Comets

I'm the boy in the white flannel gown
sprawled on this coarse gravel bed
searching the starry sky,
waiting for the world to end.

- - Stanely Kunitz, "Halley’s Comet"

Comets are most easily recognized by their glowing coma and tail. The Greek name for these beatiful objects was aster kometes, or long haired star. Images and drawings of comets going back thousands of years even associate the shape and size of the tail with certain types of disaters, or as omens of success in battle. The Bayeux tapestry depicts William the Conquerer as invading England because of the reappearance of Halley's comet in 1066. However, the brilliant come and tail are actually a very insubstantial part of comets, and a part that disappears when the comet gets far enough from the Sun.

The real heart of the comet is the nucleus. It contain silicates (rock/sand), carbon, and a mix of water, carbon dioxide, methane and ammonia ices. They also have traces of several other ices that contain carbon, nitrogen, oxygen and hydrogen, especially CN (cyanide ion), HCN (hydrogen cyanide), CO, and HCO, all of which are highly toxic organic molecules. The exposed ices sublimate when the comet gets close to the Sun. They escape the nuclues in jets and form a cloud around the nucleus - the coma. The lightweight gasses are swept away from the comet by the solar wind, forming the straight, blue, dim ion tail. The ice acts like a glue, holging the comet together. As the ice sublimates, the silicates, carbon chunks and heavier particles a released and drift off behind the comet, forming the dust tail. These particals eventually spread out along the comet's orbit, and if Earth passes through that stream we get a meteor shower. Sometimes enough ice will sublimate that the comet will actually break apart due to the gravity of the Sun or Jupiter.

There are two populations of comets. Kuiper belt comets generally have shorter periods that only take them out to the area around Neptune and Pluto. They are usually found close to the ecliptic, and their nucleus is typically only a few kilometers in diameter. For example, Halley is a somehat large short period comet, roughly 8 km by 16 km, with a period of about 76 years. Because they have already lost so much of their ice, tend to be dimmer with smaller tails. Oort cloud comets generally have much longer periods, and the typical size of the nucleus is tens of km across. These are generally the spectacular comets like Hale-Bopp, which has a period of around 3000 years and is a whopping 40 km in diameter.

The model comets have silicates (sand), carbon (graphite or charcoal), carbon dioxide, water and ammonia. They also have dark corn syrup which has fructose (C₆H₁₂O₆) in it to represent all the noxious organic molecules we can’t use.

Procedure: (May be done as a demonstration)

Caution: anyone using or in the area of someone using the hammer to break up material should wear safety glasses. Never use the side of the hammer since the hammer may shatter.

If you need to crush charcoal, place the charcoal in a fold of newspaper. Use the hammer to break up the charcoal.

Put on the heavy gloves to handle the dry ice. Wrap the dry ice in several layers of newspaper and use the hammer to break up they dry ice. The finer the ice is crushed, the better the comet will be.

Line the bowl with a garbage bag and mix together:

A large splash of ammonia

A scoop of sand

The charcoal or spoonful of graphite

A dollop of corn syrup

About 1 cup of water

Quickly mix in the dry ice. Carefully lift the bag out of the bowl and shape the comet into a ball. If there is excess water, carefully pour it off into a bucket before taking the comet out of the bowl. If the comet won’t stick together and there is no water in the bowl, add more water, a little at a time until it sticks.

Once you have a comet, the dry ice should be covered with the water, ammonia, sand, etc and be safe to handle. If there are places that appear to have exposed dry ice, pour a little water over it to cover the dry ice.

Observe the nucleus. Pay special attention to things like the color and brightness of the surface, and how the color and brightness change as it melts. Also, look for the jets of escaping gas. Compare your nucleus to the images of comet Wild 2 below. You may need to measure the comet with a small ruler. After you've had a few minutes to look at it, answer the questions on the worksheet.

The Stardust spacecraft took images of comet Wild 2 in 2004. This is a composit of two images, a short exposure that captured the solid ucleus and a long exposure that shows the inner coma and some jets. A larger, color image is also available.

last update: 3/28/08 by SAM