The small bodies of the Solar System

Between and beyond the planets are millions of asteroids and comets. This lesson sets out where they are, how far away they are and how a piece of one becomes a meteorite.

Three words, one rock

A meteoroid is a small solid body travelling through space, from a speck of dust up to about a metre across; anything larger is usually called an asteroid. A meteor is the flash of light made when a meteoroid enters the atmosphere. A meteorite is any piece that survives and lands. The words describe where the object is, not what it is made of.

Where the small bodies are

Most asteroids, rocky and metallic bodies left over from the formation of the planets, orbit the Sun in the asteroid belt between Mars and Jupiter, roughly 2.1 to 3.3 astronomical units from the Sun. One astronomical unit (AU) is the average distance from Earth to the Sun, about 150 million km.

Beyond Neptune lies the Kuiper belt, a ring of icy bodies that includes the dwarf planet Pluto. Much further out, the Sun is thought to be surrounded by the Oort cloud, a vast sphere of icy bodies stretching perhaps a light year or more from the Sun. Comets come from these icy regions.

Nearly all meteorites are pieces of asteroids. A few come from the Moon and Mars. None has yet been shown to come from a comet.

From asteroid to meteorite

Collisions between asteroids break off fragments. Over millions of years, the weak push of sunlight warming one side of a small fragment, together with repeated tugs from Jupiter's gravity, can move it into an orbit that crosses Earth's. Most meteorites spend between a few million and a few tens of millions of years drifting through space before they land.

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