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NOVA scienceNOW: Asteroid
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Viewing Ideas
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Before Watching
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.)
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)
|
Planet* |
Distance
from Sun
(AU)
|
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
|
Mercury |
0.4 |
0 |
4
+ 0 = 4
|
0.4 |
Venus |
0.7 |
3 |
4
+ 3 = 7
|
0.7 | |
Earth |
1.00 |
6 |
4
+ 6 = 10
|
1.0 | |
Mars |
1.5 |
12 |
4
+ 12 = 16
|
1.6 | |
No
planet
|
N/A |
24 |
4
+ 24 = 28
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2.8 | |
Jupiter |
5.2 |
48 |
4
+ 48 = 52
|
5.2 | |
Saturn |
9.6 |
96 |
4
+ 96 = 100
|
10.0 | |
Uranus |
19.2 |
192 |
4
+ 192 = 196
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19.6 | |
Neptune |
30.1 |
384 |
4
+ 384 = 388
|
38.8 | |
Pluto* |
39.2 |
776 |
4
+ 776 = 780
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78.0 | |
*
Pluto is now reclassified as a "dwarf planet." However, it is still useful to
include it in a discussion of the Titius-Bode Law.
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
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.
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.
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
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Approximate
Diameter (km)
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Diameter
Ratio
(compared to the moon)
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Mass
(kg)
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Mass
Ratio
(compared to the moon)
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Moon |
3,476 |
1.00 |
734.9
x 1020
|
1.00 |
Earth |
12,756 |
3.67 |
5980
x 1020
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81.37 |
Ceres
(largest asteroid)
|
933 |
0.27 |
8.7
x 1020
|
0.01 |
All
Asteroids in Asteroid Belt together
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1,500
km
|
0.43 |
23
x 1020
|
0.03 |
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.
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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
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 photojournal.jpl.nasa.gov/index.html 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
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525
km in diameter
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Molten
rock, olivine, chipped surface exposing rocky mantle
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Near-Earth
Asteroid
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1807 |
243
Ida
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Has
a moon and a heavily cratered surface
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58
x 23
|
Nickel-iron
and some silicates
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Main
Asteroid Belt
|
1880 |
433
Eros
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Peanut-shaped,
boulders on surface, giant gouge
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33
x 13 x 13
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Has
grooves, layers & boulders, may be made of stony iron
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Near-Earth
Asteroid
|
1898 |
951Gaspra |
Covered
with impact craters, mitten-like shape
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19
x 12 x 11
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Mixture
of rocky metallic minerals
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Main
Asteroid Belt
|
1916 |
4769
Castalia
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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
|
1989 |
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.
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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.
Web Sites
Asteroid and Comet Impact Hazards
impact.arc.nasa.gov/torino.cfm
Contains a chart of the Torino Impact Scale and describes why the scale is
important.
Asteroids
www.seds.org/billa/tnp/asteroids.html
Describes specific asteroids, how they are categorized, and where they are
located in the solar system.
Terrestrial Impact Craters
www.lpi.usra.edu/publications/slidesets/craters/
Provides information about and slides of several terrestrial impact craters in
our solar system.
Virtual Lab
education.spacefoundation.org/apophis
Video-based lab activity based on saving Earth from a collision with Apophis.
Books
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.
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Get more information and activities about Apophis. Also,
participate in a Virtual Lab to save Earth from a collision with Apophis. Visit
the Space Foundation's Apophis Education Web site at
education.spacefoundation.org/apophis.
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