system, past the orbit of Saturn. There is also a set
of asteroids that orbit close to the Earth. These are
called NEAs (Near-Earth Asteroids). These are also
included in a group called Near-Earth Objects
(NEOs) which also includes comets. Because of
their closeness, NEAs are dangerous to the Earth.
More than half of all NEAs have been discovered. There are still many
asteroids that are unknown. There is no way to know when such asteroids could
strike the Earth. In 1989, a NEA passed very close. However, it was only
discovered as it was going away. If it had hit the Earth, it would have been a
complete surprise. It could have had 50,000 times the destructive force compared
to the atomic bomb dropped on Hiroshima. Though the possibility of an asteroid
striking the Earth soon is not great, it is definite that the Earth will be struck
sometime.
NEAs have struck the Earth before. Most scientists believe that a NEA caused
Track 31
3
Mars --- the fourth planet from the sun; the “red” planet
4
Jupiter --- the fifth planet from the sun; the largest planet
11
solar system --- the group of objects in space including the sun and
planets near Earth
12
Saturn --- the sixth planet from the sun; the planet with rings
16
comet --- a visible asteroid due to its “tail” of gases
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30
35
40
45
50
the extinction of the dinosaurs. This sort of asteroid
would have been one or more kilometers in diameter.
If it had hit land, it would have caused a huge cloud of
dust that would have blocked out all sunlight for at
least a year. This would have caused a short ice age,
leading to the deaths of numerous plants and animals.
If the asteroid had hit the water, which is more likely,
the cloud of water vapor it produced would have had a
similar effect. Further, the shock from the impact could
have caused volcanoes to erupt, adding to the dust and vapor around the Earth.
Scientists think these kinds of events happen every million years or so.
The outlines of the craters of some of these impacts are still visible from space.
The last time a large NEA struck the Earth was in 1908, in the forests of
Tunguska, Siberia. This asteroid was between 30 and 60 meters across and
exploded in the air instead of hitting the ground. This explosion was just as
powerful as a nuclear bomb. Because Tunguska was a very remote place,
no-one was killed. However, closer to the blast, windows were smashed, and
people could feel extreme heat. Vibrations were recorded in the Earth as far as
1,000 kilometers away. The forest near the blast site had been flattened as far as
30 kilometers from the center. Some trees were burned completely. The asteroid
itself had vaporized completely.
Though people may worry about large asteroids striking the Earth, they are
easy to detect. Smaller NEAs are much harder to find, and there are many more
of them. The history of asteroids and the Earth has not ended.
Reading Time
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590 words
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block out --- to cover from sight
38
visible --- able to be seen
42
remote --- isolated; far from other people or things
44
vibrations --- small movements, like shaking
47
vaporize --- to change to gas form
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eading Comprehension
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