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Tunguska 1908 - no "Deep Impact"


Deutsche Version

by

Diane Neisius

There is much speculation about the object which crossed in 1908 the sibirian sky as a radiant fireball. When it detonated, it destroyed more than 2000 km2 pine forest. Some claimed it must have been a big meteorite, others suspected an alien spaceship accident. Still other people thought about exotic objects like "black mini-holes" or a new type of volcanic activity. Also there was discussion about new weapon tests of a "mad scientist".
It was mysterious for a long time during the 20th century what really happened in the morning of June 30, 1908 above the sibirian Taiga. Seen from outside, there were all traces of a big meteorite fall (the biggest of the 20th century). Nevertheless there is not a trace of a crater. Because observers from the region described what they saw as "white-hot fireball", "terrible detonation", "burning heat" and "ascending smoke column" which reminds us to the sadly well-known apperance of nuclear detonations, there soon came up speculations if an UFO crashed in 1908. For at this time, there was not a single nuclear apparatus on earth.
Contrary to this, "serious" scientists argued the descriptions of an nuclear-detonation-like event aren't reliable for in 1908 there couldn't have been a nuclear detonation - there is no trace of radioactivity beyond the natural values in the area.

Both, UFOlogists as well as "serious" scientists, have to stand some criticism by the opinion of the author.
UFOlogists, because not every phenomenon which reminds us humans to a technical process must have been of alien origin. Mother Nature is quite more ingenious than we can imagine.
"Serious" scientists, because men usually can well describe what the have seen (observations of the Sikhote Alin meteorite fall which fit very well to the recovered and measured traces prove that). This holds even if they due to their cultural origin use strange words to describe. If observations and scientific models doesn't fit, it's time to examine the scientific model and not to drop observation reports.

It was relatively late in the 20th century when some thought if our knowledge about meteorite falls (big meteorite equals big fireball equals big crater) maybe wasn't that complete. Thoughts tended toward an object of volatile material hit the earth, maybe a piece of comet ice. Again it was strange that there was no observation of a near-earth comet in the weeks before the Tunguska event.
Nevertheless a fragment of Encke's Comet for a long time was the best candidate for such a scenario. But when computers got more power and the simulations went into detail it was shown that a piece of ice would be vapourized by far too high in the atmosphere to cause the devastations of Tunguska.
The puzzle about Tunguska finally was solved when a known phenomenon of meteor observations was examined theoretically - the fragmentation in flight. Looking at it, only solid iron-meteoroides have a fair chance to reach earth's surface in one piece. Stony meteoroides normally fragment in middle heigths, often in severeal phases. When there were detailed enough physical models of this behaviour (e.g. Hill & Goda, see below) the veil of mystery disappeared from Tunguska.
Today we suppose a stony meteoriode of 80 meters diameter entered 1908 the earth's atmosphere by a velocity of 22 km/s and an angle of 30 degrees above horizon. Such a stony meteoroide consists of a relatively weak material; in dense layers of the atmosphere it will fragment rapidly. Small fragments are decelerated nearly instantaneous. Thus, nearly all of the kinetic energy of the meteoroide (which is well in the range of a big hydrogen bomb) is released in a short piece of its trajectory. Following up there is a detonation which has all characteristics of a very strong nuclear blast.

For this website I have reprogrammed the Hill & Goda model which simulates the energy release during the break-up / fragmentation. The pictures base upon the computations, but note that all pictures are my personal artwork (the program has no graphics front-end). Artwork also is inspired by the pictures of Michael Light (see below). In the case one of my readers wants to play around with the program, the C source code of it I put under the GPL (General Public License) for download (see below).

Two seconds to detonation the meteoroide is 40 km above earth's surface. For five seconds it now has been a luminous traveller through high layers of the atmosphere, passing more than 100 km ground path.
Observers report this phase to be "brighter than moon, but not as bright as the sun". Simulation agrees in this point. One of the observers was able to give a drawing of the roll-shaped object "nearly a quarter of the size of the moon".

(Click image to get full resolution)

Roughly one second to detonation the brightness of the object suddenly increases. The meteoroide now is 26 km above earth's surface and travelled a ground-path of 125 km. Simulation tells us at this time less than 30 cm of its surface has molten away - nearly nothing compared to its diameter of 80 m.
The reason for the flash which lets the meteoroide shine brighter than the sun now is the first break-up. At this point of its trajectory the air drag pressure surpasses material strengh of stone. (Even smaller stony meteoroides show a flash at their first break-up.)
The simulation also shows that the fragments keep close together and fly as a single coherent swarm.

(Click image to get full resolution)

The moment of detonation at a height of 16 km: the just formed fragments too can't stand the air drag pressure and rapidly break apart to smaller pieces until brick-sized to pebble-sized. This takes less than a second and sets free the kinetic energy of the meteoroid, 29 megatons TNT, in a volume about 500 m in diameter by few kilometers in length. More than half of the energy is released in only 0.2 seconds. The violent flash is 100 times brighter than sun when watched from 100 km away.
In one strike the meteoroid has changed to a lengthy plasma cloud tenthousands of degrees hot 16 km above ground.

(Click image to get full resolution)

As in every release of such a high energy in a small volume (exactly like in a nuclear detonation) the extremly hot plasma cloud emits hard UV radiation which immediately is absorbed by surrounding air. By this a rapidly growing white-hot fireball forms which grows within a second to a size about 13 km. Due to the height of the detonation it doesn't touch the ground.
Contrary to a nuclear detonation which releases its energy in a point source, the energy of the meteoroid is released along a short piece of its trajectory. Thus, the fireball is somewhat elliptic and not spherical.
By the intensive heat-radiation the forest below is set afire in a diameter of 20 km.

