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NOVA scienceNOW: Asteroid

Viewing Ideas

Before Watching

  1. Use a concept map to review asteroid-related terms. Concept maps are a way to visually show how the parts of a system relate to one another. In a concept map, nouns are used to describe the components of the system (i.e., the vocabulary term). The relationship between the different components is shown by arrows, which connect the parts. Each noun is put in a box, and the arrows are labeled with a verb describing the relationship between components. Have student pairs find the definitions for the following terms in their textbook (or other resource). As a class, discuss each term. Then, have students create a concept map that shows the relationships among the terms.

    asteroids: Rocky objects orbiting the sun that are too small to be planets. Their size can range from 10 meters in diameter to 933 kilometers in diameter. Many are not spherical. Most are found in the Asteroid Belt, located between Mars and Jupiter.

    comets: Chunks of frozen gases, ice, dust, and rocky debris usually with a nucleus between 1-10 kilometers in diameter surrounded by a gas cloud. They revolve around the sun in highly elliptical orbits. A comet's tail is made of gas and dust that has been driven off its surface by the sun's energy.

    meteoroid: A sand- to boulder-sized fragment of solid matter that drifts through the solar system rather than traveling in a regular orbit around the sun.

    meteor: When a meteoroid enters Earth's (or any celestial body's) atmosphere, it heats up and partially or completely vaporizes. The vapor trail of the disintegrating meteoroid glows. This trail of glowing vapor is called a meteor, also commonly known as a shooting star.

    meteorite: The solid portion of a meteor that survives the journey through the atmosphere and reaches the surface.

    moon: A natural satellite orbiting a planet. Moons are smaller than the planets they orbit. Our moon is thought to have formed from the rocky debris ejected when an object the size of Mars collided with Earth. Gravity eventually aggregated the debris into our moon. Some moons are thought to be asteroids captured by planets.

    planet: The International Astronomical Union recently set out three criteria for a celestial body to be called a planet: (1) it must orbit the sun; (2) it must have pulled itself into a spherical shape by its own gravity; and (3) it must have cleared other things out of the way in its orbital neighborhood. By this definition, Pluto, Xena, and Ceres are not planets. Pluto and Xena orbit among the icy bodies in the Kuiper Belt. Ceres orbits among asteroids in the asteroid belt.)

  2. Show that the spacing of the planets' orbits follows a pattern. When investigating a natural phenomenon, such as planetary motion or weather, finding a repeating pattern is an important discovery. A predictable pattern suggests that there is an underlying order that can be understood and modeled. It also enables you to test whether the pattern can explain other events. In the late 18th century, astronomer Johann Bode and mathematician Johann Titius found that the planets' orbits around the sun were spaced according to the mathematical relationship: Distance from one planet to the next = (n + 4) / 10 where n= 0, 3, 6, 12, 24, 48 ... Provide students the following chart. Have them plot the planet distances from the sun in astronomical units (AU) against the Titius-Bode values. (One AU is the distance from Earth to the sun, 93 million miles.) Then have them answer the three questions below.

    Titius-Bode Law
    (relates the average distances of the planets from the sun
    to a simple progression of numbers)


    Distance from Sun

    Titius-Bode's Sequence of Numbers
    (except for the first two, twice the value of the preceding number)

    Add 4
    (representing the orbit of Mercury in AU)

    Divide by 10




    4 + 0 = 4





    4 + 3 = 7





    4 + 6 = 10





    4 + 12 = 16


    No planet



    4 + 24 = 28





    4 + 48 = 52





    4 + 96 = 100





    4 + 192 = 196





    4 + 384 = 388





    4 + 776 = 780


    * Pluto is now reclassified as a "dwarf planet." However, it is still useful to include it in a discussion of the Titius-Bode Law.

