|
|
 |
|
|
|
register |
bbs |
search |
rss |
faq |
about
|
|
|
meet up |
add to del.icio.us |
digg it
|
|
|
|
The comet/meteorite impact in Siberia in 1908
Path: santra!tut!draken!kth!mcvax!uunet!husc6!mailrus!tut.cis.ohio-state.edu!ucbvax!hplabs!hp-pcd!hpx!gvg
From: [email protected] (Greg Goebel)
Newsgroups: sci.space
Subject: Re: Re: Asteroid Encounters
Message-ID: <[email protected]>
Date: 7 May 89 16:23:06 GMT
References: <[email protected]>
Organization: Hewlett-Packard Co., Corvallis, OR, USA
Lines: 280
Target: Tunguska
Andrew Chakin
SKY & TELESCOPE / JAN 84 / PP 18-21
* A grand dame of scientific mysteries -- the Tunguska event -- turned 75 last
summer, her charm very much intact. She continues to seduce both scientist and
charlatan alike, both hoping to explain what happened over a remote stretch of
Siberain taiga on June 30, 1908. All that can be said from direct
eyewitnesses is that a fireball nearly as bright as the Sun streaked to Earth
out a cloudless morning sky. The bolide's plunge was abruptly terminated by an
explosion so great that it registered on seismic stations across Eurasia. The
resulting shock wave circled the Earth twice.
Of course, much more has been said about the Tunguska fall by way of deduction,
educated guesswork, and outright speculation. Yet the tantalizing mystery has
endured, due to the timing of the event in the early 20th century: too late to
become lost in folklore, but a bit too early to be adequately recorded.
At 7:20 AM, a mighty noise was heard resolving into thundercracks, though the
sky was cloudless at the time. The noise caused houses to shake. Many
inhabitants saw "a fiery body like a beam" shoot from the northwest above the
ground before they heard the thunder. Immediately afterward the bang was
heard, and in the place the fiery body had disappeared, a "tongue of fire"
appeared, followed by "smoke".
Such accounts, published in July, 1908, hint at the strong impression the
meteor made on the inhabitants of central Siberia. But apparently scientists
took little note, because 13 years passed before their curiosity was stirred.
* In 1921 the Russian minerologist Leonid Kulik learned of the fall. He
included a visit to the Podkamennaya Tunguska River basin on his agenda that
year, during a survey of meteorite falls for the Soviet Academy of Sciences.
Eyewitnesses, prompted to remember the events of 13 years earlier, told Kulik
and his group tales of being knocked to the ground by the concussion, of entire
stretches of forest blown flat, and of heat so intense it could be felt at
Vanavara, 60 kilometers from the blast site.
Convinced of the enormity of the event, Kulik returned to Moscow determined to
persuade the young Soviet government to fund an expedition to the place of
fall. In 1927 Kulik's group journeyed to the "vast cauldron surrounded by an
amphitheatre of ridges and isolated summits" where they hoped to find the
remains of the meteorite. Instead they only saw the imprint of the catastrophe
-- fallen trees, scorched and stripped bare, strewn radially away from the
explosion's epicenter. Three more expeditions to the site over a decade turned
up nothing extraterrestial: Whatever had wrought such devastation apparently
left no trace of itself.
The explanations that have sprung up over the years read like a history of
scientific vogue. As fashions changed, fiery gods, antimatter meteors,
nuclear-powered UFOs, and tiny black holes were each put forth. More enduring
and more plausible has been the idea, first stated in 1930 by the British
astronomer F.J.W. Whipple, that the Earth was struck by a small comet.
Astronomers have reasoned that the dusty snowball nucleus of a comet would be
destroyed in the dense lower layers of the atmosphere. The sudden vaporization
of its ices would account for the tremendous explosion and would leave only
airborne dust. An active comet might also explain the skyglows witnessed over
much of Europe on the night of the event and for a few nights afterward.
Theorizers envision the comet's tail arriving sometime after the nucleus,
laying itself onto the atmosphere in a long, dusty cloud that would reflect
sunlight to the darkened ground.
