Conference 7.286::space

    	The two most strongly supported theories as to what the Tunguska
    event was are that either a comet or meteor struck Earth.  The other
    theories have ranged from a minicollapsar (black hole), antimatter,
    to an alien spacecraft.
    
    	Larry
    

    RM has a special on the Tunguska event a few week ago and introduced
    a new theory but I can not rember it, did anyone else pick it up.
    It was one of those night when I could not get RM very good and
    spent most of my time tuning instead of listening
    jb
    

    	The explosion occured on June 30, 1908, not 1902.
    
    	Larry
    

    >    	The explosion occured on June 30, 1908, not 1902.

    Oops, the calendar was right, I had it wrong.

Someone in my office here claims that because of the ready-alertness
of U.S and Soviet nuclear computer-controlled missles, if another
mysterious explosion were to occur similar to the one in 1908,
nuclear war could break out, because U.S. or Soviets, whoever receives
the explosion, might feel that to wait until they understand the
explosion before launching their own retaliatory missles might be too late,
because of the fear of silo-destroying missles coming in.

Has anyone heard details of this fear ?

/Eric

    	Yes, and a possible solution to the possibilty of an asteroid
    or comet (perhaps even an alien spacecraft?) which might collide
    with Earth is to have in orbit a number of satellites armed with
    nuclear missles, which would be launched at the object to destroy
    its bulk (if not the whole thing), in order to either divert it
    from Earth, or only leave relatively small pieces of harmless debris
    left to hit our planet.
    
    	See also SPACE Topic 165 for more on this subject.
    
    	Larry
    

    	Yes.  I believe that the President (and the Premier) have
    approximatly 20 minutes from warning of launch until mushroom eggs
    start to arrive and hatch.  Now there SHOULD be no cause for alarm
    if a nuclear type explosion occured somewhere in the US or the USSR.
     This is because a) the system for detecting LAUNCHES is in place
    and b) the types of nukes for which you could not detect a launch
    aren't powerful enough to knock out silos (ie. cruise missles).
     Be that as it may, one cannot determine what the human reactions
    are going to be in a pressure situation.
    	When it comes right down to it I'm far less fearful of an object
    from space setting the chain of events off than I am of some nut
    from the Mid-East setting off a device and triggering a war.  Point
    in fact, the super-powers are aware that with the increased
    proliferation of nuclear devices in the world to small countries
    (some of which place very little value on human life),
    the liklyhood of just such a terrorist act (using a nuke) also
    increases.  Now follow this carefully!  Since everyone knows that
    the chances of "nuclear terrorism" (ie. the use of nuclear weapons
    by terrorists) is increasing, this SHOULD reduce the chance of
    accidental nuclear war because a single nuclear burst would be assumed
    to have come from a terrorist.  Believe it that very few people
    in the governments of the US & USSR want a nuclear exchange
    (especially in the USSR which lost over 20 million WWII).  So perhaps
    the proliferation of nuclear weapons to smaller countires (the latest
    of which is Pakistan) will actually prevent WWIII though it certainly
    does increase the chance of onesies and twosies going off here and
    there.  It is a small comfort if you believe (as I do) that the
    greatest danger of having an all out nuclear war is that most of the 
    current life on the planet would become extinct (including man).
                 
    Rich

    A meteor would:
    - Most likely land in an ocean.
    - Have a miniscule chance of hitting something considered a military
      target.
    - Leave a radar-visible ionization trail, clearly showing it's path
      from space (especially if it's *big*). A computer calculation would
      give it's orbit, solar or interstellar. A solar-orbiting asteroid
      could *not* be easily mistaken for a weapon dropped from Earth
      orbit.
    
    Given that the first two fail, I'm shure that the orbit would be
    triple-checked before any critical decisions were made. -Tom R.

    I seem to remember reading that there was no impact crater at Tunguska.
    Am I mistaken?  Could a meteor be responsible for the explosion
    without impact?....(disintegrate in the atmosphere or something)

    	Yes - meteors have been known to explode while still in Earth's
    atmosphere (before reaching the surface).
    
    	Larry
    

    I've heard about the meteor hypothesis too. Personally,
    I am more convinced by the argument that the object
    was a comet nucleus. A comet nucleus would be vaporized
    far more quickly than a meteor would because of its
    content. The absence of a crater in the area also
    would tend to favor the comet nucleus hypothesis.
    

    RE .11 But, would the comet nucleus arrive in one piece ?
    Or would it break up 'ala' Lucifers' Hammer (Niven and 
    Pournelle) ?

    But if there is no crater there, what *is* there ?  If nothing
    is there, why call it the "Tunguska" explosion rather than the "Boston"
    explosion ?
    
    Is it that someone saw it happen ?  Or that trees were singed ?
     Or a fire broke out ?
    
    /Eric

All the trees over a very large area were blown over.  A bit like what
happened to the trees around Mt. St. Helens.

jb

    	Whatever it was exploded in mid-air over the Tunguska region
    of the Soviet Union, thus it is called the Tunguska Explosion.  If 
    it had exploded over Boston (Heaven forbid!), then it would have been 
    called the Boston Explosion.
                      
    	Larry
    

    re .15 >the boston Explosion
    
    ...or a godsend !     ;-)

    	Please correct me if I'm wrong here, but I seem to remember
    that this *damage* was in a straight path as if blown over by something
    akin to a huge jet blast, that the trees were not in a radius pattern
    as would be caused by an explosion, but in in a pattern that would
    reflect a shock wave passing by. This would tend to reflect that
    something unimaginabely large swept past in a near miss.
    