    1. Which planetary orbits match up well with the orbits predicted by the Titius-Bode Law? Mercury, Venus, Earth, Mars, Jupiter, Saturn, and Uranus. Which ones do not? Pluto and Neptune

    2. What evidence suggests that the Titius-Bode Law is imperfect? The Titius-Bode Law does not predict a planet located in Neptune's orbit. Instead, it predicts that Pluto should be the next planet after Uranus. Neptune wasn't discovered until 1846, more than 60 years after Titius and Bode presented their model. So until that discovery, the Titius-Bode Law was considered very reliable.

    3. The first asteroid was discovered in the early 19th century, decades after Titius and Bode developed their law. Yet, how does the Titius-Bode Law anticipate the existence of the Asteroid Belt, the cloud of rocks orbiting between Mars and Jupiter? The sequence predicts a planet 2.8 AU from the sun. Eventually, astronomers found objects much smaller than a planet—asteroids. Asteroids range in size from the size of a peanut to Ceres, the largest asteroid, at 933 kilometers in diameter. Astronomers named this part of the solar system the Asteroid Belt and estimate that it contains from thousands to millions of asteroids. The general consensus is that Jupiter's considerable gravity prevented the asteroids from coalescing into a planet.

  3. Compare sizes of asteroids. Asteroids range in size from 10 meters to 933 kilometers in diameter. To help students understand the difference in size, divide the class into groups and have students calculate diameter and mass ratios of the bodies relative to that of the moon. Then ask groups to use the diameter-ratio calculations to draw (using a compass) and cut out paper circles of similar diameter ratios to represent the astronomical objects listed in the table below. Have teams share their ratio models.

    How Diameter and Mass Compare to the Moon's Diameter and Mass

    Approximate Diameter (km)

    Diameter Ratio
    (compared to the moon)

    Mass (kg)

    Mass Ratio
    (compared to the moon)




    734.9 x 1020





    5980 x 1020


    Ceres (largest asteroid)



    8.7 x 1020


    All Asteroids in Asteroid Belt together

    1,500 km


    23 x 1020


    Next, model the mass ratios. Review the mass ratios that students calculated. Then have students label plates with the names of the celestial bodies. Fill each plate with salt or sand to represent the mass ratios of the bodies on the labeled plates; you will need a large tray and container for Earth's mass. (If the moon equaled 1,000 grams, then Earth would equal 8,136 grams, Ceres would equal 10 grams, and all asteroids together would equal 30 grams—smaller than our moon!) Remind students that although asteroids are small relative to Earth, an impact can be catastrophic. For example, a collision with Apophis, just 1,000 feet wide, would release the energy of 100 nuclear bombs exploding simultaneously.

  4. Calculate how tightly clustered asteroids are. There may be over one million asteroids in the main Asteroid Belt. Though some asteroids are clustered, most are widely spaced, spread out over the vast expanse between Mars and Jupiter. To help students understand how far apart "main belt" asteroids are, tell them that, on average, each asteroid has over 1 million square kilometers (1000 kilometers x 1000 kilometers) to itself. Have student teams use atlases and find an area on Earth equal to 1 million square kilometers. Have each team describe the land or ocean included in the area they identified. Egypt is just under one million square kilometers.

After Watching

  1. Make asteroid models. Divide the class into teams. Assign each one a different asteroid (which are numbered by the order of their discovery). Have them research the attributes listed in the column headings and print or draw an image of their asteroid. For close-up images of asteroids, visit NASA's photojournal at and click on "small bodies." Give each team materials (newspaper or papier mâché, colored clay (brown, red, gray); colored sand or gravel (brown, black, white, gray, red); aluminum foil) and have each team make a scale model of their asteroid. (For consistency, have the class determine a common scale before making their models.) To make the asteroids as realistic as possible, have teams add as many details as they can, such as craters and other surface features. When teams are finished, have each one present their model and findings to the class.

