Figure 1 Typical comet anatomy.

Mike Isley
Product Developer

Introduction

Comets have intrigued viewers since the beginning of civilization. Ancient viewers considered comets to be omens of death and destruction. Others thought their appearance was a message from the gods indicating that they were displeased with mankind. Today, comets are still viewed with fascination and awe.

Comets travel in wide elliptical orbits through our solar system, lighting up the night sky with long streaming tails as they approach our sun. This demonstration models the nucleus of a comet using dry ice and a mixture of common ingredients found in comets. Not only does the model look like a comet, it gives off jets of vapor simulating the tail released by real comets.

Background

Figure 2  Nucleus of comet Wild 2.

Comets are made of primordial material left over from the formation of our solar system billions of years ago. Figure 1 shows a typical comet anatomy.¹ The only solid portion of a comet is the nucleus, which is composed of frozen water and gases, rock, dust, and various organic materials. The average width of the nucleus can be from a few meters to 10 km (6 miles) wide. Halley’s Comet has a nucleus that is slightly larger. It is shaped like a potato and is about the size of Manhattan in New York City. It is 8 km (5 miles) wide and 15 km (9 miles) long.

NASA’s Stardust spacecraft in 2004 flew within 236 km (147 miles) of Comet Wild 2 and captured photos of its nucleus (see Figure 2).² The photos showed an irregularly shaped surface with plunging craters, steep cliffs, and spewing jets of gas.

As comets approach the sun, the nucleus heats up causing volatile materials of frozen gases and water to boil off and form the head of the comet, known as the coma. As the comet approaches the sun, the combined effect of the sun’s heat and the pressure of the solar wind (charged particles ejected from the sun’s corona) help to form 2 tails. One is a dust tail that is white from reflecting sunlight and is opposite the forward direction of the comet. The other tail is a faint blue ion gas tail that always points away from the sun. Both of these tails are shown in this photo of Comet Hale-Bopp (Figure 3).³ A comet’s tail increases in length as it gets closer to the sun. As it moves in its orbit away from the sun, the tail decreases in length, and it leads the way with the coma and nucleus in the rear.

Figure 3  Dust and ion tails of Comet Hale-Bopp.

Materials for comet nucleus

• Large Mixing Bowl
• Large Spoon
• Kitchen Trash Bag
• Pillow Case or Bath Towel
• Large Garbage Bag
• Pie Pan or Flat Tray
• Goggles
• Thick Work Gloves
• Dry Ice, 5 lb
• Cooler
• Mallet or Hammer
• Water, 1.5 L
• Fine Dirt, 2 cups
• Starch, 1 tbs
• Syrup or Dark Soda, 1 tbs
• Vinegar, 1 tbs
• Rubbing Alcohol, 1 tbs
• Hair Dryer and Flashlight (for extension activity)

Teacher preparation

1. The 5-lb block or pellets of dry ice should be crushed before the students arrive. Put on thick gloves and place the dry ice in a towel or pillow case. Put the contained dry ice on top of a large garbage bag lying on a table or floor.
2. Put on goggles, hold the pillow case or towel tightly closed with 1 gloved hand, and crush the dry ice into small granules or powder with a mallet or hammer. At least 50% of the dry ice should be powder. This makes your water mixture freeze and holds your comet nucleus together. After crushing, place the towel or pillow case containing the dry ice back in the cooler.
3. When students arrive, have them stand or sit at a safe distance from your demo table in case any small crystals of dry ice shoot out of the bowl when mixing.
4. Line your mixing bowl with a kitchen trash bag.
5. Add the ingredients in the following order with stirring, explaining what each represents in the comet.
   
   
   • 1 L of water (comets have lots of frozen water)
   • 1½ cups of fine dirt (represents minerals and dust)
   • 1 tbs of starch (holds nucleus together)
   • 1 tbs syrup or dark soda (represents organics)
   • 1 tbs vinegar (represents amino acids)
   • 1 tbs rubbing alcohol (represents methanol)
6. With gloved hands, mix the above ingredients thoroughly with a large mixing spoon and then add in the powdered dry ice. Continue stirring. White clouds form as the water vapor in the air above the bowl condenses.
7. After all of the dry ice is in the bowl, pull up the sides of the garbage bag with gloved hands and knead the mixture into a clump. Add more water as needed. You will feel the mixture thicken as the dry ice freezes the water. If the clump doesn’t hold together, place it in the bowl and add more water. Knead the clump inside the bag until it is firm.
8. Remove the clump, add a little more dirt on top, and place on a pie pan or tray. You now have a working model with gas jets coming off the surface.
9. View a NASA video demonstrating the above steps at Create a Comet with Dry Ice.⁴

Extension activity

You can darken the room and have a student hold together a flashlight and hair dryer (on low or cool setting) approximately 2 feet from the comet nucleus. Explain that the flashlight represents the sun and the hair dryer represents the solar wind. You should see the vapor jets moving away from the ice ball in the opposite direction, simulating the comet’s tail. As you move the nucleus farther away from the dryer, the comet’s tail should become shorter.

Note: Inform your students that the solar wind model is not scientifically correct. Solar wind is not moving air. It is the movement of charged particles (mostly electrons and protons) ejected by the sun.

Footnotes

¹Active Margin Blogspot. Available at: http://jrepka.blogspot.com/2010_08_01_archive.html. Accessed September 1, 2013.

²Composite and Stereo Images of Comet Wild 2. Available at: http://stardust.jpl.nasa.gov/news/news97.html. Accessed September 1, 2013.

³Comet Hale-Bopp. Available at: http://astroprofspage.com/wp-content/uploads/2007/11/HaleBopp.jpg. Accessed September 1, 2013.

⁴Jet Propulsion Laboratory. Available at: http://www.jpl.nasa.gov/education/videos/playVideo.cfm?videoID=17. Accessed September 1, 2013.