    	The reference to an explosion, I believe, is actually a misnomer
    given the reference to describe the event in the then current times
    of 1908

Article        10189
From: [email protected] (Richard Akerman)
Newsgroups: sci.astro
Subject: Re: The Tunguska Event
Date: 6 Nov 90 18:11:59 GMT
Organization: Physics Department, Queens University, Kingston, Ontario, Can.
 
"The most energetic, historically well-documented encounter with the Earth 
of an Earth-crossing body produced a great meteoritic fireball over the
Podkamennaya-Tunguska River region of Siberia on the morning of June 30, 1908
(Krinov 1966).  Travelling from southeast to northwest, the meteor passed 
nearly over the town of Kirensk; the endpoint of the trajectory was about 60km
northwest of the remote trading post of Vanovara, over a very sparsely 
inhabited area of the Siberian tiaga.  The meteor was observed from distances
as great as 600-1000 km from the endpoint; the atmospheric shock was audible
at still greater distances.  Trees were knocked down at distances up to 40 km
from the endpoint, and circumstantial evidence suggests that dry timber was
ignited by thermal radiation from the fireball at distances up to 15 km from
the endpoint.  Intensive investigation by expeditions from the Soviet 
Academy of Sciences carried out over many decades has shown that the Tunguska
bolide disintegrated in the atmosphere; it deposited most of its energy at an
estimated altitude of ~8.5 km (Ben-Menahem 1975).  Only microscopic spheres of
glass and magnetite, formed by ablation, reached the ground (Florensky 1963)."
(Shoemaker, 1983)
 
The force of the blast is estimate to be about 12 megatons TNT.  This would 
come from an asteroid or comet of about 100m in diameter (much smaller than
we can easily detect).  Such an event occurs every 150 - 600 years 
(300 */ 2 years).
 
The science of meteor studies is meteoritics.
 
_Short Reference_
 
Shoemaker, E. M.  Asteroid and comet bombardment of the Earth.  
    _Ann. Rev. Earth Planet. Sci._ 11, 461-494.
 
Richard Akerman, late for class...
 

From: [email protected] (Richard Akerman)
Newsgroups: sci.astro
Subject: Re: Future asteroid impact?
Date: 6 Nov 90 17:59:33 GMT
Organization: Physics Department, Queens University, Kingston, Ontario, Can.
 
I thought that it might be useful to provide a list of some of the better
references on the subject of asteroids and impacts in particular.
 
_References_
 
Alvarez, L. W., Alvarez, W., Asaro, F. & Michel, H. V.  1980  Extraterrestrial
    cause for the Cretaceous-Tertiary Extinction.  _Science_ 208, 1095-11108.
 
[the seminal paper demonstrating that an iridium anomaly of presumably e
 asteroidal origin is synchronous with the K-T boundary.  Professional-level]
[In the October 1990 _Scientific American_ there is an article by Alvarez 
 decribing the current status of the impact hypothesis and a counter-articlle
 on the volcanic hypothesis]
 
Cunningham, C.J.  1988  _Introduction to Asteroids: The Next Frontier_.
    (Richmond: Willmann-Bell, Inc.)
 
[An excellent book which examines all aspects of asteroids and asteroid studies
 very well-written, fascinating reading.  General-level. Recommended for
 anyone interested in asteroids.  Provides an extensive list of references.]
 
Gehrels, T.  1985  Asteroids and comets.  _Physics Today_ 38, 32-41.
 
[A very good general-interest article on asteroids and comets.  T. Gehrels is
 on of the major asteroid researchers.  General-level.]
 
Helin, E. F.  1987  Near-Earth asteroid searches: Status and prospects.  
    pp. 147-156 in _The Evolution of the Small Bodies of the Solar System_
    M. Fulchignoni & L. Kresak, eds. (North Holland: Amsterdam).
 
[Helin is a key researcher in this area.  Professional-level.]
 
Lemonick, M.D.  1989  Whew! That was close.  _Time_ 133(#4?), 54.
 
[The popular media take(s) a look at asteroid 1989 FC]
 
Morrison, D. & Chapman, C.R.  Target Earth: It _will_ happen.  _Sky &_
    _Telescope_ 79 (March 1990), 261-265.
 
[Very broad general overview.  They have also published a book on the topic
 called something like _Cosmic Catastrophes_.  Check last year's S&T for more
 info.; there should be an ad for it somewhere.]
 
_Physics Today_  1982  Dinosaur extinction due to asteroid?  _Physics Todday_
    35, 19-21 (Abstract).
 
Pierce, D.A.  1990  A "hollow" asteroid.  _Sky & Telescope_ 79, 272-273.
 
[near-Earth asteroid observations described by someone who has actually done
 them]
 
Shoemaker, E.M.  Asteroid and comet bombardment of the Earth.  _Ann. Rev.._
    _Earth Planet. Sci._ 11, 461-494.
 
[Comprehensive.  Shoemaker is another major asteroid researcher. 
 Professional-level.]
 
_Sky & Telescope_  1987  Pinpointing near-Earth asteroids.  _Sky &_
    _Telescope_ 74, 576 (Abstract).
 
[The difficulties of detecting near-Earth asteroids.]
 
Spratt, C.E.  The Aten-Apollo-Amor close-approach minor planets. 
    _J. Roy. Astron. Soc. Can._ 81, 8-18.
 
[General interest article with lots of information about the AAA asteroidds.]
 
Wetherill, G. W.  1979  Apollo objects.  _Scientific American_ 240, 54-655.
 
[An excellent general-interest examination of the Apollo Earth-crossing
 asteroids by one of the chief researchers in the field.]
       
Of course, the asteroid "bibles" are:
 
_Asteroids_  1979  T. Gehrels, ed. (Tucson: Univ. of Arizona Press)
_Asteroids II_ 1989  R. Binzel, T. Gehrels & M.S. Matthews, eds. (Tucson: Univ.
    of Arizona Press)
 
These are senior undergraduate/Graduate level.
 
