Asteroids

An asteroid is a minor planet of the inner Solar System. Historically, these terms have been applied to any astronomical object orbiting the Sun that did not resolve into a disc in a telescope and was not observed to have characteristics of an active comet such as a tail. As minor planets in the outer Solar System were discovered that were found to have volatile-rich surfaces similar to comets, these came to be distinguished from the objects found in the main asteroid belt. Thus the term "asteroid" now generally refers to the minor planets of the inner Solar System, including those co-orbital with Jupiter. Larger asteroids are often called planetoids.

There are lots of asteroids in our solar system. Most of them live in the main asteroid belt - a region between the orbits of Mars and Jupiter.

Asteroids hang out in other places, too. For example, some asteroids are found in the orbital path of planets. This means that the asteroid and the planet follow the same path around the sun. Earth and a few other planets have asteroids like this.

Where did asteroids come from?

Asteroids are left over from the formation of our solar system. Our solar system began about 4.6 billion years ago when a big cloud of gas and dust collapsed. When this happened, most of the material fell to the center of the cloud and formed the sun.

Some of the condensing dust in the cloud became planets. The objects in the asteroid belt never had the chance to be incorporated into planets. They are leftovers from that time long ago when planets formed.

Are all asteroids the same?

No way! Because asteroids formed in different locations at different distances from the sun, no two asteroids are alike. Here are a few ways that they differ:

• Asteroids aren’t all round like planets. They have jagged and irregular shapes.
• Some asteroids are hundreds of miles in diameter, but many more are as small as pebbles.
• Most asteroids are made of different kinds of rocks, but some have clays or metals, such as nickel and iron.

There are no known impact threats, but tiny meteors burn up in Earth’s atmosphere all the time! Watch this video to learn more from NASA asteroid expert Dr. Kelly Fast.