Tunguska event
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The Tunguska event was an aerial explosion that occurred at 60° 55′ North, 101° 57′ East, near the Podkamennaya (Stony) Tunguska River in what is now Evenkia, Siberia, at 7:17 AM on June 30, 1908. The size of the blast was later estimated to be between 10 and 15 megatons. It felled an estimated 60 million trees over 2,150 square kilometers.
Description
At around 7:15 AM, Tungus natives and Russian settlers in the hills northwest of Lake Baikal observed a huge fireball moving across the sky, nearly as bright as the Sun. A few minutes later, there was a flash that lit up half of the sky, followed by a shock wave that knocked people off their feet and broke windows up to 650 km (400 mi) away. The explosion registered on seismic stations across Eurasia, and produced fluctuations in atmospheric pressure strong enough to be detected by the recently invented barographs in Britain. Over the next few weeks, night skies over Europe and western Russia glowed brightly enough for people to read by. In the United States, the Smithsonian Astrophysical Observatory and the Mount Wilson Observatory observed a decrease in atmospheric transparency that lasted for several months.
History
Surprisingly, there was little scientific curiosity about the impact at the time, possibly due to the isolation of the Tunguska region. If there were any early expeditions to the site, their records were lost during the subsequent chaotic years ?- World War I, the Russian Revolution, and the Russian Civil War.
The first expedition for which records have survived arrived at the scene more than a decade after the event. In 1921, The Russian mineralogist Leonid Kulik, visiting the Podkamennaya Tunguska River basin as part of a survey for the Soviet Academy of Sciences, deduced from local accounts that the explosion had been caused by a giant meteorite impact. He persuaded the Soviet government to fund an expedition to the Tunguska region, based on the prospect of meteoric iron that could be salvaged to aid Soviet industry.
Kulik's expedition reached the site in 1927. To their surprise, no crater was to be found. There was instead a region of scorched trees about 50 kilometres across. A few near ground zero were still strangely standing upright, their branches and bark stripped off. Those farther away had been knocked down in a direction away from the center.
During the next ten years, there were three more expeditions to the area. 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. In 1938, Kulik managed to arrange for an aerial photographic survey of the area, which revealed that the event had knocked over trees in a huge butterfly-shaped pattern. Despite the large amount of devastation, there was no crater to be seen.
Expeditions sent to the area in the 1950s and 1960s found 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. Expeditions led by Gennady Plekhanov found no elevated levels of radiation, which might have been expected if the detonation were nuclear in nature.
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Meteorite hypothesis
Meteor airburst
In scientific circles, the leading explanation for the blast is the airburst of a meteor 6 to 10 kilometers above the Earth's surface.
Meteors are constantly entering the Earth's atmosphere from outer space, usually travelling at a speed of more than 10 kilometers per second. The heat generated by friction against the atmosphere is immense, and most meteors completely burn up or explode before they can reach the ground. Starting from the second half of the 20th century, close monitoring of the Earth's atmosphere has led to the discovery that such meteor airbursts occur rather frequently. A stony meteoroid of about 10 meters in diameter can produce an explosion of around 20 kilotons, similar to the Little Boy bomb that flattened Hiroshima, and data released by the U.S. Air Force's Defense Support Program has shown that such explosions occur at a rate of more than once a year. Tunguska-like megaton-range events are much rarer. Eugene Shoemaker has estimated that such events occur at the rate of about once every 300 years.
Blast patterns
The curious effect of the Tunguska explosion on the trees near ground zero has been observed during atmospheric nuclear tests in the 1950s and 1960s, and is due to the shock wave produced by such large explosions. (The radioactivity emitted by nuclear blasts does not have any effect on the phenomena in question.) The trees directly below the explosion are stripped as the blast wave moves vertically downward, while trees further away are felled because the blast wave is travelling closer to the horizontal when it reaches them.
Soviet experiments performed in the mid-1960s, with model forests and small explosive charges slid downward on wires, produced butterfly-shaped blast patterns strikingly similar to the pattern found at the Tunguska site. The experiments suggested that the object had approached at an angle of roughly 30 degrees from the ground and 115 degrees from north, and exploded in mid-air.
