s gravitational forces were too strong, so the material was unable to form a planet. Even if a planet had formed, it wouldn’t have been anything to write home about. It is estimated that if you put all the asteroids in the solar system together into one body, they would form an object less than half the size of our moon!
Most asteroids in the Asteroid Belt have an orbital period of about 3-6 Earth years. This means that it takes these asteroids 3-6 times longer than Earth to make a trip around the sun.
Not all asteroids are in the Asteroid Belt. Sometimes collisions between asteroids, combined with the gravitational effects of Jupiter, can cause asteroids to leave the belt. If an asteroid comes within 1.3 AU (121 million miles) of the Sun, it is called a Near-Earth Asteroid (NEA). With all these asteroids orbiting around the sun near Earth, it is reasonable to expect that, from time to time, the path of our planet and that of an asteroid will cross. In fact, scientists believe that about 65 million years ago, an asteroid about 6 miles across collided with Earth and caused the extinction of the dinosaurs. More recently, in 1989 an asteroid that was a quarter-mile in diameter came within 400,000 miles of Earth. The asteroid weighed 50 million tons and was traveling at 46,000 miles per hour. Astronomers estimate that the asteroid and the earth passed through the same point in space only six hours apart!
Here is a picture of the Near Earth Asteroid Eros, which was the destination of the Near Earth Asteroid Rendezvous (NEAR) spacecraft in 2001. This was the first in-depth study of an asteroid. It collected data on the mass, structure, geology, and compositon of the asteroid. In addition, the spacecraft captured photographs of the surface of Eros during its descent. Astronomers hope that by learning about the origins of asteroids, they may be able to find some clues about the beginnings of our own planet. The arrow points to the final landing site.