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(meteorobs) Excerpts from "CCNet, 1 December 1999"
------- Forwarded Message
From: Benny J Peiser <b.j.peiser@livjm.acdot uk>
To: cambridge-conference@livjm.acdot uk
Subject: CCNet, 1 December 1999
Date: Wed, 1 Dec 1999 09:29:38 -0500 (EST)
CCNet, 1 December 1999
-----------------------
[...]
(2) FIREBALL BREAKS UP OVER EASTERN SPAIN
Josep Ma Trigo i Rodriguez <jmtrigo@ctv.es>
(3) THE DIFFERENCE BETWEEN A FERRET AND A WEASEL: THE NATURE OF =20
THE TUNGUSKA IMPACTOR
Matthew Genge, THE NATURAL HISTORY MUSEUM
(4) THE NATURE OF THE TUNGUSKA IMPACTOR
V.A. Bronshten, RUSSIAN ACADEMY OF SCIENCE
(5) TRAJECTORY & ORBIT OF THE TUNGUSKA IMPACTOR
V.A. Bronshten, RUSSIAN ACADEMY OF SCIENCE
(6) SEARCHING FOR TUNGUSKA COSMIC BODY MATERIAL
E.M. Kolesnikov et al., MOSCOW MV LOMONOSOV STATE UNIVERSITY
[...]
======================================================================
(2) FIREBALL BREAKS UP OVER EASTERN SPAIN
>From Josep Ma Trigo i Rodriguez <jmtrigo@ctv.es>
Dear colleagues,
=20
The past November 27th at aprox. 21h30m UT was observed an impressive=20
slow moving fireball of at least -15 absolute magnitude from several=20
sites of Valencia and Castello provinces. We are preparing a detailed=20
report of this exceptional event that unfortunately happened low in the =
horizon and in a partial cloudy sky when our Network was not operative.
In this moment we have only preliminary reports from several=20
eyewitness. The fireball exhibits a spectacular break off in six=20
fragments along its long trajectory in the sky. In the next week we=20
will send additional information to Dr. Z. Ceplecha (European Fireball=20
Network, Czech Republic) and a special report to WGN, issue of the=20
International Meteor Organization. Please note that all data above=20
reported are preliminary.
*******************************************************************
Josep Ma. Trigo i Rodriguez=20
SPANISH PHOTOGRAPHIC METEOR NETWORK (SPMN)
- -Dept. Astronomy & Astrophysics, Universitat de Valencia
- -SOMYCE
E-mail: jmtrigo@ctv.es=20
Phones: (+Spain Code 34) 964 - 282584 / 282968 (office)=20
(964) 395064 (part.) Fax: 964 - 285161
Postal address: c/ Manuel de Falla 26,=20
12.560 Benicassim (Castello) SPAIN
======================================================================
(3) THE DIFFERENCE BETWEEN A FERRET AND A WEASEL: THE NATURE OF =20
THE TUNGUSKA IMPACTOR
>From Matthew Genge, THE NATURAL HISTORY MUSEUM
<M.Genge@nhm.acdot uk>
Even though it sounds as pointless as distinguishing between a ferret=20
and a weasel, evaluating whether the impactor that detonated with an=20
energy of several tens of megatonnes over the Tunguska river in 1908=20
was an asteroid or a comet is more than an esoterical exercise. These=20
smaller more frequent events pose a more tangible hazard than the=20
larger 'global killers' because there is a high probability that such=20
an event will occur in a single human lifetime (~25-70%). Despite the=20
fact that the Tunguska event resulted in no more than a biocrater, the=20
detonation of a body of this size could, given a less fortunate=20
trajectory, result in fatalities measured in tens of millions. The=20
effects and frequency of Tunguska-like events will be sensitive to the=20
impactor type. =20
In Meteoritics and Planetary Science (1999) 34, 723 V. A. Bronshten=20
presents an excellent and authoritative review of research on the=20
nature of the Tunguska bolide and based on the expectation that some=20
fragments of a stony asteroid would survive the terminal detonation=20
convincingly argues for a cometary origin. However, one less convincing =
argument presented for a cometary origin is based on the cosmochemistry =
of the ablation spheres which have a CI-chondrite like signature. The=20
CI chondrites are the most primitive known meteorites but the weight of =
scientific opinion still suggests these are derived from asteroids. =20
Suggestions that CI chondrites are cometary have been based on their=20
primitive compositions and mineralogies, and their fragile nature=20
(Campins and Swindle (1998) MAPS 33, 1201). It is important to=20
determine whether CI chondrites are asteroidal or cometary since these=20
materials have densities of ~2.1 g/cc significantly higher than the=20
~1.0 g/cc assumed for cometary materials. If some comets are composed=20
of CI chondrite-like material the behaviour of small comet fragments=20
during atmospheric entry and the outcome of small impact events will=20
differ significantly from the predictions of models based on the lower
density estimates.
