Asteroid Belt: Definition & Facts

Instructor: Elizabeth (Nikki) Wyman

Nikki has a master's degree in teaching chemistry and has taught high school chemistry, biology and astronomy.

Millions of miles away from the Earth, in between the orbits of Mars and Jupiter, hundreds of thousands of asteroids orbit the sun in what is known as the asteroid belt. Learn what defines the asteroid belt while discovering exciting facts about this region of space.


Future generations on their way to the Jovian planets may get to gaze out of the windows of their space craft and marvel at the stars. Some 170 million miles away from the sun they will enter into the asteroid belt, and they will not exit for another 220 million miles!

By definition, the asteroid belt is the area in between the orbits of Mars and Jupiter. It is populated by millions of asteroids, or small dense rocky bodies formed when the solar system was born.

Diagram of inner solar system with asteroid belt
Inner solar system with asteroid belt

In the image above, the asteroid belt is represented by a thick, speckled white band located between the orbitals of Mars and Jupiter. The color gradient is used to illustrate the areas where asteroids are most likely to be, with more intense areas of white representing locations where the asteroid belt is more densely populated.

Areas speckled with green represent asteroid populations located outside of the asteroid belt. These are known as either the 'Trojans' or the 'Greeks'. Orange dots are the 'Hildas', or asteroids that lie in the outermost regions of the asteroid belt.


More than once in science fiction history, a space ship has had to navigate the terrors of a densely packed asteroid field. While other ships crash dramatically into the space rocks, the protagonists always survive. Though it makes for an awesome and adrenaline pumping moment, our heroes wouldn't likely encounter such a challenge in our solar system's asteroid belt. When traveling through the asteroid belt, humans of the future will pass an asteroid and probably not encounter another for millions of miles.


The asteroid belt is home to a wide variety of asteroids, over 600,000 of which have been identified and named. Asteroids may range in size from hundreds of miles across to less than a mile. The largest known asteroid, Ceres, is approximately 620 miles across. Larger asteroids are likely to be more uniformly circular, like a planet, while smaller asteroids may be oblong or highly irregularly shaped. If all of the asteroids in the asteroid belt were compressed into one massive rocky body, its diameter would be less than half that of our moon!

An example of an average asteroid
average asteroid


The average age of the asteroid belt is 4.5 billion years -- the same age as the solar system. Asteroids formed from the same materials as the rest of the objects in the solar system. However, unlike the planets and moons, asteroids' chemical compositions have remained unchanged the past 4.5 billion years. Although there was enough material to form a small planet in this region when the solar system was forming, it didn't happen, most likely because of Jupiter's massive gravitational influence. Jupiter's rotation tugged unevenly on the early asteroid belt, making it difficult for the asteroids to coalesce into anything more massive than Ceres.

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