Asteroid or comet?
The composition of the Tunguska body remains a matter of controversy. In 1930, the British astronomer F.J.W. Whipple suggested that the Tunguska body was a small comet. A cometary meteorite, being composed primarily of ice and dust, could have been completely vaporized by the impact with the Earth's atmosphere, leaving no obvious traces. The comet hypothesis was further supported by the glowing skies (or "skyglows") observed across Europe for several evenings after the impact, apparently caused by dust that had been dispersed across the upper atmosphere. In addition, chemical analyses of the area have shown it to be rich in cometary material. In 1978, Slovak astronomer Lubor Kresak suggested that the body was a piece of the short-period Comet Encke, which is responsible for the Beta Taurid meteor shower; the Tunguska event coincided with a peak in that shower. It is now known such bodies regularly explode tens to hundreds of kilometres before hitting the ground, as military satellites have been observing such explosions for decades.
In 1983, astronomer Zdenek Sekanina published a paper criticizing the comet hypothesis. He pointed out that a body composed of cometary material, travelling through the atmosphere along such a shallow trajectory, ought to have disintegrated, whereas the Tunguska body apparently remained intact into the lower atmosphere. Sekanina argued that the evidence pointed to a dense, rocky object, probably of asteroidal origin. This hypothesis was further boosted in 2001, when Farinella, Foschini, et al. released a study suggesting that the object had arrived from the direction of the asteroid belt.
Proponents of the comet hypothesis have suggested that the object was an extinct comet with a stony mantle that allowed it to penetrate the atmosphere.
The chief difficulty in the asteroid hypothesis is that a stony object should have produced a large crater where it struck the ground, but no such crater has been found. It has been hypothesized that the passage of the asteroid through the atmosphere caused pressures and temperatures to build up to a point where the asteroid abruptly disintegrated in a huge explosion. The destruction would have had to be so complete that no remnants of substantial size survived, and the material scattered into the upper atmosphere during the explosion would have caused the skyglows. Models published in 1993 suggested that the stony body would have been about 60 metres across, with physical properties somewhere between an ordinary chondrite and a carbonaceous chondrite.
Christopher Chyba and others have proposed a process whereby a stony meteorite could have exhibited the behavior of the Tunguska impactor. Their models show that when the forces opposing a body's descent become greater than the cohesive force holding it together, it blows apart, releasing nearly all its energy at once. The result is no crater, and damage distributed over a fairly wide radius, all of the damage being blast and thermal.
Unexplained phenomena
There are still some aspects that have not been convincingly explained. Reports say the skyglow began the night before the explosion, and that there was strange weather and increased seismic activity for days beforehand. Attempts to apply carbon-14 dating to soil in the area gave a date "in the future"--that is, the soil was enriched in radioactive carbon-14. Some trees near the blast centre were not burnt. The site lies in the middle of an ancient volcanic eruption zone, and researchers once detected an emission of radon gas that lasted four hours. The size of bolide necessary to create such a big explosion is very unlikely in such a recent time. According to Russian geologist Vladimir Epifanov and German astrophysicist Wolfgang Kundt, some of these aspects suggest that the explosion was an explosion of methane gas which was emitted from the earth. Something similar seems to have occurred in 1994 near the village of Cando in Spain. See 'New Scientist', 7 Sept. 2002, p. 14 [1] [2].
Speculative hypotheses
Scientific understanding of the behaviour of meteorites in the Earth's atmosphere was much sparser during the early decades of the 20th century. Due to this lack of knowledge, a great many other hypotheses for the Tunguska event have sprung up, with varying degrees of credibility. The hypotheses listed below are all rejected by modern science and by skeptics who generally see them as being gross violations of Occam's Razor.
