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What Are Comets?
Comets are space rocks in our solar system. People sometimes confuse comets with asteroids and meteoroids. It is important to know the difference between these space rocks.
Comets are small, irregularly shaped space rocks that are leftover from the formation of the solar system. They are made of dust and ice, like a dirty snowball. They are often 1 to 10 kilometres across but can be up to 100 kilometres. Comets orbit the Sun. Many comets’ orbits take them very close to the Sun. When they pass the Sun, they form a bright tail that can be seen from Earth.
Asteroids are space rocks that also orbit the Sun. Like comets, they are leftover from the formation of the solar system. Asteroids are made mostly of rock and metals. Asteroids range in size from less than one kilometre across to hundreds of kilometres across. Unlike comets, asteroids never have a tail.
Meteoroids are small and rocky. They are formed when asteroids break up into smaller pieces or from dust given off by comets. They can be as small as a speck of dust or up to 10 metres across. They also orbit Earth. Sometimes meteoroids will fall into Earth’s atmosphere and burn up. When they do, they can be seen in the sky, and we call them meteors or shooting stars. If a meteor does not burn up completely and hits Earth, we call it a meteorite.
Where Do Comets Come From?
Scientists believe that comets come from two areas in the solar system: the Kuiper Belt and the Oort Cloud. The Kuiper Belt begins just past Neptune’s orbit around the Sun. It is a flat disk in the same position as the planets. The Oort Cloud is much farther away, at the very edge of our solar system. It is called a cloud because it surrounds the solar system completely.
Why Do We See Comets?
Scientists believe that the orbit, or path, of comets from the Kuiper Belt can change when they come close to the outer planets. This pulls them into the solar system. Comets from the Oort Cloud can also be pulled into the solar system but their orbit is changed by passing stars. The new orbits of these comets are elliptical, or oval-shaped. The Sun is at one end of the oval.
We can see these comets when they are close to the Sun. This is because comets reflect the light from the Sun, just as our moon does. Also, the Sun melts the ice of a comet. This causes gases and dust to flow away, forming the comet’s tail. A comet’s tail can be thousands of kilometres long. This is when we can see a comet—when it is moving toward the Sun and when it is moving away from the Sun. Most of these comets cannot be seen with a telescope, but there are some spectacular comets that people can see just by looking at the sky. Unfortunately, these do not pass by very often.
What is wonderful about comets is that when you can see one, you can see it for a long time. It does not just whiz through the sky like a meteor.
Meteors appear as fast-moving streaks of light. During a meteor shower, you might see 40 or more streaks of light every hour in the same part of the night sky. Meteor showers can happen when Earth passes through the orbit of a comet. The comet is not there, but it has left behind dust or particles (meteoroids). As Earth passes through that cloud of meteoroids, many of the meteoroids fall into the atmosphere and light up the sky.
What Are Some Famous Comets?
Comets that come from the Kuiper Belt are short-period comets. This means that they complete one orbit in less than 200 years. Long-period comets come from the Oort Cloud. Their orbits are longer than 200 years.
Halley’s comet is a short-period comet. It appears about every 75 years. The last time people could see it was in 1986. Spacecraft were sent up to take a closer look at the comet.
Hale-Bopp is a long-period comet. It is the most famous comet of recent times. Hale-Bopp was discovered in 1995. The comet’s orbit brought it closest to Earth in 1997. Hale-Bopp was unusually bright, and could be seen with the naked eye in some parts of the world for about 18 months. Scientists predict Hale-Bopp will not return for thousands of years.
You can check the Internet for some great photographs of both of these famous comets.
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1. How are comets, asteroids, and meteoroids the same?CorrectIncorrect
2. Match the description to the space rock.
Are made of dust and ice, and are from 1 to 10 kilometres
Are made from rocks and metals, and range in size from less than one kilometre across to hundreds of kilometres across
Are small and rocky, and can be tiny like a piece of dust or up to 10 metres across
3. How are the Kuiper Belt and the Oort Cloud the same? How are they different?
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4. Where did Halley’s comet come from?CorrectIncorrect
5. What does this text make you wonder about?
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