Back to Tunguska. The scary scenarios of a comet striking Earth are hypothetical, but the devastated Siberian taiga is a reality. The Soviet Academy of Sciences was determined to find an answer, and in 1954 sent Kirill P. Florenskiy, a geochemist, to survey the Tunguska site. Using modern cartographic equipment, Florenskiy made an aerial survey of the region and drew up a new, more accurate map. In 1957 the Russian mineralogist A.A. Yavnel microscopically analysed soil samples brought back by Kulik in 1929 and 1930. In some samples he discovered magnetite globules 30 to 60 micrometres in size. The samples also showed a small percentage of nickel and traces of cobalt. The results suggested that an iron meteorite had fallen in Tunguska.
These results encouraged the Academy to organise a new expedition. In the summer of 1958 Florenskiy headed the fifth Tunguska expedition, the first after the Second World War. The main aim was not to look for visible meteorite fragments but to collect micrometeorites in the soil. The expedition failed to confirm the existence of magnetite and nickel globules detected by Yavnel. It was discovered later that Kulik's samples had been contaminated with samples from the iron meteorite that fell in the Sikhote-Alin mountains in
Siberia on 12 February 1947. The soil samples from this iron meteorite - which destroyed an area of more than a square kilometre and created many craters, the largest being 26.5 metres in diameter and 6 metres deep - had been stored with Kulik's samples. In his report to the Academy, Florenskiy rejected Kulik's hypothesis that the explosion took place on the ground, and suggested that it had taken place in the air some height above.
Florenskiy headed another expedition to Tunguska in 1961-62, which analysed in detail the pattern of devastation of the forest and established the trajectory of the fireball. However, by 1960 Florenskiy was convinced that Tunguska had been flattened by a comet. 'Many facts favor the view that the colliding body was a comet: the unusually loose structure, which led to breakup in the atmosphere; the dust tail, pointing away from the sun, which caused unusual sunsets over nearly all of Europe; the nature of the orbit; and lack of big fragments', he wrote in Sky & Telescope magazine. He pinpointed the location of the explosion at an altitude of 5 kilometres, southeast of the centre of destruction on the ground, and suggested that the collision was not head-on, and nor did the comet directly follow Earth, for it struck Earth almost squarely on the side.
He explained the explosion in Earth's atmosphere by saying that the sudden stopping of a body moving at 18,000 kilometres per hour releases enough heat to vaporise it instantaneously. If the body is loosely compacted and contains volatile matter such as gases or ice, deceleration in the air may cause explosive vaporisation without the body striking the ground, he suggested.
Florenskiy established that after the explosion the Tunguska forest burned for at least five days. A weak southeasterly wind at 7.2 to 18 kilometres per hour was blowing on that day, but the fire was preceded by a dry spell and therefore spread rapidly through the treetops. The fire died out because of the unfavourable weather conditions.
'We found that branches which had been not thicker than a fountain pen before the catastrophe still retained marks of their injuries', he said. 'The damage is noticeable on the upper portions of the branches, thus making it impossible to associate these injuries with ordinary fires. The severity of these injuries diminished significantly with increasing distance from the epicentre.' He estimated that the energy required for the observed injuries would be 5 to 15 calories per square centimetre. This value could not be significantly higher, since this would lead to marked charring of the bark, and no such phenomenon was observed. 'Approximately the same energy is required to ignite dry forest debris, and this could lead to forest fire', he noted. Studies of nuclear explosions have shown that a 10-megaton explosion would produce charring similar to the Tunguska forest in fir, pine and maple bark.
In the early 1960s Vassilii Fesenkov of the Soviet Academy of Sciences' meteorite committee expanded the comet theory. He gave various reasons in support of his belief. These can roughly be grouped into four categories:
First, craters found by Kulik are now not considered to be places of the fall of fragments of the meteorite.
Despite extensive searches, no primary meteorite fragments have been found. It's easier to visualise a 'dirty iceberg' exploding to nothing than a rocky meteorite. Many magnetite and silicate globules (5 to 450 micrometres in diameter) found in the area were 'clearly of secondary origin'. They were most likely formed in the atmosphere due to rapid condensation of molten 'rain drops' as they drifted to the ground. The height of the comet's explosion -5 to 6 kilometres above the Earth's surface - has been well established by the measurement of shock waves received at observatories in Irkutsk (Siberia) and Potsdam (Germany) and the six microbarograph stations in England.
