The mysterious event appears to have been an aerial explosion that occurred near the Tunguska River in central Siberia. It took place at about 0715h local time on June 30th, 1908. The blast felled an estimated 60 million trees over andarea of some 2,150 square kilometres. Witnesses observed a huge fireball, almost as bright as the Sun, plunging across the Siberian sky, terminating in a huge explosion that registered on seismic stations across Eurasia. Vast quantities of very fine dust were scattered into the upper atmosphere allowing a newspaper to be read at night in London by scattered light. The explosive blast was later estimated to have been between 10 and 15 megatons.
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Ironically, although 60 million trees were felled by the explosion, some at the centre of the event site, where the force of the aerial blast was directed vertically downwards, have remained standing but stripped of their branches and bark.
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There was little interest in Czarist Russia to investigate the event in remote and undeveloped Siberia, while war, revolution and civil war left the country otherwise preoccupied. In 1921, the Podkamennaya Tunguska River basin was visited by the minerologist Leonid A Kulik (1883-1942) as part of a survey for the Soviet Academy of Sciences where the many consistent accounts of the 1908 event prompted him to persuage the Soviet government to fund an expedition investigating it.
The Soviet expedition reached the centre of the event's area of damaged trees about 50 km across in 1927 to find no evidence of a pre-supposed explosive crater.
Three further expeditions over the next decade to investigate the event found nothing more than Kulik's expedition of 1927.
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Kulik had found a little "pothole" bog but, after this was laboriously drained, it proved to contain old tree stumps at the bottom pre-dating the 1908 event.
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As a Red Army officer in World War II, Kulik managed to arrange for the site to be photographed from the air in 1938. The aerial survey revealed that the event had felled trees a huge butterfly-shaped pattern which provided information on the direction of the object's motion but no impact crater was rvealed.
Later Investigations
Expeditions to the event area in the 1950s and 1960s discovered microscopic glass spheres in soil siftings which chemical analysis revealed to be rich in nickel and iridium. Both elements are found in high concentrations in meteorites, indicating an extra-terrestrial origin of the material but still not defining the nature of the object which caused the explosion.
Soviet experiments of the 1960s with model forests and small explosive charges slid towards them on wires recreated the patterns of destruction revealed by the aerial survey of the event site and indicate an object travelling at 30° to the ground and 115° from north which exploded in the air.
Various explanations of the violent event have been proposed, including the Earth's collision with a small meteorite, comet or pice of anti-matter or a tiny black hole passing through the Earth, but none provides a wholly satisfactory explanation. It has even been suggested that the event might have been the malfunction and explosion of an extra-terrestrial spacecraft.
While the discovery of the microscopic nickel/iridium-rich glass spheres in the area indicates a meteorite, its origin, asteroidal or cometary, is still a mystery.
Extra-Terrestrial Spacecraft
An exploding extra-terrestrail spacecraft would have left parts of its fabric at the site, no indication of which have been found.
Antimatter
Antimatter shares mass and gravity with normal matter but all its other properties are the inverse of those of the "normal " matter to which we are accustomed. When the two forms of matter meet, they annihilate each other leaving only hard gamma radiation.
The lack of any residual radiation at the site discounts antimatter as an explanation.
A Black Hole
It has been suggested that the event might have been caused by a tiny black hole of only millimetres in size but weighing thausands of tons travelling so fast as to crash its way straight through the Earth (were it travelling slowly enough to be captured by the earth's gravity, the black hole would lodge in the centre of our planet, devouring its matter).
If such were the case, the black hole would have exited the Earth's surface in the North Atlantic on its way into space and caused atmospheric disturbances there. Searches for atmospheric disturbances in the North Atlantic at the time, including the logs of ships in the area, have produced no record of such a disturbance.
Comet
The British astronomer FJW Whipple suggested in 1930 that the event may have been caused by a small comet. Composed of ice and dust, as much of the cometary meteorite as had not been evaporated by its passage through the Earth's atmosphere could have disintegrated above ground leaving no impact crater.
It was thought that the comet was about 100 metres across, weighing about a million tons and travelling at 30 km per second (70,000 mph). Some astronomers suggested that the object may have been a piece of the short-period Comet Encke, materials from which appear to make up the stream of material that creates the Beta Perseid meteor shower - the Tunguska event coincided with a peak in that shower.
Whipple's comet theory was undermined in 1983 when astronomer Zdenek Sekanina, working at the NASA Jet Propulsion Laboratory, published a paper pointing out that eye-witnesses reported only a single explosion of the object travelling at a shallow angle and exploding some 8.5 km above the earth's surface. Sekanina argues that a body composed of ice, dust and gases could not have travelled such a distance through the atmosphere without disintegrating.
| 1838 | | Soviet minerologist and army officer aranges aerial photographic survey of the Tunguska Event site in Siberia | | 1908.Jun.30 | | Tunguska Event: Fireball in the sky hits central Siberia with a massive explosion of some 10-15 megatonnes leveling 2,000 sq km of forests and causing shockwave to travel twice round the world
Possibly the impact of a small comet | | 1921 | | Soviet minerologist Leonid Kulik surveys the Podkamennaya Tunguska River basin in Siberia and hears many consistent accounts of the Tunguska event of 1908 | | 1927 | | Soviet expedition led by minerologist Leonid Kulik to investigate the Tunguska event of 1908
Three other expeditions investigated the event over the next decade |
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