In 1978 Czech astronomer Lubor Kresak renewed interest in the cometary
hypothesis by suggesting that a piece of Comet Encke had exploded at Tunguska.
He based this idea on the fact that the June 30th event occurred near the time
of the annual Beta Taurid meteor shower, which has long been thought to derive
from this comet. But is the accumulated evidence consistent with a cometary
body? Astronomer Zdenek Sekanina, at Caltech's Jet Propulsion Laboratory,
believes not.
Sekanina's lengthy treatise on the event, published in the September 1983 issue
of the Astronomical Journal, is the most comprehensive to date,
synthesizing some five decades of published reports. Methodically reviewing
each kind of evidence -- from eyewitness reports to patterns of tree scorching
-- Sekanina ties them together with assumptions to form a physical picture of
the event. He concludes that the results do not match the expected behavior of
a comet in the Earth's atmosphere.
The majority of evidence points to only one explosion, Sekanina notes, so the
Tunguska body could not have split into fragments earlier in its flight. In
addition, the sheer magnitude of the blast argues against an early breakup,
which would have dissipated energy. Most important is that the bolide remained
intact down to a height of only 8.5 km, though it moved on a shallow trajectory
that subjected it to prolonged and ever-increasing stresses.
Sekanina believes it is "inconceivable" that a fragment of cometary material
could have survived a plunge into the lower atmosphere. Instead, it should
have behaved as the 200-ton chunk of material that caused the Sumava fireball
[where in hell is Sumava? -- gvg] which entered the atmosphere at a speed
of 26 km per second and was destroyed in little more than three seconds.
Sekanina explains that at the height the Tunguska object exploded, something
moving at such a speed, which is normal for comets, would have experienced
nearly 1000 times more resistance from the air than normally destroys cometary
meteors (such as those from annual showers). This is convincing evidence, he
says, that the Siberian meteor must have been a denser, rocky object --
probably a small member of the Apollo group of asteroids, whose orbits cross
the Earth's.
At first the idea seems to ignore the essential question: Why were no
meteorites found? For clues Sekanina turned to photographic patrol data on
fireballs. Automatic cameras of the Prarie Network in the American Midwest and
the European Network, mostly in West Germany, have recorded the flights of
thousands of meteors during the past several decades. Because the brightness
of a glowing meteor is proportional to its size, something can be said about
the behavior of different-size objects recorded by the patrols.
The trails of many small meteors suddenly become a collection of streaks as the
parent object breaks apart. Larger meteors tend not to fragment but to end
more often in brief, intense flares that indicate nearly instantaneous
destruction. A spectacular example was recorded on October 9, 1969, when a
huge chunk of probably stony material (estimates range from 10 to 500 tons)
flashed out of existence in only a tenth of a second. Harvard meteor
specialist Richard McCrosky has studied this astonishing fireball, whose
official designation is PN40503. "It's always been a puzzle to me," he admits.
"Even if it fragmented first, a tenth of a second is still an incredibly short
time."
The Tunguska meteor, whose mass has been estimated at a million tons,
apparently did the same thing. "There are strong parallels here with 40503,"
says Doug ReVelle of Northern Arizona University. Yet the means by which a
million-ton object destroys itself in an instant are not understood.
At the most basic level it appears that objects of the size estimated for
Tunguska -- about 100 meters across -- are too big to be slowed gradually by
the atmosphere but too small to survive passage. Instead, Sekanina says, they
continue to plummet virtually unimpeded until resistance from the air becomes
so great that "the atmosphere acts as a wall."
For Tunguska that point came 8.5 km above the taiga when the body
disintegrated into a cloud of particles. Slowing from 10 km per second to a
complete stop, the debris released its kinetic energy in an explosion
equivalent to a 12-megaton nuclear blast [roughly the size of the most
powerful class of nuclear weapon and two orders of magnitude greater than a
typical warhead]. Sekanina notes that pressures experienced by the meteorite
when it broke apart were within the range at which ordinary chondrites give way
in laboratory tests. [Just like smashing an aspirin with a hammer.]