There are, however, very good reasons for believing that CI chondrites=20
are derived from asteroids rather than comets.
(1) Large unmelted micrometeorites (>50 microns in diameter) include=20
CI-like particles. These materials have survived atmospheric entry=20
heating and therefore must be derived from sources with low geocentric=20
velocity and thus are most likely to be asteroidal.
(2) CI chondrite clasts have been found in the Nilpena ureilite. The=20
ureilites are igneous meteorites derived from inner main belt asteroids
(Brearley and Prinz, 1992 GCA 56, 1373).
(3) The extensive aqueous alteration of anhydrous silicates in=20
CI-chondrites to clay minerals could not occur on cometary nuclei since =
these cannot support free water. The presence of sulphate veins in CI
chondrites clearly indicates that aqueous alteration was a parent body
process.
(4) There is evidence for a continuous range in aqueous alteration=20
between CI and CM2 chondritic meteorites (Zolensky et al., 1998 GCA 61, =
5099). The latter group are relatively abundant meteorites and are=20
undoubtedly derived from asteroids.
(5) The Antarctic CI-chondrite Yamato-82162 contains heated=20
phyllosilicates that suggest metamorphism on the parent body to=20
temperatures of 700-800C that would not be possible on a cometary body.
An asteroidal origin for CI chondrites does not, however, imply that=20
the Tunguska impactor was a fragment of an asteroid since there are=20
likely to be strong chemical affinities between CI chondrites and=20
comets since both are thought to be highly primitive materials. It does =
however suggest that primitive C-type asteroids can have material=20
densities of 2.1 g/cc or lower.
Dr Matthew J. Genge
Researcher (Meteoritics)
Department of Mineralogy, The Natural History Museum
Cromwell Road, London SW7 5BD, UK.
Tel: Int + 020 7 942 5581
Fax: Int.+ 020 7 942 5537
email: M.Genge@nhm.acdot uk
Staff internet page http://www.nhm.acdot uk/mineralogy/genge/genge.htm
======================================================================
(4) THE NATURE OF THE TUNGUSKA IMPACTOR
V.A. Bronshten: The nature of the Tunguska meteorite. METEORITICS &=20
PLANETARY SCIENCE, 1999, Vol.34, No.5, pp.723-728
RUSSIAN ACADEMY OF SCIENCE, COMM METEORITES,MOSCOW,RUSSIA
Arguments in favor of the cometary origin of the Tunguska meteorite=20
(sic!) are adduced along with reasons against the asteroidal=20
hypothesis. A critical analysis is given for the hypotheses by Sekanina =
(1983) and Chyba et al. (1993). On the basis of the azimuth and=20
inclination of the trajectory of the Tunguska body with plausible=20
values of the geocentric velocity, the semimajor axis of the orbit and=20
its inclination to the ecliptic plane are calculated for this body. It=20
is noted that the theory of the disintegration of large bodies in the=20
atmosphere put forward by Chyba et al. (1993) is crude. Applying more=20
accurate theories (Grigoryan, 1979; Hills and Goda, 1993) as well as=20
taking into account the realistic shape of the body yield for the=20
cometary body lower disruption heights than obtained by Chyba et al.=20
Numerical simulations carried out by Svettsov et al. agree well with=20
the cometary hypothesis and the analytical calculations based on=20
Grigoryan's theory. The asteroidal hypothesis is shown not to be=20
tenable: the complete lack of stony fragments in the region of the=20
catastrophe, cosmochemical data (in particular, the results of an=20
isotope analysis), and some other information contradict this=20
hypothesis. It is shown that stony fragments that would have originated =
in the explosive disruption of the Tunguska body would not be vaporized =
by the radiation of the vapor cloud nor as a result of their fall to=20
the Earth's surface. Copyright 1999, Institute for Scientific=20
Information Inc.
