The Why Files The Why Files --

Comets: the universal recipe?

Make a Comet in the Classroom
Copyright 1985 Dennis Schatz, Pacific Science Center.

The coma, tail and nucleus of a comet in a diagram.
Comets don't have many moving parts. Courtesy of NASA

A dramatic and effective way to begin a unit on comets is to make a comet in front of the class. The ingredients are easy to find and watching a comet being "constructed" is something the students will remember for a long time. Students make good baker's assistants!


Dry ice is usually available from ice companies -- (look under "ice" in the Yellow Pages). Most ice companies have a minimum sale, maybe 5 pounds, but extra dry ice helps because some will sublimate, and you should practice this activity at least once. Day-old dry ice works best, so try to buy it in advance. Transport the dry ice in an ice chest, and store in the freezer overnight.

(Recipe makes one (1) six-inch comet. To make the nucleus of Comet Halley, increase all ingredients x 300 trillion, +/- 20%.

Other equipment

The recipe

  1. Arrange all ingredients and utensils in front of you.
  2. Cut open one garbage bag and line your mixing bowl.
  3. Place water in mixing bowl.
  4. Add sand or dirt, stirring well.
  5. Add dash of ammonia.
  6. Add dash of organic material, stirring until well mixed.
  7. Nest three garbage bags inside each other, and place dry ice inside. Don't get burned: Wear gloves while handling dry ice!
  8. Crush dry ice by pounding with hammer.
  9. Add dry ice to the mixing bowl, stirring vigorously.
  10. Continue stirring until mixture is almost totally frozen.
  11. Lift the comet from the bowl with the plastic liner, and shape it like a snowball.
  12. Unwrap the comet as soon as it is frozen enough to hold its shape. Display the comet so students can watch during the day, as it begins to melt and the carbon dioxide sublimates (turns directly from solid to a gas. Carbon dioxide sublimates at room temperature; water ice sublimates in the vacuum of space when a comet is heated by the Sun).

The comet is reasonably safe to touch without danger of frostburn, but it is still best to ask students to examine it with a spoon or a stick. As the comet begins to disintegrate, point out small gas jets, where gaseous carbon dioxide escapes through small holes in the still-frozen water. Jets on real comets can be strong enough to adjust the comet's orbit!

After several hours, the comet will become a crater-filled ice ball, because the more volatile carbon dioxide will sublimate before the water ice melts.

Got a comment on our comet bibliography?


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