The Siberian meteorite, it seems, was doomed by a combination of
characteristics. According to Sekanina, if it had been made of iron, like the
projectile of similar size that excavated Arizona's meteor crater, chunks of it
might have survived. Or, if it had been smaller, it might have been saved by a
breakup at a greater altitude where pressures were lower and deceleration more
gentle.
Agreement with Sekanina's hypothesis is not universal. Some researchers
question the validity of such a detailed analysis based on the sketchy and
sometimes conflicting information about the event. Eyewitness accounts, for
example, vary widely as to the direction and brightness of the bolide, so
Sekanina had to make certain assumptions in order to resolve contradictions.
McCrosky feels the available data is just too sparse: "You can't make a
sophisticated model from poor data," he says. McCrosky even believes his own
fireball data, used by Sekanina, is equally limited. "It seems everything he
assumes must be true for his conclusion to be right."
That Sekanina seems to have taken data reduction to great lengths reflects the
great inertia of the cometary hypothesis. "I think the reason he wrote such a
long paper," ReVelle offers, "is that people have been saying for 50 years that
Tunguska was the head of a small comet, and it just doesn't behave that way.
There is a very high probability that this thing was a type II (chondritic)
fireball."
What can be said about the path followed by the Tunguska body in space?
Sekanina deduces some basic characteristics of the orbit, and finds
disagreement with cometary paths. From the breakup height he believes the
meteorite was not traveling at much more than the Earth's escape velocity of 11
km per second when it entered the atmosphere. A much faster speed would have
brought greater aerodynamic stress, hastening the destruction. From this
constraint, Sekanina estimates the object's farthest (aphelion) distance from
the Sun as 1 to 1.5 astronomical units, within the range of both short-period
comets and asteroids. But the former, he points out, have perihelia and
aphelia near the orbital plane of Jupiter, whose gravity has tugged them into
their current paths. Asteroids show no such relationship, and Sekanina finds
the Tunguska body doesn't, either.
George Wetherill of Washington's Carnegie institute agrees. Wetherill, who
with ReVelle has attacked the problem of distinguishing cometary and asteroidal
meteors on Prarie Network photographs, says flatly, "The only evidence for
Tunguska being cometary was Kresak's idea [linking it with Beta Taurid
meteors]. That's circumstantial evidence." Sekanina, who specializes in comet
studies, is more emphatic. "The orbit of Encke simply does not intersect the
Earth's," he declares. So misaligned are Tunguska's and Encke's paths that
Sekanina dismisses the idea that they could be related.
Kresak, nevertheless, is standing firm. Speaking from his home in Bratislava,
he maintained: "If you take [Sekanina's] trajectory for the body and his light
curve, you find that the object would be invisible from many points from which
it was observed."
Kresak believes the discrepancies between Tunguska's calculated path and that
of comet Encke are no greater than that between other comets and the meteors
associated with them. "It doesn't only apply to small particles, [such as] the
Beta Taurids, which are subject to nongravitational forces, but also to larger
bodies -- some of the Prarie Network meteoroids and so on."
"I don't say I can explain the difference. But if one doesn't accept this
difference between Encke and Tunguska, one should not also accept the
difference between the Taurids and Comet Encke." Kresak is currently drafting
a reply to Sekanina's treatise.
Yet even if Kresak is wrong, if evidence really is stacking up against comets
in this cosmic whodunit, an additional burden of proof for asteroid advocates
lies in accounting for the "bright nights" of 1908. This appears not to be
very difficult. A significant fraction of the meteorite's million tons could
have been injected into the upper atmosphere, according to ReVelle, if not by
the blast wave then by eddies induced in the surrounding air.
Atmospheric scientist Richard Turco notes that the wind was blowing in the
right direction on June 30th for stratospheric dust to reach Western Europe.
