Image of Halley's Comet nucleus from the Giotto mission
Image of the nucleus of Comet Halley as viewed by Giotto

Build a Comet

An activity by Padi Boyd

related to the song:
'A Little Bit of Rock'

If you're a comet, it's what you're made of that matters. The chemical composition of comets - the ratio of ices to dust - helps place limits on theories of their origin. To contain so much ice they must have formed in the cooler, outer regions of the early solar system. Their spectacular tails in the night sky are the result of sublimation of the ices, and of outgassing from the irregularly shaped nucleus. Each time a comet passes by the Sun, more gas is stripped from the nucleus than dust. Since ice is very bright and reflective, while carbon dust is dark, older comets have darker nuclei. The nucleus of a comet is not very dense: current estimates are about 1.5 g/cm3 - not much denser than ice!

Since most of the materials in a real comet are easy to obtain, it's straightforward, and fun to build a scale model of a comet in your classroom!

This recipe makes a 15-cm comet with chemical composition very close to that found in actual comets in the solar system. It is a scale model. You would need to scale the diameter by 100,000 to reach the size of the nucleus of Halley's comet.

Materials Needed Before You Start:

  1. 450 grams Dry Ice. Try to get flaked, as opposed to pelleted, dry ice. This consistency works best when forming the comet. Dry ice is carried by most ice companies.
  2. A cooler in which you can store the dry ice
  3. A mallet or hammer to have on hand to break up any large dry ice chunks
  4. A large plastic mixing bowl for each group making a comet
  5. Thick black plastic lawn bags, two for each comet group
  6. One pair of thick work gloves for each comet group
  7. One wooden spoon for each comet group
  8. Lots of clean-up materials (newspaper, paper towels, etc.)
  9. One pedestal per comet group. This can simply be a slab of wood, with a wooden dowel attached
Each group making a comet should collect the following comet components:

Making the comet:

NOTE: Everyone handling the dry ice during mixing should wear gloves!

  1. Line the mixing bowl with one large leaf bag. Cut some of the excess, but leave a large 'lip' of plastic bag hanging over the edge of the bowl.
  2. Mix water and dirt in bowl very well.
  3. Add ammonia and mix well.
  4. If ice is not flaked, place it in a double-bagged leaf bag (or wrap it several times in one bag) and use the mallet or hammer to crush the pieces so there are no clumps. Remember, wear gloves! Dry ice can burn skin.
  5. Now, while stirring vigorously and constantly, add the dry ice to the other ingredients in the mixing bowl.
  6. Keep stirring. Within several minutes the entire mixture will be nearly totally frozen.
  7. You will now be able to lift the bag out of the bowl, with the comet nucleus inside. With gloved hands, compact the nucleus more tightly. Remember, it is an icy, dirty snowball after all. Shape it to the consistency of a snowball.
  8. At this point the comet nucleus should be able to hold its form when it is removed from the plastic bag.
  9. Place the comet on a pedestal. Right away you will see the sublimation as the dry ice escapes from the comet, going directly from solid form to gas form. If you have access to a fan, place one comet in a fan and notice how the trail of gas leaves the nucleus.
  10. As the day progresses, note how the darkness of the comet changes. What's left over at the end of the day? Compare this with actual comets. After a few hours you should see 'cratering' on your comet's surface, where the carbon dioxide in the comet has sublimated before the ice melts. You may even see small jets of gas coming from small holes in the comet. Ice and carbon dioxide will sublimate from a real comet every time its orbit brings it close to the Sun. Eventually comets may reduce down to a small rocky core or break into pieces, and the same things should happen to the classroom comets you or your students have made!


This activity was adapted from a comet recipe developed by NOAO and found at: