Fortunately,
Asteroids and other NEO (Near Earth Objects) have a very small
chance of actually colliding with the Earth because the magnitude
of the disaster could be enormous - ranging from the destruction
of a small city to a large percentage of the planet. Scientists
at Sandia National Laboratories simulated
what would happen if a 1-kilometer asteroid or comet struck
the ocean (here is a short animation).
Their calculations showed that the energy released would be
equivalent to 300 gigatons of TNT or about 10 times the explosive
power of ALL nuclear weapons on Earth during the height of
the cold war. While scientists say there is very little chance
of such an event, similar impacts have happened in the past.
For example, scientists believe that a large impact approximately
65 million years ago resulted in planetary weather changes
that eventually wiped out the dinosaurs.
Small
objects such as dust particles and other small pieces of debris
do hit the Earth's atmosphere thousands of times every day.
Most of these are so small they burn up before reaching land.
You can see this happen at night when you see a "shooting
star" but it happens during the day too. At certain times
of the year, the Earth passes through a large debris field
left by a passing comet. For example, the Perseids
(remnants of the comet Swift-Tuttle) can be seen every year
in August and are a popular meteor display.
Sometimes
somewhat larger objects do not burn up entirely and actually
make it to the surface. These are called meteorites and usually
come from asteroids and sometime comets. Most of these fall
in the oceans. Since about 75% of the Earth is water, there
is a 75% chance that any meteor will not fall on land. However,
some do. One of the most famous is "Meteor Crater"
(or the Barringer
Meteorite Crater) in Winslow, Arizona. Around 50,000 years
ago a large meteor about 150 feet in diameter and weighing
around 300,000 tons crashed into the ground in what is now
Arizona with an explosive force equal to about 20,000,000
(20 million) tons of TNT. It left a large hole or crater a
little under a mile in diameter and 570 feet deep.