    Asteroid Number and Name

    Shape and Special Features

    Size or
    Dimensions (km)

    Surface Features and Composition

    Location Grouping

    Year Discovered

    4 Vesta

    Corn-kernel-like shape, geologically diverse with light and dark areas, giant impact crater

    525 km in diameter

    Molten rock, olivine, chipped surface exposing rocky mantle

    Near-Earth Asteroid


    243 Ida

    Has a moon and a heavily cratered surface

    58 x 23

    Nickel-iron and some silicates

    Main Asteroid Belt


    433 Eros

    Peanut-shaped, boulders on surface, giant gouge

    33 x 13 x 13

    Has grooves, layers & boulders, may be made of stony iron

    Near-Earth Asteroid



    Covered with impact craters, mitten-like shape

    19 x 12 x 11

    Mixture of rocky metallic minerals

    Main Asteroid Belt


    4769 Castalia

    Has a rocking rotation and a dumbbell shaped

    Widest point is 1.8 km, 2 lobes are about 0.75 km across

    Both lobes have similar composition and roughness

    Near-Earth Asteroid


  2. Research Earth's impact craters. All terrestrial (i.e., rocky) bodies and their moons in the solar system have been bombarded with objects, such as meteoroids, asteroids, and comets. Ask students what evidence suggests that such bombardment actually happened. Impact craters are found on the terrestrial planets and on many moons, and some impacts have been observed as they happened, including the Shoemaker-Levy comet that hit Jupiter in 1994.

    Earth has some well-known impact craters such as Meteor Crater in Arizona, USA; Reis Crater in Germany; Sudbury Crater in Ontario, Canada; Manicouagan Crater in Quebec, Canada; and Chicxulub crater in Mexico's Yucatan coast. Ask students why many craters are visible on the moon but not as many are visible on Earth. (Earth's water and plant-life can obscure craters and, over time, weathering and geologic activity destroys the craters or makes them difficult to identify.) Divide the class into teams and assign each one a different crater to research. Have teams answer the questions below and share their findings with the class.

    • Describe the specific location of the impact.
    • What is the suspected origin of the crater?
    • When did the impact possibly occur?
    • What are the dimensions (diameter and volume if possible) of the crater?
    • What effect did the impact likely have on Earth, Earth's climate, and living things?
    • Describe any interesting features of the crater or of the impact.
  3. Discuss the probability that an asteroid will hit Earth. The probability of an asteroid hitting Earth is based on the number of estimated paths an asteroid could take that might lead to an impact. For example, if astronomers watch the movement of an asteroid and determine that only one out of a thousand possible paths would cause the asteroid to collide with Earth, then the odds of the asteroid hitting Earth are one thousand to one. Often, when an asteroid is first detected, it is far away and its orbit is poorly understood. However, the actual orbit of an asteroid becomes more certain after close monitoring for weeks or months. So, astronomers are able to refine their predictions about the chances of an asteroid hitting Earth after monitoring its orbit over time.

    Review probability by putting different-colored marbles (or jelly beans) into a jar. Use the following equation to calculate the chances of selecting a marble of a particular color.

    Number of chances possible for the event
    (e.g., the number of a certain color marble)

    Number of total chances
    (e.g., the total number of marbles)

    Remind students that the probability of an event happening is expressed as a fraction or decimal from zero to one. Zero probability means the event will not happen; one means it is certain to happen.

Links and Books

Web Sites

Asteroid and Comet Impact Hazards
Contains a chart of the Torino Impact Scale and describes why the scale is important.

Describes specific asteroids, how they are categorized, and where they are located in the solar system.

Terrestrial Impact Craters
Provides information about and slides of several terrestrial impact craters in our solar system.

Virtual Lab
Video-based lab activity based on saving Earth from a collision with Apophis.


The Atlas of Space
by Jack Challoner. Copper Beech, 2001.
Provides information about the Big Bang and the universe and includes several photographs.

DK Guide to Space
by Peter Bond. Dorling Kindersley Publisher, 1999.
Contains photos and illustrations of the celestial bodies in our solar system including moons, comets, and asteroids.

Eyewitness Astronomy
by Kristen Lippincott. Dorling Kindersley Publisher, 2000.
Covers aspects of the history of astronomy and includes information on space exploration.

Teacher's Guide
NOVA scienceNOW: Asteroid