Education and
Public Engagement
at the Lunar and Planetary Institute
Explore! Jupiter's Family Secrets

The Pull of the Planets

Overview

The Pull of the Planets is a 30-minute activity in which teams of children model the gravitational fields of planets on a flexible surface. Children place and move balls of different sizes and densities on a plastic sheet to develop a mental picture of how the mass of an object influences how much effect it has on the surrounding space.

This activity should be conducted after Heavyweight Champion: Jupiter!, which allows the children to discover the force of gravity in the solar system. These concepts involve more advanced science than previous activities in Jupiter's Family Secrets, and they explore more deeply the science of the Juno mission and the rich information it will return to us. Facilitators who choose to undertake this activity should have a firm grasp of the scientific basis so that misconceptions are not introduced to the children.

This series is appropriate for children ages 10 to 13.

What's the Point?

Materials

For each group of up to 30:

For each group of four children:

For each child:

For the facilitator:

Preparation

Activity

1. Ask the children to connect what they have learned about gravity to the motions of objects in the solar system.

Facilitator's Note: There are many different misconceptions about gravity; children may think that it is related to an object's motion, its proximity to Earth, its temperature, its magnetic field, or other unrelated concepts. Guide conversations cautiously and listen carefully to what the children say to avoid supporting their misconceptions.

2. Tell the children they will make a model of how objects — like planets — interact in space.

Explain that space can act much like the surface of the trampoline. The indentations made on the surface represent the "gravity wells" created by massive objects in space.

3. Invite the children to experiment with the same effects on smaller–scale models. Separate the children into groups and give each group a prepared embroidery hoop, suspended in the air on bricks or books. Explain that they will use marbles and Play-Doh balls to model the effects of gravity on objects in space.

Facilitator's Note: Gravity is a universal force, like magnetism and electricity. However, it becomes important only at large scales. Gravity determines the interactions stars, planets, and moons.

In the model, the balls are too small to exert a significant gravitational pull on each other. However, they are gravitationally pulled toward Earth! They move toward each other because the weights of heavier objects distort the sheet and lighter objects roll "downhill."


4. Invite the children to experiment with their models of space
by placing and dropping the marbles (together and separately) onto the sheet

5. Ask the groups to each add a large, 2" round ball of Play-Doh to represent a large "planet" alone on the sheet. Ask the children to hypothesize what will happen if the marbles are dropped onto the sheet, and have them record their thoughts in their journals before they test them. After they have dropped the marbles onto the sheet, share that this "pull" toward the "planets" is a model of gravity.

Facilitator's Note: The Play-Doh and Styrofoam balls used in steps 5–7 serve to create test "wells" on the sheets. They should remain stationary while the children roll the marbles to see how they move at each step. Encourage the children to only roll marbles, as the Play-Doh is sticky and will not model the motion accurately.

6. Ask the groups to place a very small round ball of Play-Doh (about half of the size of a marble), which represents a small asteroid, alone on the sheet. Have them note their predictions in their journals and then test what will happen to marbles added to the sheet.

7 Ask the groups to place the Styrofoam ball alone on the sheet and, keeping records in their journals, experiment with its gravitational pull.

Remind the children that the gravitational pull of a planet depends on its mass and size. Saturn is large in size, but it does not have nearly as much mass packed into its volume as Jupiter does.

Facilitator's Note: Saturn does have plenty of mass, and as they explored in Heavyweight Champion: Jupiter!, it does have gravity. However, because it is not dense, a person standing in its cloud tops would only weigh about as much as they weigh on Earth. Saturn's cloud tops are far above the planet's bulky — and gravitationally strong — center.Because the force of gravity depends on both mass and distance, planets that are puffy and less dense have less gravity at their cloud-tops or surfaces, which are far above the bulk of the mass in their interiors. This is why planets like Saturn appear to have less gravity than Neptune, despite Saturn's greater mass. You may need to remind the children of what they learned in Dunking the Planets in order for them to understand these difficult concepts.


8. Invite the groups to experiment with dropping the marbles in different locations, and with different amounts of Play-Doh or the Styrofoam ball, in various locations on their gravity field. 

9. After the children have finished experimenting, discuss their findings.

Ask the children to draw in their journals, based on their models, how deep a gravity well the Moon, Earth, and Jupiter each create in space. Have them describe how their differences in gravity relate to each object's size and mass.

10. Invite the children to describe how this model of gravity resembles real gravity and how it fails.

Facilitator's Note: Children also may not understand that the planets are not being significantly pulled toward each other. They are strongly pulled toward the Sun, but since they are also moving, they move around the Sun in stable orbits. Smaller objects like comets and asteroids may have less circular orbits that cross the paths of planets — sometimes resulting in a collision. Be careful when identifying the objects in this activity not to introduce misconceptions regarding planets' orbits and collisions.

Conclusion

Explain that the Juno mission to Jupiter will experience Jupiter's gravity in much the same way as a very, very small marble might in our model. Show a picture or video animation of Juno orbiting Jupiter. (Juno will orbit Jupiter, however, rather than falling into it.) Juno's instruments will keep careful track of how Jupiter's pull on the spacecraft changes as the spacecraft passes over the planet's surface. In this way, Juno will be able to measure how Jupiter's gravity is different from place to place. By measuring the slight changes in Juno's trajectory, scientists will learn where exactly Jupiter keeps the bulk of its mass in its deep interior. Scientists can then infer details about the composition of Jupiter's unseen lower layers and core.

If possible, build on the children's knowledge by offering them a future Jupiter's Family Secrets activity. Invite the children to return to wrap-up their investigations of Jupiter by attending the concluding activity, My Trip to Jupiter, where they create scrapbooks to document their own journeys into Jupiter's deepest mysteries!