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Tunguska event

The Tunguska Event, or Tunguska explosion, was a powerful explosion that occurred near the Podkamennaya (Lower Stony) Tunguska River in what is now Krasnoyarsk Krai of Russia, at around 7:14 a.m. (0:14 UT, 7:02 a.m. local solar time) on June 30, 1908 (June 17 in the Julian calendar, in use locally at the time).

Although the cause is the subject of some debate, the explosion was most likely caused by the air burst of a large meteoroid or comet fragment at an altitude of 5–10 kilometres (3–6 miles) above Earth's surface. Different studies have yielded varying estimates for the object's size, with general agreement that it was a few tens of metres across.

Although the meteor or comet burst in the air rather than directly hitting the surface, this event is still referred to as an impact. Estimates of the energy of the blast range from 5 megatons to as high as 30 megatons of TNT, with 10–15 megatons the most likely - roughly equal to the United States' Castle Bravo thermonuclear explosion set off in late February 1954, about 1,000 times as powerful as the bomb dropped on Hiroshima, Japan and about one third the power of the Tsar Bomba, the largest nuclear weapon ever detonated. The explosion knocked over an estimated 80 million trees over 2,150 square kilometres (830 square miles). It is estimated that the earthquake from the blast would have measured 5.0 on the Richter scale, which was not yet developed at the time. An explosion of this magnitude is capable of destroying a large metropolitan area. This possibility has helped to spark discussion of asteroid deflection strategies.

Although the Tunguska event is believed to be the largest impact event on land in Earth's recent history, impacts of similar size in remote ocean areas would have gone unnoticed before the advent of global satellite monitoring in the 1960s and 1970s.

Description

At around 7:17 a.m. local time, 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 sound similar to artillery fire. Eyewitnesses closer to the explosion reported the sound source moving 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. Eyewitness accounts differ as to the sequence of events and their overall duration.

The explosion registered on seismic stations across Eurasia. Although the Richter scale was not developed until 1935, in some places the shock wave would have been equivalent to an earthquake of 5.0 on the Richter scale. It also produced fluctuations in atmospheric pressure strong enough to be detected in Great Britain. Over the next few weeks, night skies were aglow such that one could read in their light, caused by dust suspended in the stratosphere by the explosion. In the United States, the Smithsonian Astrophysical Observatory and the Mount Wilson Observatory observed a decrease in atmospheric transparency that lasted for several months, also from the suspended dust.

The Guinness Book of World Records (1966 edition) states that because of the rotation of Earth, if the collision had occurred 4 hours 47 minutes later, it would have completely destroyed the Imperial Russian capital, Saint Petersburg.

Selected eyewitness reports

  • Testimony of S. Semenov, as recorded by Leonid Kulik's expedition in 1930.

"At breakfast time I was sitting by the house at Vanavara trading post (65 kilometres/40 miles south 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. Chekaren and I 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!

"Chekaren and I had some difficulty getting out from under 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."

History

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, the records were likely to have been lost during the subsequent chaotic years — World War I, the Russian Revolution of 1917, and the Russian Civil War.

The first recorded expedition 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 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 miles) 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 arranged 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 silicate and magnetite spheres in siftings of the soil. Similar spheres were predicted to exist in the felled trees, although they could not be detected by contemporary means. Later expeditions did identify such spheres in the resin of the trees, however. Chemical analysis showed that the spheres contained high proportions of nickel relative to iron, which is also found in meteorites, leading to the conclusion they were of extraterrestrial origin. The concentration of the spheres in different regions of the soil was also found to be consistent with the expected distribution of debris from a meteorite airburst. Later studies of the spheres found unusual ratios of numerous other metals relative to the surrounding environment, which was taken as further evidence of their extraterrestrial origin.

Chemical analysis of peat bogs from the area also revealed numerous anomalies considered consistent with an impact event. The isotopic signatures of stable carbon, hydrogen, and nitrogen isotopes at the layer of the bogs corresponding to 1908 were found to be inconsistent with the isotopic ratios measured in the adjacent layers, and this abnormality was not found in bogs located outside the area. The region of the bogs showing these anomalous signatures also contains an unusually high proportion of iridium, similar to the iridium layer found in the K–T boundary. These unusual proportions are believed to result from debris from the impacting body that deposited in the bogs. The nitrogen is believed to have been deposited as acid rain, a suspected fallout from the explosion.

