A mysterious object that exploded high above the Tunguska river in Siberia in 1908 was a stony asteroid 30 metres in diameter, and not, as many thought, a comet. According to the calculations of Christopher Chyba of the NASA Goddard Space Flight Center in Greenbelt, Maryland, only a stony meteorite would explode at an altitude of 10 kilometres, the commonly agreed height of the Tunguska blast. A comet would disintegrate much higher in the atmosphere and cause less damage on the ground.

Concern that a similar catastrophic impact might occur today has revived interest in the Tunguska event. The force of the explosion, equivalent to that of 10 to 20 megatons of TNT, felled trees over an area of about 2200 square kilometres. It was by far the largest impact in recorded history.

Luckily, the Tunguska region was largely uninhabited at the time. But if a similar impact occurred today, it could devastate an urban area ('Will we catch a falling star?', New Scientist, 7 September 1991).

The idea that it was a comet that exploded above Tunguska was undermined by measurements of the density of Halley's Comet. In order to explode at a height of 10 kilometres, an incoming object would have to have a density of between 0.001 and 0.01 grams per cubic centimetre. But Halley's Comet had a density of between 0.6 and 1 gram per cubic centimetre.

Chyba's calculations are the first to incorporate the aerodynamic forces acting on an incoming body. He found that these would shatter a stony object, effectively reducing its density so that it would explode at an altitude of around 10 kilometres (Nature, vol 361, p 40).

Comets and carbonaceous meteorites, which are more weakly held together than the stony variety, would explode higher in the atmosphere, while stronger iron meteorites might reach the ground. According to Jay Melosh of the University of Arizona in Tucson, this may explain why iron meteorites, which are rare, have formed most craters between 1 and 2 kilometres across.

Because no one has found meteorites at Tunguska, this had been taken as evidence for a comet explosion. But the blast may have vaporised the object. It was 20 years before anyone searched for fragments.

Chyba's fragmentation calculations are still fairly crude, but they indicate that the fate of an incoming object depends critically on its size and composition. The atmosphere shields the ground from some objects: stony asteroids between 10 and a few hundred metres across explode, and so do comets and carbon-rich asteroids up to several hundred metres across. However, kilometre-sized objects of any composition are too large to react to the atmosphere before they hit the ground, says Chyba.

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