(Click image to get full resolution)

A fireball of this size is luminant well over a minute before fadeout. That fact is known from the nuclear tests of the 1950ies to 1960ies. Driven by its enormous heat, the fireball begins to rise, barely visible due to its big diameter. Below it pulls air from lower layers with it which forms plate-shaped condensation layers, a phenomenon well known from the 1950ies and 1960ies.
30 seconds after the detonation the shock wave reaches ground, where it blows out the just forming massive forest fires. At ground zero, the shock arrives directly from above so some trunks still stand, just stripped off all their branches. Those are the weird "telegraph pole forest" the soviet researcher Leonid Kulik found in 1923 at his first expedition to the area.

(Click image to get full resolution)

One minute after detonation the cloud still is red-hot. Its rise still is barely visible due to the size.
The violent pull the ascending fire-cloud causes behind itself (and to its inside) sucks more and more air upwards. The condensation layers raise bellshaped until they touch the main cloud. The well-known mushroom cloud of such strong detonation forms.
Possibly the air pull is strong enough to suck ashes, dust and smoke from near ground zero upwards. Meanwhile at the ground the shockwave has blown out all fires leaving only charred trunks in the middle area. The wave runs outwards, breaking trees so trunks lie away from center of the explosion.

(Click image to get full resolution)

Some minutes after the event the fireball has faded out and ascended more and more. The mushroom cloud has grown to a multiple of its original size. If there weren't the smoke trail along the original trajectory of the meteoroide, it won't be different from one of the mushroom clouds seen above so much unlucky pacific atolls or sibirian ice sea islands during the development of the hydrogen bomb.

In Irkutsk, 900 kilometers away, the meteorologic station registered an earthquake.

Fortunalely at the barely settled area only a few men were hurted or killed. A number of families of the Tungus tribes which inhabit the area lost reindeer herds, winter storage, tipees and huts. The shamans of the Tungus declared the devasted area as enchanted by the gods. It was for a long time forbidden to go there.

(Click image to get full resolution)

The detonation cloud ascended to an estimated height of 60 km and spread about 200 km until it dissolved. In the evening and following nights luminous clouds of fine dust in the high atmosphere were watched in Europe.

It is somewhat threatening to imagine what would heve happened if the Tunguska-meteoroide had hit the earth just a few decades later during the "Cold War". The atmospheric effect of a mushroom cloud definitely would have been interpreted as an enemy thermonuclear strike. A worldwide thermonuclear war would have been followed up. Maybe, too late, a scientist would have found out there was no radiactivity at the site of the first strike... we all lived at knive's edge without knowing it.
In the 1970ies and 1980ies from time to time early warn satellites registered flashes and fireballs in the earth's atmosphere which were interpreted as illegal nuclear tests (most above sea, Israel and South Africa obscured). But there was never a trace of a true nuclear test. It is probable that they were of meteoroide origin, too.

What if...

One can ask what happens if still bigger stone meteoroides hit the earth. The result of simulations is somewhat weird.
Very big (and thus very rare) asteroids of stone or iron or very big comets of ice reach earth's surface in one piece and leave a big crater. The not so rare small and middle-sized asteroids of stone all break up in the atmosphere where the blast is much more effective in destruction than at an impact.
The energy of the celestial object increases much faster than one would believe. For example, if a stony meteoroide of 200 m diameter hits the earth at 30 km/s (which is not that far more than the Tunguska meteoroide had) it has an energy of about one gigaton TNT (1000 megatons TNT), more than the 30fold of Tunguska. It would detonate at a height of 6 km forming a fireball of 42 km diameter. The flash in 100 km distance would be as 10,000 times bright as the sun. Possibly the fireball would melt a crater into the ground even if the remains of the meteoroide never reach it.
In a circle of 140 km diameter, there would be worst devastations, in hundreds of kilometers the effects still would affect. Due to the big distances the shockwave would need a long time to blow out all fires. In some cases this could mean there is already enough glowing charcoal to re-ignite. Apokalytic firestorms on large areas would develop. A catastrophe of this kind could wipe out a small state totally.
The fireball of the detonation would break out of the earth's atmosphere, forming a layer of dust, smoke and gas around the earth which would weaken the sunlight for weeks or months. A "nuclear winter" would be probable.

Conclusion

To start a global catastrophe, it needs no celestial body which strikes into the earth's surface like in "Deep Impact". Common stony asteroides of middle sizes have much more kinetic energy than biggest man-made nuclear blasts - and they release their energy in very desastrous atmospheric explosions.
A crater isn't caused by this, but nevertheless a fireball, shock wave, mushroom cloud and their destructive follow-ups are similar to a big thermonuclear detonation. Just radioactivity is not released.
The Tunguska event of 1908 no longer is a mystery to us. But despite of scientific curiousity we should hope we will not have to stand such an event again.


References

[1] Florensky, K. P., in Meteoritica 23(1963), http://abob.libs.uga.edu/bobk/tungmet.html
[2] Florensky, K. P., in: Sky and Telescope, Nov. 1963, p. 268
[3] Hills, J. G. und Goda, M. P., in: Astronomical Journal 105 (1993), p. 1114
[4] Gallant, R. A., in: Sky and Telescope, Jun. 1997, p. 38
[5] Light, M.: 100 Suns. German issue, Knesebeck Verlag, Munich 2003.


Download

Download tunguska.tar.gz (meteoroide fragmentation simulator)
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Download Impactors Add-on for Celestia (includes Tunguska)
(Screenshot 1)
(Screenshot 2)
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Created: 10-Sep-2004
© 2004 Diane Neisius