NASA - National Aeronautics and Space Administration

Teacher Section

Objective: To construct a scale model of the Earth-Moon-Mars system in terms of planetary size, and to discover how far one might have to travel to get to the Moon or Mars.

Grade Level: 5-8
Subject(s): Space Science, Mathematics
Prep Time: < 10 minutes
Duration: One-two class periods
Materials Category: Special

National Education Standards

Science
Earth and Space Science
        Structure of the Earth system

Mathematics
Compute fluently and make reasonable estimates
        Select appropriate methods and tools for computing with fractions and decimals from
        among mental computation, estimation, calculators or computers, and paper and
        pencil
Understand patterns, relations, and functions
        Represent, analyze and generalize a variety of patterns with tables, graphs, words
        and, when possible, symbolic rules
        Relate and compare different forms of representation for a relationship
        Identify functions as linear or nonlinear and contrast their properties from tables,
        graphs or equations

Materials (for a class of thirty):

• One bag of blue balloons (at least 10 per bag)
• One bag of white balloons
• One bag of red balloons
• Rulers or measuring tape
• Student Page

Pre-lesson Activities

• Obtain balloons. If you cannot find red, white and blue balloons, you can substitute a few other colors. Earth can be blue or green, Mars can be red or orange, and the Moon can be white or yellow. Balloons should be available at your local grocery or party supply store.
• Duplicate the Student Pages (one per group).

Background Information

How big is the Moon? How far from Earth is the Moon? Earth science and astronomy books depict a Moon that is much closer and much larger than in reality. The example to the right is typical of what is found in textbooks.

Image to right: The Sun is larger than the Earth, and the Earth is larger than the Moon. Credit: NASA

This balloon activity will allow students the opportunity to construct a scale model of the Earth-Moon system, both in terms of planetary sizes and distances. In addition, students make a scale model of Mars and discover how far one might have to travel to visit the most Earth-like planet in our solar system. This activity is also a good introduction at the beginning of a unit on the solar system to pre-assess student knowledge of planetary distances.


Guidelines

1. Discuss Earth's size relative to the Moon and to Mars. Determine what misconceptions students may have.
   
2. Divide students into three groups and distribute Student Pages and balloons. One group should have "Earth" balloons (blue), one group should have "Mars" balloons (red), and one group should have "Moon" balloons (white).


3. Tell students that the Earth balloon should have a diameter of 20 centimeters (cm). Have them follow the instructions on their Student Pages to determine the scale. Ask students with Earth balloons to inflate their models to this scale. (Obviously, the balloon is not a perfect sphere, but neither is the Earth.)


4. Ask students to look at the Student Pages and to calculate the sizes of the Moon and Mars using the same scale as the Earth model. The Moon should be about 5 cm and Mars about 11 cm.


5. Have students inflate their Mars or Moon balloons.


6. Ask students, "At this scale, how far apart are the Earth and the Moon?" The diagrams in common textbooks might lead many of them to suggest that the Moon balloon should be held less than a meter from the Earth balloon.


7. Have students use the information on the Student Pages to calculate the distance from the Earth to the Moon at the same scale as the balloon models. The distance is about 6 meters. Have students holding the Earth models stand at one side of the room and students with the Moon models about 6 meters away.


8. Point out to students that they now have a scale model of the Earth-Moon system. The distance between the two is the distance traversed by the Apollo astronauts who went to the Moon in the 1960s and '70s.


9. Compare the size of the Mars model with the Earth and Moon models. Look at the distance between the Earth and the Moon.


10. Ask students how far away they think Mars will be at this scale. Have students attempt to demonstrate it in the classroom.


11. Have students use the information on the Student Pages to calculate the distance to Mars at this scale. (Note: This is an average distance to Mars. This distance varies depending on the relative positions of Earth and Mars in their orbits around the Sun.) The answer is about 120,000 cm, or close to three-quarters of a mile! Have students identify a local landmark that is about three-quarters of a mile away.

Discussion/Wrap-up

• Discuss the relative distance between the Earth and Mars in the context of a human trip. How long did it take for Apollo astronauts to get to the Moon? Three days. How long would it take for astronauts using similar technology to get to Mars? Mars Pathfinder, which launched December 1996, arrived at Mars on July 4, 1997 (seven months). Mars Global Surveyor, which launched in November 1996, arrived at Mars in September 1997 (11 months).

Extensions

• Ask the students to make models of one of the martian moons, Phobos, at the same scale as the balloon models. They can calculate its scale diameter using the information on the Student Pages. It turns out that at this scale, Phobos is about the same size as a small grain of sand!
• Research careers of interest and create plans for reaching those careers.
• Have students convert all metric measurements into the U.S. system.

Earth, Moon and Mars Balloons Student Page