Earth impactor

Meteoroid airburst

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

Meteoroids enter Earth's atmosphere from outer space every day, usually travelling at a speed of more than 10 kilometres per second (6 miles/sec or 21,600 mph). Most are small but occasionally a larger one enters. The heat generated by compression of air in front of the body (ram pressure) as it travels through the atmosphere is immense and most meteoroids burn up or explode before they reach the ground. Since the second half of the 20th century, close monitoring of Earth's atmosphere has led to the discovery that such meteoroid airbursts occur rather frequently. A stony meteoroid of about 10 metres (30 ft) in diameter can produce an explosion of around 20 kilotons, similar to that of the Fat Man bomb dropped on Nagasaki, 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 rarer. Eugene Shoemaker estimated that such events occur about once every 300 years.

Blast patterns

The explosion's effects 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 knocked over 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.

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" or "bright nights") observed across Europe for several evenings after the impact, possibly explained by dust and ice that had been dispersed from the comet's tail across the upper atmosphere. The cometary hypothesis gained a general acceptance amongst Soviet Tunguska investigators by the 1960s.

In 1978, Slovak astronomer Ľubor Kresák suggested that the body was a fragment 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, and the approximate trajectory of the Tunguska impactor is consistent with what would be expected from such a fragment. 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 to have been 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.

During the 1990s, Italian researchers extracted resin from the core of the trees in the area of impact, to examine trapped particles that were present during the 1908 event. They found high levels of material commonly found in rocky asteroids and rarely found in comets.

Lake Cheko

In June 2007 it was announced that scientists from the University of Bologna had identified a lake in the Tunguska region as a possible impact crater from the event. They do not dispute that the Tunguska body exploded mid-air, but believe that a one meter fragment survived the explosion and impacted the ground. Lake Cheko is a small bowl shaped lake approximately 8 kilometres north-north-west of the hypocenter. The hypothesis has been disputed by other impact crater specialists. A 1961 investigation had dismissed a modern origin of Lake Cheko, saying that the presence of metres thick silt deposits at the lake's bed suggests an age of at least 5,000 years, however more recent research suggests that only a meter or so of the sediment layer on the lake bed is "normal lacustrine sedimentation", a depth indicating a much younger lake, about 100 years. Acoustic-echo soundings of the lake floor provide support for the hypothesis that the lake was formed by the Tunguska event. The soundings revealed a conical shape for the lake bed, which is consistent with an impact crater. Magnetic readings indicate a possible meter-sized chunk of rock below the lake's deepest point, which may be a fragment of the colliding body. Finally, the lake's long axis points to the hypocenter of the Tunguska explosion, about 7.0 km away. Work is still being done at Lake Cheko to determine its origins.

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 lack of scientific data about Tunguska due to Soviet secrecy during the Cold War, a great many other hypotheses for the Tunguska event have sprung up, none of which are accepted by the scientific community.

End of the World?

Perhaps the earliest widely-held theory for the Tunguska explosion was that the world was about to end. As the minutes passed, this theory was dropped in favour of other, less final theories, until today one is hard-pressed to find anyone who truly believes the world ended on the morning of 30 June 1908...

According to G. K. Kulesh, head of the Kirensk Meteorological Station

The peasants of the village [of Korelina] were so stunned by the crashes that they sent a deputation to town to the local archpriest to ask if the end of the world was beginning, [and] how they were preparing for it in Kirensk.

Natural H-bomb

In 1989, D'Alessio and Harms suggested that some of the deuterium in a comet entering the Earth's atmosphere may have undergone a nuclear fusion reaction, leaving a distinctive signature in the form of carbon-14. They concluded that any release of nuclear energy would 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 in its composition, could have exploded as a natural hydrogen bomb, generating most of the energy released. The sequence would be first a mechanical or kinetic explosion, triggering a thermonuclear reaction. These proposals are inconsistent with knowledge both of the composition of comets and of the temperature and pressure conditions necessary for initiating a nuclear fusion reaction. Studies have found the concentration of radioactive isotopes in the blast region to be inconsistent with those expected following a nuclear explosion, fusion or otherwise.

Black hole

In 1973, Albert A. Jackson and Michael P. Ryan, physicists at the University of Texas, proposed that the Tunguska event was caused by a "small" (around 1017 kg to 1019 kg) black hole passing through the Earth. This hypothesis fails, as there was no so-called "exit event" — a second explosion occurring as the black hole, having tunnelled through the Earth, shot out the other side on its way back into space, which would have happened in the Southern Ocean, about 10 degrees west of the Drake Passage — nor were there the continuous seismic disturbances that would occur along the hole's path through the mantle.

