Active Learning Exercises in Planetary
and Solar Astronomy

Copyright 1996
National Optical Astronomy Observatories
Volume 2, Number 1


This exercise was presented as part of a NASA IDEA grant titled Active Learning Exercises in Planetary and Solar Astronomy for K-3 Students. NOAO astronomers worked with students and teachers of the Satori School in Tucson, AZ, to present eight topics in the elementary classrooms. This writeup is intended to be a resource for other astronomers venturing into classrooms. It describes our experiences presenting the classic "Build Your Own Comet" exercise and includes additional facts about comets that might be useful to astronomers who don't specialize in this field.

For more information on this module, or others developed through this program, contact the NOAO Educational Outreach Office at


This was the second of four presentations made to students on the topic of planetary astronomy. It was the most exciting and talked about of the presentations, but care has to be taken to minimize misconceptions. This activity is useful and fun for all grade levels, with the presentation tailored to the age and interest of the students. In each class, the expected learning outcomes included:

  • the composition of comets
  • why comets have a tail
  • the size of comets

Depending on the age of the students and the teacher's comfort level with the materials, students can also be expected to understand:

  • the process of sublimation (going from a solid phase directly to a gas phase)
  • the orbits of comets
  • the differences in comets, asteroids, and meteoroids
  • the concepts of radiation pressure and solar wind, and why a comet's tail is always pointing away from the sun (anti-sunward).

To demonstrate these concepts, we started with an interactive slide show of comet images with lots of questions and discussion. In our discussion, we compared and contrasted planetary orbits and the orbits of comets. After students understood something about cometary orbits, they could draw conclusions about why we see a given comet only once over a period of many years. We then compared the size and composition of asteroids, planets, and comets. In our slide show, students were encouraged to observe the comet tail in each picture. We had the students hypothesize as to why comets have tails and why the tails get longer as the comet approaches the sun in its orbit. We then had each class create a "comet" from pans of materials. Students worked in groups of four, with one adult per group.

Safety: Students will be working with dry ice and ammonia. Both of these materials must be handled responsibly. Depending on the age group you are working with, you need to decide if the students can pour the ammonia and work with the dry ice themselves. If each student has a pair of rubber gloves, the danger of dry ice burns is minimized. For younger students, an adult should be in charge of the ammonia to minimize risks.

We used a comet recipe developed by Dr. Dennis Schatz of the Pacific Science Center, found in the Project ASTRO Universe at Your Fingertips Resource Book, which is available from the Astronomical Society of the Pacific. The recipe is also available on-line from the K-12 Educational Outreach Activities link on the NOAO Home Page.

The original recipe, as given on the last page of this brochure, makes a 6 inch comet. We had good results cutting all ingredients in half, making the final comets smaller in size and easier for the elementary students to handle. It worked best when each student had their own pair of rubber gloves. The recipe was pretty reliable: if the comet didnít compact well enough, just adding a bit more water and squeezing harder always made it come together. We also displayed a poster board whole-language recipe card for the students to follow along as the groups assembled their comets.

Most real comets have a much higher ratio of "dirt" than the student comets. Halley's Comet has about 50% dirt. Because of the high dirt content, comets are very dark and absorb a lot of light. The students' comets will not be dark because this recipe does not work well with the higher ratio of dirt.

The most common misconception was that real comets contained Karo syrup, the ingredient used in Dr. Schatzís recipe to represent organic materials. Although it was pointed out that Karo Syrup only represented the sort of molecules found in comet nuclei, post-testing showed several students misunderstood this point. Next time, we would pour the syrup out of an intermediate container, rather than pouring it directly out of the bottle in view of the students.


These facts about comets are contributed by NOAO/NASA Scientist, and IDEA Grant participant, Dr. Nalin Samarasinha.

