Tunguska event

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Trees felled by the Tunguska blast. Photograph from Kulik's 1927 expedition
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Trees felled by the Tunguska blast. Photograph from Kulik's 1927 expedition
This article contains verbatim references to dates from the obsolete Julian calendar. Be aware that dates might not agree with dates from the Gregorian calendar.

The Tunguska event was an explosion that occurred at 60°55′N 101°57′E, near the Podkamennaya (Under Rock) Tunguska River in what is now Evenk Autonomous Okrug, at 7:17 AM on June 30, 1908. The event is sometimes referred to as the Great Siberian Explosion.

The explosion was probably caused by the airburst of an asteroid or comet 5 to 10 kilometers (3–6 mi) above the Earth's surface. The energy of the blast was later estimated to be between 10 and 20 megatons of TNT, which would be equivalent to Castle Bravo, the most powerful nuclear bomb ever detonated by the US. It felled an estimated 60 million trees over 2,150 square kilometers (830 sq mi). An overhead satellite view centered at 60.917N 101.95E (near ground zero for this event) shows an area of reduced forest density, with a fully visible irregular clearing of somewhat less than one square kilometer in area.

In recent history, the Tunguska event stands out as one of the rare large-scale demonstrations that a full doomsday event is a real possibility for the human race.

Approximate location of the Tunguska event, in Siberia.
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Approximate location of the Tunguska event, in Siberia.

Contents

[edit] Description

At around 7:15 a.m., Tungus natives and Russian settlers in the hills northwest of Lake Baikal observed a column of bluish light, nearly as bright as the Sun, moving across the sky. About 10 minutes later, there was a flash and a loud "knocking" sound similar to artillery fire that went in short bursts spaced increasingly wider apart. Eyewitnesses closer to the explosion reported the sound source moving during each barrage, east to north. The sounds were accompanied by a shock wave that knocked people off their feet and broke windows hundreds of miles away. The majority of eyewitnesses reported only the sounds and the tremors, and not the sighting of the explosion; to different eyewitnesses the sequence of events and their overall duration is also different.

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 days, night skies were aglow such that one could read in their light. In the United States, the Smithsonian Astrophysical Observatory and the Mount Wilson Observatory observed a decrease in atmospheric transparency that lasted for several months.

[edit] Select Eyewitness Reports

At breakfast time I was sitting by the house at Vanavara trading post (65 kilometres or 40 miles north of the explosion), facing North. [...] I suddenly saw that directly to the North, over Onkoul's Tunguska road, the sky split in two and fire appeared high and wide over the forest (as Semenov showed, about 50 degrees up - expedition note). The split in the sky grew larger, and the entire Northern side was covered with fire. At that moment I became so hot that I couldn't bear it, as if my shirt was on fire; from the northern side, where the fire was, came strong heat. I wanted to tear off my shirt and throw it down, but then the sky shut closed, and a strong thump sounded, and I was thrown a few yards. I lost my senses for a moment, but then my wife ran out and led me to the house. After that such noise came, as if rocks were falling or cannons were firing, the earth shook, and when I was on the ground, I pressed my head down, fearing rocks would smash it. When the sky opened up, hot wind raced between the houses, like from cannons, which left traces in the ground like pathways, and it damaged some crops. Later we saw that many windows were shattered, and in the barn a part of the iron lock snapped.
  • Testimony of Chuchan of Shanyagir tribe, as recorded by I.M.Suslov in 1926.
We had a hut by the river with my brother Chekaren. We were sleeping. Suddenly we both woke up at the same time. Somebody shoved us. We heard whistling and felt strong wind. Chekaren said, "can you hear all those birds flying overhead?" We were both in the hut, couldn't see what was going on outside. Suddenly, I got shoved again, this time so hard I fell into the fire. I got scared. Chekaren got scared too. We started crying out for father, mother, brother, but no one answered. There was noise beyond the hut, we could hear trees falling down. Me and Chekaren got out of our sleeping bags and wanted to run out, but then the thunder struck. This was the first thunder. The Earth began to move and rock, wind hit our hut and knocked it over. My body was pushed down by sticks, but my head was in the clear. Then I saw a wonder: trees were falling, the branches were on fire, it became mighty bright, how can I say this, as if there was a second sun, my eyes were hurting, I even closed them. It was like what the Russians call lightning. And immediately there was a loud thunderclap. This was the second thunder. The morning was sunny, there were no clouds, our Sun was shining brightly as usual, and suddenly there came a second one!
Me and Chekaren had some difficulty getting under from the remains of our hut. Then we saw that above, but in a different place, there was another flash, and loud thunder came. This was the third thunder strike. Wind came again, knocked us off our feet, struck against the fallen trees.
We looked at the fallen trees, watched the tree tops get snapped off, watched the fires. Suddenly Chekaren yelled "Look up" and pointed with his hand. I looked there and saw another flash, and it made another thunder. But the noise was less than before. This was the fourth strike, like normal thunder.
Now I remember well there was also one more thunder strike, but it was small, and somewhere far away, where the Sun goes to sleep.
On the 17th of June, around 9 in the AM, we observed an unusual natural occurrence. In the N Karelinski village (200 verst N of Kirensk) the peasants saw to the North-West, rather high above the horizon, some strangely bright (impossible to look at) bluish-white heavenly body, which for 10 minutes moved downwards. The body appeared as a "pipe", i.e. a cylinder. The sky was cloudless, only a small dark cloud was observed in the general direction of the bright body. It was hot and dry. As the body neared the ground (forest), the bright body seemed to smudge, and then turned into a giant billow of black smoke, and a loud knocking (not thunder) was heard, as if large stones were falling, or artillery was fired. All buildings shook. At the same time the cloud began emitting flames of uncertain shapes. All villagers were stricken with panic and took to the streets, women cried, thinking it was the end of the world.
The author of these lines was meantime in the forest about 6 verst N of Kirensk, and heard to the NE some kind of artillery barrage, that repeated in intervals of 15 minutes at least 10 times. In Kirensk in a few buildings in the walls facing north-east window glass shook.
When the meteorite fell, strong tremors in the ground were observed, and near the Lovat village of the Kansk uezd two strong explosions were heard, as if from large-caliber artillery.
Kezhemskoe village. On the 17th an unusual atmospheric event was observed. At 7:43 the noise akin to a strong wind was heard. Immediately afterwards a horrific thump sounded, followed by an earthquake which literally shook the buildings, as if they were hit by a large log or a heavy rock. The first thump was followed by a second, and then a third. Then - the interval between the first and the third thumps were accompanied by an unusual underground rattle, similar to a railway upon which dozens of trains are traveling at the same time. Afterwards for 5 to 6 minutes an exact likeness of artillery fire was heard: 50 to 60 salvoes in short, equal intervals, which got progressively weaker. After 1.5 - 2 minutes after one of the "barrages" six more thumps were heard, like cannon firing, but individual, loud, and accompanied by tremors.
The sky, at the first sight, appeared to be clear. There was no wind and no clouds. However upon closer inspection to the North, i.e. where most of the thumps were heard, a kind of an ashen cloud was seen near the horizon which kept getting smaller and more transparent, and possibly by around 2-3 p.m. completely disappeared.

