Total ablation

Vladimir Svetsov of the Institute for Dynamics of Geospheres in Moscow removed a layer of mystery from the Tunguska event, when in 1996 he showed that the entire mass of the Tunguska body vaporised before it could reach the ground. Ablation - the loss of material from a cosmic body through evaporation or melting caused by friction within the atmosphere - of the Tunguska debris was total.

According to Svetsov, when a Tunguska-sized body enters deep into the atmosphere, it is broken into a large number of fragments, the maximum size of which is 10 centimetres. As the body decelerates, these fragments are separated from each other. Mathematical simulations show that stony fragments fully ablate either inside or outside the fireball due to high temperatures. All vaporised material does not reach the ground; it moves upwards in the atmosphere. Although Svetsov admitted that fragmentation processes were complex and certainly needed further investigation, he declared his scenario 'quite plausible'.

As for the Tunguska body, Svetsov said that it would have been heated to 15,000 degrees Celsius. This temperature was high enough to create an explosion 'quite comparable with that of a nuclear explosion'. Upon explosion the body broke into a vast number of fragments, typically 1 to 3 centimetres across but no larger than 10 centimetres. But the temperature was high enough to melt these fragments until nothing remained. Some of this microscopic debris condensed in the atmosphere and then was scattered over the Tunguska forest. 'The apparent absence of solid debris is therefore to be expected following the atmospheric fragmentation of a large stony asteroid', Svetsov concluded.

He also said that, similar to the 1994 impact of comet Shoemaker-Levy 9 on Jupiter, Tunguska debris was probably also widely scattered because of the turbulent wake of the asteroid. Svetsov also suggested that the microscopic particles recovered from the resin of trees by the Italian researchers 'might be recondensed material precipitated in the general vicinity of the impact site'.

Svetsov did leave some hope for Tunguska meteorite hunters: if some larger fragments accidentally gained significant speeds at altitudes between 15 and 20 kilometres, sizeable remnants could reach the ground. But they would have fallen to the ground at a distance of 5 to 10 kilometres southeast from the blast's epicentre.

Svetsov's calculations were based on the assumption that the Tunguska fireball was a 15-megaton asteroidlike object that hit Earth with a speed of 54,000 kilometres per hour and at an angle of 45 degrees. What if the object was a comet? Obviously, this analysis would then make no sense. NASA scientist Kevin Zahnle provides an argument that comet partisans would find hard to demolish. His argument goes something like this: all small impact craters on Earth are almost always produced by the relatively rare iron meteorites. The 1.2-kilometre-wide Meteor Crater in Arizona, for example, was produced by an iron body of essentially the same energy as the Tunguska explosion. The smallest known crater made by a stony meteorite is the 3.4-kilometre New Quebec crater. This raises a problem for the comet theory: if comets with energies of 15 megatons can reach the higher atmosphere before exploding, then the much more numerous asteroid, which most astronomers agree will penetrate deeper, should be cratering the land every 1,000 years. 'If Tunguska was a comet, where are all the Meteor Craters made by rocks?' Zahnle asks.

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