University of Michigan - Department of Astronomy




Version: intro


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.

- - Stanley Kunitz, "Halley’s Comet"

Real Comets

Comets are most easily recognized by their glowing coma and tail. The Greek name for these beautiful 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 disasters, or as omens of success in battle. The Bayeux tapestry depicts William the Conqueror as invading England because of the reappearance of Halley's comet in 1066. However, the brilliant coma and tail that are so easily visible are actually very insubstantial. In fact, they only appear when the comet is close to the Sun. The real heart of the comet is the nucleus.

The size and shape of the nucleus varies widely. For example, Halley is 16 x 8 x 7 km - similar to a slightly squashed football. Wild 2 on the other hand (shown below) is roughly spherical, and about 2 km across. After its discovery in 1997, astronomers used the Hubble Space Telescope to image the nucleus of comet Hale-Bopp. They estimated it was 30 -40 km across, making it one of the largest known comets.

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

The nucleus primarily contains silicates (rock/sand), carbon, and a mix of water, carbon dioxide, methane and ammonia ices. The nucleus of large comets may have a rocky core. Comets also have traces of several other ices that contain carbon, nitrogen, oxygen and hydrogen, especially CN (the 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 nucleus 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, holding 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.

Comet Hale-BoppIn this image of comet Hale-Bopp, you can make out the large dust tail and dimmer, blue ion tail. The nucleus is completely hidden inside the coma when comets are viewed at optical wavelengths from Earth.

The particles from the dust tail 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. As the comet gets farther from the Sun again, the gas in the coma re-freezes to the surface.



Comet Shoemaker-Levy 9 fragments

Comet Shoemaker-Levy 9 broke into about two dozen pieces shortly before hitting Jupiter in 1994, as shown in the Hubble Space Telescope image.

There are two populations of comets, separated by their orbital periods. The short period comets generally come from the Kuiper belt, which is also where Pluto and the other icy dwarf planets reside. The long period comet come from the Oort cloud.

Kuiper belt comets generally have orbits close to the ecliptic and shorter periods that only take them out to the area around Neptune and Pluto. The short period means they have made many orbits around the Sun already, so they have already lost a lot of their ice. Without as much ice, they tend to have smaller comas and shorter tails. Halley is a somewhat large short period comet, with a period of about 76 years. At its last pass near Earth, it never became bright enough to see naked eye from Ann Arbor.

Oort cloud comets generally have much longer periods, and can come in from any angle. since they don't get close to the Sun very often, they tend to be bigger, and have a lot of ice to make very spectacular coma and tails. Hale-Bopp for example has a period of roughly 2500 years. When it came in to the inner solar system in 1997, it was clearly visible even from light polluted skies.

The Model

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 add:

Mix well, then quickly mix in the dry ice. Add more water until the stuff in the bowl seems damp but not drowning. Carefully lift the bag out of the bowl and, pushing on the outside of the bag, 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 image of comet Wild 2. 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.

Additional Resources:

You may find the Pluto, Kuiper belt, and Comets lithographs from NASA's "Our Solar System" lithograph set useful.



last update: 4/19/12 by SAM

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