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...provides an explanation for why no substantial fragments of the =20
object reached the ground to provide meteorite samples which would have=20
enabled us to determine its nature a long time ago. Svetsov shows that=20
the radiation field within the fireball would have vaporized all the=20
small fragments into which the object would be expected to have=20
dispersed as it disintegrated.  All meteor scientists will know that=20
fragmentation as an important phenomenon at all meteoroid sizes. The =20
author of the following paper is one of the world's leading experts
on meteoric phenomena, and he emphasizes this fact.
=20
(2) V.A. Bronshten, 'Fragmentation and crushing of large meteoric bodies
in an atmosphere', Solar System Research, 29, 450-458 (1995).

Abstract:  Three theories of large meteoroid fragmentation in an=20
atmosphere are compared: Grigoryan, Hills-Goda and Chyba-Thomas-Zahnle.=20
It is shown that the Grigoryan and Hills-Goda theories are virtually=20
identical, although the first is better justified.  The Chyba-Thomas-=20
Zahnle theory appreciably ranks below the first two theories, since it=20
does not take into account fragmentation by the splitting mechanism.=20
Therefore, the destruction heights for bodies of various natures are=20
sometimes overestimated by 10-12 km.  Arbitrary assumptions accepted in=20
all three theories are discussed: a neglect of evaporation and an=20
oriented flight of an idealized-shape body.  It is shown that an=20
inclusion of evaporation slightly increases crushing heights (from=20
fractions of a kilometer to several kilometers).  Arguments are=20
presented in favor of a rapid chaotic rotation of irregular-shaped=20
bodies coming into the atmosphere.  The inclusion of such rotation=20
retards the disruption of the body as compared to the oriented flight.

(3) J.E. Lyne, M. Tauber & R. Fought, 'An analytical model of the
atmospheric entry of large meteors and its application to the Tunguska
Event', Journal of Geophysical Research, 101, 23207-23212 (1996).

Abstract: The atmospheric entry of a meteor [sic 1] is quite complex,=20
with the body losing kinetic energy both from atmospheric drag and from=20
mass loss due to aerodynamic heating.  Moreover, high pressures on the=20
windward side of the body result in enormous compressive stresses which=20
may exceed the yield strength of the material and cause rapid=20
fragmentation of the meteor.  While ablative mass loss is not important=20
for extremely large objects, it must be accurately estimated
to correctly predict [sic 2] the trajectories of objects that are several
tens of meters in diameter.  The current paper describes [sic 3] a
computer model which performs calculations [sic 3] of shock layer
conditions, accounting for the time varying temperature distribution,
radiative cooling of the shocked gases, and blockage of surface heating
by ablation products.  Application of the model to the well-known
Tunguska Event indicates that the responsible bolide was probably a
carbonaceous chondrite, although a stony asteroid or a cometary body
cannot be conclusively ruled out.
=20
[sic 1] A meteor is an atmospheric phenomenon, so that it cannot "enter"
        the atmosphere.
[sic 2] Split infinitive.
[sic 3] Personification twice in one sentence.
=20
Such minor gripes aside, in this paper the authors have clearly advanced
our understanding of the Tunguska event, although it will be interesting=20
to see what the Russian experts have to say about the paper/model in the
fullness of time.
=20
Note also that Svetsov, and Lyne et al., and various other authors=20
including Grigoryan, have papers in the recent special Tunguska issue=20
of Planetary and Space Science (volume 46, April 1998).  In particular=20
see Figure 1 of the paper by Grigoryan, in which he indicates that an=20
icy body could penetrate to an altitude of 10 km or less.  (Although=20
myself I favour a comet-derived carbonaceous chondrite.)
=20
Having read through all of the above, it is perhaps best to recall the=20
words of Sir Arthur Stanley Eddington: "It is also a good rule not to=20
put overmuch confidence in the observational results that are put =20
forward until they are confirmed by theory."  I think that he had his
tongue in his cheek, as when he spoke of a "perfectly spherical elephant,
whose weight may be neglected."
=20
Duncan Steel

=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
(2) TUNGUSKA REMINDER IN TURKMENISTAN: METEORITE IMPACT ON 20 JUNE