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Re: (meteorobs) Tunguska bolide & Beta taurid stream


Jure,
Sometime last year over on the meteorite mailing list, Bernd Pauli posted a 
little ditty about the pros and cons of the tunguska incident in reference to 
it being of astroidal origin or cometary. Without permission I'm posting it 
here:

" Whereas some scientists believe that a small piece of an asteroid struck in 
Siberia, other scientists favor a cometary impact. The protagonists in this 
discussion were Kresak(cometary origin) and Sekanina (meteoritic origin).

Zedenek Sekanina from Caltech's JPL disagrees with the cometary hypothesis. 
He strongly favors a meteoritic origin. Kresak based his idea on the fact 
that the June 30 event occurred near the time of the annual Beta Taurid 
meteor shower has long been thought to derive from comet Encke. 
...a fragment of comet encke, separated from it thousands of years ago.

The Beta Taurid meteor shower has a radiant only 10 deg from the Tunguska 
object's radiant.

Here are some of their opposing view:

Points in favor of a cometary origin (Kresak):

1) No trace of itself - low mechanical strength and density- fragmented and 
brought to a halt before hitting the ground.

2) vaporization of it's ices - tremendous explosion - only airborne dust.

3) skyglows (cometary dust)

4) Nearness of the Tunguska event to the time of the Beta Taurid meteor 
shower.

5) The Beta Taurid meteor shower is thought to derive from Comet Encke.

6) The Beta TAurid meteor shower's radiant is only 10 deg from the Tunguska 
object's radiant.

7) Energy comparable to that expended in forming meteor crater but no sizable 
crater was formed - this favors a low density impacting body.

8) The radiant position of the Tunguska object is uncertain to at least 10 
deg.

9) Observed ozone depletion at the time about 30 % which was caused by NO.

POINTS IN FAVOR OF A METEORITIC ORIGIN (Sekanina)

1) The majority of evidence points to only one explosion - so no 
fragmentation earlier in its flight.

2) The sheer magnitude of the blast argues against an early break up which 
would have dissipated energy.

3) The bolide remained intact down to a height of only 8.5 km.

4) A fragment of cometary material could not have survived a plunge into the 
lower atmosphere.

5) The resistance of the air at a height of a 8.5 km is 1000 times as strong 
as is necessary for the destruction of a comet entering the atmosphere at a 
speed of 26 km/s which is normal for comets.

6)ca. 6 km to the northwest of the "central impact point", there are two 
circular lakes - diameter about 100 meters.

7) Favorable position of the Tunguska object in the evening sky for several 
weeks before the encounter- a comet should have been discovered.

8) Statistics of observed comets seem to indicate that there are no active 
objects with nuclear diameters as small as a hundred meters.

9) From the breakup height the object ws not traveling much more than 11 km/s 
when it entered the atmosphere.

10) The object's farthest aphelion distance of 1 -1 1/2 A.U. is within the 
range of short-period comets and asteroids but the Tunguska object has no 
perihelion or aphelion near the orbital plane of Jupiter.

11) Microscopic spheres of metal and glass were sifted from the soil in the 
late 1950's and early 1960's.

12) Soviet researchers found abnormal concentrations of nickel in the 
samples, indicative of meteoritic origin.

13) New Jersey researcher Ganapathy discovered enrichments of iridium, which 
is cosmically abundant but rare on earth.
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