I hope this is somewhat useful.  There are zillions more references
but I tried to select the most useful. 
 
Richard Akerman

From: [email protected] (John Tender)
Newsgroups: sci.astro
Subject: Re: The Tunguska Event
Date: 12 Nov 90 04:52:31 GMT
Organization: The NSS BBS, Pittsburgh PA (412) 366-5208
  
 DM> Has anybody heard of this??  The Tunguska Event ... It's an 
 DM> 'event' that happened in the early 1900's.. 1906 or so.
 
     It occurred on June 30, 1908. The accepted explanation for the
event is the air burst of a comet about 5 miles above the surface of
Earth.  Seisemic activity and effects of the atmospheric dust were
evident in Britain and Europe.  Unfortunately, due to the inaccessability 
of the Tunguka area and the political situation in Russia, no scientific 
expedition was dispatched until 1921. 
 
     Here's some sources of info:
 
Baxter, John and Thomas Atkins, (1976). The Fire Came By. Macdonald's and
    Jane's (London).
 
Brown, John C.  and David W.  Hughes, (1977). "Tunguska's Comet and
    Non-thermal C12 Production in the Atmosphere", Nature, Vol. 268, pp.
    512-14.
 
Corliss, William R. , (1983). Handbook of Unusual Natural Phenomena.
    Arlington House, Inc. (New York). (0-517-60523-6)
 
Crannel, (1975). "Experiments to Measure the Anti-matter Content of the
    Tunguska Meteorite", Nature, Vol. 248, pp. 396-8.
 
Furneaux, Rupert, (1977). The Tungus Event. Panther.
 
Jackson, and M.P.  Ryan, (1973). "Was the Tungus Event Due to a Black
    Hole? ", Nature, Vol. 245, pp. 88-9.
 
Ridpath, Ian, (1977). "Tunguska - The Final Answer", New Scientist, Vol.
    1977, Iss. Aug. 11, pp. 346-7.
 
Sagan, Carl, (1980). Cosmos. Random House (New York). (0-345-33135-4)
 
Stonely, Jack, (1977). Tunguska - Cauldron of Hell. Star Books (London).
 
Taylor, John G. , (1977). Black Holes - The End of the Universe? . Random
    House (New York).
 
***
Welfare, Simon and John Fairley, (1980). Arthur C.  Clarke's Mysterious
    World. A & W Publishers, Inc. (New York). (0-84979-075-7)
 
Wick and J.D.  Isaacs, (1974). "Tunguska Event Revisited", Nature, Vol.
    247, pp. 139.
  
     The best description of the event and the observed effects I've
come accross can be found in "Arthur C. Clarke's Mysterious World".
The book is based on a BBC series that the Discovery Channel is
currently rebroadcasting, although I'm not sure whether the Tunguska
episode has been already been shown. 
 
     Clarke suggests that the comet may have been associated with the
Beta Taurid meteor shower, which occurs each June 30. 
  
--- Via Silver Xpress V2.28
--  
John Tender - via FidoNet node 1:129/104
UUCP: ...!pitt!nss!John.Tender
INTERNET: [email protected]

Article        17001
From: [email protected] (Nick Szabo)
Newsgroups: sci.space,sci.astro
Subject: Earth-crossing comets
Date: 19 Sep 91 10:10:36 GMT
Organization: TECHbooks of Beaverton Oregon - Public Access Unix
 
In article <[email protected]>
[email protected] writes: 
 
>>  As mentioned, the Tungaska event, a large 
>>fireball in 1908, is believed to have been caused by such an object, 
>>massing about 100,000 tons, calved off of P/Encke.  
>
>Any more info on this? 
 
(Morrison & Owen 1987) say the following: "Since there was no crater
formed...[it] broke up at an altitude of 8 km... It has been suggested
on orbital grounds that it might have been a fragment associated with
Comet Encke."  It also describes it as "having a mass of more than
100,000 tons. For an icy body this implies a diameter of about fifty
meters..." 
 
Another suggestion is a weak stony meteorite.  (Lewis 1987) states
that it "was about as strong as a sedimentary rock such as sandstone."
(Whipple, 1986) mentions a study using small metallic spheres found
near the Tungaska fall and microparticles found in the Antartic ice
cores for 1908.  Their conclusion was a stony object of 7 million
tons.  However, such an object would have caused an explosion far
larger than the estimated 10 megaton blast.  On the other hand, Zdenek
Sekanani at JPL has estimated that a comet could not have penetrated
as far down as 8 km to produce the blast.  An icy body would have
broken up and vaporized much higher in the atmosphere, spreading out
harmlessly. 
 
Unfortunately, we come back to the salient fact that we just don't
know.  We don't know the size distribution of particles in the comet
"dust" band and meteor shower orbits, because nobody has pointed a
good IR telescope like IRAS at them as Earth passes by.  IRAS got
some good pics from a distance, and discovered the new phenomenon of
narrow comet dust bands in the orbits of P/Encke and P/Tempel.  But
IRAS was designed to look at stars; the new comets and asteroids it
discovered have been lost.  We don't know how fast large pieces
evaporate, because the surface can range from exposed ice to dust to
black tar.  We don't know how many Earth-crossing asteroids are burnt
out comets, and if they are how much ice remains.  We know that some
meteor showers are associated with "asteroids", for example the
Geminid meteor shower and 1983TB.  (Lewis 1987) says estimates range
from 10% to 90%, with Gene Shoemaker's best guess at 60%.  We just
don't know. 
 
The issue of Earth-crossing volatiles is important to discovering the
origins of the solar system, and central to the issue of bootstrapping
space industry with native materials.  Our ignorance, like Europe's
ignorance of the western oceans in the 15th century, is one of the
main problems of the space frontier. 
 
