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.
HOW BIG ARE ASTEROIDS?
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.
WHERE ARE ASTEROIDS IN SPACE?
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.
HOW DO ASTRONOMERS STUDY ASTEROIDS?
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.
COULD AN ASTEROID HIT EARTH?
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.