[edit] History

Surprisingly, there was little scientific curiosity about the impact at the time, possibly owing to the isolation of the Tunguska region. If there were any early expeditions to the site, the records were lost during the subsequent chaotic years — World War I, the Russian Revolution of 1917, 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 meteoritic iron that could be salvaged to aid Soviet industry. The iron would more than pay for the expedition alone.

Photograph from Kulik's 1927 expedition.
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Photograph from Kulik's 1927 expedition.

Kulik's party 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 (30 mi) 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, hinting that they were of extraterrestrial origin. But there are problems in assigning the particles to the Tunguska event, especially as Tunguska occurred in the ancient volcanic region with iridium abundance.

Detailed systematic eyewitness reports began to be gathered as late as 1959, when interviews were conducted with many of the indigeneous people who had been within 100 kilometres (60 mi) of the explosion. Most of these accounts claimed that the local people had been covered with boils after the explosion, with whole families dying off. The medical scientists attached to the expedition concluded that there had been an epidemic of smallpox in the area at the time. Expeditions led by Gennady Plekhanov found no elevated levels of radiation, which might have been expected had the detonation been nuclear in nature.

[edit] Earth Impactor hypothesis

[edit] Meteoroid airburst

In scientific circles, the leading explanation for the explosion is the airburst of a meteoroid 6 to 10 kilometers (4–6 mi) above the Earth's surface.

Meteoroids enter the Earth's atmosphere from outer space every day, usually travelling at a speed of more than 10 kilometers per second (6 mi/sec). Most are small but occasionally a larger one enters from space. The heat generated by compression of air in front of the body as it travels through the atmosphere is immense and most meteoroids burn up or explode before they 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 meteoroid airbursts occur rather frequently. A stony meteoroid of about 10 meters (30 ft) in diameter can produce an explosion of around 20 kilotons, similar to that of the Little Boy bomb dropped on Hiroshima, and data released by the U.S. Air Force's Defense Support Program indicate that such explosions occur high in the upper atmosphere more than once a year. Tunguska-like megaton-range events are much more rare. Eugene Shoemaker estimated that such events occur about once every 300 years.

[edit] Blast patterns

The curious effect of the Tunguska explosion on the trees near ground zero was replicated during atmospheric nuclear tests in the 1950s and 1960s. These effects are caused by the shock wave produced by large explosions. 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 (made of matches) 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 had exploded in mid-air.

[edit] 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, the analysis of samples from the area has shown it to be rich in cometary material.

In 1978, Slovak astronomer Ľubor Kresák 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 that bodies of this kind explode at frequent intervals tens to hundreds of kilometres above the ground. Military satellites have been observing these explosions for decades.

