Near-Earth objects
Some scientists believe a huge crater, beneath the ocean in the Gulf of Mexico, is proof that 65 million years ago an asteroid collided with Earth, wiping out the dinosaurs and most other life on this planet.
Scientists take the threat of another devastating impact very seriously. An asteroid one kilometre in diameter could kill a billion people. The last asteroid of this size collided with Earth 900,000 years ago.
Asteroids of 100-250m in diameter are more frequent and a direct impact could destroy a city. A 100m rock strikes on average every 50 years, a 250m rock every 3000 years.
Scientists can precisely predict the time and location of potential impacts and there is a realistic chance that disaster could be averted by deflecting the rock away from a collision course.
The Near-Earth Objects Information Centre in Leicester, UK, is monitoring asteroids and comets large enough to cause substantial damage to planet Earth.