[meteorite-list] 100 Years of Space Rock: The Tunguska Impact

[Sterling K. Webb]

Hi, List,

> The resulting seismic shockwave registered with
> sensitive barometers as far away as England...

    Where ARE all you geologists? Do you measure seismic shock
with a barometer? How you do that? There is an entanglement of
two data sets here.

    "...the seismographic center at Irkutsk, 550 miles to the south,
registers tremors of earthquake proportions. Vibrations travel
3,000 miles through the ground to other stations in Moscow
and the capital, St Petersburg; and the earthquake observatory
at Jena, Germany, 3,240 miles away, records strong seismic shocks.
Even as far away as Washington, DC, and Java seismographs
are activated..."

    Barometers measure pressure, I believe...

    At a distance of 375 miles to the south-southwest, strong gusts
wind rattle buildings in Kansk, a station town on the Trans-Siberian
Railway. Two additional waves of seismic shock strike the town.
People working nearby on rafts are hurled into the river, while
the Trans-Siberian Express train is jarred and shakes wildly on
its visibly vibrating tracks; the train is halted. There is a "black
rain" of dust, dirt, tiny debris and carbonized particles.

    "Within five hours of the blast, turbulent air waves travel west
beyond the North Sea, causing strong oscillations at meteorological
stations in England. During a span of twenty minutes, sudden
fluctuations in atmospheric pressure are detected by recently
invented self-recording barographs at all six stations between
Cambridge, 50 miles north of London, and Petersfield, 55
miles south.

    Much later analysis of the barographic record shows that the
detectible pressure wave passed completely around the planet
and was registered again. (Traces of a third pass are disputed.)
The flash of the explosion was seen 225 miles away and the
sound was reported from 575 miles away

    In 1930, in the Royal Meteorological Society Quarterly
Journal, Spenser Russell gives an account of the odd colors
he observed over England in 1908 on the nights of June 30
and July 1: 'A strong orange-yellow light became visible in the
north and northeast ... causing an undue prolongation of twilight
lasting to daybreak on July 1st, when the eastern sky was an intense
green to yellow-gold hue. ... The entire northern sky on these two
nights, from the horizon to an altitude of 40º, was of a suffused
red hue varying from pink to an intense crimson. There was a
complete absence of scintillation or flickering, and no tendency
for the formation of streamers, or a luminous arch, characteristic
of auroral phenomena. ... Twilight on both of these nights was
prolonged to daybreak, and there was no real darkness. ... The
phenomenon was reported from various places in the United
Kingdom and on the Continent, from Copenhagen, Konigsberg,
Berlin, and Vienna.'

    According to the London Times of July 4, 1908, 'The remarkable
ruddy glows which have been seen on many nights lately have
attracted much attention, and have been seen over an area extending
as far as Berlin.' ..." On July 5, there is a New York Times story
from Britain entitled "Like Dawn at Midnight." And so on, you
read newspapers at midnight all over Europe...

    The outside world knew nothing about the event, of course. The
newspaper in Tomsk heard and reported a garbled account of a
meteorite falling "near the tracks" at Kansk in July, 1908. They sent
a reporter to Kansk who found no meteorites and concluded that if
one fell it was far away from Kansk (which it was).

    A week later the Tomsk newspaper, still dubious about that
meteorite story, suggested that the event near Kansk had been
an earthquake, followed by "a subterranean crash and a roar as
from distant firing. Doors, windows, and the lamps before icons
were all shaken. Five to seven minutes later a second crash followed,
louder than the first, accompanied by a similar roar and followed after
a brief interval by yet another crash."

    It was finding clippings of this and other newspaper stories
12 years later that got Kulik interested in locating "the fall." Getting
to the area soon showed him the "fall" was elsewhere and he widely
circulated flyers asking for reminisences. (I think he may have beat
Nininger to this trick; it was 1921.)

