You lie back on the ground and zip up your sleeping bag to stay warm. Out here, under the dark and clear desert skies with very little cloud cover, every bit of space looks like it is studded with stars. A streak of light passes right through the middle of the sky—a falling star. But what you’ve just seen isn’t really a star falling from the sky. It’s a meteor: a rock from space that is traveling through Earth’s atmosphere. When it hits the ground, it is not a meteor anymore. Then it is called a meteorite.

Where do meteorites come from?

Most meteorites come from asteroids in the asteroid belt between Mars and Jupiter. Asteroids are rocky or metal bodies (objects) that revolve around the Sun. Asteroids are mostly leftovers from the processes that happen when planets are formed. They can range in size from that of a boulder to hundreds of miles across.

Objects smaller than asteroids are called meteoroids, and they are usually much smaller – about the size of pebbles. Some meteorites may also come from Mars or the Moon. When the orbit or path of an asteroid or meteoroid crosses Earth, it starts its journey through the Earth’s atmosphere (meteor) until it lands on the surface (meteorite). Then scientists can try to trace back where the meteorite came from.

What are meteorites made of?

Are all meteorites the same? No! They can come from different planets, asteroids, or meteoroids, or even different parts of the same object. Because of this, they can look different and can be made of different materials.

All meteorites can be put into one of three main categories, depending on the materials from which they are made. Iron meteorites are made mostly of iron and nickel. Stony-iron meteorites are made of iron and rock silicates. Stony meteorites are made mostly of rock silicates. The materials from which a meteorite is made greatly affect how heavy a meteorite is.

How heavy or dense a material is can be important for where it ends up. We can see how important this is if we travel back to what happened when our solar system and planets first formed.

Why are meteorites made of different materials?

Our solar system started out as hot gas. When gas clouds condensed, the Sun was formed and solids combined to form planets. The leftover solids that did not fall into the Sun or form planets became asteroids. Some of these asteroids had enough heat due to radioactive decay to melt. The metal (mostly iron and nickel), being heavier, sank to the center of the molten mass and formed the metallic core of the body. The lighter rocky material floated on top and formed the mantle and crust. It is similar to how oil (lighter) floats on water (heavier).

All terrestrial or inner planets (Mercury, Venus, Earth, and Mars) and some asteroids have such a layered, or differentiated structure. You can think of it as the layered structure of an onion. Some of these asteroids with different layers were hit by other asteroids and broken apart. That is how we have meteorites from different layers of a body. Such meteorites are called achondrites. So, an achondrite can be an iron meteorite if it comes from the core of the body or a stony meteorite if it comes from the crust or mantle of the body. Or it could be a stony-iron meteorite if it comes from the boundary between the core and the mantle.

But not all asteroids melt. Some asteroids do not have enough radioactive heat to melt them. In such asteroids, the minerals (metal, silicates, etc.) are all mixed together. Meteorites from such asteroids are called chondrites and they are stony meteorites.

Additional images via Wikimedia Commons. Chebarkul lake meteorite image by Denis Panteleev.