Black Hole
In 1973, Jackson and Ryan proposed that the Tunguska event was caused by a "small" (around 10²³ kg) black hole passing through the Earth. Unfortunately for this hypothesis, there is no evidence for a second explosion occurring as the black hole exited the Earth and it has not gained wide acceptance. Furthermore, the subsequent discovery by Stephen Hawking that black holes radiate energy indicates that such a small black hole would have evaporated away long before it could encounter the Earth.
Antimatter
In 1965, Cowan, Atluri, and Libby suggested that the Tunguska event was caused by the annihilation of a chunk of antimatter falling from space. However, as with the other hypotheses described in this section, this does not account for the mineral debris left in the area of the explosion. Furthermore, there is no astronomical evidence for the existence of such chunks of antimatter in our region of the universe. If such objects existed, they should be constantly producing energetic gamma rays due to annihilation against the interstellar medium, but such gamma rays have not been observed.
Electromagnetism
Some hypotheses link the Tunguska event to the magnetic storms similar to those that occur after thermonuclear explosions in the stratosphere. For example, in 1984 V. K. Zhuravlev and A. N. Dmitriev proposed a "heliophysical" model based on "plasmoids" ejected from the Sun. Valeriy Buerakov has also developed an independent model of an electromagnetic "fireball".
UFOs
UFO aficionados have long claimed that the Tunguska event is the result of an exploding alien spaceship or an alien weapon going off to "save the Earth from an imminent threat". This hypothesis appears to originate from a science fiction story penned by Soviet engineer Aleksander Kazantsev in 1946, in which a nuclear-powered Martian spaceship, seeking fresh water from Lake Baikal, blew up in mid-air. This story was inspired by Kazantsev's visit to Hiroshima in late 1945.
Many events in Kazantsev's tale were subsequently confused with the actual occurrences at Tunguska. The nuclear-powered UFO hypothesis was adopted by TV drama critics Thomas Atkins and John Baxter in their book The Fire Came By (1976). The 1998 television series The Secret KGB UFO Files, broadcast on Turner Network Television, referred to the Tunguska event as "the Russian Roswell" and claimed that crashed UFO debris had been recovered from the site. In 2004, a group of Russian scientists from the Tunguska Space Phenomenon Public State Fund claimed to have found the wreck of an alien spacecraft at the site [3].
The proponents of the UFO hypothesis have never been able to provide any significant evidence for their claims. It should be noted that the Tunguska site is downrange from the Baikonur Cosmodrome and has been contaminated repeatedly by Russian space debris, most notably by the failed launch of the fifth Vostok test flight on December 22, 1960. The payload landed close to the Tunguska impact site, and a team of engineers was dispatched there to recover the capsule and its two canine passengers (which survived).
The Wardenclyffe Tower
It has also been suggested that the Tunguska explosion was the result of an experiment by Nikola Tesla at his Wardenclyffe Tower, performed during Robert Peary's second North Pole expedition. Tesla had claimed that the tower could be used to transmit electromagnetic energy across large distances. Allegedly, he had sent a communication to Peary, advising him to be on the alert for unusual auroral phenomena encountered as he attempted to reach the North Pole.
However, the workings behind Tesla's Wardenclyffe Tower are not well understood, and it is not certain if Tesla ever used the facility for this purpose. It is unknown if the facility could produce energy and transmit it longitudinally to produce the Tunguska event, an equivalent to a thermonuclear explosion; the atomic nucleus was not even discovered until the following decade (though in 1891, on the structure of the aether and electromagnetics, Tesla stated that there was "an infinitesimal world, with molecules and their atoms spinning and moving in orbits, in much the same manner as celestial bodies carrying with them static charges").
Regardless if it was possible for the facility to produce such an effect, the main contention that Tesla was not responsible for the Tunguska event is that it occurred at about 7:17 AM. Given accounts (if they can be trusted) of when Tesla performed his experiments in the evening of June 30th. Tesla's facility was 1/4 a day ahead of Tunguska time. Tesla's Wardenclyffe experiment would then be the day after the Tunguska event (which occurred around 1:00 in the morning on June 30th [New York time]).
http://en.wikipedia.org/wiki/Tunguska_event
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