Tunguska event

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[[sv:Tunguska-h�ndelsen]]

The Tunguska event was a mysterious aerial explosion that occurred near the Tunguska River in what is now Evenkia, in Siberia. It took place at 7:17 AM on June 30, 1908. The blast felled an estimated 60 million trees over [[1 E9 m�|2,150 square kilometres]].

Contents

History

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. For several nights afterwards, night skies over Europe and Western Russia glowed brightly enough to read by. The size of the blast was later estimated to be between 10 and 15 megatons. Had the object responsible for the explosion hit the earth a few hours later, Oslo, which lies on the same latitude as Tunguska, would likely have been obliterated.

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.

Theories

Scientific understanding of the behavior 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 hypotheses have sprung up, with varying degrees of credibility.

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, and 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 "ground zero" in 1927. Much to their surprise, there was no crater, just a 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. 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. 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. However, even this clue could not pin down the nature of the object precisely.

Airburst

In scientific circles, one of the leading explanation for the blast is the airburst of a meteorite 6 to 10 km above the Earth's surface. 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.

With closer monitoring of the Earth's atmosphere, it has been found that meteorite impacts occur rather frequently. Shoemaker has estimated that impacts in the kiloton range occur at the rate of about once a year, and that impacts in the megaton range (such as the Tunguska event) approximately once every 300 years. For details, see the article on impact events.

Whether the Tunguska event was of an cometary origin is still a matter of controversy.

Impacts

In 1930, the British astronomer F.J.W. Whipple suggested that the Tunguska event was produced by the impact of a small comet that has gained some acceptance. A cometary meteorite, being composed primarily of ices and dust, could have been completely vaporized by the impact with the Earth's atmosphere, leaving no obvious traces. The idea of a comet impact was 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 showed it to be enriched in cometary material.

In 1983, an astronomer named Zdenek Sekanina, working the NASA Jet Propulsion Laboratory, published a paper 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 body composed of ice and gases could not have travelled such a distance without disintegrating. Proponents of the comet theory have suggested, in reply, that the object might have been an extinct comet with a stony mantle that allowed it to penetrate the atmosphere.

Whether the Tunguska event was of an asteroidal origin is still a matter of controversy. The comet idea remains popular, with some astronomers speculating that it might have been a piece of the short-period Comet Encke. Materials from Encke appear to make up the stream of space debris that create the Beta Perseid meteor shower, and the Tunguska event coincided with a peak in that shower.

Another explanation is that the Tunguska meteorite was a stony asteroid. In 2001, Farinella, Foschini, et al. released a study suggesting that the object had arrived from the direction of the asteroid belt, working from eyewitness accounts, seismic records, and samples from a 1999 expedition to the area.

The chief difficulty in the asteroid theory 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. However, it remains an open question why the meteorite should have disintegrated so abruptly.

Electromagnetics

A number of alternative theories focus on the similarities between the Tunguska event and the magnetic storms that occur after thermonuclear explosions in the stratosphere. Anomalous concentrations of electrical energy in the region could have produced an explosive releases of energy. Electromagnetic fireballs, spherical plasmoids, and ball lightning have been reported to exhibit the same phenonomena. Other plasma and geomagnetic theories have been formed. For example, in 1984 V. K. Zhuravlev and A. N. Dmitriev proposed the "heliophysical" model as the explaination of the Tunguska event. This model would indicate that the event was a result of plasmoids ejected from the Sun. Valeriy Buerakov also develops an independent model of an electromagnetic "ball" that could deliver such force.

Some have 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 stated it was an evolution of his magnifying transformer and could concentrate electromagnetic energy output over long distances. Tesla, in March 1907, did state that he was capable of "projecting wave energy" and, in April 1908 (in the Wireless Telegraphy and Telephone), Tesla expounds on the possiblities of "direct application of electrical waves without the use of aerial engines or other implements of destruction" (eg. direct electrical energy to any point on the globe).

Wardenclyffe tower was designed as a high voltage source strong enough create such chain reactions, i.e., an amount of energy equivalent to that released by a thermonuclear explosion. However, the workings behind Tesla's Wardenclyffe Tower are not well understood and it is not certain if Tesla ever used the Wardenclyffe facility for this manner. (ed. Skeptics have expressed doubt that the facility could have been used as such so many years before the mechanisms of nuclear energy were elucidated. Ernest Rutherford did not discover the atomic nucleus until the following decade.)

Reports of 1908 have Tesla testing the facility during Peary's second North Pole expidition. Reportedly, Tesla operated the Wardenclyffe facility to send enormous power to an area west of the Peary expedition. Tesla's associate, George Scherff, witnessed these evernts at the Tesla Tower. During the test, Wardenclyffe tower emitted a faint soft glow and killed an animal. Analysis of Peary's position and Tunguska deviates by 2 degrees on a straight line from the Wardenclyffe facility. Shortly afterward, Tesla stopped speaking of this type of possibility for Wardenclyffe-type installations (though he does introduce the concept of teleforce).

Black hole and antimatter

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 theory, there is no evidence for a second explosion occurring as the black hole exited the Earth and 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.

In 1965, Cowan, Alturi and Libby suggested that the Tunguska event was caused by the annihilation a chunk of antimatter falling from space. However, this theory does not explain the mineral debris left in the area of the explosion (nor, in fact, do the other theories described below.) Furthermore, astronomical studies indicate that antimatter does not exist in our region of the universe in significant amounts; if it did, its annihilation with the interstellar medium would have been observed. It is thus highly implausible that a chunk of the postulated size could have existed and this theory has not gained wide acceptance.

UFOs

UFO aficionados have claimed that the Tunguska event is the result of an exploding alien spaceship. This theory appeared 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 Baykal, blew up in mid-air. Unfortunately, many events in this story have since been confused with the actual occurrences at Tunguska. There does not appear to be any supporting evidence for this theory and is not usually seriously considered.

See also

Images sources

External links and references


Adapted from the Wikipedia article, "Tunguska event" December 3, 2003

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