Build a Comet

This is a science demo to highlight comets: what they're made of, where they live, why they have tails, etc. Much of this information is also available in the attached PDF.

Instructions

For full instructions on how to make a comet, please go to this website.

Background

Comets are cosmic snowballs of frozen gases, rock and dust, roughly the size of a small town. When a comet's orbit brings it close to the sun, it heats up and spews dust and gases into a giant glowing head larger than most planets. The dust and gases form a tail that stretches away from the sun for millions of kilometers.

Short-period comets (comets that orbit the sun in less than 200 years) reside in the icy region known as the Kuiper Belt, beyond the orbit of Neptune from about 30 to 55 astronomical units (an astronomical unit, or AU, is equal to the distance between Earth and the sun--about 93 million miles). Long-period comets (comets with long, unpredictable orbits) originate in the far-off reaches of the Oort Cloud, which is five thousand to 100 thousand AUs from the sun.

Each comet has a small frozen part, called a nucleus, often no larger than a few kilometers across. The nucleus contains icy chunks, frozen gases with bits of embedded dust. A comet warms up as it nears the sun and develops an atmosphere, or coma. The sun's heat causes the comet's ices to change to gases, so the coma gets larger. The coma may extend hundreds of thousands of kilometers. The pressure of sunlight and high-speed solar particles (solar wind) can blow the coma dust and gas away from the sun, sometimes forming a long, bright tail. Comets actually have two tails -- a dust tail and an ion (gas) tail.

Supplies for Making a Comet

• Two pairs leather work gloves
• Two pairs safety goggles
• Mallet for crushing dry ice
• Pillowcase for holding dry ice while crushing
• Large and small plastic bowls
• Small plastic bags
• Pitcher for water
• Measuring cups
• Corn starch
• Corn syrup
• White vinegar
• Rubbing alcohol / ammonia
• Hairdryer
• Cooler
• Dry ice - 10 - 20 lbs (not included but we can buy it at Joe’s Ice or from Caltech Chemistry)

Procedures

1. Crushing the dry ice will take some time and can be done ahead of the demo. To prep the dry ice, put on thick gloves and place the dry ice in a towel or pillowcase. You can purchase dry ice in blocks. A 5-pound block will be almost intact after several hours as long as it's kept in the cooler when not in use!. In either case, use the mallet to break the ice into small bits. You need to have at least 50 percent of your dry ice as a powder, which will make the water freeze and hold your comet together. Keep the dry ice in a cooler after you have crushed it. After crushing, store the dry ice back in your insulated container.
2. Once students arrive, explain that you will be making a model of a comet that will show jets coming from the comet model. Explain that they must stand a safe distance back since dry ice can cause injuries. Tiny pieces of dry ice will shoot out, especially when you crush it and while you add water. That is why you have goggles and gloves.
3. Line the bowl with a plastic bag.
4. Add to the lined bowl: 1 liter of water, dirt, starch, corn syrup or soda, vinegar and alcohol/ammonia. As you do this, explain that comets have lots of ice and water. In our comet model, dirt represents the dust, minerals and water found in comets; starch helps hold the model together; the syrup or soda are organics and give the comet a dark appearance; vinegar represents amino acids in a comet and rubbing alcohol represents methanol found in comets. Do not add too much alcohol as it has an antifreeze effect. In general, I recommend adding mostly water and then just little dashes of all the other ingredients.
5. Mix ingredients and stir in the dry ice. Students will love this part since a murky white cloud puffs up as moisture in the air is being frozen out by the gas that is coming out of the dry ice.
6. Once all of the dry ice is in the bowl, pick up the sides of the bag and use them to form the mixture into a large clump. Add more water as needed. The mixture will start to thicken as the dry ice freezes the water. You can feel the clump forming through the plastic bag. If the mixture does not hold together, add more water. This part takes a bit of experience. However, you may get lucky on the first try. You will certainly have a good feel for the proportion of ice-to-water on the third try.
7. Once you see you have a clump, take it out of the bag and show it to students. You should see gas jets coming off the comet. If your model falls apart, it’s OK. Comets frequently disintegrate as they come closer to the sun!
8. For added effect, you can gently blow on the comet and generate a week "tail". Talk about how tails are generated as comets approach the sun from the outer solar system and begin to heat up / sublimate.
9. You can leave your comet sitting out, but please put it in a bowl or something so that it doesn't melt all over the table and make a big mess.
10. Please keep dry ice in cooler when not in use, and cooler in the shade to minimize sublimation! This demo doesn't work if we run out of dry ice.

Relevant notes

• Dry ice sublimation point -110F (-80C)
• Comet nuclei are a few miles in size while their tails can extend to millions
• Short period (P<200 yr) comets form in Kuiper Belt (30-50 AU)
• Long period (P>200 yr) comets form in Oort Cloud (5000-100000 AU)
• Halley’s Comet’s period is 75 years (1986, 2061), and first recorded in 467 BC
• Shoemaker-Levy comet disintegrated into Jupiter in 1992
• Hale-Bopp 1997

When you are done with the demo

• Rinse things off (get the mud/ammonia off the gloves and bowls)
• Let things dry for a day BEFORE putting them back in a sealed plastic box!! (mildew/rot)
• Note/replace any items that you used up

Link back to home page.