Dirty Snowballs: What is a Comet Made Of?

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Author: Janice VanCleave


What is a comet made of?


  • large white cotton ball
  • school glue
  • One-quarter teaspoon (1.25 ml) dirt (garden soil)


  1. Unroll the cotton ball and spread it into a square measuring about 3 by 3 inches (7.5 by 7.5 cm).
  2. Pull a dime-size piece off one corner of the cotton square. Roll this piece into a ball.
  3. Place 8 to 10 dots of glue on the rolled cotton, then sprinkle it with dirt.
  4. Place the dirty cotton in the center of the cotton square. Wrap the cotton square around the dirty cotton to make a large ball about 1 inch (1.25 cm) in diameter.
  5. Place 20 to 25 dots of glue on the cotton, then sprinkle it with dirt.


A model of a comet's head is made.


A comet is a celestial body made of dust, gases, and ices (mainly water and carbon dioxide) that moves in an extremely elongated orbit about the Sun. The head of a comet is made of a nucleus (the central part of a material) surrounded by a fuzzy starlike principal part called the coma. The nucleus may be only a few miles (km) in diameter and is made of ices, which are mainly water and carbon dioxide mixed with dust. The nucleus is described as a dirty snowball. In the model of a comet's head, the nucleus is the small, compact, dirty co1ton ball surrounded by a coma, the large, fluffier, dirty cotton ball.

Dirty Snowballs

Far out in space, away from the Sun, the comet exists as only the nucleus, a dirty snowball. As the comet approaches the Sun, the ices sublimate (change from a solid to a gas without becoming a liquid), creating a cloud of dust and gas around the nucleus—the coma. A comet's head can have a diameter of 30,000 miles (48,000 km).

Let's Explore

When a comet is 2 AU from the Sun, solar wind blows some of the gas and dust away from the comet's coma. Solar wind is an energetic stream of particles constantly moving away from the Sun. The stream of gas and dust blown away from the coma generally divides, forming two comet tails. One of the tails, called an ion tail, is composed of gas that is ionized (electrically charged) by the solar wind. This ionized gas is blue in color and is lightweight enough to be swept in a line away from the Sun. The other tail, called a dust tail, is made of heavier solid dust particles separated from the coma as the comet orbits the Sun. The larger the dust particles, the less they are swept away from the curved orbital path. The shape of the dust tail is an indication of the size of the dust particles. The more curved it is, the larger the particles.

Complete the comet model by adding the two tails to the comet's head made in the previous experiment. Use blue and white paper, such as 2-inch (5-m)-wide crepe paper. Cut two 6-foot (1.8-m) strips of each color. Fold each strip of paper in half and cut along the fold to form four 1-inch (10-cm)-wide strips of each color. Glue the strips together at one end, first the blue strips, then the white strips on top. Glue the comet head to the glued end of the strips. Allow the glue to dry. This should take 15 to 30 minutes.

Ask an adult to tape the comet's head to the wall, at least 6 feet (1.8 m) from the floor. Allow the blue paper strips to hang straight down, but tape the white strips to the wall so that they curve to one side. The blue strips represent the ion tail, the white strips the dust tail.

Show Time!

The scale of the comet model is 1 inch (2.5 cm) : 30,000 miles (48,000 km). Thus, the tails are 2.16 million miles (3.46 million km) long. Use a yardstick (meterstick) to mark off inches (cm) on a 2-inch-by-6 foot (5-cm-by-l.8-m) piece of adding machine tape. Secure this tape next to the comet model. Take a photograph of the model and display an enlarged copy of the photograph.

Dirty Snowballs

Check It Out!

Some comets are bright because of how close they are to Earth, such as Comet IRAS-Araki-Alcock, which passed 0.031 AU from Earth in May 1983. Other comets are bright because of their size, such as the Great Comet of 181l. Comet Halley was a spectacular sight in the sky in 1910. The apparent length of its tail on that occasion was the longest recorded for any comet. Why was Halley's comet less spectacular in 1986? What things other than size and distance from Earth affect the brightness of comets? Find out more about what makes a comet "great." For information, see pages 39–47 in Alan Hale's Everybody's Comet (Silver City, NM: High-Lonesome Books, 1996).

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