Asteroids, Comets, and Meteors: Three Kinds of Sky Object

Look up at a clear night sky and you might see a streak of light flash across it in less than a second. People often call this a "shooting star," but it is not a star at all. It is a pebble — sometimes no bigger than a grain of rice — burning up as it slams into the top of Earth's atmosphere at tens of kilometers per second. That streak has a name: a meteor.

The pebble itself, while it was still floating through space, had a different name: a meteoroid. And if any chunk of it survives the fiery plunge and lands on the ground, that surviving rock gets a third name: a meteorite. Same object, three names, depending on where it is in its journey. This naming trick is the key to the whole topic, so hold onto it.

Now, where do these space pebbles come from? Mostly from two larger families of objects: asteroids and comets.

An asteroid is a rocky, sometimes metallic body left over from the early solar system — leftover building material that never managed to clump together into a planet. Most asteroids live in the asteroid belt, a wide ring between Mars and Jupiter. Jupiter's enormous gravity kept stirring the rocks there, so they kept smashing each other apart instead of merging. Asteroids range from boulder-sized to nearly a thousand kilometers across. They look like dark, cratered potatoes — dry, dusty, and lifeless.

A comet is different. Comets formed farther from the Sun, where it was cold enough for water, carbon dioxide, and other gases to freeze solid. So a comet is mostly ice mixed with dust — picture a dirty snowball a few kilometers wide. Out in the cold, far reaches of the solar system, that snowball just drifts. But when a comet's orbit swings it close to the Sun, the ice on its surface warms up and turns directly into gas. That gas, along with dust shaken loose by the escaping vapor, streams away from the comet and forms two glowing tails that can stretch for millions of kilometers. The tails always point roughly away from the Sun, because the solar wind pushes the gas and dust outward.

Here is where the three categories connect. As a comet loops past the Sun, it leaves a trail of dust behind it along its orbit. When Earth's own orbit crosses that dust trail — which happens at the same time each year — we plow through the debris and see dozens of meteors an hour. That is a meteor shower. So the bright streak you see overhead may have started as a grain of ice-cemented dust shed by a comet decades ago.

Asteroids feed the supply too. When two asteroids collide in the belt, fragments scatter, and some eventually wander into Earth's path. Most meteorites that scientists pick up and study turn out to be chips of asteroids — which is why asteroids are sometimes called time capsules of the early solar system.

So the three words sort themselves out like this. Asteroids and comets are objects: rocky ones close in, icy ones from farther out. Meteoroids are the small pieces that break off either of them. And meteor is not really a thing at all — it is an event, the brief, brilliant moment when one of those pieces hits our air and burns.