Planetary Defenders

Rocky Rubble Piles


Investigate asteroid models to see which are solid rock and which are piles of rubble. This activity (for children ages 10 and up) demonstrates how scientists can infer the structure and density of asteroids using telescopes and spacecraft instruments.

What's the Point?

  • Some asteroids may be solid rock, but others are made of pieces of rock and dust, loosely held together by gravity.
  • The interior structure of an asteroid can be difficult to determine by direct observation; scientists must infer the structure from their observations.
  • We need to study asteroids and their characteristics to defend Earth from them.


For the facilitator:

  • 3 hand-sized rocks (about 2in x 3in x 3in)
  • about 3 cups of sand
  • a large liquid measuring cup (big enough to put one of the rocks inside)
  • water
  • a freezer (best) or cooler filled with ice
  • aluminum foil
  • a marker that will write on foil
  • Background information:

For each child or group of children:

  • Data Sheet
  • 1 pencil or pen
  • Digital infrared laser thermometer (Note: if this is being conducted as a group demonstration, only one thermometer is needed.)


  • Review the background information and the instructions for the digital laser thermometer.
  • If the thermometer uses a laser, read all safety precautions and prepare a plan to ensure that it will not be pointed at anyone’s face. If conducting this activity with younger children, ensure constant adult supervision for each thermometer.
  • Prepare “asteroids” using the facilitator materials listed above:
    • Measure the size (volume) of each rock by placing it in the measuring cup, adding enough water to cover it (for example, filling it to 2 cups), then removing the rock and measuring how much the water level drops. Make a note of the volume for each rock (for instance, 1/3 cup, ½ cup, 2/3 cup, or 75 ml, 100 ml, 125 ml). Measurements can be rounded off to a rough approximation.
    • Measure an equivalent amount of sand corresponding to each rock (1/3 cup of sand, for instance) and wrap each in aluminum foil (matte/ dull side facing out) into an “asteroid”, keeping track of their sizes.
    • Wrap each rock in aluminum foil (matte/dull side facing out), keeping track of their sizes.
    • Number the rocks and sand “asteroids” in a random order, and label them with the marker. Write down which asteroid model corresponds to which size.
  • Put the asteroid models in the freezer for at least an hour. If a freezer is not available, fill the bottom of a cooler with ice , place the asteroid models inside, and cover with more ice or with dry ice, for at least an hour.


