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# Space Rocks: How Can Astronomers Determine the Shape of an Asteroid?

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Author: Janice VanCleave

### Problem

How can astronomers determine the shape of an asteroid?

### Materials

• transparent tape
• sheet of typing paper
• sheet of brown construction paper
• pencil
• flashlight

### Procedure

1. Tape the typing paper to a wall at about shoulder height. This will be your screen.
2. Crumple the brown paper into a loose ball.
3. Tape the ball of brown paper to the pencil.
4. Darken the room and hold the flashlight in one hand to the side of the paper screen with its bulb pointing away from the screen. The screen and the flashlight should be at an angle of about 45°.
5. Holding the pencil in the other hand, position the brown paper ball in front of the flashlight so that light reflects off the ball and onto the screen. Adjust the angle of the flashlight if necessary.
6. Slowly rotate the pencil and observe the light reflected on the screen.

### Results

The amount of light that is reflected changes as the paper ball rotates.

### Why?

Asteroids are relatively small, irregularly shaped, rocky chunks of matter which rotate as they orbit the Sun. They are also called minor planets. In the experiment, the brown paper ball represents an asteroid, and the flashlight represents the Sun. The brightness of the light reflected off the paper ball fluctuates (changes continuously). Reflected sunlight bounces in a similar way from an asteroid to Earth. This is because asteroids, like the paper ball, are irregularly shaped and rotate. Different parts of the asteroid reflect different amounts of light. Astronomers study the different amounts of light reflected by an asteroid to determine its shape.

### Let's Explore

1. The amount of light reflected from an asteroid gives clues to its composition. About 95 percent of asteroids can be separated into two classes by composition, bright and dark. The bright asteroids are called S-type asteroids (stony) and the dark asteroids are called C-type asteroids (carbon). Repeat the experiment, using the brown paper ball to represent an S-type asteroid and a black paper ball to represent a C-type. Observe the difference in the light intensity of the two colors.
2. About 5 percent of asteroids have some metal composition and are called M-type asteroids. Prepare an M-type by repeating the experiment using a piece of aluminum foil. Science Fair Hint: Use clay as a stand for the three types of asteroids. Fold an index card in half to make a stand-up label for each type. Use photographs of the asteroid models as part of your display.

### Show Time!

1. The largest asteroid, Ceres, has a diameter of about 600 miles (960 km). The state of Texas is about 740 miles (1,184 km) wide. Prepare a model to compare Ceres to the state of Texas. Cut out a road map of Texas and secure it to a piece of cardboard. Using the scale for the
2.
1. Most asteroids orbit the Sun in a region called the asteroid belt; which lies between Mars and Jupiter. This belt is between 197 million miles (315 million km) and 309 million miles (495 million km) from the Sun. Design a 3-D model to represent the location of the asteroid belt. The model could be cardboard with loops of string glued on to represent the orbits. For planets and the Sun, attach different sizes of painted Styrofoam balls that have been cut in half by an adult. Gravel can be glued on to represent the asteroid belt. For information about why asteroids are in the belt, see page 24 in Sten Odenwald's Astronomy Cafe (New York: Freeman, 1998).
2. Two groups of asteroids that do not lie in the asteroid belt, but move along the orbital path of Jupiter, are called Trojan asteroids. On Jupiter's orbital path, one of the groups lies about 60° in front of Jupiter and the other lies about 60° behind it. Add the Trojan asteroids to the model made in the previous experiment.

### Check It Out!

Not all asteroids are found beyond Mars. Some, such as the Apollo asteroids and the Aten asteroids, have orbits that cross Earth's. One theory of the extinction of dinosaurs is that a huge meteorite, which was all or part of an asteroid that crossed Earth's orbit, collided with Earth. Find out more about the effect of this collision. What evidence is there that a collision occurred? For information, see pages 113–114 in Isaac Asimov's Guide to Earth and Space (New York: Fawcett Crest, 1991).