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Tunguska event

The Tunguska event is a mysterious aerial explosion that occurred near the Tunguska River in Siberia, at about a quarter after seven on the morning of June 30, 1908, which felled an estimated 60 million trees over 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 all across Eurasia. The size of the blast was later estimated to be between 10 and 15 megatons of TNT.

In the absence of an obvious explanation, numerous fanciful theories have been offered, such as a small black hole passing through the earth, an impact from a piece of antimatter, and even UFO activity. In scientific circles, the favored explanation for the blast is the impact of a meteorite. A related suggestion is that a meteorite exploded just above the Earth's surface. Whether the object was of cometary or asteroidal origin is still a matter of controversy.

The early expeditions Surprisingly, there was little scientific curiosity about the impact at the time, and due to the subsequent occurrence of war, revolution, and civil war in Russia, it wasn't until the 1920s that anyone performed a serious investigation of what had happened in Siberia in 1908.

In 1921, the Russian mineralogist Leonid Kulik[?] visited the Podkamennaya Tunguska River basin as part of a survey for the Soviet Academy of Sciences. Locals told him of the great blast, of huge stretches of forest being flattened, of people being blown over by the shock.

The reports were basically consistent with each other, and Kulik was able to persuade the Soviet government to fund an expedition to the Tunguska region. His group reached the "ground zero" of the "event" in 1927. Much to their surprise, there was no crater, just a great region of scorched trees about 50 kilometers across. The trees pointed away from the center of the event, with a few still bizarrely standing upright at ground zero, their branches and bark stripped off.

Over the next ten years, there were three more expeditions to the area, and none of them discovered anything much different from what Kulik and his people had found. Kulik found a little "pothole" bog that he thought might be the crater, but after a laborious exercise in draining the bog, he found there were old stumps on the bottom, ruling out the possibility that it was a crater.

Kulik did manage to arrange an aerial photographic survey of the area in 1938, a few years before his death as a Red Army officer in the Great Patriotic War (Second World War). The aerial survey revealed that the event had knocked over trees in a huge butterfly-shaped pattern that provided information on the direction of the object's motion. It found no crater, despite the large amount of devastation.

Later expeditions and laboratory experiments Soviet experiments performed in the mid-1960s with model forests and small explosive charges slid downward on wires that duplicated this pattern suggested that the 1908 object had approached at an angle of roughly 30 degrees from the ground and 115 degrees from north, and exploded in mid-air.

Expeditions sent to the area in the 1950s and 1960s did find microscopic glass spheres in siftings of the soil. Chemical analysis showed that the spheres contained high proportions of nickel and iridium, which are found in high concentrations in meteorites, and indicated that they were of extraterrestrial origin. However, even this clue could not pin down the nature of the object precisely.

Many ideas have been proposed for what happened at Tunguska, such as an impact of an antimatter meteor, the passage of a tiny black hole, or even the catastrophic destruction of a nuclear-powered alien spacecraft.

There has never been much evidence for such exotic ideas, and simpler theories were available. In 1930, the British astronomer F.J.W. Whipple[?] suggested that the Tunguska event was the impact of a small comet, which vaporized itself in the explosion and left no obvious trace. Comets are generally believed to be "dirty snowballs" of ices and dust, and one would be quickly destroyed by an impact with the Earth's atmosphere. The idea of a comet impact was reinforced by the fact that there were "skyglows" in the evenings across Europe for several days after the impact, obviously caused by dust dispersed through the upper atmosphere.

The comet idea remained popular for over 50 years, with some astronomers speculating that it might have been a piece of the short-period comet Encke[?]. Materials from Encke apparently make up the stream of sky junk that create the "Beta Perseid" meteor shower, and the Tunguska event coincided with a peak in that shower.

However, in 1983 an astronomer named Zdenek Sekanina[?], of the US National Aeronautics and Space Administration's Jet Propulsion Laboratory (NASA JPL), published an article that undermined the comet theory. Sekanina pointed out that eyewitness accounts and other evidence point only to one explosion, and that the object passed through the atmosphere at a shallow angle, remaining intact to an altitude of 8.5 kilometers. A dirty snowball of ice and gases would have not got that far in one piece.

Sekanina proposed that the object was a stony "chondritic" asteroid that rammed through the atmosphere until pressures and temperatures reached a point that caused it to abruptly disintegrate in a huge explosion, something like what would happen on a much smaller scale to an aspirin pill smashed with a hammer. The destruction was so complete that no remnants of substantial size survived. The material scattered into the upper atmosphere from the event would have caused the skyglows.

Sekanina's theory was appealing, but it was based on very limited information. Said one critic: "You can't make a sophisticated model from poor data." Sekanina admitted there was "a lot of handwaving" in his ideas. The comet theory still has its partisans, who reply that chemical analyses of the area have showed it to be enriched in cometary material, and suggest that the comet might have been extinct and had formed a tough "mantle" that allowed it to penetrate the atmosphere. In the absence of conclusive evidence, the debate seems likely to continue, but at least nobody is seriously suggesting it was a UFO any more.

In 2001, Farinella, Foschini, et. al released a study based on eyewitness accounts, seismic records, and samples from a 1999 expedition to the area. Their data suggests that the object originated from the asteroid belt. However, it is still not understood how a stony object can totally disintegrate in the atmosphere.

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