Terminology
Asteroid, meteor,
meteorite - which is which?
The same rock can be an asteroid, a meteor, and a meteorite - depending entirely on where it is and what it is doing at the moment. Here is a clear breakdown of all the terms.
Near-Earth objects explained →Quick reference
A small rocky or metallic body in space
Typical size: Up to ~1 metre
A rocky or metallic body orbiting the Sun
Typical size: > 1 metre (up to ~1,000 km)
An icy body that develops coma and tail near the Sun
Typical size: Typically 1–50 km nucleus
The light phenomenon caused by a body entering the atmosphere
Typical size: Grain of sand to tens of metres
An exceptionally bright meteor
Typical size: Typically centimetres to metres
A space rock that survives to the ground
Typical size: Grams to tonnes
One rock, multiple names
The naming reflects location and state, not composition. A specific object - say, a piece of rock drifting through the inner solar system - is an asteroid when it is orbiting the Sun in space. If it is small enough (under about 1 metre), it is technically a meteoroid.
When that rock enters Earth's atmosphere, friction with air molecules heats its surface to incandescence. The glow and streak of light is a meteor. The rock itself is still the same object - it is just burning. Most objects in the centimetre-to-metre range vaporise completely during descent. Nothing reaches the ground.
Larger objects - those starting at a few kilograms or more - may survive the descent partially or largely intact. What reaches the surface is a meteorite. The original asteroid (or meteoroid) is now on the ground, slightly smaller, and is renamed accordingly.
The Chelyabinsk event illustrates this: the incoming object was a meteoroid (too small to be a properly catalogued asteroid), it produced a superbolide (an exceptionally bright meteor/fireball) as it disintegrated in the atmosphere, and numerous meteorite fragments were subsequently recovered from the surrounding region - including one from the bottom of Lake Chebarkul.
What meteorites are made of
About 85% of meteorites are chondrites - stony meteorites containing small spherical structures called chondrules, which formed during the earliest stages of the solar system. These are among the oldest solid material available for scientific study, preserving conditions from 4.6 billion years ago.
Iron meteorites (about 5% of falls) are fragments of the metallic cores of differentiated bodies - asteroids that were large enough to heat up internally, allowing denser metals to sink to the centre. Stony-iron meteorites (about 2%) represent material from the boundary between a differentiated asteroid's core and mantle.
A small fraction of meteorites originate from the Moon or Mars, ejected by ancient impacts with enough force to escape those bodies' gravity. These are identified by their composition, which matches the geological signature of their source.
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