Second, according to all evidence, this meteorite moved around the Sun in a retrograde direction - it was moving from south to north at a time when Earth was moving generally from north to south -which is impossible for typical meteorites. Meteorites rarely hit Earth in the morning, because the morning side faces forward in the planet's orbit. Usually the meteorite overtakes Earth from behind, on the evening side. Comets, on the other hand, have a wide range of orbits and speeds and could collide with Earth on the morning side, hitting head-on with a speed of about 145,000 kilometres per hour. The Sun's glare prevented any sighting of the comet before it hit Earth, because its direction and the angle of strike towards Earth were from behind the Sun.
Third, the most probable explanation for the brilliant night skies observed in the British Isles, Europe and Western Russia would be that the meteorite was actually a little comet with a dust tail pointing away from the Sun. The dissipation of this tail in the atmosphere greatly increased the night sky brightness. The dust particles causing the night glows were at a height of several hundred kilometres above the Earth's surface and did not behave as ordinary meteor showers.
Fourth, a marked decrease in the air's transparency, recorded two weeks after the explosion, was caused by the loss of several million tonnes of dust from the comet during its flight through the atmosphere.
Fesenkov told The New York Times on 20 November 1960 that 'the recent study suggests that explosions took place over at least three points in the area'. He said that in one of the Soviet studies, an experimental model was used to simulate the forest and miniature explosions were set off in the air. These experiments suggested that the head of the comet, a 'small one', consisted of dust and frozen gas in one or more extremely dense clouds several kilometres in diameter. 'The total weight is thought to have been more than 1,000,000 tons', The Times reported. 'When it hit the atmosphere the resulting explosion is thought to have been comparable in force to that of an equal amount of TNT.' It's worth noting that Fesenkov's estimate of the blast's energy - 1 megaton - is similar to the estimate made by Astapowitsch in 1933.
Further support for the comet theory came in 1975 from Ari Ben-Menahem, an Israeli scientist, who re-analysed the old seismographs of the Tunguska explosion and compared them with a series of air explosions from nuclear tests at the USSR test site at Novaya Zemlya. He concluded that the explosion took place 8.5 kilometres above the ground and had energy of about 12.5 megatons.
A year later, David Hughes, a British astronomer, estimated the comet's nucleus diameter at about 40 metres, much smaller than the diameter estimated for visual comets. The small diameter explained why the comet wasn't seen as it approached on its collision course to Earth. 'A cometary nucleus of this size will hit Earth about every 2,000 years, the rarity of the event giving ample justification for visiting Tunguska again', he said.
In a further study, Hughes and his colleague John Brown noted that, although the temperature produced by the burn-up of the comet in the atmosphere would have been no more than a few million degrees, too low for nuclear reactions, this temperature was high enough to produce nuclear-like effects such as the production of X-rays, gamma rays and highly accelerated charged particles. Even if the Tunguska body did cause nuclear effects, it does not mean it was not a comet. 'The Tunguska explosion was by an impacting small comet and [...] nothing more exotic needs to be invoked', Hughes and Brown concluded.
After a more recent analysis of the unusual sky glows, Vitalii Bronshten of the Committee of Meteorites of the Russian Academy of Sciences concluded that 'the cause of the glow was secondary scattering of sunlight by the dust constituting the envelope of the Tunguska comet'. Bronshten, one of the main contemporary supporters of the comet theory, first calculated the volume density of the dust ejected by the nucleus of a comet like Halley's at a distance of several thousand kilometres, and then its transfer to the west in Earth's gravitational field, taking into account deceleration caused by the atmosphere. He showed that the bigger dust would reach the British Isles in six hours. Smaller particles could cover greater distances but they could not produce a noticeable scattering of light. This is the reason why the glowing area had a western border: night glows were limited to the British Isles, Europe and Western Russia.
If the Tunguska object was indeed a comet, then it must be a comet known to astronomers. In 1978 the Slovak astronomer Lubar Kresak suggested that a piece of comet Encke had exploded at Tunguska. He based this idea on the fact that the fireball exploded at the peak of one of the most intense annual daytime meteor showers in late June, which has long been thought to derive from this comet. Encke's comet is named after the mathematician who investigated its orbit. The German astronomer Johann Encke was born five years after the comet was discovered in 1786, but he showed in 1822 that it has a period of three years and four months, the shortest known.
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