References: 

_The Planetary System_, Morris & Owen, pg. 217-18
_Space Resources: Breaking the Bonds of Earth_, Lewis & Lewis, pg. 239
_The Mystery of Comets_, Whipple, pg. 245-46
-- 
[email protected]  ...!{tektronix!nosun,uunet}techbook!szabo
Public Access UNIX at (503) 644-8135 (1200/2400) Voice: +1 503 646-8257
Public Access User --- Not affiliated with TECHbooks

Article: 19404
From: C09630GK@WUVMD (Gary Kronk)
Newsgroups: sci.astro
Subject: Re: COMET IMPACT
Date: 9 Jan 92 17:02:03 GMT
Organization: Washington University, St. Louis
 
The Tunguska event occurred on June 30, 1908. The meteor stream it is
apparently related to is the Taurid meteor shower which is visible
during November. However, the Taurids also produce a meteor shower
during June, which is only detectable during daylight hours. I think
it is called the Beta Taurids, but since I am doing this from memory I
could be incorrect on the name. The object that came down over Siberia
did so during daylight hours. 
 
The Taurid meteor complex is produced by the periodic comet Encke,
which was discovered in 1786. The comet returns to perihelion every
3.3 years, and has apparently been contributing material to the Taurid
complex for a few hundred years. In addition, there are theories that
Encke may once have been a part of a much larger comet. There have
been several minor meteor showers which have been identified as
probable older branches of the Taurid Complex which would indicate an
origin from a body much larger than Encke. Also, the Taurid Complex is
very large, probably the largest known meteor stream in the solar
system, even though observers might remark that the low hourly rates
of 10-15 would imply a fairly unimpressive stream. 
 
Duncan Steel of Australia is probably the best authority on the Taurid
meteor complex, as he has written a few articles that have brought
many of the above links (and others not discussed here) to light. 
 
Gary W. Kronk
Washington University
St. Louis, Missouri

Article: 21927
From: [email protected] (Nick Szabo)
Newsgroups: sci.astro,sci.space
Subject: Tungaska C&A
Date: 23 Apr 92 09:27:19 GMT
Organization: TECHbooks of Beaverton Oregon - Public Access Unix
 
>In article <[email protected]> [email protected]
(Nick Szabo) writes: 
 
>>...A large object, about 100,000 tons, hit Siberia in 1908...  
 
In article <[email protected]>
[email protected] (Peter Cash) writes: 

>Can that be right? The Tungaska (spelling?) event didn't result in a
>crater; I would think anything weighing 100K tons would leave a hole of
>some sort. I thought it hadn't been settled just what caused this
>phenomenon. 
 
It hit the atmosphere over Siberia and exploded, mowing down a forest,
but no piece large enough to cause a crater reached the ground.
Therefore, it was not very dense (< 1 g/cm^3) and fairly fragile,
fitting the profile of most fireballs.  Zdenek Sekanina calculated
that it was not fragile enough to have been primarily ice: a 100,00
ton chunk of ice condensed in space would have split up from tidal
forces and dispersed without causing harm, according to his
controversial numbers.   There have also been silicate and metal
droplets found in Antartic ice cores, tagged as extraterrestrial and
dated to 1908.  Must have been one heck of a boom! 
 
The object was probably either a comet consisting primarily of dust or
a loosely packed asteroid but you're right, it hasn't been settled. 
As we learn more about C&A (comets & asteroids, for the guy who asked
for a shorthand :-) we will get a better idea about what happened in
1908 and what might happen next time. 
-- 
[email protected]  Public Access User --- Not affiliated with TECHbooks
Public Access UNIX and Internet at (503) 644-8135 (1200/2400, N81) 

Article: 21946
From: [email protected] (Millard Edgerton)
Newsgroups: sci.astro
Subject: Re: comet approaching?
Date: 23 Apr 92 18:23:17 GMT
Organization: NASA Ames Research Center
 
[email protected] (Yaron Sheffer) writes:
 
>>This has lead to some speculation (always fun ;-) that maybe what hit
>>Siberia was a small (pinhead size) black hole!
 
>Well then, we can shift our focus from the southern Atlantic to northern
>Siberia... Maybe the 1908 event was a combined Israeli/South African
>nuclear experiment? Or maybe the 1978 event was a black hole hit? Or maybe
>it's the same object with a period of 70 years? (You can work on a model:-)
 
>But seriously, I think they've found small grains that are remnants of the
>object, believed to be a cometary fragment, rather that a dense meteorite,
>hence not resulting in any cratering. There was also a mention of relationship
>to the June Taurids, and also Comet Encke.
 
>Ronny
  
According to David Morrison, NASA, speaking to the Ames Amateur Astro-
nomical Associan( our local group), the 1908 Siberian event broke up
at about 8 km altitude.  The force is estimated approx. Hiroshima size. 
 
Millard
 
***************************************************************************
*  "To be conscious that you are ignorant  | Standard disclaimer(s)       *
*  of the facts is a great step to         | Millard J. Edgerton, WA6VZZ  *
*        knowledge".                       | [email protected]     *
*                 -Benjamin Disraeli       |                              *
*                         1804 -1881       |                              *
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*            Fax 415-604-6475             |      Voice 415-604-3480       *
*      Employed by Sterling Software at NASA Ames Research Center.        *
***************************************************************************

Article: 21956
From: [email protected] (Jim Scotti x2717)
Newsgroups: sci.astro
Subject: Re: comet approaching?
Date: 23 Apr 92 20:58:38 GMT
Sender: [email protected]
Organization: Lunar & Planetary Laboratory, Tucson AZ.
 
