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About 2-4 au (186-370 million miles) away from the Sun, between the orbits of Mars and
Jupiter, is a region called the Asteroid Belt. This region is a ring of tens of thousands of
relatively small rocky objects called Asteroids. Asteroids can vary greatly in size. The
smallest are the size of small pebbles, while the largest known asteroid, Ceres, is about
600 miles in diameter. Even Ceres is small compared to the planets of our solar system:
about 500 Ceres-sized objects could fit inside the earth!
Why does our solar system have an Asteroid Belt? One theory that astronomers have is
that 4.6 billion years ago, when our solar system was being formed, a tenth planet tried to
form between Mars and Jupiter. However, Jupiter’s gravitational forces were too strong,
so the material was unable to form a planet. Even if a planet had formed, it wouldn’t
have been anything to write home about. It is estimated that if you put all the asteroids in
the solar system together into one body, they would form an object less than half the size
of our moon!
Most asteroids in the Asteroid Belt have an orbital period of about 3-6 Earth years. This
means that it takes these asteroids 3-6 times longer than Earth to make a trip around the
Not all asteroids are in the Asteroid Belt. Sometimes collisions between asteroids,
combined with the gravitational effects of Jupiter, can cause asteroids to leave the belt. If
an asteroid comes within 1.3 au (121 million miles) of the Sun, it is called a Near-Earth
Asteroid (NEA). With all these asteroids orbiting around the sun near Earth, it is
reasonable to expect that, from time to time, the path of our planet and that of an asteroid
will cross. In fact, scientists believe that about 65 million years ago, an asteroid about 6
miles across collided with Earth and caused the extinction of the dinosaurs. More
recently, in 1989 an asteroid that was a quarter-mile in diameter came within 400,000
miles of Earth. The asteroid weighed 50 million tons and was traveling at 46,000 miles
per hour. Astronomers estimate that the asteroid and the earth passed through the same
point in space only six hours apart!