Until recently, most astronomers thought the craters which cover the surface of the Moon were of volcanic origin. Although there are indeed a few volcanic features, it is now universally accepted that its appearance is the product of over four billion years of impacts. Over that period the size and frequency of these collisions has reduced... but not stopped.
During the last few years there has been considerable excitement at the confirmed sighting of the flashes of grit-sized meteoroids striking the Moon during the Leonid meteor shower. There has also been heightened awareness that much larger objects still lurk in our part of the solar system, and that the impacts of such objects with the Earth have caused devastating extinctions such as at the end of the Cretaceous and Permian eras.
The massive collision of comet Shoemaker-Levy with Jupiter showed how real the threat is, and slowly governments are awakening to the need to guard against such a catastrophe. This chilling artists impression illustrates the possible consequences of ignoring the warning.
Image: Don Davis, NASA.