Asteroids

     Some 200 years ago, astronomers began searchind for a "missing planet" between Mars and Jupiter. They found not one cut countless rocks- some large, some small - orbiting the Sun. They called them asteroids. Asteroids means starlike in Greek.

    Ceres was the first and biggest asteroid to be discovered. It was discovered in 1801 by Giuseppi Piazzi. This is how it was discovered: on January 1, 1801, the Italian astronomer Giuseppi Piazzi turned his telescope to the constellation Taurus. Suddenly, he noticed a light that wasn't listed on his star chart. By the next evening, the light had moved. First, he thought it was a comet, but after watching it for three weeks, he thought he had found the lost planet. There isn't a planet there. Instead, he had discovered the first asteroid. He called it Ceres, after the Roman goddess of agriculture.

    Scientists believe that there are more than one million asteroids, but most of them are too small to be seen from Earth. In fact, if you put them all together into one big asteroid, its total mass would be less than that of the Moon.

    Asteroids frequently collide with one another as they speed around the Sun. They break into smaller pieces or join together into strange shapes. Sometimes, chips from these collisions hit the Earth as meteorites.

    Some of the major groups of asteroids are the Trojans, Amos, Apollos, and Atens. The Trojans are really two clusters of asteroids that share the same orbit of Jupiter. The Amors cross the orcit of Mars. The last two are the ones to worry about. They are the Apollos and Atens. They have elliptical orbits that cross Earth's orbit, bringing them very close to Earth. There are only a few of these asteroids, though, and one crashes into Earth only about once every 500,000 years.

    One of the most amazing discoveries ever made about an asteroid happened in 1993 when the space probe Galileo flew by the asteroid Ida. One of the pictures sent from Galileo showed that Ida has a moon. Scientists named the moon Dactyl. It is believed that other asteroids may have moons as well.