In 1983, astronomer Zdeněk 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.

[edit] Unexplained phenomena

There are still some circumstances that have not been convincingly explained. 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. Attempts to apply carbon-14 dating have shown that the soil was enriched in radioactive carbon-14 [citation needed].

The Russian geologist Vladimir Epifanov and German astrophysicist Wolfgang Kundt have suggested that the explosion was of methane gas emitted from the earth. Something similar seems to have occurred in 1994 near the village of Cando in Spain. Compare to Cando event. See 'New Scientist', 7 Sept. 2002, p. 14 [1] [2].

The Russian physicist Dr. Ol'khovatov points to problems with asteroidal and cometary interpretations, and inclines to think that Tunguska was a geophysical event (see his web-article, and his published abstract).

[edit] 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, as well as a paucity of scientific data about Tunguska due to Soviet secretiveness during the Cold War, 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.

[edit] Black hole

In 1973, Albert A. Jackson IV and Michael P. Ryan, Jr., physicists at the University of Texas, proposed that the Tunguska event was caused by a "small" (around 10-20 g to 10-22 g) 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, Stephen Hawking's subsequent hypothesis that black holes radiate energy via Hawking radiation indicates that such a small black hole would have evaporated away long before it could encounter the Earth.

[edit] 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.

[edit] Natural H-bomb

In 1989, the astronomers D'Alessio and Harms suggested that some of the deuterium in a comet entering the Earth atmosphere may have undergone a nuclear fusion reaction, leaving a distinctive signature in form of Carbon-14. They concluded that the release of nuclear energy may have been almost negligible. Independently, in 1990, César Sirvent proposed that a deuterium comet, i.e., a comet with an anomalous high concentration of deuterium into its composition, may have exploded as a natural hydrogen bomb, generating most of the energy released. The sequence would be first a mechanical or kinetic explosion, and instants later a thermonuclear reaction generated by this first explosion.

[edit] 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".

[edit] 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 one of Robert Peary's North Pole expeditions. It is claimed that Tesla 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, by the time of the Tunguska event most work at Wardenclyffe had already ended and the site was mostly abandoned. In addition, it is by no means apparent how the small energy input at Wardenclyffe could be responsible for such a large energy output elsewhere. Also this event happened on June 30, 1908 and Peary didn't leave New York for the North Pole until July 6, 1908. Long distance didn't take that long even in 1908.

[edit] Location of event site disputed

Another hypothesis asserts that a meteor fell in a different area of Siberia.

Russian mineralogist Leonid Kulik identified the place of impact in a forest near the Podkamennaja River (coordinates 60° 53' 40" N latitude and 101° 53' 40" E longitude.) Between 1921 and 1938 Kulik organized 5 expeditions to the area, but neither a crater nor other evidence of the impact was found.

The photos of the blasted forest and felled trees, made by Kulik in 1927 and 1928, are not convincing: they appear to be in a perfect state of preservation 20 years after the event, while the only trees still alive are young saplings that can hardly be more than a few years old. The trees photographed by Kulik were probably felled by the Evenki, the local inhabitants, in order to create pasture for reindeer, to construct their characteristic conical log huts, and to collect firewood.

In addition, other evidence suggests the craters found were a natural formation caused by melting frost, and a large rock originally identified as a meteorite was later recognized to be a common morainic stone. Kulik and his associates, however, strongly asserted that they had found the exact spot at which the event had occurred so as not to damage their reputations as competent scientists and researchers.

[edit] Tunguska event in literature, comics, games, film, TV, and songs

Main article: Tunguska event in fiction

[edit] TV shows

[edit] References

  • Lerman, J. C., Mook, W. G. & Vogel, J. C. Nature, 216, 990–1 (1967).
  • Cowan, C., Atluri, C. R. & Libby, W. F. Nature 206, 861–865 (1965).
  • Brown, J.C, and Hughes, D.W. Nature 268, 512 - 514 (1977)
  • Ol'khovatov, A.Yu. Earth, Moon and Planets, v.93, pp.163-173 (2003)

[edit] Books on Tunguska

  • John Baxter and Thomas Atkins, The Fire Came By: The Riddle of the Great Siberian Explosion, Macdonald and Jane's, London 1975
  • Rupert Furneaux, The Tungus Event, Nordon Publications, New York, 1977
  • Roy A. Gallant, The Day the Sky Split Apart: Investigating a Cosmic Mystery, Atheneum Books for Children, New York, 1995
  • E.L. Krinov, Giant Meteorites, trans. J.S. Romankiewicz (Part III: The Tunguska Meteorite), Pergamon Press, Oxford, 1966
  • Jack Stoneley, Cauldron of Hell: Tunguska, Simon and Schuster, New York, 1977
  • Surendra Verma, The Tunguska Fireball: Solving One of the Great Mysteries of the 20th Century, Icon Books, Cambridge, 2005

[edit] See also

[edit] External links

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