CHRIS BUTLER / SCIENCE PHOTO LIBRARY CHRIS BUTLER / SCIENCE PHOTO LIBRARY
Asteroid approaching Earth. Illustration of an asteroid passing close to Earth. The asteroids (also called minor planets) are lumps of rock measuring 1-1000km wide. Over 4000 have been catalogued so far, but astronomers reckon the Solar System contains over 40,000. Some asteroids follow an elliptical orbit, occasionally passing dangerously close to Earth. In 1989 a 300 metre wide asteroid came within 690,000km of Earth. If a rock of this size hit the planet it could explode with the force of 100,000 nuclear bombs.
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