Making a Comet in the Classroom by Dennis Schatz, Pacific Science Center Copyright 1985 by Dennis Schatz
A dramatic and effective way to begin a unit on comets 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.
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.
To create the nucleus of Comet Halley from the
following recipe, all ingredients must be increased by a factor of 300
trillion, +/- 20%.
The 12 step program
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 barren asteroid. Ready to join CA (Comets Anonymous)? Check out our bibliography! |