Explainer: What are asteroids?

Unless they’re ‘trojans,’ most of these space rocks fly in a belt between the orbits of Mars and Jupiter

Hello, Vesta. The NASA spacecraft Dawn spent a year orbiting this asteroid before leaving in July 2012.


The solar system contains millions of asteroids. They may be round or oblong. Some have even stranger shapes, as though molded in play dough and left in space to harden. All are made from the same stuff as the planets. However, unlike rocks on Earth, those that make up asteroids have not been shaped by erosion, heat or intense pressure.

All asteroids are fairly small. Their diameters tend to range from less than a kilometer (a little more than half a mile across) to nearly 1,000 kilometers (621 miles across). Together, all of the asteroids in our solar system have a combined mass that is less than that of Earth’s moon.

Some asteroids resemble small planets. More than 150 of them have their own moon. Some even have two. Still others orbit with a companion asteroid; these pairs race circles around each other as they orbit the sun.

The orbits of most fall in a swath of space between Mars and Jupiter. It’s known, naturally enough, as the asteroid belt. But that’s still a lonely neighborhood: An individual asteroid is usually at least a kilometer (0.6 mile) away from its nearest neighbor.

Asteroids called trojans don’t inhabit the belt. These rocks may follow a larger planet’s orbit around the sun. Scientists have identified nearly 6,000 trojans that follow along in Jupiter’s orbit. Earth has only one known trojan.

When zooming through space, these rocks are called asteroids. When one — or a chunk of one — plummets into Earth’s atmosphere, it becomes a meteor. Most meteors will disintegrate as they burn up from the friction of passing through the atmosphere. But those that survive to reach Earth’s surface are called meteorites. And some have left big pock marks, called craters, across Earth’s surface.

Stephen Ornes lives in Nashville, Tenn., and his family has two rabbits, six chickens and a cat. He has written for Science News Explores since 2008 on topics including lightning, feral pigs, big bubbles and space junk.

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