In article <[email protected]>
[email protected] (Millard Edgerton) writes: 

>According to David Morrison, NASA, speaking to the Ames Amateur Astro-
>nomical Associan( our local group), the 1908 Siberian event broke up at
>about 8 km altitude.  The force is estimated approx. Hiroshima size.
>
>Millard

The estimated impact energy for the Tunguska event is estimated at
something more like 5-10 Megatons of TNT equivalent - much larger than
the Hiroshima blast which was "only" something like 17 kilotons of TNT
equivalent.  A 5-10 meter asteroid deposits about a Hiroshima worth of
energy into the atmosphere, of course mostly higher up in the
atmosphere as it fragments.  As I remember Krinov or earlier
investigators at Tunguska found what they thought was a small impact
crater near ground zero, but they were unable to locate any large
fragments in it and may have dismissed it as a pre-impact natural
depression. The Tunguska progenitor is thought to have been only about
50 meters in diameter and the latest estimates at impact rates for
this sized object is on the order of once per 50-100 years! 
 
If Morrison is saying Tunguska was Hiroshima sized, he is off by 2-3
orders of magnitude!  I suspect you might have mis-heard what he said
about the energy. 
 
Jim.
---------------------------------------------
Jim Scotti 
{[email protected]}
Lunar & Planetary Laboratory
University of Arizona
Tucson, AZ 85721 USA
---------------------------------------------

Article: 21957
Newsgroups: sci.astro,sci.space
From: [email protected] (R. Cage)
Subject: Re: comet approaching?
Organization: Ford Motor Company Scientific Research Labs, Dearborn, MI
Date: Thu, 23 Apr 1992 14:13:00 GMT
 
In article <[email protected]>
[email protected] (Peter Cash) writes: 

>Can that be right? The Tungaska (spelling?) event didn't result in a
>crater; I would think anything weighing 100K tons would leave a hole of
>some sort. I thought it hadn't been settled just what caused this
>phenomenon. 
 
It's hard to settle it with 100% certainty without having the actual
characteristics of the parent body (difficult to obtain now ;-), but
current thinking is that comets are sufficiently fragile that small
ones would break up under the air pressure of atmospheric entry.  As
the body breaks up, its frontal area increases dramatically (and even
more so as the sub-bodies break up in turn).  This causes a very rapid
conversion of the kinetic energy of the parent body into heat/light,
and dumps the momentum into millions of tons of air.  What is thought
to reach the ground is a big heat flash, a shock wave, and a very fast
air current carrying a bit of chondrite dust which survived the
deceleration phase.  If nothing solid hits the ground, it's not going
to make a classical crater. 
-- 
Russ Cage              [email protected]  russ%[email protected]
  *  When Ford pays me for my opinions, THEN they can call them theirs. *
 
        "Science is the only self-correcting human institution, but it
    also is a process that progresses only by showing itself to be wrong."

			       - Allan Sandage

Article: 28247
Newsgroups: sci.astro,sci.space
From: [email protected]
Subject: TUNGUSKA/SWIFT-TUTTLE
Organization: University of Western Ont, London
Date: Tue, 3 Nov 1992 13:42:40 GMT
Sender: [email protected] (USENET News System)
 
The following are comments from Duncan Steel on some recent postings
on sci.astro and sci.space. Please address responses to his email
address. I am posting this for him as he does not have direct access
to UseNet. 

Peter Brown 
=======================================================================
 
SOME COMMENTS ON RECENT DISPARATE POSTINGS W.R.T. TUNGUSKA AND P/SWIFT-TUTTLE
 
(1) Tunguska object being stoney: The work referred to is by Chris Chyba
   (now at NASA-Goddard SFC), Paul Thomas (U. Wisconsin-Eau Claire) and
   Kevin Zahnle (NASA-Ames RC).  They have a paper prepared which shows that
   the Tunguska event is consistent with the entry of a stoney asteroidal
   object about 50 m in size (i.e. there is NO NEED for a fluffy underdense
   object like a "comet" fragment - as if we know anyway what a comet is
   like; hell we don't even know P/Halley's density to within a factor of 3).
   I imagine that they would have given a paper on this at the DPS in Munich.
 
