London missed obliteration by four hours

IF YOU were in London on 30 June 1908, you are astronomically lucky to be alive. That day a comet vaporised over Tunguska, Siberia, releasing as much energy as a 12-megaton atom bomb - 1,000 times more powerful than Hiroshima's blast.

Had the comet entered the atmosphere four hours earlier it would have exploded over the British capital, which lies on the same latitude, and killed millions of people. In Siberia, 2,000 sq km of forest were destroyed.

Such "comet hits" are not rare, occurring on average about once every 100 years. We may get only one year's warning that such an object is threatening the Earth, and none if it approaches us from the southern part of the sky, said Dr Matthew Genge of the Natural History Museum.

"The effect of a comet breaking up in the atmosphere would be like a nuclear airburst," said Dr Genge. "In Tunguska, the comet exploded 4km (2.5 miles) above the ground, and the trees were burnt for miles around."

Had that comet, which was about 70m in diameter, fallen towards London, people on the ground would first have seen bright lights in the sky. Moments later the heat from the comet's friction with the air would have meant anything combustible - including skin - would have burst into flame.

Within seconds the ice at its heart would have turned into superheated steam and exploded, sending out a shockwave and flattening buildings for miles. The whole event would take less than a minute.

"It would have left no crater, because nothing would have actually hit the Earth," said Dr Genge. "It would have posed quite a problem trying to understand what had happened."

The Tunguska event is thought to have killed only a few woodcutters. It still counts as one of modern humanity's narrowest escapes from extraterrestrial destruction. Comets, made of ice and mud, pose a greater risk to us than asteroids, which are usually solid iron or rock and would fall without breaking up.

"We know that fragments of comets about 10m across enter the atmosphere four or five times a year, because US military spy satellites began detecting them - and mistaking them for 20-kiloton nuclear bombs going off - in the 1970s," said Dr Genge.