People were very superstitious in ancient times. They believed that fuzzy, white streaks that sometimes appeared in the night sky meant an important event was about to happen. They believed that these comets were omens (signs) that bad things were about to happen. Astronomers now know that a comet is a ball of ice, rock, and dust. A comet is like a dirty snowball speeding though space.

WHAT ARE COMETS?

Astronomers in the 1500s and 1600s began to study comets. They found that comets are bodies in space, not part of the atmosphere.

A British astronomer named Edmond Halley discovered that comets actually orbit the Sun. They return again and again, but they go so far from the Sun that their returns take a long time. He studied a comet that showed up in 1682. He proposed that it was the same comet that had appeared in 1607 and 1531. Halley predicted that the comet would return again around 1758. The comet returned as he had said it would. The comet came to be known as Halley’s Comet. The orbit of Halley’s Comet takes it close to the Sun about every 76 years. The last time Halley’s Comet came near the Sun was in 1986.

Astronomers now study comets with telescopes and spacecraft. They are learning more about what comets are made of, where they come from, and how they move in space.

WHAT DOES A COMET LOOK LIKE?

A comet looks like a fuzzy, white ball with one or more fat, white tails trailing behind it. A comet has three main parts. It has a center made of ice and rock. A hazy cloud of gas called the coma surrounds the center. A comet can also have one or more tails that trail behind it.

Comets only have tails when they get near the Sun. When a comet comes near the Sun, heat makes some of the ice turn to gas. The comet gets very bright. This gas streams away and carries dust with it, making one or more tails. The tails always point away from the Sun.

HOW DO COMETS TRAVEL IN SPACE?

Comets orbit the Sun just as the planets do. That is why astronomers see the same comet return over and over again. But comets have huge oval-shaped orbits. They swing far out to the edge of the solar system.

It takes Earth 365 days to go around the Sun. The orbits of some comets are so big that it takes them hundreds of years to go around the Sun once.

WHERE DO COMETS COME FROM?

Comets come from places in the outer solar system called the Kuiper Belt and the Oort Cloud. The Kuiper Belt and the Oort Cloud are made up of chunks of ice and rock. Comets that orbit the Sun in less than 200 years come from the Kuiper Belt. The Kuiper Belt is just beyond the planet Neptune. Comets that take longer than 200 years to go around the Sun come from the Oort Cloud. The Oort Cloud is far out at the edge of the solar system, beyond the planet Pluto.

Astronomers think that comets formed soon after the solar system began about 5 billion years ago. The solar system formed from a big cloud of gas and dust. The Sun formed in the center of the cloud. Gas and dust farther out clumped together to make the planets. Comets may have formed from leftover gas and dust at the edge of the solar system where it is very cold.

COULD A COMET HIT EARTH?

We know that comets can hit planets. In 1994, telescopes and spacecraft watched Comet Shoemaker-Levy 9 crash into the planet Jupiter. The crash sent up fireballs that were bigger than Earth!

Some scientists think that comets hit Earth long ago. A comet or asteroid (space rock) crashing into Earth 65 million years ago may have killed off the dinosaurs. The crash would have sent up a huge cloud of dust. The dust could have blocked out the Sun’s light. Earth would have become cold and dark enough to kill off the plants that dinosaurs ate.

On a dark, clear night you can sometimes see streaks of light shooting across the sky. Some of these shooting stars are bits left behind by comets. They give off light as they burn up while falling through the atmosphere. So in a way, comets are constantly falling on Earth!

Source: Microsoft ® Encarta

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