    S.V. Obruchev, a geologist conducting research along the Stony
Tunguska River in the summer of 1924, encountered such superstitious
awe among the natives about the blast, which he presumed had been
caused by the impact of a large meteorite, that he wrote, "In the
eyes of the Tungusi people, the meteorite is apparently sacred,
and they carefully conceal the place where it fell." As Kulik was
later to discover on his second Siberian journey, many Tungus were
afraid to talk about the explosion and some completely denied its
existence. Others reluctantly admitted to Obruchev only that a
huge area of "flattened forest" could be found by traveling three
or four days northeast of Vanavara to a wild and almost inaccessible
part of the country near the Chambe and Khushmo rivers. Another
local report sent to Kulik stated that, according to the Tungus,
at least a thousand reindeer had been killed and several of their
nomadic villages had vanished during the explosion.

    At a distance of 25 miles from the site is the closest where merely
injured individuals and destroyed and damaged buildings (huts) are
found. According to... Akulina, who was questioned in 1926 by
ethnographer I. M. Suslov, the entire family in the tent was thrown
into the air and several knocked out by the explosion. The tent was
approximately 25 miles southeast of the blast site. When Akulina and
her husband woke up, Suslov reported, they saw "the forest blazing
around them with many fallen trees. There was also a great noise."
Suslov spoke to an elderly Tungus who had been sharing the tent with
the family and recorded this story:

    "Vasily had been sleeping at the moment when the tent was torn
away and had been thrown to the side by a powerful jolt. He had not
lost consciousness. He said that he heard an unbelievably loud and
continuous thunder; the ground shook, burning trees fell, and all
around there was smoke and haze. Soon the thunder stopped, the wind
ceased, but the forest continued to burn."

    In 1926, A. V. Voznesensky, former head of the Irkutsk Observatory,
using information acquired by Kulik and Obruchev, as well as earlier
seismic data from Irkutsk and other Russian stations and observations
of acoustical phenomena throughout central Siberia, attempted to
trace the path of the body and determine its impact point. He found
that the effects of the explosion had been seen and heard by people
over an incredibly immense geographical area, one larger than France
and Germany combined. The "fiery object" racing through the cloudless
sky had been observed by thousands from the southern border of
Siberia to the Tunguska region, while the noise of the explosion,
the heavy claps, and rumblings "like thunder" were audible for a
radius of 500+ miles. From these reports and the seismic data, he was
able to gauge the time of the blast at about 7:17 A.M. on June 30,
1908. The place of the fall, he estimated, was in the territory north
of Vanavara. He calculated a hole bigger than Arizona's Meteor Crater.
He gave his results to Kulik, who set off in 1927.

    There's actually a lot of useful data in the witness reports. For
example, If you look at the fired forest at 25 miles, the feeling that
one's shirt had burst into flame (but hadn't) at 40 miles and the reported
secondary shadows half as dense as the morning sun's shadows at a
station 125 miles away, there have three data points: the infra-red flux
at 25 and 40 miles and the visible light flux at 125 miles. This is
sufficient to construct a rough "black-body" curve of the event (with
about +/- 20% accuracy) and determine the energy and peak temperature
of the event.

    The barometric traces from England were compared to nuclear
airbursts of 15 and 25 megatons respectively, measured at the same
distance (5270 km), by the meteologist E. L. Deacon in 1982. The
traces show stronger and sharper excursions than either nuclear event.
Deacon suggests the range of 30 to 40 megatons is the best fit.
The "black body" calculation (above) yields a result of 32.7 (+/-5.6)
megatons.

    On April 13, 1927, Kulik, one assistant, and his Tungus guide, all
suffering from scurvy, reached the banks of the small Makirta River
and the edge of 830 square miles of flattened forest that stretched to
the horizon, a circle roughly 15 to 20 miles across.

    Kulik wrote: "I still cannot sort out my chaotic impressions
of this excursion. Above all, I cannot really take in the whole
majestic picture of this unique meteorite fall. A very hilly,
almost mountainous, region stretches away tens of versts
towards the northern horizon. In the north the distant hills
along the River Khushmo are covered with a white shroud
of snow half a metre thick. From our observation point no
sign of forest can be seen... One has an uncanny feeling when
one sees 20 to 30-inch thick giant trees snapped across like
twigs, and their tops hurled many metres away to the south.

