Tunguska event
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The Tunguska event
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.
(Trees felled by the Tunguska
blast. Photograph from Kulik's 1927 expedition.)
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.
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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.
(Photograph from
Kulik's 1927 expedition.)
Meteorite
hypothesis
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.
This hypothesis
is discounted by mainstream scientific opinion.
http://en.wikipedia.org/wiki/Tunguska_event
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