======================================================================
(5) TRAJECTORY & ORBIT OF THE TUNGUSKA IMPACTOR
V.A. Bronshten: Trajectory and orbit of the Tunguska meteorite=20
revisited. METEORITICS & PLANETARY SCIENCE, 1999, Vol.34, No.SS,=20
pp.A137-A143
RUSSIAN ACADEMY OF SCIENCE,COMM METEORITES,MOSCOW,RUSSIA
A critical survey is presented of all determinations of the azimuth and =
inclination of the Tunguska meteorite's (sic!) trajectory based either=20
on eyewitness testimonies or on the mathematical treatment of the=20
forest-leveling field in the area of the catastrophe. The eyewitness=20
testimonies collected in the neighborhood of the Nizhnyaya Tunguska=20
River indicate the most probable azimuth of the trajectory projection=20
to be 104 degrees from the north to the east, which is close to the=20
most recent azimuth estimate from the forest-leveling field, 99=20
degrees. For the most part of the trajectory, its inclination could not =
exceed 15 degrees. However, it is seen from aerodynamic calculations=20
that the combined action of the gravity field and a nonzero aerodynamic =
lift could increase the inclination to 40 degrees as the end of the=20
trajectory was approached. Meteoroid orbits are calculated for a broad=20
family of trajectories with azimuths ranging from 99 degrees (Fast et=20
al., 1976) to 137 degrees (Krinov, 1949) and geocentric velocities=20
ranging from 25 to 40 km/s. Orbits with large azimuth values (120=20
degrees and larger) are shown to belong to the asteroidal type. They=20
are succeeded by the orbits of short-period and long-period comets,=20
whereas very small azimuth values and large geocentric velocities=20
correspond to the region of hyperbolic orbits. Certain restrictions on=20
the possible trajectory azimuths and geocentric velocities of the=20
Tunguska body are imposed by this study. Copyright 1999, Institute for=20
Scientific Information Inc.
======================================================================
(6) SEARCHING FOR TUNGUSKA COSMIC BODY MATERIAL
E.M. Kolesnikov*), T. Boettger, N.V. Kolesnikova: Finding of probable=20
Tunguska Cosmic Body material: isotopic anomalies of carbon and=20
hydrogen in peat. PLANETARY AND SPACE SCIENCE, 1999, Vol.47, No.6-7,=20
pp.905-916
*) MOSCOW MV LOMONOSOV STATE UNIVERSITY,FAC GEOL,MOSCOW 119899,RUSSIA
Method of a search for traces of Tunguska Cosmic Body (TCB) material=20
using layer-by-layer analysis of the isotopic composition of light=20
elements in peat has been offered. Four peat columns sampled at the=20
explosion epicentre indicated significant carbon and hydrogen isotopic=20
effects in its 'near catastrophic' layers. The shifts, opposite in=20
direction, for carbon (Delta(13)C reaches +4.3 parts per thousand) and=20
hydrogen (Delta D reaches -22 parts per thousand) cannot be attributed=20
to any known terrestrial reasons (fall-out of terrestrial dust and fire =
soot; emission from the Earth of oil-gas streams; climate changes,=20
humification of peat, and so on). Moreover, the isotopic effects are=20
clearly associated with the area and with the time of the 1908 event.=20
They are absent in the uppermost and the lowest peat layers and also in =
the control peat columns sampled at the remote places. Since calculated =
delta(13)C value for an admixture of carbon (+51-64 parts per thousand) =
is very high, these effects may not be explained by contamination of=20
peat with material similar to ordinary chondrites or achondrites, too.=20
Such heavy carbon occurs in the most primitive CI and CM types of=20
carbonaceous chondrites. However, C/Ir ratio in a cosmic admixture is=20
10,000 times as many as in CI chondrites that points to cometary nature =
of the TCB. The isotopic effects are in agreement with the increase of=20
the Ir content observed in peat, but, at the same time, small content=20
of Ir points to the low content of dust in the Tunguska comet that=20
sharply differs it from Halley's comet. (C) 1999 Elsevier Science Ltd.=20
All rights reserved.
[...]
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