S e c o n d e d I t I o n 1 Reading for the Real World 1

solar 
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
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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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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