What are comets?
Have you ever looked up and saw a comet zoom in and out of view? Did you wonder what it was made of, or why it glowed?
Most comets are very dim and can only be seen with a telescope because comets donít give off light of their own. The light from the Sun shines on them and makes them look like they are glowing. The closer a comet gets to the Sun, the brighter it glows.
A comet is like a large, icy rock that travels in an orbit (path) around the Sun. A cometís orbit is very long and elliptical (like a circle squished into an oval).
Most comets have a rocky center called a nucleus. Around the nucleus are bits of dust and pebbles held together by ice and frozen gases. When a comet gets closer to the Sun, the ice and frozen gases start to melt. This creates a hazy cloud around the nucleus. The hazy cloud is called a coma. The nucleus and coma make the head of the comet.
As the comet gets closer and closer to the Sun, the coma starts to grow larger and stretch out behind the comet. As the coma stretches, a tail is formed. The closer the comet gets to the Sun, the longer the tail gets.
How is the tail created?
A comet gets a tail as it gets closer to the Sun because atomic particles are streaming out from the Sun. Atomic particles are so small that you canít even see them with a regular microscope! These atomic particles create a solar wind.
When the comet is about 150 million miles from the Sun (a little farther from the Sun than Mars), the solar wind is strong enough to push some of the melted ice and gases away from the comet. This melted ice and gas create the cometís tail.
A comet can have different types of tails that can stretch over millions of miles. The different types are:
The tail is spread so thin that there is barely anything there! If you could fill a bucket with comet tail and compare it to a bucket filled with air, then the bucket filled with air would have billions of more matter than the one filled with comet tail.
As the comet moves away from the Sun, its tail starts to shrink, and its nucleus starts to freeze again.
Every time a comet goes around the Sun it loses a little bit of gas, ice, and dust. These pieces spread over the orbit of the comet until the entire orbit has bits of dust, gas, and ice over it. Soon all that is left of the comet is the rocky center. Comets that do not have a rocky center completely break up over time.
Unless otherwise noted, all images courtesy of NASA. Permission for use at http://www.nasa.gov/gallery/photo/guideline.html.
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