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It's out there, lurking. It's several miles wide. It's solid stone and iron, and it's hurtling through space on a collision course with Earth. Sometime in the future it will enter the Earths atmosphere traveling 50,000 miles an hour. It will heat up to 5000 degrees as it cuts its way through the atmosphere and takes aim at the planet surface. Pressure from atmospheric gasses will prevent it from flying apart, but it will build up almost inconceivable pressure as it gets hotter and hotter. On impact it will knife its way deep into the ground, and then explode with the force of millions of atomic bombs, blowing superheated dirt and debris into the atmosphere, choking off the sun and setting everything in a 500 mile radius on fire. Virtually every living thing on Earth will die in the devastation that ensues. It happened 65 million years ago and it will certainly happen again, unless we stop it. Fortunately, asteroid impacts are the one type of natural disaster that are actually preventable.

There are literally millions of asteroids in our solar system, ranging in size from 1000 kilometers to a few centimeters across. The vast majority are confined to an orbit that lies between Mars and Jupiter known as the asteroid belt, and pose no threat to the Earth. The problem is the rogue asteroid in an orbit around the sun that brings it close enough to the Earth to threaten impact, and there are many thousands of those.

The Earth is constantly bombarded by debris from space. Scientists estimate that three to four million pieces enter our atmosphere every year, and virtually all burn up before they reach the planet surface. An asteroid has to be at least 40 meters across to have any chance of making it all the way to the ground. The larger the asteroid the more significant the damage at the impact site will be. In 1908 an asteroid about 50 meters wide exploded over an area known as Tunguska in Siberia. It devastated an area of about 700 square miles. The impact of an object over one kilometer in diameter will throw enough debris into the atmosphere to block the sun and create an "impact winter". Anyone not killed in the initial impact will slowly starve, as the cold and darkness destroy crops, and society breaks down under the strain.

Until recently the possibility of an asteroid impact received very little attention from scientists. After it was discovered that such an impact probably wiped out the dinosaurs researchers began investigating the likelihood of a

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