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A meteoroid striking the Earth becomes a meteor or "shooting
star" as it burns up in the atmosphere. Most meteors are from
particles the size of a sand grain, but occasionally larger
objects enter the atmosphere and produce a spectacular display
called a fireball. If the object is large enough, it can survive
its passage through the atmosphere and end up on the ground as a
meteorite. When the Earth encounters a rich lode of concentrated
debris left over from a comet, a meteor shower occurs, and there
are several well known meteor showers visible throughout the
year.
The Moon and other Solar System bodies are heavily cratered from
massive bombardment of asteroids and comets in their past. Such
heavy bombardment is rare today, but a modest sized asteroid or
comet impact is still possible, and it could wreck havoc on the
Earth, even ending civilization as we know it (Cook, 2004;
Morrison).The primary risk to the globe and its major
ecosystems is from an object large enough to disturb the Earth’s
climate by injecting massive quantities of dust into the
stratosphere. A large object striking the Earth at 20 +
kilometers per second (a typical meteoroid velocity) has an
enormous kinetic energy equal to its mass times its velocity
squared. It would vaporize upon contact and excavate a large
crater. The object’s vapor and the material excavated from the
crater would then be thrown into the atmosphere creating a
suffocating mass of dust that rapidly envelopes the globe. This
dust could block sunlight for months lowering temperatures
around the world causing worldwide crop failures and global
starvation. An asteroid or comet mass of several billion tons
entering the atmosphere at 10-60 km/sec would be necessary to
cause such destruction. This would be the equivalent of a
million megaton explosion of TNT.
An asteroid 1-2 kilometers in diameter is large enough to
render such havoc, and smaller objects in the order of tens of
meters in diameter could easily destroy a large metropolitan
area (Morrison). The crater record on the Earth is sparse,
because plate tectonics and weathering erase most craters in
short geological time spans. It took until the 1960’s for
geologists to completely accept that some craters on the Earth
were formed by impacts (Barringer). To estimate the Earth impact
rate for various sized bodies, a number of different parameters
need to be examined. These include counts of meteor craters on
the Moon, paleontological evidence of mass extinctions on the
Earth, studies of orbits of asteroids and comets, and satellite
measurements of explosions in the upper atmosphere from large
meteoroids (Cooke, 2004). |
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