(2) Frequency of Tunguska-type impacts.  The thing which MAY be wrong with
   most of the discussions is that it is generally assumed that such objects
   hit Earth randomly in time.  This is daft in that we know that a good
   fraction - perhaps even the majority - of the mass influx of smaller
   meteoroids hit the Earth in showers (meteor showers).  These occur as
   the Earth passes through the the meteoroid stream produced by an 
   asteroid/comet.  They recur each year since the smaller particles produced
   by the cometary decay are spread around its orbit by ejection speeds from
   the comet nucleus, radiative forces etc.  However, there is a concenration
   close to the cometary nucleus (cf. cyclic meteor storms such as Leonids,
   Draconids, Perseids in 1992 - and in 1993/94?).  The large particles are
   also grouped close to the nucleus since ejection velocities are small,
   and radiative forces insignificant.  What if a giant comet fragments,
   leaving some km-sized lumps but many more smaller ones (50-100m)? These
   will be mostly in a clump.  At such time as the orbital precession/
   evolution gives a node at 1 AU, a shower of large lumps may occur: but
   not every year, only when the clump is at the right (wrong?) point in
   the orbit, like the Leonids recurring every 33 yr (the orbital period
   of the parent comet).  Anyhow, the above would lead to the following
   occurring: a few random incoming Tunguska-type objects every so often
   (every few centuries), but every millenium or so (time depending upon
   exact orbit because of precession rate) there will be a phase of a
   century or so in which every few years/few decades there is a large number
   of Tunguska-type events spread over a week or so, and this dominates the
   long-term (sporadic) influx.  This I call "Coherent Catastrophism", and
   catastrophic it would be.  Indeed, I believe, it "has been", judging
   from the historical record, since this is what is going on at the moment,
   with us now (late 20th Century) being in a hiatus between mass influxes.
   I would refer one and all again to V.Clube and B.Napier, "The Cosmic
   Winter", Blackwells, Oxford & NY, 1990, for the historical stuff.  For
   the nitty-gritty w.r.t. the meteoroids (small ones in showers observed
   now) see Steel, Asher & Clube, Mon Not Roy Astron Soc, 251, 632, 1991.
   Since then we have looked at the asteroids which we believe to be members
   of the complex (see Steel, The Observatory, 112, 120, June 1992) and
   shown that at the >95% confidence level there are 8/9 objects in the
   inventory of known Apollo asteroids which are members of this complex
   (Asher, Clube & Steel, presented at Meteoroids and their parent bodies,
   Slovakia, July 1992 and Mon Not Roy Astron Soc, submitted).  This would
   imply that there are many more smaller objects (50-100m?) which have
   orbits which do not have a node at 1 AU at present, but will precess
   so as to do so before too long!  Outliers in that respect include the
   Tunguska object itself, the 1975 lunar impactors (also detected on
   Earth as ionospheric disturbances: Kaufmann et al, Science in 1989 I
   think), and 1991 BA (that 5-10m near-miss asteroid/meteoroid).  As I
   have said before, I do not know when the world will end but I would
   bet that it'll be in the last week of June one year.  You all might
   like to note that the above ideas on Coherent Catastrophism were
   totally excluded from the NASA report on how to search for Earth-crossing
   objects (Report of the Near-Earth-Object Detection Workshop, ed. 
   D. Morrison) despite my best efforts as one on the committee members
   to have it included.
 
(3)  There has been a lot of bull written in these columns about
   P/Swift-Tuttle and whether it will hit the Earth and what the
   consequences might be.  Here is what led to the news stories.
   In IAU Circ 5636 dated October 15th Brian Marsden pointed out that
   with present orbital solutions and their acknowldeged uncertainties
   an impact on 2126 August 14th is/was possible.  Whenever something
   topical comes up I do some radio interviews with Australian Broadcasting
   Corp stations here, to keep the Australian public abreast of what is
   going on: good for astronomy, good for space, good for science all
   round.  I also informed a few newspapers here.  To that extent the
   Australian public knew all about this story a week or more ahead of
   the populace elsewhere, and with that in mind I would encourage others
   spread all over to do something to keep the public informed about what
   is happening; hell, they (mostly) pay for our salaries, equipment,
   education.  Getting back to Oz, I did my first radio interviews on
   Oct 16th (Darwin & Adelaide), then every other capital city on Oct
   19-21.  The first story in print was, I believe, in The Australian
   newspaper dated Oct 20.  That weekend there was a 3-day conference
   (National Space Development Conference) in Sydney and I gave a talk
   on our program here searching for near-Earth asteroids, NASA plans,
   international plans, SDI involvement, etc.  The case of P/Swift-Tuttle
   was/is an excellent example: a possible impactor, > 100 yrs notice,
   requires observations over the next 5/6 yr from the southern hemisphere
   for better knowledge of whether an impact is likely. (Despite that, it
   turns out that we have no funding as from December so that our program
   will be closed down, resulting in no southern hemisphere searches; I
   would also point out that the US searches have depended upon us for
   follow-up of objects moving south but we will be unable to do this from
   now on.  This is due to the idiocy of the Australian government; we
   have had great support from our north American co-workers in trying to get
   funding here).  Anyhow, back to the talk in Sydney.  The local Reuters
   correspondent was there and after discussions with me on the topic he
   put out a wire story which led to the furore.  Indeed he says that no
   story that he has previously filed has got as wide a response.  So far
   as I know the story he put out was accurate (saying that the CHANCE of
   an impact existed) except that he got the year wrong: 2116 instead of
   2126.  You would not believe where the story was carried: for example
   front page of THE TIMES (London) with an editorial, and a reply by
   Marsden (Oct 30), page 3 of THE SUN (London) with a semi-naked lady,
   front page of a paper in Fort McMurray (Canada) sharing top spot with a
   story about a moose.  I fielded phone calls from all over the world: 
   most of them based upon errors and misconceptions. "Such is life" (the
   last words of Australian anti-hero Ned Kelly).
 
(4) Impact velocity by P/Swift-Tuttle: this is known accurately.  If the
   comet does not hit this next time but it does at some time in the
   future then whenever that occurs the speed would be between 60 and 61
   km/sec (see my paper MNRAS, 227, 501, 1987) unless its orbit (a,e,i)
   changes appreciably.  The 60-61 km/sec ariation is due to
   the eccentricity of the Earth's orbit and hence terrestrial speed.
   For 2126 August 14 the speed can be found exactly, but I haven't done
   it; it's not important since the mass is so poorly estimated.
 
(5) Using an assumed sphere of diameter 5 km, density 1 gm/cm3, I get
   an energy of above 20 million Megatonnes, or around 1 billion times
   Hiroshima.
 
Duncan Steel,
Anglo-Australian Observatory,
Coonabarabran, NSW 2357.
 
"[email protected]" 
=====================================================================

Regarding Airburst Explosions, which apparantly happenned at Tunguska:

From physics experimentation & nuclear bomb testing, it's known that the
blast effects at Ground Zero (directly beneath the explosion) to trees &
strong buildings can be less than to same things further out from ground
zero.  This is because the blast force there is almost directly downward,
thus parallel to tree trunks & other things perpendicular to the ground.
This of course depends on the severity and/or altitude of the blast.

