Thousands of meteorites enter Earth's atmosphere and fall harmlessly to the ground each year. One widely accepted theory is that the impact of an asteroid 6 miles (10kilometers) in diameter could have been responsible for the extinction of many life forms, including the dinosaurs 65 milion years ago.
On June 30, 1908,a small asteroid of approximately 200 feet (60 meters) in diameter exploded at an altitude of 5 miles (8 kilometers) over the remote region of Tunguska in Siberia. The explosive energy from the meteor was so great that half a milion acres of Siberian forest were devastated, and some trees were knocked over at distances as far as 25 miles (40 kilometers) away.
In order to protect against such events (or at least reduce their effects), NASA has developed ways to monitor Near-Earth Objects (NEOs). For example, the Near-Earth Asteroid Rendezvous (NEAR) is spacecraft that monitors Eros, one of the largest asteroids being studied. Eros is scheduled for a near-Earth approach on January 31, 2012. This huge flying rock is the shape of a potato, and its size is 170 square miles (442 square kilometers). Scientists believe that it might strike Earth in 1.5 to 5 milion years. The probability of one of these Near-Earth Objacts hitting Eath over the next hundred years is virtually negligible. However, scientists are concerned, because at present, there are adout 900 asteroids circulating our solar system. If an asteroid the size of Eros fell on Eath in the middle of the Atlantic, it would have catastrophic effects.
But there are alos other hazards from space. Thousands of nuts, bolts, and other debris from space missions form an orbiting garbage dump. Some of these bits and pieces fall back into Earth's atmosphere and behave just like any other meteor, lighting up the sky like a "falling star." According to NASA, there must be over 100,000 objects with a daimeter of one to ten centimeters floating around speac. The number of fragments less than one centimeter could be in the milions. The U.S. Space Command is awaer of the threat of an orbital impact during space missions and monitors this junk constantly. The agency estimates that there might be more than 3,000 satellites orbiting Earth. In February 2008, for example, the United States blew up a satellite that contained toxic fuel. It was exploded over the Pacific Ocean so that it could not fall and humans. Some scientists speculate that one day there might be orbiting "garbage collector" machines to pick up the debris.
Since the space surveillance mission begen, almosphere either burn up or reenter over water. As a result, the risk of someone getting hit on the head by one of these objects is very remote. Nearly three-quarters of the planet is covered by water, and large areas of Earth's land are uninhabited.