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June
30, 2008: The year is 1908, and it's just after seven
in the morning. A man is sitting on the front porch of a trading
post at Vanavara in Siberia. Little does he know, in a few
moments, he will be hurled from his chair and the heat will
be so intense he will feel as though his shirt is on fire.
That's
how the Tunguska event felt 40 miles from ground
zero.
Today,
June 30, 2008, is the 100th anniversary of that ferocious
impact near the Podkamennaya Tunguska River in remote Siberia--and
after 100 years, scientists are still talking about it.
"If
you want to start a conversation with anyone in the asteroid
business all you have to say is Tunguska," says Don Yeomans,
manager of the Near-Earth Object Office at NASA's Jet Propulsion
Laboratory. "It is the only entry of a large meteoroid
we have in the modern era with first-hand accounts."
Above:
Trees felled by the Tunguska explosion. Credit: the Leonid
Kulik Expedition. [more]
While
the impact occurred in '08, the first scientific expedition
to the area would have to wait for 19 years. In 1921, Leonid
Kulik, the chief curator for the meteorite collection of the
St. Petersburg museum led an expedition to Tunguska. But the
harsh conditions of the Siberian outback thwarted his team's
attempt to reach the area of the blast. In 1927, a new expedition,
again lead by Kulik, reached its goal.
"At
first, the locals were reluctant to tell Kulik about the event,"
said Yeomans. "They believed the blast was a visitation
by the god Ogdy, who had cursed the area by smashing trees
and killing animals."
While
testimonials may have at first been difficult to obtain, there
was plenty of evidence lying around. Eight hundred square
miles of remote forest had been ripped asunder. Eighty million
trees were on their sides, lying in a radial pattern.
"Those
trees acted as markers, pointing directly away from the blast's
epicenter," said Yeomans. "Later, when the team arrived
at ground zero, they found the trees there standing upright
– but their limbs and bark had been stripped away. They looked
like a forest of telephone poles." Such
debranching requires fast moving shock waves that break off
a tree's branches before the branches can transfer the impact
momentum to the tree's stem. Thirty seven years after the
Tunguska blast, branchless trees would be found at the site
of another massive explosion – Hiroshima, Japan.
Kulik's
expeditions (he traveled to Tunguska on three separate occasions)
did finally get some of the locals to talk. One was the man
based at the Vanara trading post who witnessed the heat blast
as he was launched from his chair. His account:
Suddenly
in the north sky… the sky was split in two, and high above
the forest the whole northern part of the sky appeared covered
with fire… At that moment there was a bang in the sky and
a mighty crash… The crash was followed by a noise like stones
falling from the sky, or of guns firing. The earth trembled.
The
massive explosion packed a wallop. The resulting seismic shockwave
registered with sensitive barometers as far away as England.
Dense clouds formed over the region at high altitudes which
reflected sunlight from beyond the horizon. Night skies glowed,
and reports came in that people who lived as far away as Asia
could read newspapers outdoors as late as midnight. Locally,
hundreds of reindeer, the livelihood of local herders, were
killed, but there was no direct evidence that any person perished
in the blast.
Above:
The location of the Tunguska impact.
"A
century later some still debate the cause and come up with
different scenarios that could have caused the explosion,"
said Yeomans. "But the generally agreed upon theory is
that on the morning of June 30, 1908, a large space rock,
about 120 feet across, entered the atmosphere of Siberia and
then detonated in the sky."
It
is estimated the asteroid entered Earth's atmosphere traveling
at a speed of about 33,500 miles per hour. During its quick
plunge, the 220-million-pound space rock heated the air surrounding
it to 44,500 degrees Fahrenheit. At 7:17 a.m. (local Siberia
time), at a height of about 28,000 feet, the combination of
pressure and heat caused the asteroid to fragment and annihilate
itself, producing a fireball and releasing energy equivalent
to about 185 Hiroshima bombs.
"That
is why there is no impact crater," said Yeomans. "The
great majority of the asteroid is consumed in the explosion."
Yeomans
and his colleagues at JPL's Near-Earth Object Office are tasked
with plotting the orbits of present-day comets and asteroids
that cross Earth's path, and could be potentially hazardous
to our planet. Yeomans estimates that, on average, a Tunguska-sized
asteroid will enter Earth's atmosphere once every 300 years.
"From
a scientific point of view, I think about Tunguska all the
time," he admits. Putting it all in perspective, however,
"the thought of another Tunguska does not keep me up
at night."
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Editor: Dr.
Tony Phillips | Credit: Science@NASA
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