ESA Science & Technology05-Jul-2005 16:50:09

Asteroid Encounters

Near Earth Asteroids

When asteroids collide, some of them may be knocked out of the Main Belt and into the inner Solar System. Others may be disturbed by Jupiter's gravity. These objects cross the orbits of Mars and the Earth, occasionally striking the planets themselves.

Studies of these stray asteroids, or fragments of asteroids, are particularly important for the future of humanity, since many such objects have struck the Earth in the past, altering geological history and the evolution of life. Over 150 large impact craters have been identified on the Earth, but many more may lie hidden on the ocean floor. Many scientists believe that one 10 km-wide object caused the extinction of the dinosaurs when it struck the Gulf of Mexico 65 million years ago.

One of the most recent impact events occurred on 30 June 1908, when a small asteroid 100 m in diameter exploded over the remote region of Tunguska in Siberia, devastating more than half a million acres of forest.

Near misses are taking place all of the time. One close call occurred on 23 March 1989, when a 400 m wide asteroid came within 640 000 km of Earth. Surprised scientists estimated that Earth and the asteroid, which weighed 50 million tonnes and was travelling at 74 000 km per hour, had passed the same point in space just six hours apart.

Until recently, scientists thought the population of large near-Earth asteroids with diameters of more than one km was between 1000 and 2000. However, recent studies suggest that there are between 500 and 1000 in this size category.

In the past, astronomers found asteroids by looking for streaks on photographic plates of the night sky. Today, most near-Earth asteroids are detected using robotic cameras such as a 1.2 m U.S. Air Force telescope on top of Mount Haleakala on Maui, Hawaii. Data gathered by the asteroid tracking system are processed by computer.

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