How can astronomers find out whether a large object
from space is going to strike our planet?
View the video segment
“Big Dish” to learn how
astronomers use the
largest radio telescope
on Earth.
662 Unit 6: Space Science
The streak of light in the photograph above was produced by a tiny
particle from space burning up in Earth’s atmosphere. Shown to the
left is Barringer Crater in Arizona.
Collisions in Space
In the summer of 1994, telescopes all over the world were aimed
at Jupiter. For the first time in history, astronomers had warning
of a collision in space. Jupiter’s gravity had split a comet named
Shoemaker-Levy 9 into more than 20 large pieces. As the rocky
objects collided with Jupiter’s atmosphere, they exploded
spectacularly.
Astronomers have found evidence of impacts closer to home.
The craters that cover much of the Moon’s surface were caused
by collisions with space objects billions of years ago. In 1953 an
astronomer even caught on film the bright flash of an object hitting
the Moon. Other solid bodies in space also have impact craters.
Little evidence of impacts remains on Earth because its surface is
always changing. Fewer than 200 craters are still visible.
Earth’s atmosphere protects us from collisions with small
objects, which burn up in the air. However, when a large object
strikes Earth, the atmosphere can spread the effects of the impact
far beyond the crater. A large collision may throw dust high into
the air, where it can be carried around the globe. The dust can
block sunlight for months and sharply lower global temperatures.
About 65 million years ago, a large space object struck Earth.
The dust from this collision can be found around the world in a
layer of rock that was forming at the time. At about the same time,
most species of organisms died out, including the dinosaurs. Many
scientists think that the collision caused this global devastation.
Frontiers in Science 663
The Risk of a Major Collision
When will the next space object hit Earth? A
collision is probably occurring as you read this
sentence. Tiny particles hit Earth’s atmosphere all
the time. Some of these particles have enough
mass to make it through the atmosphere. Objects
that reach Earth’s surface are called meteorites.
Most meteorites splash harmlessly into the ocean
or hit unpopulated areas. Every few years a
meteorite damages a home or other property.
However, there is no known case of a meteorite’s
killing a person.
Collisions that cause widespread damage
happen less often because the solar system
contains fewer large objects. In 1908 a large
object from space exploded above a remote
region of Russia. The explosion knocked down
trees across an area more than half the size of
Rhode Island. Even this impact was small in comparison with major collisions that affect the
entire world. Such collisions happen on average
about twice every million years. Events that kill
off many species occur even less often.
View the “Big Dish” segment
of your Scientific American
Frontiers video to learn how
astronomers are using the giant
Arecibo radio telescope to
explore the universe.
IN THIS SCENE FROM THE VIDEO
You see a close-up of the
Arecibo telescope’s dome and
one of its antennas.
EXPLORING ASTEROIDS An asteroid’s crashing
into Earth may seem like the subject of a science
fiction movie. Yet asteroids pose a real danger to
humans. Some asteroids could cause widespread
destruction if they struck our planet.
664 Unit 6: Space Science
Tracking Asteroids
Although Earth is unlikely to have a major collision
with a space object anytime soon, the danger is
too great to ignore. Scientists are using telescopes
to find large, rocky space objects called asteroids.
After locating an asteroid, they use computer
models to predict its path centuries into the future.
Scientists expect that by 2008 they will have found
almost all of the asteroids that could cause global
devastation on Earth.
Locating objects that may threaten life on
Earth is just the first step. Scientists also want to
Astronomers are tracking
these asteroids to determine how close they will
pass to Earth in the future.
Asteroids are too faint
to be viewed clearly with
optical telescopes on
Earth. However, radio
telescopes can provide
detailed images of asteroids. Inside the dome of
the Arecibo telescope is
the world’s most powerful
radar transmitter. The transmitter can bounce a
beam of radio waves off the telescope’s dish to
reach an asteroid millions of miles away. The
telescope picks up returning signals, which are
converted into images.
As you study this unit, work alone or
with a group on one of these projects.
Shown at left is the Arecibo
telescope. Data collected with
the telescope were used to make
this model of the asteroid
Toutatis.
Observe the Sky
Choose a space object or part of the
distant sky to observe over a month.
Keep an observation journal of what
you see and think.
• Pay special attention to any changes
relative to other objects in the sky.
learn about the characteristics of asteroids. The
Arecibo telescope in Puerto Rico is an important
tool for studying asteroids. The largest radio dish
in the world, it allows scientists to determine the
motions and shapes of asteroids. Computer models
and tests with real materials provide additional
information about the mass, materials, and
structure of each asteroid.
If scientists ever find an asteroid headed
toward Earth, these studies may help us change
the asteroid’s course safely. Remember the comet
that struck Jupiter in many pieces? If an asteroid
broke apart before reaching Earth, pieces hitting
different locations could cause even more damage than a single impact. Before using a bomb or
laser to change the course of an asteroid, governments must make sure that the asteroid will not
break apart. Fortunately, scientists would have
decades to study a dangerous asteroid and figure
out what action to take.
Scientists are learning about the risk of an asteroid’s colliding with Earth. The more we learn about
collisions in space, the more questions we have.
• What methods can be used to change the
course of an asteroid that threatens Earth?
• How can we make sure that an asteroid will
not break apart because of our efforts to
change its course?
• How many smaller but still dangerous objects
may be headed toward Earth?
• Look up information or construct tools
to help you observe.
• Copy your best drawings for a display
board. Explain your observations.
Multimedia Presentation
The Arecibo telescope is not used only for
studying asteroids. Prepare a multimedia
presentation on other research that is
being carried out with the giant radio
telescope.
• Find information about the research
from Internet sites and other sources.
• Prepare both audio and visual components for your presentation.
Map a Space Object
Use a large potato to represent a newly
explored space object. Draw lines of
latitude and longitude. Then identify
features, and make a flat map.
• Use roller-ball pens to mark poles, an
equator, and lines of longitude and latitude. Try not to pierce the potato’s skin.
• Do the potato’s eyes seem like craters
or volcanoes? Decide how to name the
different types of features.
• Make a flat map of the space object.
CAREER CENTER
CLASSZONE.COM
Learn about careers in astronomy.
Frontiers in Science 665