What was the the Tunguska event?

Veröffentlicht auf 19. Dezember 2011 von Chris

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.

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 almost two decades 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 further 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.

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

Meteorites 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 meteorites completely burn up or explode before they can reach the ground. 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.

Starting from the second half of the 20th century, close monitoring of the Earth's atmosphere has led to the discovery that kiloton-range meteorite airbursts occur rather frequently. 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 tests of airburst nuclear weapons. 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 ices 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 Z. 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 shown that the event is consistent with a stony meteorite [1]. Their models show that when the forces opposing the body's descent become greater than the cohesive force holding it together, it simply 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 therma.

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.

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, this hypothesis does not explain the mineral debris left in the area of the explosion (nor, in fact, do the other hypotheses 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 hypothesis has not gained wide acceptance.

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 —a story inspired by the Hiroshima blast (the author had visited Hiroshima in late 1945). Unfortunately, many events in this story have since been confused with the actual occurrences at Tunguska. For example, 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 August, 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. Their claim was greeted with scepticism by mainstream scientists who demanded clearer evidence ([2]). The Tunguska site is downrange from the Baikonur Cosmodrome and has been contaminated repeatedly by Russian space debris, most notably by the December 22, 1960 failed launch of the fifth Vostok test flight. The payload landed close to the 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. The Wardenclyffe Tower was designed to utilize the largest version of Tesla's patented magnifying transmitter, popularly known as the Tesla Coil, to transmit electrical power into the earth as well as the upper atmosphere.

In 1908, Tesla allegedly sent a cryptic communication to the American explorer, Robert E. Peary, advising him to be on the alert and make notes of any unusual auroral phenomena encountered as he attempted to reach the North Pole. Allegedly Tesla fired up his transmitter for a trial run and attempted to generate and direct his ethereal oscillations toward the North Pole in the hope of stimulating the polar aurora and perhaps attracting world attention to his invention. It is alleged that Tesla's trial run coincided with the Tunguska event in Siberia.

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.

Had the object responsible for the explosion hit the Earth a few hours later, it would have exploded over Europe (most probably Scandinavia) instead of the sparsely-populated Tunguska region, producing massive loss of human life and changing the course of human history.