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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
_______ minutes _______ seconds
590 words
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block out --- to cover from sight
38
visible --- able to be seen
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remote --- isolated; far
from other people or things
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vibrations --- small movements, like shaking
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vaporize --- to change to gas form