The hypothesis was used by Larry Niven in his science fiction story The Borderland of Sol, and by David Brin, in his novel Earth, as well as by Bill DeSmedt in his novel Singularity. It also figures in Dan Simmons's Hyperion Cantos novels.

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.

UFO crash

Various UFO aficionados have claimed that the Tunguska event was the result of an exploding alien spaceship or even an alien weapon going off to "save the Earth from an imminent threat". These claims appear to originate from a science fiction story penned by Soviet engineer Alexander 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 (Phenomenon: The Lost Archives), 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 from the Tunguska Space Phenomenon Public State Fund claimed to have found the wreckage of an alien spacecraft at the site.

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 Soviet space dogs (who survived).

Geophysical hypotheses

Astrophysicist Wolfgang Kundt has suggested the Tunguska event was caused by the sudden release and subsequent explosion of 10 million tonnes of natural gas from within the Earth's crust. The similar Verneshot hypothesis has also been suggested as a possible cause of the Tunguska event.

Similar events

The Tunguska event is the strongest, but not the only, significant meteorite airburst in recent history. A selection of similar events follows. This list is quite biased, since recording of meteorite explosive yields is relatively recent:

Date Place Yield of explosion (TNT equivalent) Height of explosion Remarks
June 30, 1908 westnorthwest of Vanavara, at 60°53' 09"N, 101°53' 40"E in Krasnoyarsk Krai, Imperial Russia 10–15 Mt Tunguska event
August 13, 1930 Curuçá River Area, Amazonas, Brazil 0.1-1.0 Mt
May 31, 1965 Southeast Canada 600 t material from meteorite found
September 17, 1966 Lake Huron, Michigan, United States 600 t No material from meteorite found
February 5, 1967 Vilna Alberta, Canada 600 t Two very small fragments found - and . Stored at University of Alberta, in Edmonton.
September 22, 1979 Southern Indian Ocean 2 kt
January 19, 1993 Lugo, Northern Italy > 10 kt > -20a
January 18, 1994 Cando, Spain
June 6, 2002 Mediterranean Sea between Libya and Greece 26 kt
September 25, 2002 Bodaybo, Russia 0.5 – 5 kt

Tunguska event in fiction

See also

References

  • Baxter, John and Thomas Atkins, The Fire Came By: The Riddle of the Great Siberian Explosion, Macdonald and Jane's, London 1975. ISBN 044689396X.
  • Brown, J.C, and Hughes, D.W. Nature 268, 512–514 (1977)
  • Furneaux, Rupert. The Tungus Event, Nordon Publications, New York, 1977. ISBN 058604423X.
  • Gallant, Roy A. The Day the Sky Split Apart: Investigating a Cosmic Mystery, Atheneum Books for Children, New York, 1995. ISBN 0689803230.
  • Gasperini, Luca, Bonatti, Enrico and Longo, Giuseppe. The Tunguska Mystery 100 Years Later, Scientific American, June, 2008
  • Krinov, E. L. Giant Meteorites, trans. J.S. Romankiewicz (Part III: The Tunguska Meteorite), Pergamon Press, Oxford, 1966
  • Morgan, J. Phipps, Ranero, C.R., Reston, T.J. Contemporaneous mass extinctions, continental flood basalts, and ‘impact signals’: are mantle plume-induced lithospheric gas explosions the causal link?, Earth and Planetary Science Letters. 217. 263-284 (2004)
  • Lerman, J. C., Mook, W. G. & Vogel, J. C. Nature, Effect of the Tunguska Meteor and Sunspots on Radiocarbon in Tree Rings, (9 December 1967) | doi:10.1038/216990a0; 216, 990–1 (1967)
  • Ol'khovatov, A.Yu. Earth, Moon and Planets, v.93, pp.163–173 (2003)
  • Stoneley, Jack. Cauldron of Hell: Tunguska, Simon and Schuster, New York, 1977. ISBN 0671229435.
  • Verma, Surendra. The Tunguska Fireball: Solving One of the Great Mysteries of the 20th century, Icon Books, Cambridge, 2005. ISBN 1840466200.
  • Verma, Surendra. The Mystery of the Tunguska Fireball, Icon Books, Cambridge, 2006. ISBN 1840467282.

Notes

External links

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