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Comets

Comets Comets have been called big, dirty snowballs because they are composed of rock dust wrapped around a big ball of ice. The rock dust and ice may be matter left over from the time the solar system was formed.

Some astronomers think that there may be a huge grouping of about a billion comets on the outskirts of our solar system. They call this the Oort Cloud. Comets in the Oort Cloud are too far away for even astronomers to see. It is only when they travel toward the sun in their orbit that they become visible. As a comet gets closer to the sun, the heat of the sun starts to melt the ice of the comet and turn it to gas. The gas and the dust are released and form a cloud that reflects sunlight (a comet has no light of its own), and the comet becomes visible. This cloud of dust surrounding the nucleus of the comet is called the coma. The nucleus and the coma together are called the head of the comet. The comet grows in size and brightness as more and more gas and dust are released from its coma. The solar winds cause the dust and gas to gather behind the comet, giving it a tail. In its long elliptical orbit, after the comet loops around the sun, it begins its long journey back to the distant part of the solar system. It gradually appears smaller and dimmer and it loses its tail as it cools down and freezes. Eventually, it disappears from view.

Most comets follow huge orbits in the solar system and may not return to view in earth's skies for thousands of years if ever. However, some comets appear at regular and more frequent intervals. They follow orbits that stay within or close to the orbits of the planets. These comets are called periodic or regular comets. The most famous one is Halley's Comet which appears every 75-76 years. It is named after the English astronomer Edmond Halley.

People have been aware of comets since ancient times. Some people believed that comets were made of poisonous gases and that if the earth passed through the tail of a comet, earth's inhabitants would be killed. During Halley's Comet's 1910 visit, merchants even sold comet pills to protect people from the gases.

Sample these internet sites about comets and complete the activities for each one. Then decide if you would have been one of the persons in 1910 who purchased comet pills.

Comets

  1. What are the parts of a comet?
  2. Comets are invisible except when they are near what?
  3. What happens after a comet passes by the sun about 500 times?

 

Comets

  1. According to this site, about how many comets do scientists think orbit the sun?
  2. About how long can the tails of comets get? (The figure is given in kilometers. Convert it to miles. A kilometer is .62 miles.)
  3. As a comet approaches the sun, does its head or tail lead the way? As it moves around from the sun, which parts leads?

 

Halley's Comet

  1. For how long have people on earth been observing Halley's Comet?
  2. On what famous medieval artpiece does Halley's Comet appear?

 

Halley's Comet

  1. When will Halley's comet next be visible from earth?

 

Comets in History

  1. Comets used to inspire fear in earthly observers. What happened in Europe in 1066 with the appearance of Halley's Comet?
  2. What was Halley's Comet blamed for in 1456?
  3. What was Halley's Comet blamed for in 1835-36?

 

Comet Hale-Bopp Home Page

  1. The name of this comet sounds like a song from the 50's. How did it get its name?

 

Ask the Astronomer

  1. Is the earth in the way of any comets?

 

The Official Site of the Houston Comets

  1. What are these comets famous for?

 

Longest Visibility of Ancient Comets

  1. List the dates and names of the brightest comets ever seen from the earth.

 

Stardust

  1. What are the objectives of the Stardust Mission?

 

Asteroid and Comet Impact Hazards

  1. What are NEOs?
  2. Are any NEOs predicted to hit the earth?

 

Bibliography

Kerrod, Robin. The Solar System. Marshall Cavindish : New York, 1994.

Levy, David. Stars & Planets. Time Life Books, 1996.

Marsh, Carole. Asteroids, Comets, and Meteors. Twenty-first Century Books : New York, 1996.

Sipiera, Paul. Comets and Meteor Showers. Children's Press : New York, 1997.

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Author: LINDA MOSBACKER - Email linda.mosbacker@slc.k12.ut.us