Special Issue on Halley's Comet
Making A Comet
in the Classroom
by Dennis Schatz,
Pacific Science Center, Seattle
A dramatic and
effective way to begin a unit on Halley's Comet is to make your own comet right
in front of the class. The ingredients for a comet are not difficult to find
and watching a comet being "constructed'' is something the students will
remember for a long time.
for a six-inch comet are:
Other materials you
should have on hand include:
- 2 cups water
- 2 cups dry ice
(frozen carbon dioxide)
- 2 spoonfuls
of sand or dirt
- a dash of ammonia
- a dash of organic
material (dark corn syrup works well)
Dry ice is available
from ice companies in most cities (look under "ice'' in the Yellow Pages
for a local source.) Day-old dry ice works best, so you might want to buy it the
afternoon before the day you do the activity. Keep the dry ice in an ice chest
when transporting it and in your refrigerator's freezer compartment overnight.
Most ice companies have a minimum on the amount of ice they will sell (usually
5 pounds). But having extra dry ice on hand will be useful because some will evaporate
and also because it is advisable to practice this activity at least once before
doing it with the class.
- an ice chest
- a large mixing
bowl (plastic if possible)
- 4 medium-sized
plastic garbage bags
- work gloves
- a hammer, meat
pounder, or rubber mallet
- a large mixing
- paper towels
Here are the steps
for making a 6-inch comet (students make good baker's assistants for this exercise):
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 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.)
- Cut open one
garbage bag and use it to line your mixing bowl.
- Have all ingredients
and utensils arranged in front of you.
- Place water
in mixing bowl.
- Add sand or
dirt, stirring well.
- Add dash of
- Add dash of
organic material (e.g. corn syrup), stirring until well mixed.
- Place dry ice
in 3 garbage bags that have been placed inside each other. (Be sure to wear
gloves while handling dry ice to keep from being burned.)
- Crush dry ice
by pounding it with hammer.
- Add the dry
ice to the rest of the ingredients in the mixing bowl while stirring vigorously.
- Continue stirring
until mixture is almost totally frozen.
- Lift the comet
out of the bowl using the plastic liner and shape it as you would a snowball.
- Unwrap the comet
as soon as it is frozen sufficiently to hold its shape.
The comet is reasonably
safe to touch without getting burned by the dry ice, but it is still best to
have a spoon or a stick for the 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 water 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
note: Dennis Schatz is the author of a marvelous new student (and teacher)
activity book about Halley's Comet discussed elsewhere in this issue.
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