    It took Kulik weeks to reach the epicenter, even though
every tree down pointed at it and a small stand of dead trees
with all their branches stripped marked the spot. There was,
as we know, no crater to be found there. Kulik made more
expeditions, dug up the river valley, took along a motion picture
photographer, gave lectures...

    For the next decade, Kulik obstinately persisted in his conviction
that under the swamp lay "crushed masses of ... nickeliferous iron,
individual pieces of which may have a weight of one or two hundred
metric tons." The original meteorite, he estimated, probably weighed,
before falling into the earth's atmosphere, "as much as several
thousands of metric tons." His companion on the second expedition,
Sytin, guessed that the value of the metal might be between 100 and
200 million dollars, chiefly for the iron and platinum. Following
his 1928 trip to central Siberia, Kulik had given a lecture, accompanied
by Strukov's motion picture of the Tunguska destruction, to a Moscow
audience that, according to the report of the New York Times, "shivered"
as he outlined one of the more alarming implications of the event:

    "Astronomers and geologists know that this was an exceptional
circumstance. But they know also that there is no reason whatever
why a similar visitation should not fall at any moment upon a more
populous region. Thus, had this meteorite fallen in Central Belgium,
there would have been no living creature left in the whole country;
on London, none left alive in South [of] Manchester or East [of]
Bristol. Had it fallen on New York, Philadelphia might have escaped
with only its windows shattered, and New Haven and Boston escaped,
too. But all life in the central area of the meteor's impact would
have been blotted out instantaneously."

    In 1933, Nininger urged an American expedition to Tunguska
to search for "the meteorite." He couldn't get the funding... Kulik
(like Barringer) never found his vast mass of "nickeliferous iron."
In 1938-39 he attempted aerial mapping (unsuccessfully), cut a road
through the forest to the site, and built an airstrip.

    On July 5, 1941, at the beginning of the Nazi advance into
Russia, Kulik "volunteered" for the Moscow People's Militia,
a home guard unit composed chiefly of older men like himself
with little military training. Despite the Soviet Academy's request
that, because of his achievements for the Committee on Meteorites,
he be excused from service, Kulik remained in the home guard.
In October, while taking part in a battle on the front line, Kulik
was wounded in the leg and captured by the advancing German
Army. Imprisoned in a Nazi camp in Spas-Demensk, in the
Smolensk district, the fifty-eight-year-old scientist contracted
typhus and died on April 24, 1942.


Sterling K. Webb
-----------------------------------------------------------------------------
----- Original Message ----- 
From: "Ron Baalke" <baalke at zagami.jpl.nasa.gov>
To: "Meteorite Mailing List" <meteorite-list at meteoritecentral.com>
Sent: Friday, June 27, 2008 7:20 PM
Subject: [meteorite-list] 100 Years of Space Rock: The Tunguska Impact



http://www.jpl.nasa.gov/news/features.cfm?feature=1769

100 Years of Space Rock: The Tunguska Impact
Jet Propulsion Laboratory
June 27, 2008

At around 7:17 on the morning of June 30, 1908, a man based at the
trading post at Vanavara in Siberia is sitting on his front porch. In a
moment, 40 miles from the center of an immense blast of unknown origin,
he will be hurled from his chair and the heat will be so intense he will
feel as though his shirt is on fire. The man at the trading post, and
others in a largely uninhabited region of Siberia, near the Podkamennaya
Tunguska River, are to be accidental eyewitnesses to cosmological history.

"If you want to start a conversation with anyone in the asteroid
business all you have to say is Tunguska," said 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."

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 a few yards. 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.

"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. On this 100th anniversary of
the Tunguska event, does that mean we have 200 years of largely
meteor-free skies?

"Not necessarily," said Yeomans. "The 300 years between Tunguska-sized
events is an average based on our best science. I think about Tunguska
all the time from a scientific point of view, but the thought of a
another Tunguska does not keep me up at night."

------------------------------------------------------------------------

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