Asteroid Belt

Artist's depiction of the asteroid belt between Mars and Jupiter. Credit: David Minton and Renu Malhotra
The asteroid belt is a region of space between Mars and Jupiter. It is generally called the main belt to distinguish it from the Kuiper belt and the scattered disc. More than half the mass of the belt is is composed by Ceres, Vesta, Pallas, and Hygiea. Ceres makes up nearly 25% of the asteroid belt's mass by itself. The asteroids range from the size of Ceres down to many that are as small as a dust particle.

The asteroid belt is so thinly populated that several unmanned spacecraft have been able to move through it – this isn't the dense field of asteroids you see in movies and television. The next mission to pass through the asteroid belt will be the Dawn space mission. Despite these space craft moving safely through the asteroid belt, several of the larger asteroids have collided and formed “families” with the same orbital characteristics and make up.

In 1802 Heinrich Olbers theorized that the asteroid belt was formed by an ancient planet exploding. This planet having either suffered an internal explosion or a cometary impact many million years ago. The large amount of energy that would have been required to destroy a planet, combined with the belt's low combined mass tends to destroy that theory. Today it is believed that the planets were formed by a process of accretion and the asteroid belt are just pieces that have never joined a planet. The larger asteroids are on the verge of being classified as dwarf planets. It is believed that they were formed by the same process of accretion.

The first spacecraft to make a journey through the asteroid belt was Pioneer 10 in 1972. There have been a total of 12 missions that have made it through the belt. The Dawn space mission will be the first to be launched to specifically explore two asteroids from orbit. If the craft is still usable, it may be sent on to other destinations. The Dawn will make history whatever happens on the mission.

The asteroid belt has many mysteries. Although there are many things that are known about the belt, there hasn't been much exploration. The Dawn space mission will change all of that in the next few years. Many of the things that are currently known come from meteorites that have fallen to Earth. Perhaps we will learn more if we ever decide to try to mine asteroids.

We have written many articles about the asteroid belt for Universe Today. Here's an article about an alien asteroid belt, and another about how Jupiter and Saturn plowed through the asteroid belt.

Want more resources on asteroids? Here's NASA's Lunar and Planetary Science Page on asteroids. And here's NASA's World Book on Asteroids.

We have recorded two episodes of Astronomy Cast about asteroids. There's Episode 55: The Asteroid Belt, and here's Episode 29: Asteroids Make Bad Neighbors.

Filed under: Astronomy