A comet consists of the following parts:

  • nucleus - a comet's distinct center
  • coma - a hazy cloud of gas and dust that surrounds the nucleus (The comet head consists of both the nucleus and coma.)
  • tail - the coma, pushed by radiation pressure and solar wind away from the nucleus

Ices in the nucleus of a real comet sublimate as they approach the sun. The gas and dust released during this process form the coma. Solar radiation pressure pushes the dust away from the sun forming a dust tail, while solar wind (and associated magnetic field lines) causes the cometary ions to form a plasma tail. The tail of a comet will always be anti-sunward (away from the sun), not opposite the direction of comet motion. The gasses produced during the demonstration are representative of sublimation, going from a solid state directly to a gas phase (state), Students can discuss the three phases of matter: solid, liquid, gas, and the changes between these states.

According to the "Catalogue of Cometary Orbits 1993" there were least 855 individual comets observed until that year, with 174 of them being short period comets, that is, periodic comets with periods less than 200 years. Comet Halley and Comet Encke are examples of short period comets. Comet Halley has a period of 76 years.

The following physical properties of comet nuclei are based mainly on observations of periodic comets:

  • Typical albedo (the fraction of incident sunlight that is reflected back is few percent. That is, comets are dark and good absorbers of light. Therefore, the surface of a typical short-period cometary nucleus most probably consists of a dark mantel (crust) made up of dust grains with few active areas (vents). The outgassing is mainly confined to these active areas. (Note: Do not confuse the surface mantel of the comets with the refractory organic mantels of the dust grains; crust may be a better word for the cometary mantels).
  • Typical radii are of few kilometers (Halley is relatively large at about 5.2 km). Chiron has a radius of about 90 km and is currently the largest known comet.
  • Comets are not round, they are elongated (e.g., peanut shaped). Further, they are not smooth. Just like the students' samples, comets are rough and have vents where the gasses escape.
  • Indications for density imply less than 1 gram/cm3. Note that density is never measured directly, but inferred based on mass estimates derived from non-gravitational forces caused by sublimation.

Chemical composition of the nucleus by number, based on coma observations:

  • H2O ice is the main component (80-90%). CO ice is next with 7-15%. The other major parent molecules include CO2, CH4, NH3, N2, H2CO (formaldehyde), and HCN.

Ultimately, all of a comet's light comes from the sun, either from the scattering of radiated sunlight by dust particles or the reemission of absorbed sunlight by gas molecules as fluorescence.

The corn syrup in the recipe is representative of organic compounds found in comets. Organics are carbon based molecules, molecules which are common to all known forms of life.

The coma of bright comets extends well over 100,000 km. The Hydrogen coma extends well over 1,000,000 km from the nucleus. The coma can be approximated as spherical. Typical speeds of molecules in the coma exceed 1 km/sec. Also, a cometary coma is thinner than the best vacuum we can produce on earth (except when very close to the cometary nucleus).

Outgassing rates: For Halley, each orbit, it sheds about 1 meter of its surface. The peak production of water near the perihelion is about 30,000,000 grams/sec (30 tons per sec). Also, the outgassing gas carries dust grains with it (For Halley, the dust to gas ratio is about 1; for other comets it can be easily differ by factors of a few or even by an order of magnitude).


Support for this work was provided by NASA through Grant number ED-90020.01-94A from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. This work was carried out through the Educational Outreach Office of the National Optical Astronomy Observatories (NOAO). NOAO is based in Tucson, AZ, and operates facilities for ground-based astronomical research including Kitt Peak National Observatory near Tucson, the National Solar Observatory, with facilities on Kitt Peak and on Sacramento Peak in New Mexico, and the Cerro Tololo Inter-American Observatory in Chile. NOAO is operated by the Association of Universities for Research in Astronomy, Inc., under agreement with the National Science Foundation. This brochure was produced in February, 1996, by the NOAO Educational Outreach Office, P.O. Box 26732, Tucson, AZ, 85726.


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Support for this work was provided by NASA through grant number ED-90020.01-94A from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555.

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