Leftover bricks
What Are Asteroids?
They are the rubble of a planet that never managed to form.
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Step 1 of 5
The solar system began as a disc of rubble.
Four and a half billion years ago the Sun was ringed by dust and rock chunks. Gravity pulled the chunks together into bigger and bigger lumps.
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Step 2 of 5
Between Mars and Jupiter, the clumping failed.
Jupiter is so massive that its gravity kept stirring that region, shoving rocks into each other too hard to stick. Nothing ever grew into a planet.
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Step 3 of 5
So a belt of rubble was left behind.
Millions of rocks still orbit there, from pebbles to Ceres, which is nearly 1,000 km across. Together they are the asteroid belt.
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Step 4 of 5
The belt is almost entirely empty.
Films show ships dodging boulders. Actually the average gap between asteroids is over a million kilometres — spacecraft cross without seeing one.
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Step 5 of 5
They are frozen samples of the early days.
Asteroids never melted into layers like a planet did, so their insides are 4.5-billion-year-old original material — a time capsule of how everything started.
One roughly 10 km wide struck Earth 66 million years ago and ended the dinosaurs.
The short version
Asteroids are the bricks left over from planet-building, mostly parked between Mars and Jupiter.
Try it yourself
Try rolling loose gravel into a ball. It only sticks if you press gently — press hard and it scatters. That is the asteroid belt's problem.
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