Educator's Guide to Kitchen Comets
Courtesy of the Jet Propulsion Laboratory
Make a Comet Nucleus
Comets are made up of some of the original material from which
the solar system formed. Orbiting far from the Sun, this
primordial material has survived in an unaltered state for
billions of years.
When a comet nucleus is gravitationally drawn into the inner
solar system it begins to heat up. The volatile materials from
which it is made boil off to form the head and tail(s) that have
amazed, baffled, and frightened people throughout history. This
tremendous light show is produced from just the small solid
nucleus measuring only 15 or 20 kilometers long. Think of it as
a very dirty iceberg! This view was confirmed by the spacecraft
flybys of Halley's Comet by Japanese, Soviet and European
spacecraft in 1986.
You can make an accurate model of a comet nucleus easily and
inexpensively. Unfortunately it is difficult to do it neatly.
Here is what you need:
- Dry Ice (5 lbs) available from ice companies or ice cream
parlors; CAUTION: Dry ice is -79 degrees C (-110 degrees F). Any
more than brief exposure will cause "burns. Be careful when
handling it.
- Water (around half a gallon) in pitcher;
- Ammonia (a few drops or sprays of window cleaner);
- Dirt (fine grained, one handful);
- Corn Starch, or Worcester Sauce (a couple of pinches or
drops);
- Trash Bags (2);
- Large Bowl or Small Pot;
- Water Proof Gloves (the better insulated the warmer your
hands will remain);
- Cloth Towel;
- Paper or Cloth Towels;
- Hammer;
- Mixing Spoon or Stick.
These ingredients are either actual components or handy analogous
ones. The dry ice is frozen carbon dioxide. Water, ammonia,
organic (carbon based) molecules, and silicates are all present
on comet nuclei. They have been identified through spectral
measurements of comet tails and the collection of tiny ice
particles by very high flying research aircraft.
Here is the recipe:
Line the bowl with a trash bag. Place the other trash bag on the
floor. Pour about a pint of water into the bowl. Add the corn
starch or Worcester sauce, ammonia, and some of the dirt; mix a
bit.
Put on the gloves. Wrap the dry ice in a cloth towel;
place it over the trash bag on the floor. Use the hammer to
grind up the dry ice into a powder. Gradually pour the dry ice
powder into the water, mixing as you pour. There will be lots of
vapor formed. The dry ice, water and other ingredients should
form a thickening slush. Keep stirring for a few seconds as it
thickens.
Now, using the trash bag to lift the slush away from the sides of
the bowl, use your gloved hands to pack the slush into a ball.
Keep packing and forming until the ball solidifies as a big lump.
Peel back the trash bag. Scatter some more dirt over the lump.
Pour some of the remaining water over the lump, turning it as you
do so, so that a layer of water ice forms over the entire lump.
Observe the behavior of your miniature comet nucleus. It can be
handled without gloves if the water ice coating is intact. If a
spot feels sticky, pour water on the spot. It hisses and pops as
carbon dioxide sublimes (goes from the solid state directly into
a gas) and forces its way through weak spots in the water ice
crust. On real nuclei this results in slight jetting forces that
can cause the nucleus to spin, slightly alter its orbit, or split
apart (or "calve").
Note: Get three or four pounds of dry ice for each nucleus you
plan to make. You can purchase it the afternoon or evening prior
to the demonstration and store it in a freezer or ice chest.
Place an inch or so of newspaper below the dry ice to prevent
cracking of the surface on which the dry ice rests. Try the
demonstration first to get an idea of the correct amount of water
to use.
It's fun, it's a mess, and it's one of the most memorable and
scientifically accurate demonstrations in astronomy!
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