Dirty Snowballs: How a Comet's Size Affects How Fast It Melts
AbstractComets—big lumps of rock, ice, and frozen gases that orbit the Sun—are among the most amazing heavenly objects seen in the night sky. The glowing tail behind the comet's nucleus inspires wonder. But did you know that a comet's tail is evidence that it's melting? As a comet passes by the Sun on its orbital path, it starts to melt. But do bigger comets melt faster than smaller comets? In this astronomy science project, you will investigate how the size of the comet affects the rate at which it melts and by how much.
Michelle Maranowski, PhD, Science Buddies
This project is based on one of the Science Buddies — Lick Observatory Astronomy Contest winning projects: Boulos, Bassam N. (2011). "Comets"
To model comets using ice forms and determine how weight affects the rate of melting.
A comet streaking across the sky, its tail stretching behind, is an amazing sight. Because comets are not visible all the time, they are usually special and even spectacular for people on Earth. In olden times, comets were considered as both good and bad omens. But what is a comet? Where do comets come from? And what is a comet made of?
A comet is basically a "dirty snowball" of dust and sometimes rock, frozen water, and frozen gases like ammonia, methane, carbon monoxide, and carbon dioxide. Comets travel in an orbit; a short orbital-period comet is one that takes 200 years or less to go around the Sun, and a long orbital-period comet takes more than 200 years—often thousands or even millions of years. Halley's Comet is a short orbital-period comet that can be seen from Earth every 75 to 76 years. Comet Hale-Bopp, which was visible to the naked eye for 18 months spanning 1996 and 1997, is a long orbital-period comet that won't be visible from Earth again for several thousand years. Short orbital-period comets are thought to originate from the Kuiper Belt, while long orbital-period comets are believed to be from the Oort Cloud. The Kuiper Belt lies beyond the planet Neptune (between 30 astronomical units [AU] and 55 AU from the Sun), and the Oort Cloud, which is thought to be a spherical cloud surrounding the solar system, lies about 50,000 AU from the Sun. Objects in the Oort Cloud are made up of frozen water, frozen ammonia, and frozen methane.
Comets are made up of a nucleus, a coma, and a tail, as shown in Figure 1. A comet's nucleus can range from hundreds of meters to tens of kilometers across.
Figure 1. This simple diagram shows the different parts of a comet. (HEASARC, n.d.)
Occasionally, the gravitational influence of one of the outer planets or, in the case of the Oort Cloud, the gravitational influence of nearby stars, causes a comet to enter an elliptical orbit that carries it closer to the sun. The nucleus is volatile because it is made up of unstable matter that evaporates easily. As the comet orbits closer to the Sun, solar radiation—the energy that the sun gives out—begins to melt the nucleus and causes the volatile material and dust to come out. (We also feel solar radiation, in the form of sunlight, on our skin.) The dust surrounds the nucleus to form the coma. Solar winds push the melted material away from the nucleus, causing the tail to form. Some comas can be nearly a million kilometers wide while tails can be more than 100 million kilometers long. Later, when the comet moves away from the Sun to the outer solar system and beyond, the coma and tail disappear and the nucleus remains frozen. Figure 2 shows a comet as seen through a telescope. Sometimes, we can see comets from Earth without telescopes when they pass close to the Sun because their tails and comae reflect sunlight.
Figure 2. This color image of Comet C/2001 Q4 (NEAT) was taken at the WIYN 0.9-meter telescope at Kitt Peak National Observatory near Tucson, Ariz., on May 7, 2004. It is a composite picture, assembled by combining images taken through blue, green, and red filters. (T. Rector [University of Alaska Anchorage], Z. Levay and L. Frattare [Space Telescope Science Institute], and WIYN/NOAO/AURA/NSF)
As a comet travels towards the Sun, how does it melt? Do bigger comets melt faster than smaller comets? In this astronomy science project, you will model comets of different sizes and then use a hair dryer as a heat source (to mimic the Sun and other "warm" planetary objects) to determine how the size of a comet affects melting.
Terms and Concepts
- Orbital periods (short and long)
- Kuiper Belt
- Astronomical units
- Oort Cloud
- Elliptical orbit
- Solar radiation
- Solar wind
- How long is an astronomical unit?
- How long is a light year?
- How many astronomical units are in a light year?
- What causes comets to move out from the Oort Cloud?
