November 23, 2012 11:52 am

Asteroids are rocks in space that never quite made it as planets. Astronomers think that our solar system began as a cloud of gas and dust. Gravity pulled parts of the cloud together to make the Sun and the nine planets. Astronomers think that the asteroids formed in that cloud but never grew large enough to be planets.
There are thousands of asteroids, and they come in all sizes. The biggest asteroid ever found is called Ceres. Ceres is more than 600 miles (1,000 kilometers) wide. Astronomers have found about 200 asteroids that are more than 60 miles (100 kilometers) across. All the other asteroids are much smaller. Some are only a few feet wide.
Astronomers wonder if once there were just a few big asteroids. The big asteroids may have crashed into each other. The crashes would have broken them into smaller pieces, making all the asteroids we see today.
Some asteroids are round. Some asteroids are long and bumpy. Some asteroids even have tiny moons going around them.
Asteroids go around, or orbit, the Sun just like planets. Most asteroids orbit in the asteroid belt. The asteroid belt is farther out from the Sun than Earth’s orbit. It lies between the orbits of the planets Mars and Jupiter.
Sometimes asteroids change orbits and move out of the asteroid belt. These asteroids cross the orbits of planets as the planets go around the Sun. A few cross Earth’s orbit.
Asteroids are normally too small and far away to see with your eyes. Astronomers study asteroids with telescopes. They have also sent spacecraft for close-up looks at several asteroids. A spacecraft named Near-Earth Asteroid Rendezvous (NEAR) Shoemaker actually landed on an asteroid in 2001.
Astronomers have found that most asteroids are made mainly of stone. Some asteroids are made mostly of metals. Astronomers think that asteroids made of metal must have melted at some time in the past. The liquid metal clumped together at the center of the asteroid. Most of the rocky part later broke off from the asteroid, leaving the metal behind. What melted these metal asteroids is still a mystery.
Astronomers think that several thousand asteroids have orbits that might one day make them strike Earth. Asteroids have certainly hit Earth in the past. People have found thousands of meteorites (stones from space) that have crashed into Earth. Most meteorites are pieces of asteroids. There is a giant crater (hole in the ground) in Arizona that is more than half a mile (about a kilometer) wide! The crater was formed when a meteor crashed into Earth.
An asteroid crashing into Earth may have killed off the dinosaurs 65 million years ago. Scientists have found a big meteorite crater around Mexico’s Yucatán Peninsula. They think the asteroid that created this crater may have killed the dinosaurs. After the asteroid crashed, a huge cloud of dust would have darkened Earth. It could have become very cold on Earth, and plants that dinosaurs ate might have died. As the plant-eating dinosaurs died from lack of food, meat-eating dinosaurs would have run out of food and died as well.
Scientists are setting up a system to warn us of asteroids coming toward Earth. If they find one they might be able to blow up the asteroid. They might be able to attach a rocket to the asteroid and push it just enough to miss Earth.
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