Asteroids: the parent bodies

Meteorites are samples of asteroids, and asteroids are leftovers from the building of the planets. To understand meteorites, we first need to understand their parents.

Leftovers from planet building

About 4.6 billion years ago, the Sun formed from a collapsing cloud of gas and dust. Around the young Sun was a flattened, spinning disc of gas and dust called the protoplanetary disc. Dust grains stuck together into pebbles, pebbles into larger bodies, and some of these grew into objects kilometres to hundreds of kilometres across, called planetesimals.

Many planetesimals collided and merged to build the planets. But in the region between Mars and Jupiter, the strong gravity of Jupiter disturbed their orbits, so they crashed together too fast to merge easily. The survivors became today's asteroid belt.

The whole asteroid belt now contains surprisingly little material: its total mass is only about 3 per cent of the mass of our Moon.

Parent bodies

The asteroid that a meteorite originally came from is called its parent body. Different meteorite types come from different parent bodies, and some parent bodies were very different from others.

Some asteroids never got hot enough to melt. Their insides still preserve dust and grains from the early disc, and they give us the meteorites called chondrites. Other asteroids formed early, when they contained more of a short-lived radioactive form of aluminium, aluminium-26. Its decay produced enough heat to melt them. In those asteroids, metal sank to form a core and rock floated above it, just as happened on Earth. These melted, or differentiated, asteroids give us iron meteorites from their cores and certain stony meteorites from their crusts and mantles.

Scientists estimate that known meteorites sample more than a hundred different parent bodies.

From the belt to Earth

How does a chip knocked off an asteroid end up crossing Earth's orbit? The answer involves two important effects.

First, Jupiter's gravity creates zones in the asteroid belt called resonances. An asteroid in a resonance gets a regular gravitational tug from Jupiter at the same point in its orbit, like a child on a swing pushed at just the right moment. Over time its orbit becomes stretched until it crosses the paths of the inner planets.

Second, small bodies are slowly pushed by sunlight. As an asteroid absorbs sunlight and re-emits the heat, the escaping heat gives a tiny push called the Yarkovsky effect. Over millions of years, this can drift fragments into a resonance.

Together, these processes deliver a steady trickle of fragments to near-Earth space.

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