Killer asteroids are not just the invention of disaster movies. Asteroid strikes are real possibilities, and they have actually happened in the past. The chances are small that we will get hit, but if the asteroid was large enough, it could do enough damage to end life on Earth.
Many scientists believe that a collision with an asteroid or a comet may have been responsible for the extinction of dinosaurs. A giant crater on the Yucatán Peninsula in Mexico marks the spot where a comet or asteroid struck Earth about 65 million years ago. This is about the same time as the disappearance of the last of the dinosaurs. A collision with an asteroid large enough to make the Yucatán crater would have sent so much dust and gas into the atmosphere that sunlight would have been blocked for months or years, killing much of Earth’s vegetation and interrupting the food chain the dinosaurs depended on.
The most recent major encounter was in 1908 in Siberia. The force of the encounter flattened over 500,000 acres of pine forests and killed thousands of reindeer.
Most asteroids are in the Asteroid Belt between the orbits of Mars and Jupiter. It is the home to thousands of small, rocky bodies that orbit the Sun. These asteroids are also called "minor planets," and they are thought to be remains of a planet that tried to form millions of years ago.
An asteroid can leave its orbit and travel into the inner solar system. This change may be caused by a collision with another asteroid, the gravitational pull from Mars, or the Sun’s heat.
Tracking Down Asteroids
In 1995, NASA and the U.S. Air Force began a project called Near-Earth Asteroid Tracking (NEAT). NEAT has an observatory in Hawaii to search for asteroids with orbits that might threaten Earth. Astronomers have found more than 200 asteroids with orbits that cross Earth’s orbit. Some scientists think several thousand of these Earth-crossing asteroids may exist and that as many as 1,500 could be large enough to cause a global disaster if they collided with Earth. Still, the chances of such a collision average out to only one collision about every 300,000 years. Astronomers believe that tracking programs like NEAT would probably help the world by giving us decades or even centuries of warning time before the asteroid hits.
Scientists have suggested several strategies for knocking asteroids off their course. If the asteroid is far enough, we could use a nuclear warhead to blow it up without much danger of pieces of the asteroid causing significant damage to Earth. Another way is to attach a rocket engine to the asteroid and direct the asteroid off course without breaking it up. Both of these methods require that the asteroid be far away from Earth. If an asteroid exploded near Earth, chunks of the asteroid would probably still cause major damage. Any effort to push the asteroid off course would also require years of work. Asteroids are much too big for a rocket to push quickly.
Level of Asteroid Damage
Unless otherwise noted, all images courtesy of NASA. Permission for use at http://www.nasa.gov/gallery/photo/guideline.html.
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