Re: Impact Scenario/New Ref.

[Willa25743@aol.com]

>From: swf@ElSegundoCA.ATTGIS.COM (Stan Friesen)

> From: Willa25743@aol.com
 >> In Steel's opinion, the Tunguska event of 1908 is directly linked to the 
 >> twice-annual Taurid Complex of meteor showers associated with 
>>Comet Encke. He believes an ice-coated stony cometary fragment 
>>exploded above Siberia, as described in impacting models only 
>>recently proposed.

swf replies:
>Only recently proposed?  The idea that the Tsunguska blast was an
>exploding comet or cometary fragment has been around for at least
>10 years.  In fact I have long been convinced that this is the
>only viable explanation, all other alternatives failing to explain
>all the facts.

No, I said "in impacting models only recently proposed." The cometary
hypothesis has been around for decades, just as car wrecks have been
around for quite some time. However, if auto makers try to study and
predict such collisions, they may attempt to model such incidents
with vehicles of varied composition, physical makeup and velocities.
Such modeling is discussed by Steel based on "The 1908 Tunguska 
explosion: Atmospheric detonation of a stony asteroid" by Chyba, Thomas
and Zahnle in Nature, v.361, pp 40-44, 1993.

He also refers to another recent (1993) study by Hills and Goda in 
Astronomical Journal, v. 105, pp. 1114-1144, "The fragmentation of 
small asteroids in the atmosphere." 

Duncan Steel appears to disagree with these scientists in that he 
makes no sharp delineation between comets and asteroids. A classic
"dirty snowball" comet seems different from a classic rocky asteroid, 
but he thinks there as a wide range of mixtures in between. To him,
the modeling suggests an ice-coated stony cometary fragment, as 
opposed to the rock or snowball, or a metallic body or organic "tar"-
coated sludge ball.

>Now, as to whether it was a fragment of Encke or an otherwise unknown
>independent comet is a different issue.  What evidence does he present
>to support this idea?
 
He points out that the Tunguska event occurred at the time of the peak of
one of the most intense annual daytime meteor showers. He also quotes
the work of Slovak astronomer Lubor Kresak, who published in 1978
a paper "The Tunguska object: A fragmentof comet Encke?" (Bulletin of
the Astronomical Institutes of Czechoslovakia, v. 29, pp.129-134).
Kresak studied eyewitness reports and found that the arrival direction 
(what meteor watchers call the "radiant") of the object appeared to be 
slightly west of the sun in the constellation Taurus. This corresponds 
with the radiant of the meteor shower as determined by radar. Kresak
also pointed out that this meteoroid swarm is known to be linked to
comet Encke.

[Incidentally, Encke or Comet Halley is never linked with the K/T 
extinction. Since even long period comets are not believed to last 
longer than a few million years, the dinosaurs probably never saw
the same comets seen by humanity.]

>> Moreover, he suggests that about 5,000 years ago the Taurids burst into a 
 >> period of meteor storms and Tunguska-like explosions that inspired the 
 >> building of Stonehenge(!) and the pyramids of Egypt.

>This seems unlikely to me.  Pyramid building in Egypt developed
>gradually, over a period of a hundred years or more.  And the designs
>used changed over time....

I agree that his conclusions seem far-fetched. But in his defense, he does
contend that such catastrophism was not random but instead in surges 
that lasted for hundreds (maybe 1,000s) of years. And that the effects 
may have ranged from displays resembling fireworks, multi-megaton
blasts or merely glowing pyramid-shaped zodiacal light. 

Indeed, notes Steel, "disaster" is derived from words meaning "evil star"!