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Dirty Snowballs: How a Comet's Size Affects How Fast It Melts

Time Required Short (2-5 days)
Prerequisites None
Material Availability Readily available
Cost Average ($50 - $100)
Safety Always exercise caution when using electrical equipment. Adult supervision is recommended.


Comets—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.


To model comets using ice forms and determine how weight affects the rate of melting.


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"

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Last edit date: 2013-02-16


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 below. A comet's nucleus can range from hundreds of meters to tens of kilometers across.

Science Fair Astronomy project This simple diagram shows the different parts of a comet. (HEASARC, n.d.)
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 below 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.

Science Fair Astronomy project  <B>Figure 2.</B> This 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.  (T. Rector [University of Alaska Anchorage], Z. Levay and L. Frattare [Space Telescope Science Institute] and WIYN/NOAO/AURA/NSF)
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
  • Nucleus
  • Coma
  • Tail
  • Elliptical orbit
  • Volatile
  • EvaporateSolar radiation
  • Solar wind
  • Sublimation


  • 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?


Materials and Equipment

  • Balloons (1 package)
  • Butcher's twine, cotton (1 roll)
  • Scissors
  • Volunteer to help make the comets
  • Baking sheet
  • Hair dryer
  • Small pot
  • Kitchen timer
  • Lab notebook
  • Ruler

You will also need to purchase, or borrow from your science teacher, these pieces of laboratory equipment. Or, with a little bit of problem-solving, you may find a way to rig up your own way of hanging the comets rather than using the support stand and ring clamp.

  • Graduated cylinder, 100 milliliter (1). This can be purchased online at www.scientificsonline.com, Item # 3037525-GRP
  • Support stand (1). Can be purchased online at www.scientificsonline.com, Item # 6308003
  • Ring clamp (1). Can be purchased online at www.scientificsonline.com, Item # 6308504

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Experimental Procedure

Making the Comets

  1. 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.
  2. 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.
  3. Fill the graduated cylinder with 100 milliliters (ml) of water. Ask your volunteer for help. Carefully pour the water into the balloon. Repeat two more times so that you have 300 ml of water in the balloon. Then, fill the graduated cylinder with 50 ml of water and pour that into the balloon, for a total of 350 ml of water in the balloon. Tie the end of the balloon with an additional piece of string, making sure that no water can leak out, and place it on the baking pan.
  4. Repeat step 3 two more times so that you have three balloons filled with 350 ml of water. These will be the large comets.
  5. Repeat steps 3 and 4 with the remaining balloons, except fill them with only 250 ml of water, to make a total of three small comets.
  6. 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

  1. Make a table in your lab notebook, like the one shown below, so that you can record data.
Comet Trial Amount of Water Melted into Pot
(in Milliliters)
Large (350 ml)
Small (250 ml)
Table 1. Milliliters of melted water for large and small comets
  1. It is important to do at least three trials of the experiment so that you know that your results are accurate and repeatable.
  2. Put together the support stand and attach the ring clamp to the post. Place the pot underneath and directly below the ring clamp.
  3. Plug in the hair dryer.
  4. 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.
  5. Tie the loose end of the string to the ring clamp so that the comet hangs directly over the pan.
  6. 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.
  7. Once 2 minutes have elapsed, turn off the hair dryer and set it aside. Pour the water from the pan into the graduated cylinder and measure how much water melted from the large comet. Record the data in your lab notebook.
  8. Take the comet off the ring and discard it.
  9. Repeat steps 5 through 9 with the two other large comets. Record the data in your lab notebook.
  10. Now repeat steps 5 through 10 with the three small comets. Record the data in your lab notebook.

Analyzing Your Data

  1. Now for both sizes of comets, average the amount of liquid melted over the three trials and also figure the average percentage of melted water in terms of the total amount of water for both comet sizes. Record your data in a table like the one shown below.


Average Amount of Water Melted into Pot (ml)

Percentage Comet Melted (%)
(350 ml)
(250 ml)
Table 2. Average amount of water melted in large and small comets and its percentage in terms of the total amount of water
  1. To calculate an average, add the numbers of the three trials and then divide by three. Equation 1 shows how to calculate an average.

Equation 1.
Average = X1 + X2 + X3

X1= The amount of water melted into pot, trial 1
X2= The amount of water melted into pot, trial 2
X3= The amount of water melted into pot, trial 3
  1. Now calculate the percent melted for each comet size. Divide the average amount of water that melted into the pot by the amount that was in the comet originally, either 350 ml or 250 ml, and then multiply by 100. Equation 2 shows how to calculate the percentage the comet melted.

Equation 2.
Percentage Comet Melted (%) = Average Amount of Water Melted
Amount of Water in Comet
× 100

Amount of Water in Comet= 350 ml or 250 ml
  1. Based on the data in your tables, which comet melts the most and the fastest? What did you learn from this project?

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  • 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. Try this comet recipe. 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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