As we pass by Mars, we notice one of the most important characteristics of our solar system. In contrast to the few steps separating the four inner planets in the model solar system, the walk from Mars to Jupiter covers more than 50 meters–over half the length of a football field. As we look ahead, we see that increasingly large distances separate the remaining planets. Thus, the solar system has two major regions: the inner solar system, where Mercury, Venus, Earth, and Mars orbit the Sun relatively closely, and the outer solar system, where the remaining planets are widely separated. In between we find the asteroid belt, where thousands of asteroids orbit the Sun.

Asteroids are chunks of rock and metal left over from the formation of our solar system–bits and pieces that never became part of a moon or a planet. The largest asteroid, Ceres, is about 1,000 kilometers (600 miles) across. But most asteroids are far smaller and would be microscopic on our 1-to-10-billion scale. Despite their large number, the total mass of the asteroids is less than that of any of the inner planets. The average distance between asteroids is millions of kilometers–so the real asteroid belt does not at all resemble the crowded rock fields depicted in many science fiction films.

Not all asteroids are confined to the asteroid belt, and those that orbit elsewhere in the solar system may collide with moons or planets; impact craters are the scars of such collisions. (Impact craters may also result from comet collisions.) Asteroid impacts may have played an important role in the development of life on Earth. For example, an asteroid impact may have been responsible for the extinction of the dinosaurs some 65 million years ago. Smaller pieces of asteroids fall to the Earth quite often; we call these fragments meteorites.