Serendipity An explosive theory

LAST WEEK I visited the "Full Moon" exhibition at the Hayward Gallery in London, which displays just a tiny fraction of the 32,000 photographs taken by the Apollo astronauts between 1967 and 1972. It is a spectacular array of images, dismissing the myth that grey is necessarily dull. Unfortunately, the exhibition closes today, and so if you want to see it, you will have either to dash down to the South Bank this afternoon, or else buy the book, entitled Full Moon, by Michael Light.

While at the exhibition, I was reminded that the craters that litter the lunar landscape were at the centre of a vigorous debate that lasted for decades. The argument related to their origin. Initially, the majority of scientists argued that they were caused by volcanoes, but there were also those who favoured a theory based on meteorite impacts. The greatest problem for the supporters of the meteorite theory was that the meteorites would have struck the moon at a variety of angles, and it seemed obvious that a meteorite coming in from a low angle would generate an oval crater. In contrast, all the Moon's craters are circular, which only seems possible if all the meteorites fell vertically.

However, evidence in favour of the meteorite theory emerged by chance from the devastation of the First World War. It became clear that mines caused craters identical to lunar craters. For example, a mine that exploded at La Boiselle in 1916 formed a crater that was 85m in diameter and 25m deep, with a 5m tall crater rim. This led astronomers to speculate that meteorites had hit the Moon at high speed, had buried themselves underground, and then, regardless of their impact angle, they exploded like mines, creating circular craters. The theory was only proved correct when astronauts were able to examine the Moon's surface in 1969.

Thanks to Hollywood, we are now all aware of how meteorites and asteroids not only hit the Moon, but also hit the Earth. This summer, the International Astronomical Union met in Turin and defined the Torino scale, an astronomical version of the Richter scale. Asteroids are rated on a scale from zero to 10 - an asteroid rating zero is completely harmless, an asteroid rating five has a significant chance of hitting the Earth and causing a regional catastrophe, and an asteroid rating 10 has Armageddon written all over it, because it would be on a direct course with Earth and large enough to cause global devastation. So far, only two asteroids have registered on the Torino scale, 1997 XF11 and 1999 AN10. Each briefly scored one on the Torino scale, before further analysis meant that both could be downgraded to zero.

Simon Singh is the author of `The Code Book - the Science of Secrecy from Ancient Egypt to Quantum Cryptography', Fourth Estate, pounds 16.99