Asteroid Belt

In the great stretch of space between Mars - the fourth planet from the Sun - and the next planet Jupiter, is a region filled with several million rocky objects, known as the Asteroid Belt. These objects, the asteroids, are mostly small and just a few hundred yards across. Some are much bigger, a few hundred are more than fifty miles across, and the very largest is Ceres, which is some 600 miles across and is really a dwarf planet rather than an asteroid. All of the asteroids orbit around the Sun, just like the planets, but most of their orbits are not perfect circles and instead are ellipses, and many orbits are tilted so that the asteroids are constantly moving above and below most of the planets.

A map of known asteroid positions looking ‘sideways’ at the solar system – yellow and white dots are asteroids, the orange-red line is where the planets orbit.

Think About It
Even though there are millions of asteroids in the Asteroid Belt it is easy to fly a spaceship through because there is so much space between Mars and Jupiter that all these rocky objects hardly take up any room at all ! Although the asteroids do sometimes collide with each other it’s a very rare event, a million years can go by without any of the larger ones bumping into each other.

Where did the Asteroid belt come from ?
The asteroids are the type of objects that gang up to build planets, but in the space between Mars and Jupiter they were never able to collide enough to stick together and make bigger and bigger pieces. At least part of the reason is that Jupiter is so massive that its gravity is constantly stirring the asteroids, like a huge spoon, and that prevents them from making anything very big.

What are the asteroids made of ?
About three-quarters of the asteroids are made of light rock, composed of silicon, carbon, oxygen and sulfur. They also contain a lot of more complex ‘organic’ chemicals, these are compounds that we might see on Earth, and involve carbon. They also contain some water – spread thinly inside the rock. Other asteroids are made of different rock types, including a lot of silicon, and some contain a lot of metal elements like iron and nickel. These metal rich asteroids are thought to have come from the inside of small planets that formed billions of years ago and were then destroyed in huge collisions with other small planets!

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