- What are some other examples of short orbital-period and long orbital-period comets?
- What is the lifespan of a comet? Do they last forever?
- What is the cycle of comet Hale-Bopp in years? When will it be seen from Earth again?
- What is the source of the Perseid meteor showers?
- Wikipedia Contributors. (2011, September 2). Comet. Wikipedia: The Free Encyclopedia. Retrieved September 8, 2011.
- Harvey, S. (2011, March 25). Solar system exploration: Comets. Retrieved September 9, 2011.
- Coffey, J. (2009, September 20). Parts of a comet. Retrieved September 9, 2011.
- Regents of the University of California. (2000). The comet's tale: Comet orbits. Retrieved September 14, 2011.
Materials and Equipment
- Balloons (1 package)
- Make sure you have a way to fill the balloons with water. For example, for large party balloons, you may be able to stretch the neck of the balloon around the faucet of a sink. Smaller water balloons may require a special nozzle or pump to fill.
- Butcher's twine, cotton (1 roll)
- Volunteer to help make the comets
- Baking sheet
- Kitchen scale that can measure in grams, like this one available from Amazon.com.
- Hair dryer
- Small pot
- Kitchen timer
- Lab notebook
- Support stand with ring clamp, available from Amazon.com. You may be able to borrow this equipment from your science teacher, or come up with another creative way to suspend your comets for testing.
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Making the Comets
- Take six balloons, stretch them out and confirm that there are no holes in them. For this project, you will make three large comets and three small comets.
- Cut twelve lengths of butcher's twine that are each 12 inches long. Push one end of a string as far as it can go into a balloon. Repeat this with the other five balloons.
- Fill three balloons up all the way with water, so the balloons start to stretch out. Do not overfill the balloons so much that you make them pop. These will be your "large" comets.
- Fill three balloons up partially with water, so they start to form a spherical shape but do not stretch out the membrane much. Try to make them about half the size of your large balloons. These will be your "small" comets.
- Place the baking pan with the six balloons (comets) in the freezer and wait until the water in the balloons is completely frozen.
Testing the Comets
- Make a table in your lab notebook, like the one shown, so that you can record data.
|Comet size||Trial number||Initial mass
|Decrease in mass |
|% Comet melted||Average % melted|
- It is important to do at least three trials of the experiment so that you know that your results are accurate and repeatable.
- Put together the support stand and attach the ring clamp to the post. Place the pot underneath and directly below the ring clamp.
- Plug in the hair dryer.
- Remove one of the large comets from the freezer. Carefully untie and remove the balloon so that you have just the ice and the hanging string left.
- Use your kitchen scale to measure the mass of the comet in grams (g). Record this value in the "Initial Mass (g)" column of your data table.
- Tie the loose end of the string to the ring clamp so that the comet hangs directly over the pan.
- Set the timer for two minutes. Turn the dryer on high heat and then hold it 2 inches away from the comet for two minutes. Use a ruler to figure out how far away two inches is.
- Once 2 minutes have elapsed, turn off the hair dryer and set it aside.
- Untie the string from the ring clamp and use your kitchen scale to measure the mass of the comet again. Record this value in the "Final Mass (g)" column of your data table.
- Repeat steps 5 through 10 with the two other large comets. Record the data in your lab notebook.
- Now repeat steps 5 through 11 with the three small comets. Record the data in your lab notebook.
Analyzing Your Data
- Now, for each trial, calculate the amount of water that melted off the comet and enter the result in your data table. Use this formula:
Now calculate the percentage of each comet that melted and enter the results in your data table. You do this by dividing the mass of water that melted by the initial mass of the comet:
Now calculate the average percent melted for each type of comet (large and small), and record the results in your data table. Do this by adding the percentages for each trial and dividing by 3. For example, if the percentages melted for 3 trials were 10, 15, and 20%, the average would be:
- Make a graph of your data with the type of comet (large or small) on the horizontal axis and the average % melted on the vertical axis.
- Based on the data in your table and graph, which type of comet melts the fastest? What did you learn from this project?
Ask an Expert
- How long does it take for each size of comet to completely melt? How big is the difference in time?
- Make your comets more realistic. Add a given number of small rocks and a consistent, measurable, and identical quantity of sand to each of the comets before freezing and proceed with the experiment. How do these more realistic comets sublimate and how does size affect the sublimation?
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