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(meteorobs) Excerpts from "CCNet 97/2000 - 28 September 2000"
*Note in particular* the final item on the heating mechanism for meteoroids.
This was fascinating to me, as it appears to contradict some of the "state of
current knowledge" as I'd derived it from reading McKinley and Oepik. Can
anyone comment further on this item?
Clear skies,
Lew Gramer <owner-meteorobs@jovian.com>
------- Forwarded Message
From: Peiser Benny <B.J.Peiser@livjm.acdot uk>
To: cambridge-conference <cambridge-conference@livjm.acdot uk>
Subject: CCNet, 28 September 2000
Date: Thu, 28 Sep 2000 14:05:54 +0100
CCNet 97/2000 - 28 September 2000
---------------------------------
[...]
(4) NATURE AND DESTRUCTION OF THE TUNGUSKA COSMICAL BODY
V.A. Bronshten
[...]
(6) NEW 2.5 KM IMPACT STRUCTURE DISCOVERED IN NW AUSTRALIA?
J.D. Gorter & S.W. Bayford
[...]
(11) MEA CULPA! [Meteoroid heating in the atmosphere]
Phil Plait <badastro@badastronomy.com>
[...]
=================
(4) NATURE AND DESTRUCTION OF THE TUNGUSKA COSMICAL BODY
V.A. Bronshten: Nature and destruction of the Tunguska cosmical body
PLANETARY AND SPACE SCIENCE 48: (9) 855-870 AUG 2000
The problem of the nature of the Tunguska Cosmical Body (TCB) is closely
related to the
mechanism of its disintegration which ended with an explosive disburse. The
only hypothesis capable of explaining such a process is the idea that TCB
was a fragment of a comet. Three alternative hypotheses are considered here.
They assume the TCB to be: (i) a fragment of a
stony asteroid; (ii) a porous snowball; and (iii) a plasmoid. It is shown
that the first of
them is not plausible because the fragmentation and explosion of an asteroid
should result in
the scattering of numerous fragments over the ground. They, however, have
not been found on the terrain for many years by many tens of explorers who
have carried out a cartful analysis of
the soil and peat. Bodies like a porous snowball or a plasmoid cannot exist
in the Solar
system (they should be unstable). The history of the cometary hypothesis is
followed in the article, starting from its original formulation in a book by
the well-known astrophysicist H. Shapley (1930. Flights from Chaos. A Survey
of Material Systems from Atoms to Galaxies. N.Y.: McGraw-Hill, p. 57-58.
Russian translation: From Atoms to Milky Ways. Moscow: ONTI, 1934),
who has priority over F. J. W. Whipple [the latter having put this
hypothesis 4 years later (Whipple, 1934. On phenomena related to the Great
Siberian meteor. Quart. Journ. of the Roy. Meteorol. Sec. 60, 505-513)].
Simultaneously with the evolution of this hypothesis our ideas
of the structure of cometary nuclei have also varied. Estimates of the main
parameters of the
TCB - its initial mass m, initial velocity v and energy of explosion E-e -
are examined. The following quantities are assured to be the most plobable:
m = 2 x 10(6) t, v = 31 km/s,
E-e = 5 x 10(23) erg. The mentioned value of v corresponds to the
Zotkin-Kresak hypothesis
that the TCB was a fragment of the Encke comet. Sekanina's criticism against
this hypothesis
is examined. During recent years, three analytical theor ies of the
sequential disintegration
of large bodies in the atmosphere have bern put forward by Grigoryan (1979:
Motion and disintegration of meteorites in the planetary atmospheres. Cosmic
Res. 17(6), 875-893), Hills
and Goda (1993. The fragmentation of small asteroids in the atmosphere.
Astron. J. 105(3) 1114-1144) (their theory is physically equivalent to
Grigoryan's theory), and by Chyba et al. (1993. The Tunguska 1908 explosion:
atmospheric disruption of a stony asteroid. Nature, 36 (1) 40-44.). The
comparison of all the three theories is presented. It is shown that the
theory of Chyba et al. overestimates the altitudes of disruption of the
meteoroid compared to other theories. The results of the numerical
simulations of the process in the free-Lagrangian and the Eulerian
approximation (Svettsov V.V., Nemchinov I.V., Teterev A.V., 1995.
Disintegration of large meteoroids in Earth's atmosphere: theoretical
models. Icarus 116, 131-153) are also analysed.
The results of numerous studies examined either do not contradict the
cometary hypothesis of
the TCB nature or the discrepancies (e.g. in the case of Chyba et al., 1993)
can simply be explained. An interpretation is presented fur the anomalous
sky glow observed after the TCB
fall, west of the treefall epicenter, in Russia and in West Europe. It was
first put forward
by the author (Bronshten, 1991. Nature of the anomalous illumination of the
sky related to
the Tuuguska event. Solar System Res. 25(4), 490 504) and considers the
secondary scattering
of solar light by the dust of the head of the comet that entered the Earth's
atmosphere simultaneously with the TCB. (C) 2000 Elsevier Science Ltd. All
rights reserved.
Addresses:
Bronshten VA, 16-130 Varshavskoye Shosse, Moscow 113105, Russia.
Russian Acad Sci, Comm Meteorites, Moscow 117975, Russia.
Copyright ) 2000 Institute for Scientific Information
================
(6) NEW 2.5 KM IMPACT STRUCTURE DISCOVERED IN NW AUSTRALIA?
J.D. Gorter & S.W. Bayford: Possible impact origin for the Middle Miocene
(Serravallian)
Puffin Structure, Ashmore Platform, Northwest Australia. AUSTRALIAN JOURNAL
OF
EARTH SCIENCES 47: (4) 707-714 AUG 2000
The Puffin Structure is interpreted from high-quality 3D seismic data as a small
multiringed impact structure formed by collision of a meteorite or small
asteroid with unconsolidated, water-saturated shallow-marine shelf carbonates
during the Middle Miocene (mid to late Serravallian). The impact created a
dish-shaped structure about 2.5 km in diameter with annular rings and no central
uplift.
Addresses:
Gorter JD, British Borneo Australia Ltd, POB 1265, W Perth, WA 6872, Australia.
British Borneo Australia Ltd, W Perth, WA 6872, Australia. Copyright ) 2000
Institute for Scientific Information
[...]
===================
* LETTERS & OPINION
===================
[...]
(11) MEA CULPA! [Meteoroid heating in the atmosphere]
>From Phil Plait <badastro@badastronomy.com>
In the CCNet from 22 September 2000, James Oberg says
about heating meteor[oid]s:
>Actually, friction has nothing to do with it -- although
>this is a common, careless, misconception. The source of
>heat on an entering object is the shock compression of the
>air piled up in front of it
Mea Culpa!
He is correct. The hypersonic passage of the meteoroid compresses the
air tremendously, which heats it. Radiant energy from the shocked air
heats the forward surface of the meteor, melting it. There is a
standoff zone of relatively dead air in front of the meteor, and
friction with that ablates off the melted material.
I would define friction as the force of air moving laterally over a
surface, so the heating is not really friction-induced. References
on the actual heating mechanism of the air in front of a meteor
(at least, the ones I could find) are maddeningly vague, but John Lewis
at the University of Arizona set me straight, and I thank him for it. If
there are any remaining errors in my comments, they are mine and not his.
Phil
* * * * The Bad Astronomer * * * *
Phil Plait badastro@badastronomy.com
The Bad Astronomy Web Page: http://www.badastronomy.com
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