BUILD YOUR OWN COMET
Active Learning Exercises in Planetary
and Solar Astronomy
Copyright 1996
National Optical Astronomy Observatories
Volume 2, Number 1
INTRODUCTION
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 outreach@noao.edu.
DESCRIPTION:
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
didnt 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. Schatzs 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.
COMET FACTS:
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).
ACKNOWLEDGMENTS:
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.
COMET LINKS:
For further browsing you may begin with the following sources:
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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