http://en.rian.ru/analysis/20080306/100847639.html
Tunguska meteorite: a warning from outer space
MOSCOW. (Alexander Bagrov, for RIA Novosti) - Almost a century ago, on June 17
(30), 1908, a massive explosion occurred near the Podkamennaya Tunguska River,
in what is now Russia's Krasnoyarsk Territory, Central Siberia.
The residents of the Vanavara trading post, 65 kilometers (40 miles) south of
the blast site, later claimed that the ground trembled violently when attacked
by a huge ball of fire, followed by a terrible storm that destroyed everything
in its wake.
The explosion was most likely caused by the airburst of a large meteoroid or
comet fragment at an altitude of 5-10 kilometers (3-6 miles) from the Earth's
surface. Studies have yielded varying estimates for the object's size, with
most experts agreeing that it measured several dozen meters in diameter.
Estimates of the energy of the blast range from 5 megatons to as high as 30
megatons of TNT, with 10-15 megatons being the most likely yield. The blast,
about 1,000 times more powerful than the Hiroshima bomb, felled an estimated 80
million trees over 2,150 square kilkometers (830 square miles). The earthquake
caused by the blast measured 5.0 on the Richter scale. The region has never
completely recovered.
The Tunguska blast was the largest meteoroid impact in the Earth's recent
history, and demonstrated the awesome destructive power of near-space objects.
An explosion of the scale of the one in Tunguska could destroy large
metropolitan areas. It is this possibility that has helped to spark discussion
of asteroid deflection strategies.
Due to the rotation of Earth, if the collision had occurred 4 hours 47 minutes
later, it would have completely destroyed the Imperial Russian capital, St.
Petersburg. A little later still, and the Tunguska meteorite would have wreaked
chaos and destruction in densely populated Europe. Although scientists have
advanced over 80 theories explaining the Tunguska event, none of them offers
any conclusive evidence. Moreover, it is now impossible to verify them.
When I became an astronomer 30 years ago, I believed that the mystery of the
Tunguska meteorite would never be solved. However, astronomers have since then
obtained additional information about the origin of celestial bodies, and can
offer more convincing explanations for the Tunguska blast.
I am going to tell an upcoming international conference in Moscow to mark the
100th anniversary of the Tunguska event that it was most likely caused by a
comet fragment consisting of the Solar System's primary matter.
There are two kinds of comets in our Solar System. "Primary" comets consist of
micron-sized inter-stellar dust and gas, whereas "secondary" comets feature
meteorite substance. A disintegrating "secondary" comet forms the meteor
showers that are frequently observed from the Earth. Some of their fragments do
not burn up during reentry and can be recovered. On February 12, 1947, a large
meteorite disintegrated spilled fragments over a 1.3-sq. km. area in the
Sikhote-Alin range, some 440 km from Vladivostok in the Russian Far East.
Subsequent expeditions recovered many iron fragments there.
The Astronomy Institute's experts and I believe that if the Tunguska meteorite
were a "secondary" comet, then it would have contained several metric tons of
meteor substance, and some of it could have been recovered.
It would be therefore logical to assume that the Tunguska meteorite was a huge
gas-and-dust snowball, whose tiny fragments vaporized after hitting the
terrestrial atmosphere. The remaining comet-nucleus also vaporized instantly,
causing the loud and powerful air burst that was registered by many
observatories all over the world.
When a "primary" comet blows up, surviving comet-nucleus particles, including
tiny hard-to-melt dust, are dispersed in the Earth's atmosphere, subsequently
becoming embedded in tree bark. Unfortunately, scientists in 1908 lacked modern
methods for detecting space-bolide substances.
The Tunguska meteorite was a milestone in terrestrial history, highlighting the
dangers posed by killer asteroids. The other two known large meteorites that
fell in the sparsely populated Sikhote-Alin range on February 12, 1947 and on
August 13, 1930 in the Brazilian Amazonas region (approximately latitude 5° S.
and longitude 71.5° W.), near the Peruvian frontier, caused no substantial
damage.
Such narrow escapes are either a coincidence, or God's Providence. At the same
time, we know very little about meteorites falling into the world's oceans.
With the onset of the Space Age, humankind realized that the Earth was under
constant threat from celestial bodies. Asteroid-and-comet studies have soared
in popularity as people have become aware that these space objects have the
capability to wipe out human civilization. Even now, numerous near-Earth
satellites designed to detect ballistic-missile launches or unauthorized
nuclear tests annually register 10-15 airbursts equivalent to a megaton of TNT.
Most scientists believe that these explosions are the result of mini-comets
burning up in the atmosphere.
Although we still lack the technology to deflect approaching asteroids and
comets, we can watch the skies and evacuate the population of threatened areas
in case of an impending strike.
Hollywood films notwithstanding, it would be unforgivably dangerous to use
weapons to destroy an incoming asteroid. The resulting fragments could cause at
least as much damage as the original object. Probably the best solution would
be to land a small rocket on the asteroid's surface, exerting just enough force
to gently change its trajectory.. This would be far less spectacular, but could
save thousands of lives, as well as humankind's material and spiritual
heritage.
Doctor of Physics and Mathematics, Alexander Bagrov, is a leading research
associate of the Astronomy Institute of the Russian Academy of Sciences.
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