Making a Comet in the Classroom

by Dennis Schatz

Pacific Science Center

A good way to begin a unit on comets or astronomy is to make a comet in the class. Older students can get into groups and make comets themselves.

Ingredients for a six-inch comet:

2 cups water

2 cups dry ice

2 spoonfuls of sand or dirt

a dash of ammonia

a dash of organic material - dark corn syrup or Coke

Other materials you should have on hand:

an ice chest (keep the dry ice in)

a large mixing bowl

work gloves

goggles

4 medium - sized plastic garbage bags

a hammer (break up the dry ice)

a wooden spoon

paper towels

paper plate (place the finished comet on)

Steps for making a comet

  1. 1. Cut open one garbage bag and use it to line your mixing bowl.
  2. 2. Place water in mixing bowl.
  3. 3. Add sand or dirt, stirring well.
  4. 4. Add dash of ammonia.
  5. 5. Add dash of organic material stirring until well mixed.
  6. 6. Place dry ice in 3 garbage bags that have been place inside each other.

Be sure to wear gloves while handling dry ice to keep from being burned.

  1. 7. Crush dry ice by pounding it with a hammer.
  2. 8. Add the dry ice to the rest of the ingredients in the mixing bowl while stirring vigorously.
  3. 9. Continue stirring until mixture is almost totally frozen.
  4. 10. Use gloves - Lift the comet out of the bowl using the plastic bag and shape it as you would a snowball.
  5. 11. Unwrap the comet as soon as it is frozen sufficiently to hold its shape.
  6. 12. Place the comet on a paper plate.

Now you can place the comet on display for the students to watch during the day as it begins to melt and sublimate(turn directly from a solid to a gas,which is what carbon dioxide does at room temperature and comets do under the conditions of interplanetary space when they are heated by the Sun).

 

The comet is reasonable safe to touch without getting burned by the dry ice, but it is still best to have a spoon or a stick for students to use while examining it. As the comet begins to melt, the class may notice small jets of gas coming from it. These are locations where the gaseous carbon dioxide is escaping through small holes in the still-frozen water. This type of activity is also detected on real comets, where the jets can sometimes expel sufficient quantities of gas to make small changes in the orbit of the comet.

 

After several hours, the comet will become a crater-filled ice ball as the more volatile carbon dioxide sublimates before the ice melts. Real comets are also depleted by sublimation each time they come near the Sun. Ultimately, old comets may break into several pieces or even completely disintegrate. In some cases, the comet may have a solid, rocky core that is then left to travel around the comet's orbit as a dark barren asteroid.

   

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