Wind speeds of 80 meters per second (about 180 miles per hour) would have been
needed, but these are common in the stratosphere. Alternatively, he suggests
high-altitude ice clouds (which can brighten night skies by reflecting
sunlight) could have formed if the meteor were rich in water vapor.
Unfortunately, Turco thinks a chondrite would have been too dry to provide the
water needed for such clouds.
Sekanina admits the question is still open. "I tell you quite frankly that
there is a lot of handwaving in this. There isn't any quantitative theory
available that would give you so many tons of material at such and such a
height above the Earth's surface." Nevertheless, he is quick to add, the old
comet-tail hypothesis is untenable. Even the dustiest tail would not
contribute enough material to cause the glows seen.
While the finest particles drifted westward from Tunguska, some of the
vaporized remains of the meteorite condensed into particles a fraction of a
millimeter in size. They rained onto the devastated terrain, and microscopic
spheres of metal and glass were painstakingly sifted from the soil by
expeditions during the late 1950s and early 1960s. Lest there be any doubt of
their extraterrestial nature, Soviet researchers soon found abnormal
concentrations of nickel in the samples, indicative of meteoritic origin.
More recently, Ramachandran Ganapathy, a researcher with the JT Baker Chemical
CO in Phillipsburg NJ analyzed trace elements in several spheres and discovered
enrichments of Iridium, which is cosmically abundant but rare on Earth. "There
is no question about he," he declares, "the spheres are extraterrestial." He
adds that his finding of identical elemental ratios in each sample confirms the
origin of the spheres from one body.
As to the nature of their source, Ganapathy cautions that no distinction
between comets and asteroids can be made on his data. Only inorganic compounds
common to all chondrites and possibly included in comet nuclei would have been
included in the spheres, he says, because these elements would have been the
first to condense from the hot, vaporous remains of the meteorite.
Recently Ganapathy uncovered a Tunguskan imprint in the remote ice of
Antartica. Searching for additional manifestations of the fall, he scrutinized
ice laid down during the first 20 years of this century. A sample
corresponding to 1909, once melted, left microscopic residues that were rich in
meteoritic material -- submicron-size debris sticking to dust grains. If
derived from the 1908 event, the find could mean previous researchers
underestimated the size of the Siberian body. Using the South Pole data as a
clue to the total amount of atmospheric fallout from the event, Ganapathy
estimates the object was a seven-million-ton, 160-meter-diameter monster.
If you have read this far, whether a comet or an asteroid came to visit three
quarters of a century ago may suddenly seem less important than the prospects
for a future intrusion. Either type of body can do the planet considerable
harm, and researchers like Eugene Shoemaker of the US Geological Survey have
put considerable effort into calculating the odds. In a recent review of
terrestial bombardment, Shoemaker details a relationship in which the frequency
of occurrence varies inversely with the energy of the event.
Each year, Shoemaker estimates, a meteorite arrives with the energy equivalent
of 20 kilotons of TNT. [This is in the yield range of the bomb that destroyed
Hiroshima ... ONCE a YEAR?!] One example would be the Revelstoke fireball of
1965, when a carbonaceous chondrite was obliterated over the snows of Canada.
Fifty-megaton events -- 10 times the magnitude of the impact that formed Meteor
Crater -- are a once-in-a-millenium occurrence.
For a 12.5-megaton blast like Tunguska, the frequency is roughly 300 years
(within a factor of two). In other words, Shoemaker notes, there is a 12- to
40-percent chance of another Tunguska in the next 75 years. With that in mind,
it seems safe to make plans for future anniversaries of this remarkable
punctuation mark in history.
[<>]
|
|
|
To the best of our knowledge, the text on this page may be freely reproduced and distributed.
If you have any questions about this, please check out our Copyright Policy.
totse.com certificate signatures
|
|
|
About | Advertise | Bad Ideas | Community | Contact Us | Copyright Policy | Drugs | Ego | Erotica
FAQ | Fringe | Link to totse.com | Search | Society | Submissions | Technology
|
|
|
|
|
|
