Where do meteorites come from?

Meteorites are visitors from far away. Most come from a ring of rocks called the asteroid belt, and a few come from the Moon and Mars.

The asteroid belt

Between the orbits of Mars and Jupiter there is a huge, wide region full of asteroids. It is called the asteroid belt. Asteroids are rocky and metal leftovers from the time when the planets were forming. They never joined together to make a planet, partly because the strong pull of the giant planet Jupiter kept stirring them up.

The asteroid belt is not crowded like it looks in films. The asteroids are usually very far apart, often more than a million kilometres. Spacecraft have flown straight through it without bumping into anything.

Even so, over millions of years asteroids sometimes crash into each other. The crashes break off chips and chunks. Some of those pieces are nudged into new paths that cross Earth's orbit. Those are the pieces that can become meteorites.

Pieces of the Moon and Mars

Most meteorites come from asteroids, but not all. When a big asteroid hits the Moon or Mars, the crash can be so powerful that it throws rocks off into space. A few of those rocks drift through space for millions of years and eventually land on Earth.

Scientists can tell where these rocks came from by comparing them with samples. Astronauts on the Apollo missions brought rocks back from the Moon, so we know exactly what Moon rocks look like inside. For Mars, scientists compared tiny bubbles of gas trapped inside some meteorites with measurements of the Martian air made by spacecraft that landed there. They matched!

Moon and Mars meteorites are very rare. Only a few hundred of each are known, out of many thousands of meteorites.

A very long journey

A meteorite's trip to Earth is usually long and slow. After a piece is knocked off an asteroid, it can spend millions of years travelling around the Sun before it finally meets our planet.

While it is in space, the rock is hit by fast-moving particles called cosmic rays. These change a few of the atoms inside the rock. By measuring those changed atoms, scientists can work out how long a meteorite has been flying through space on its own, rather than hidden deep inside its asteroid.

So each meteorite has three stories to tell: how it formed billions of years ago, how it was broken off its parent asteroid, and how long its journey through space took. Scientists call this time in space the cosmic-ray exposure age.

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