  1. Invite the children to share what they know about asteroids.
    • What are asteroids? (Asteroids are rocky objects, smaller than planets, orbiting our Sun.)
    • What are asteroids made of? (Asteroids are primarily rock and dust, and some also have ice. Some asteroids appear to be solid rock, and others are made of piles of rock and dust.)
    • Could they be dangerous? (Yes, if they hit the Earth.)
    • Share that we are studying asteroids so that we can safely move them away from Earth’s orbit.
    Facilitator’s Note: : If a child or other participant’s statements are incorrect, consider asking additional questions to the group that allow them to share what they know, rather than immediately correcting them. As much as possible, encourage the children to offer information and to respond to others’ questions, rather than answering them yourself. Use phrases like “What do the rest of you think?” “Do you agree with ____?” “Do you have any additional ideas?”
  2. Share that NASA’s missions and observatories are studying asteroids and collecting information about them.
    • Are there different tools we can use to study asteroids? (We can use telescopes and can send robotic spacecraft to asteroids.)
    • How can we study the inside of an asteroid? (No instruments can "see" inside an asteroid. We need to use indirect methods to study asteroid interiors.)
    • Why is it important that we know what asteroids are made of? (Our plans to deflect asteroids may work differently, depending on whether they are solid or not.)
    • Describe OSIRIS-REx and NEOWISE missions: The OSIRIS-REx mission is studying the asteroid Bennu, and measuring its physical properties, including its internal structure. OSIRIS-REx will return a sample of Bennu’s surface to the Earth in 2023 for study. The NEOWISE project is a telescope orbiting the Earth that is finding warm asteroids by the infrared energy they are emitting into space. Many other telescopes on Earth are also collecting data about asteroids. These missions and observatories can measure sizes, positions, and even temperatures!
  3. Tell the children that they are going to explore models of asteroids, to understand how we can learn about the insides of asteroids.
    • Explain that some of the models are made of sand, and others are made of rock, but they all look similar on the outside.
    • How could we tell which ones are made of sand and which are rock? Accept all answers, adding that scientists can’t break open or even touch most asteroids in real life.
    • Which warms up faster: a glass filled with small pieces of ice, or a glass filled with a large solid piece of ice? (Smaller pieces of ice warm up faster.) If the children struggle with this, ask which cools off faster from the oven: cupcakes or a single large cake? (Small cupcakes will cool off faster.) Invite the children to share their experiences with cooking (or eating) food that warms up faster or cools off faster. Repeat at the end that a group of smaller pieces will cool off or warm up faster than a larger object.
    • How can we determine if our asteroid models are made of lots of small pieces of sand or if they’re made of solid rock? (We can measure how quickly they warm up or cool down.)
    • Let the children know that you have frozen the models of asteroids, and they will measure their temperatures over the next 10 minutes as the asteroids warm up.
  4. Give each child or group of children a Solid or Rubble Notes sheet and pen or pencil, and a laser infrared thermometer.
    • Divide the children into groups. Each group will measure at least one asteroid model (or each group could move around and measure all of the asteroid models).
    • Tell the children the sizes you recorded for each of the asteroid models, and ask them to write the sizes for each of the models they will measure on their notes sheet.
    • Show the children how to take a temperature using the thermometers. If the thermometer uses a laser that could cause eye damage, carefully monitor the children’s uses of it and reiterate that they cannot point it at anyone’s face. If children are mature enough to use the thermometer safely, have them take turns practicing taking the temperature of the tables, windows, and other objects in the room; otherwise, model how the thermometer works by taking and sharing the temperatures of objects in the room for the children.
  5. Take the asteroid models out of the freezer or cooler and place them on tables.
    • Do not let the children pick up or handle the asteroids. (Doing so will affect the temperature measurements.)
    • Either invite the children to take turns using the thermometers to take the temperatures of the asteroids every minute or two or take them yourself. Ask the children to record the temperatures of the asteroid models on the Data Sheet. Note: Temperatures will be different on different sides of the “asteroids.” Take the temperatures at the center of the top surface or as close to that point as possible.
    • Halt the measurements after 10 minutes or when the asteroids reach room temperature.
  6. Invite each group to share their data and observations with the others.
    Discuss their initial findings:
    • Did all of the asteroids warm up at the same rate?
    • Did the size of the asteroids make a difference?
    • What ideas do they have about why some asteroids warmed up faster than others?
    Invite the children to pick up the asteroid models, and compare their weight.
    • Now, why do they think some asteroids warmed up faster than others?
    • Which asteroid models do they think were made of sand?


Thank the children for their research! Ask whether it was easy to determine which asteroid models were made of rocks and which were made of sand. Ask whether they were surprised by their findings. Invite the children to share how it felt to be conducting research and making discoveries.

Reiterate that scientists are studying the characteristics of asteroids, including determining which ones are likely to be solid and which are likely to be rubble piles, to learn more about our solar system and to help up understand how to defend Earth from asteroids.

Consider following up with other Planetary Defenders activities, such as the Space Rocks game or Edible Rocks.

Activity Background Information:

This activity serves as a model of how we study planetary objects. Scientists are able to directly observe some of an object's characteristics, such as location in the solar system, size, mass, external composition, and more.

Scientists study the interiors through models they create, which are based on an object's observable characteristics. Some asteroids, particularly larger asteroids, have solid interiors (sometimes called monolithic). Other asteroids, typically smaller asteroids, are not solid. Rather they are collections of smaller individual boulders held together by gravity and friction. These asteroids are commonly referred to as “rubble piles.” One method for determining if an asteroid is solid all the way through or is a “rubble pile” is to determine the asteroid’s thermal inertia, or its resistance to temperature changes which it experiences as it rotates (and alternate sides face the Sun) and as its orbit carries it closer to and further from the Sun. Solid materials, like a rock, typically take longer to heat-up and cool down than more porous materials, like a pile of gravel.