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Tunguska event
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Tunguska event

The Tunguska event was an 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 2,150 square kilometres.

At around 7:15 AM, Tungus natives and Russian settlers in the hills northwest of Lake Baykal observed a huge fireball, almost as bright as the Sun, moving across the sky. 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 400 miles away. The explosion registered on seismic stationss 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.

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, it would have exploded over Europe instead of the sparsely-populated Tunguska region, producing massive loss of human life.

History

Surprisingly, there was little scientific curiosity about the impact at the time, possibly due to the isolation of the Tunguska region. Any records of expeditions to the site were destroyed during the unrest of those years - World War I, the Russian Revolution, and the Russian Civil War. It took until the 1920s for the next expedition to arrive.

In 1921, the Russian mineralogist Leonid Kulik visited the Podkamennaya Tunguska River basin as part of a survey for the Soviet Academy of Sciences. After studying local accounts, he reasoned that the explosion was caused by a giant meteorite impact, and persuaded the Soviet government to fund an expedition to the Tunguska region, based on the prospect of meteoric iron that could be salvaged for 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 kilometers 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.

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.

Theories

Meteorite hypothesis

In scientific circles, 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.

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, Czech astronomer Lubor Kresak suggested that the body was a piece of the short-period Comet Encke, which is responsible for the Beta Perseid meteor shower; the Tunguska event coincided with a peak in that shower.

In 1983, astronomer Z. Sekanina published a paper criticizing the comet theory. 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 theory 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 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.

Alternative 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 other hypotheses for the Tunguska event have sprung up, with varying degrees of credibility.

A 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 theory, 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, Alturi, and Libby suggested that the Tunguska event was caused by the annihilation of 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.

Electromagnetism

Some theories link the Tunguska event to the magnetic stormss 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. This theory 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 Baykal, blew up in mid-air. Unfortunately, many events in this story have since been confused with the actual occurrences at Tunguska. For example, the nuclear-powered UFO theory 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.

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. 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 manner. Furthermore, it seems unlikely that the facility contained an energy source strong enough to produce the Tunguska event, equivalent to a thermonuclear explosion; the atomic nucleus was not even discovered until the following decade.

External links and references