The further out the trees or structures are, the more at an angle the blast
force is, & the more likely to blow over or snap off trees trunks, walls,
etc.  So trees right underneath a blast might be stripped of branches, and
buildings might have their roofs caved in, while further out these same
things are flattenned.  Of course a really strong explosion especially
if not too high will cause just as severe a destruction beneath it, too.

Anyway, has anyone read or heard of this sort of evidence at Tunguska, an
area of trees found not to have been blown over, but perhaps stripped of
branches, and left standing?  That would've been Ground Zero.

- Tom

The June issue of Sky and Telescope has a great article on Tunguska, with 
one of a number of pictures of blasted trees on the front cover.  Many
are shown still standing, but mostly stripped, but I think these are not
at the epicenter, as all around them are other trees, blown over and all
lying in the same direction.  I don't recall from reading it that any such
site as you mention was actually found, but I may have just missed it.
Actually, the first paragraph kind of answers that question. I qoute,
without permission, from Roy A Gallant's article:
  "On the morning of June 30, 1908, a fireball cascaded down the Siberian
sky and exploded with 2000 times the force of the nuclear blast that devastated
Hiroshima, Japan. Weighing some 100,000 metric tons, the cosmic missle cut
into the atmosphere at about a 30 degree angle above the horizon and an
azimuth of 110 degrees out of the southeast.  Observers described a fiery tail
some 800 kilometers long.  At an altitude of about 6 km, the object shattered
in a rapid series of bursts and vaporized, felling trees in a radial pattern
over an area  of 2,150 square kilometers and incinerating a central area half
that size."

Eyewitness accounts from survivors discuss such things as herds of reindeer
bursting into flames.  So, there may have been no trees left standing 
in the epicenter (and if it was a series of bursts, there may have not been
ONE clear center) and seeing as how it came in at an angle, there may have
been no direct downward force, at least not one unaffected by other
forces of the blast and entry.  If you get a chance, read this article.
This issue also has a neat article speculating what the world would look
like if we had eyes that were designed for starlight.

PeterT

Article: 698
From: [email protected] (Peter Venetoklis)
Newsgroups: sci.space.science
Subject: Re: Earth-Comet Defense System ?
Date: Mon, 23 May 1994 17:07:59 GMT
Organization: Northrop Grumman Corporation
 
In article <[email protected]> [email protected] (John Parres) writes:

>How rare is it that comets impact planets?  Extinctions aside, does the 
>upcoming event with Jupiter give anyone else pause?
>
>I wonder, if the Earth were in danger, what would be the best way to 
>handle the situation?  Nukes?  Do we have enough? 
>
>[Mod Note: tech followups to sci.space.tech please -gwh]
 
There was a conference on this subject in January, 1993 on this subject.
A textbook covering this subject is in final edit now.  It is called
"Hazards Due to Comets and Asteroids," and is being put together by the
Lunar and Planetary Laboratory at the University of Arizona.
 
Going by memory...
There is a 1/10,000 chance of a big rock hitting the Earth in the next 100
years.  Big means big enough to cause substantial damage if it hits in
the wrong place.  A number of methods for mitigating the threat have been
proposed, some involving large nuclear bombs to divert the rocks.  An
effort called Spaceguard is trying to catalog the Earth-crossing asteroids,
since apparently one of the biggest problems is early warning.  Lots
of interesting science was covered, in addition to mitigating technologies.
 
E-mail me if you have specific questions.
 
-- 
____________________________________________________________________
Peter Venetoklis                           [email protected]
Senior Engineer - Mission Analysis      Northrop Grumman Corporation
Opinions are mine, not Grumman's, not Northrop's, not anyone else's.

Article: 699
From: [email protected] (Jeffrey A Foust)
Newsgroups: sci.space.science
Subject: Re: Earth-Comet Defense System ?
Date: 24 May 1994 16:55:37 GMT
Organization: Massachusetts Institute of Technology
 
In article <[email protected]> [email protected] (Peter
V.Vorobieff) writes: 

>In article <[email protected]>
[email protected] (Nick Haines) writes: 
>
>>We can be fairly sure (99%) that no full-size comet (or even a large
>>fragment) has hit the Earth in the last few millenia (3 or 4).
>
>A large something (a comet for the reasonable, an ET ship for the romantics)
>hit the Earth in 1908. That resulted in an explosion equivalent to an 
>100-megaton H-bomb. If instead of the middle of nowhere in Siberia the comet
>hit Netherlands, the entire country would be history.
 
Current thinking is that the Tunguska explosion was caused by a stony
asteroid, not a comet.  A paper by Chyba et al. in Nature last year
ruled out comets and carbonaceous asteroids because an object of the
size corresponding to the appropriate energy would have disrupted too
high in the atmosphere.  The only objects of that size that would have
survived down to altitude where the Tunguska explosion took place
(approx. 10 km) are stony asteroids. 
 
Here's the reference to the Nature paper, if you're interested in the details:
 
THE 1908 TUNGUSKA EXPLOSION - ATMOSPHERIC DISRUPTION OF A STONY ASTEROID
by CHYBA-CF THOMAS-PJ ZAHNLE-KJ
 
NATURE  VOL:361 (6407):40-44(1993)
 
(sorry about the all caps; this came straight off a computer catalog :)
 
-- 
Jeff Foust
EAPS Dept., MIT      | "Ninety percent of the time things will turn out worse
[email protected]  |  than you expect. The other 10 percent of the time you 
[email protected]       |  had no right to expect so much." -- Augustine's Law #37

Article: 700
From: [email protected] (Kevin R. Boyce)
Newsgroups: sci.space.science
Subject: Re: Earth-Comet Defense System ?
Date: Tue, 24 May 1994 13:05:33 -0400
Organization: NASA/GSFC
 
[email protected] (Peter V.Vorobieff) wrote:
 
>  [email protected] (Nick Haines) writes:
> 
> >We can be fairly sure (99%) that no full-size comet (or even a large
> >fragment) has hit the Earth in the last few millenia (3 or 4).
> 
> A large something (a comet for the reasonable, an ET ship for the romantics)
> hit the Earth in 1908. That resulted in an explosion equivalent to an 
> 100-megaton H-bomb. If instead of the middle of nowhere in Siberia the comet
> hit Netherlands, the entire country would be history.
 
