Time to build a comet!
If you have adult or older-student assistants, ask them to take charge of crowd control; that is, keeping the audience from crowding around the demonstration. Everyone will get to see the comet! Spare a minute for a brief lecture on the hazards of dry ice. You may have participants who know that dry ice can “burn”, but not all will understand that idea at first. However, no one wants to get hurt. Mention that you will be protecting your hands with gloves and your eyes with safety goggles (or safety-rated eyeglasses).
Participation opportunities include: helping move the materials and equipment to a mess-tolerant location, measuring ingredients, and smashing dry ice. The trauma of allotting slots to help out is one important reason to try the exercise at least twice. (Crowd-control tip: sometimes it helps to announce “I’ll only choose helpers from those who do not raise hands and call out to volunteer.”) As a first step, take one of your plastic bags and cut it open along one side, then use it to line your mixing bowl. Take 2 other bags and put one inside the other to make a double-thickness bag.
In the first stage, your chosen helpers will take turns measuring all the “safe” ingredients into the bag-lined mixing bowl. Working with the dry ice needs closer control, so keep your supply of CO2 off to one side for now. As you introduce each ingredient, explain why it’s being included. You can use the short explanations provided here as a starting point, adding your own facts or curriculum tie-ins, but remember to keep it brief or you’ll lose your audience’s attention.
Let’s start with water: most comets are composed primarily of water ice. During the early formation of the solar system, the planets were bombarded by comets—so some of the water you will use in this experiment may have actually originated in the Kuiper Belt! (For a popular-science overview, check out this article from Time Magazine.) Your helper will add 2 cups of water.
Next, add sand or gravel: most comets incorporate at least some rocky material. Have your helpers measure out about 2 Tablespoon (TB) of grit.
Next, you’ll add ammonia: real comets typically contain NH3, the active ingredient in this cleaning solution. (Regrettably, few, if any, comets show up to help when it’s time to clean house.) If you’re using a squirt bottle to store the solution, your helper just needs to add one “squirt” of ammonia solution. Otherwise, your helper should measure in 1 Tablespoon.
And, for our last step before major excitement sets in, stir in a touch of ice-cream topping: these contain organic molecules, which are a normal component of comets. The organic molecules in real comets are not this delicious–they include hydrogen cyanide and formaldehyde–but comets often contain complex and interesting compounds such as amino acids. Researchers at NASA’s Ames Research Center have shown that amino acids from comets striking Earth long ago during the Solar System’s early eons would not only survive impact but would form even more important compounds for life under the heat of impact. So it may be that we are here to enjoy ice cream (and sugary toppings) thanks to ancient comets. Let your helper squirt in one squeeze-worth (it will be about a tablespoon).
Now, finally, it is time to add the dry ice. Comets contain significant quantities of frozen gases, especially carbon dioxide, which just happens to be the gas that we call “dry ice” when frozen. This stage of your demonstration is a two-step process. First, you will put on safety goggles and work gloves and use the hammer to tap off about 2 pounds of dry ice (1/4 to 1/3 of your supply). Place the chunks into the doubled plastic bag and twist the opening closed. Then, and only then, one lucky volunteer will be asked to don a set of goggles and, once protected, may proceed to smash the contained dry ice with the hammer.
Have your crusher use a two-handed grip (this helps deflect the temptation to also handle the bag of dry ice and also limits the range of motion, protecting bystanders from the crusher’s swing) and turn the hammer sideways, to smash with a broader surface area.
Once that stage is completed, ask the crusher to rejoin the group. Make sure that the wooden stirring spoon is at hand and that you are still wearing your work gloves and goggles. Then open the bag and quickly scoop out roughly two cups of crumbled dry ice.
Give the mixture a stir and then swiftly add the dry ice, stirring vigorously. There will be some dramatic vaporization of CO2 and in moments the dry ice will freeze the water solution to a slushy slurry. Quickly wrap the plastic bag around your slushy mass and—keeping those gloves on—form the contents into a snowball, using firm pressure to shape the contents.
You will feel the mass harden as you form your iceball. At that point, it is time to unwrap the comet and reveal it to your onlookers. You will have something that looks surprisingly like the common description of a comet—“a dirty snowball”. You may even want to use your snowball-making skills to firm up the comet a bit once you remove it from the bag–remember to keep your gloves on!
Set the comet aside on a cold-safe surface, in a location where the eventual water-ice-melt will not damage anything. The comet will continue to outgas CO2 vapor. If you are working outdoors, any breeze will push this plume into a fair imitation of a comet’s tail.
Your experiment team will undoubtedly want to repeat this process. A typical group of students will demand about four comets. After 2 or 3 builds, it will be time to set up fresh plastic bags for mixing and crushing. If the group is larger, find ways for students to share participation tasks. For instance, two students can take turns as dry-ice crusher, two can each measure one cup of water into the mix, and so on. As you proceed, instead of repeating the descriptive information yourself, invite the students to call out more of what they remember about the components represent.
These model comets will last a long time, up to a few hours depending on their size and the conditions. You can explain that the comets which get our attention are much larger–Comet Halley is estimated to be about the size of Manhattan Island–and between visits to the inner Solar System, they orbit back to where it is too cold for water, ammonia, or CO2 to be anything other than solids. By no means do you need to make any effort to create spherical, smooth comets. In fact, as you create successive comets, allow them to be different, irregular, and, well, messy. Here are a few samples from a few of my comet-making sessions:
If your schedule permits, allow some time to pass and return to look at the comets after they have lost more material, as if you are checking in on a comet as it approaches the sun and some of its ice has been drawn off under the combined forces of the sun’s radiation and the solar wind…forming the comet’s tail.