OverviewLearn how to build an icy model of a comet, complete with shooting jets. This activity provides an opportunity for students to make observations, discuss changing states of matter and demonstrate measuring skills.
ManagementWatch the "DIY Space: Create a Comet with Dry Ice" video tutorial at the top of the page for instructions on creating the comet demo.
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.
- 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 or as inch-size granules. A 5-pound block will be almost intact after several hours. 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.
- 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.
- Line the bowl with a plastic bag.
- Add to the lined bowl: 1 liter of water, dirt, starch, corn syrup or soda, vinegar and alcohol. 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.
- 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.
- 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.
- Once you see you have a clump, take it out of the bag and show it to students. At this time, measure the mass of the comet (this can be done using a basic kitchen scale, or on a simple balance if you do not have a triple-beam balance or some other measuring device). Record your data. You should see gas jets coming off the comet. Repeat the mass measurement in five-minute increments. If your model falls apart, it’s OK. Comets frequently disintegrate as they come closer to the sun!
- For added effect, have students take turns holding the flashlight and hairdryer close together and pointing at the comet. The flashlight represents the sun and the dryer is the solar wind. You should see the jets moving away as the heat from the dryer pushes the gas away.
- To clean up, place your comet in a sink and let it melt. Continue to measure the mass of the comet in five-minute intervals up to 30 minutes. After the table is complete, have students plot the data in the graph. The y-axis is unlabeled, as using different ingredients will yield comets with different masses.
There are a number of processes at work both in the creation of the comet and the melting of the comet. Many of these are visible processes. Students should be asked to describe what they think will happen before ingredients are mixed together, what they predict will happen after the mixing process is complete, and what they are observing once the comet chunk is removed and exposed to the open air, as well as the light and hairdryer.
A few key points to ask students about or to bring into the discussion that will help build student understanding.
- Water freezes at 0 degrees Celsius (32 degrees Fahrenheit)
- Dry ice is carbon dioxide, frozen into a solid at -79 Celsius (-109 degrees Fahrenheit)
- While the water will freeze into a solid as it is added to the dry ice and then melt into a liquid as it warms, the solid carbon dioxide skips the liquid state and goes directly from a solid to a gas as heat energy is added in a process called sublimation.