Actually, the latest thinking is that a comet wouldn't have made it that
close to the ground.  There was a report in Nature sometime last year by
some folks here at Goddard and some Wisconsin people about the Tunguska
blast.  They concluded that a stony meteorite about 30 meters in diameter
would do just what whatever-it-was did.  Carbonaceous meteors and comets
would have broken up higher in the atmosphere, and iron meteors would make
it to the ground.
 
Peter's point remains, of course.  Scary, isn't it?
-- 
Kevin      [email protected]
Would you feed your daughter to the Soft Machine?

    Re.-1:
    It amazes me that an object as small as 30 meters could cause all the
    damage done in the Tunguska area!  I don't even want to think about what
    an object a couple miles in diameter could do!
    
    The idea of a space watch system, to give early warning (perhaps a couple
    years?) of an impending collision with a comet or meteor is a good one,
    but, what could we do about it if we knew?  Worry a lot & panic, probly.
    
    Kind of reminds me of the 1950's movie, 'When Worlds Collide', in which
    the earth collides with huge, planet sized body.  Knowing this is coming,
    a group of scientists & engineers have time to build a space ship to save
    about 50 people or so, and some animals, a kind of Celestial Noah's Ark.
     	(I think they do it in about 2 yrs.!!  Geniuses, I guess!)
      
    A scary situation to think about, 'cause I don't think we could do much
    about an object of any real size, especially anything like what the people
    in the movie did, not anytime soon, anyway.  Maybe we could hit it with
    a big enough explosion to divert it's path just enough to miss us?
    					   (If it's not too big!)
    	- Tom

  Some of the account in .25 sounds a bit overblown to me. Eyewitness accounts
of herds of reindeer bursting into flames? If someone was an eyewitness to that,
how did they survive themselves? Especially since this was a forested region; IF
the flash was bright enough to incinerate a large animal, it would certainly
have also set fire to the trees for a large area around as well. So even if an
observer was sheltered from the direct flash, they would still have to escape
a forest fire which would almost certainly rapidly produce a firestorm, due to
the simultaneous ignition of material over a large area. But would the flash
really be powerful enough to do that? I didn't think the temperatures in this
sort of event went anywhere near the millions of degrees that produces the
intense flash of a nuclear explosion. The mass of material in the fireball is
much greater, so the temperature is correespondingly lower.

  I have certainly read of the remains of a herd of reindeer being found there;
I think it most likely that they were killed by blast/falling trees, and then
cooked by forest fires. It is perhaps not even the case that the explosion
caused the fires. The large quantity of dead felled timber would have dried out,
and could have been ignited much later, perhaps by lightning strikes.

  I haven't read the article quoted, so my comments may be well off-base. But
the mention of graphic eye-witness accounts makes me suspicious. Ground zero
of the Tunguska explosion is so remote that no scientific expedition reached it
for nearly 30 years.

Ken

It's a kind of curious article.  The reindeer burning up is not part of the 
main article itself, but is in a map of the area showing the generally affected
blast site with markers showing the direction the trees blew down and location
of various hunter or herder camps in the area, some within the zone of
greater damage.  Boxes are pointed to each encampment with an eyewitness
account from the sites.  One goes "God, in his displeasure with us tore the 
sky apart. In the nomad camp of Ivan Dzhenkoul all 200 reindeer in a single
instant were incinerated.  All of his stores of furs, food, and other goods
were likewise destroyed."    So basically this is coming probably second or
third hand and there may well have been some exaggeration or distortion.

But it sounds kind of neat ;-)

PeterT

From:	US3RMC::"[email protected]" "Astronomy Discussion List" 
        17-SEP-1994 19:09:58.62
To:	Multiple recipients of list ASTRO <[email protected]>
CC:	
Subj:	Re: Hawking and comet

Jack, I suggest that you read "Death from the Sky," Astronomy,
December,1993, by Christopher Chyba. 

The graph on p. 42 says it all-- (rough approx. below)

                         -----------EXPLOSION-------------
    Body Type            Altitude    Energy         Effect
Long-Period Comet          30km      2800Ktonnes    Flaky debris high in air
Short-Period Comet         22km      3200Kt         Too little, too high
Carbonaceous Asteroid      14km      3000Kt         Close, but no cigar!
STONY ASTEROID ---------->  8KM      5000KTONNES    BEST FIT FOR OBS. DATA!!!
Iron Asteroid               0km       300Kt         Too dense, big hole,
                                                    small explosion.

There is another model that supports the stony asteroid model (but I can't
remember the reference)-- It has to do with the fire storm.

A high-altitude burst (i.e. comets) would produce a big firestorm that would
have _burned down_ all of the forests.

A ground impact would cause a crater and would have knocked down all the trees.

A low-altitude burst (i.e. stony asteroid) would have started the firestorm,
and then the shock wave would have _blown_ the fires out and knocked down all
the trees.

And it is this last model that best fits the data -- thousands of acres of
trees blown down away from a central area with evidence of charring on the
side towards the epicenter.

This pretty well convinces me of a best fit scenario (at least, until better
data is discovered!).

Clear Skies, Dennis

% ====== Internet headers and postmarks (see DECWRL::GATEWAY.DOC) ======
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% Subject:      Re: Hawking and comet
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