PLEASE NOTE:


*

CCNet, 1 December 1999
-----------------------


      QUOTE OF THE DAY

      "Even though it sounds as pointless as distinguishing between a
      ferret and a weasel, evaluating whether the impactor that
      detonated with an energy of several tens of megatonnes over the
      Tunguska river in 1908 was an asteroid or a comet is more than an
      esoterical exercise. These smaller more frequent events pose a
      more tangible hazard than the larger 'global killers' because
      there is a high probability that such an event will occur in a
      single human lifetime (~25-70%). Despite the fact that the
      Tunguska event resulted in no more than a biocrater, the
      detonation of a body of this size could, given a less fortunate
      trajectory, result in fatalities measured in tens of millions.”

               -- Matthew Genge, 1 December 1999


(1) NATIONAL SPACEGUARD CENTRE
    Jonathan Tate <fr77@dial.pipex.com>

(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  
    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

(7) IMPACT CRATERS OF NORTHEASTERN EURASIA
    V.L.. Masaitis, KARPINSKY GEOL INSTITUE


===================
(1) NATIONAL SPACEGUARD CENTRE

From Jonathan Tate <fr77@dial.pipex.com>

30 November 1999

Dear Benny,

You will, no doubt, have seen the recent ripples in the press
concerning the proposal to establish a National Spaceguard Centre in
the UK. My initial reaction to Jonathan Leake’s piece in the Sunday
Times was that there were some glaring inaccuracies (the set-up cost is
nearer 65K than 500K for example, and had the Tunguska object been
800 metres in diameter, I think it unlikely that we would be having
this debate now!), and that it could endanger the much improved
relationship between Spaceguard UK and the BNSC. It could look like an
attempt to pre-empt the BNSC report.

However, on reflection, I think that the three items (Sunday Times on
28 Nov, BBC Radio 4 on 29 Nov and a short piece in today’s Daily
Telegraph) serve as a timely reminder to Lord Sainsbury and the BNSC
that this is a subject that requires action, and that it will not just
go away. Public interest is sufficient to warrant press interest, and
is growing. As Spaceguard UK activities continue to ramp up, public
interest is growing steadily, and the increasing rate of NEO discovery
will inevitably lead to repeats of the XF11 and AN10 affairs.

I am confident that when our facts and figures are validated by the
BNSC the “grown ups” will see that the point that we have been making
for the past three years is reasonable and deserving of suitable
resource allocation. Quite when the BNSC report will be submitted to
the minister remains an unknown, as does the decision resulting from
it.  If the government proves to be unwilling to support the proposal,
I have some other irons in the fire. However, as I have already said, I
am confident that the case speaks for itself, and that our scientists
and politicians must be too savvy to ignore it, thereby missing a
golden opportunity. It is not often that the UK has the chance to
assume a leadership role these days, but this is one area in which we
could so easily become ground breakers.

Jay Tate
Spaceguard UK

======================
(2) FIREBALL BREAKS UP OVER EASTERN SPAIN

From Josep Ma Trigo i Rodriguez <jmtrigo@ctv.es>

Dear colleagues,

The past November 27th at aprox. 21h30m UT was observed an impressive
slow moving fireball of at least -15 absolute magnitude from several
sites of Valencia and Castello provinces. We are preparing a detailed
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
eyewitness. The fireball exhibits a spectacular break off in six
fragments along its long trajectory in the sky. In the next week we
will send additional information to Dr. Z. Ceplecha (European Fireball
Network, Czech Republic) and a special report to WGN, issue of the
International Meteor Organization. Please note that all data above
reported are preliminary.
*******************************************************************

Josep Ma. Trigo i Rodriguez
SPANISH PHOTOGRAPHIC METEOR NETWORK (SPMN)
-Dept. Astronomy & Astrophysics, Universitat de Valencia
-SOMYCE
E-mail: jmtrigo@ctv.es
Phones: (+Spain Code 34)  964 -  282584  / 282968  (office)
(964) 395064 (part.) Fax: 964 - 285161
Postal address: c/ Manuel de Falla 26,
12.560 Benicassim (Castello) SPAIN

===========================
(3) THE DIFFERENCE BETWEEN A FERRET AND A WEASEL: THE NATURE OF  
    THE TUNGUSKA IMPACTOR

From Matthew Genge, THE NATURAL HISTORY MUSEUM
<M.Genge@nhm.ac.uk>

Even though it sounds as pointless as distinguishing between a ferret
and a weasel, evaluating whether the impactor that detonated with an
energy of several tens of megatonnes over the Tunguska river in 1908
was an asteroid or a comet is more than an esoterical exercise. These
smaller more frequent events pose a more tangible hazard than the
larger 'global killers' because there is a high probability that such
an event will occur in a single human lifetime (~25-70%). Despite the
fact that the Tunguska event resulted in no more than a biocrater, the
detonation of a body of this size could, given a less fortunate
trajectory, result in fatalities measured in tens of millions. The
effects and frequency of Tunguska-like events will be sensitive to the
impactor type. 

In Meteoritics and Planetary Science (1999) 34, 723 V. A. Bronshten
presents an excellent and authoritative review of research on the
nature of the Tunguska bolide and based on the expectation that some
fragments of a stony asteroid would survive the terminal detonation
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
CI chondrites are the most primitive known meteorites but the weight of
scientific opinion still suggests these are derived from asteroids. 
Suggestions that CI chondrites are cometary have been based on their
primitive compositions and mineralogies, and their fragile nature
(Campins and Swindle (1998) MAPS 33, 1201). It is important to
determine whether CI chondrites are asteroidal or cometary since these
materials have densities of ~2.1 g/cc significantly higher than the
~1.0 g/cc assumed for cometary materials. If some comets are composed
of CI chondrite-like material the behaviour of small comet fragments
during atmospheric entry and the outcome of small impact events will
differ significantly from the predictions of models based on the lower
density estimates.

There are, however, very good reasons for believing that CI chondrites
are derived from asteroids rather than comets.

(1) Large unmelted micrometeorites (>50 microns in diameter) include
CI-like particles. These materials have survived atmospheric entry
heating and therefore must be derived from sources with low geocentric
velocity and thus are most likely to be asteroidal.

(2)  CI chondrite clasts have been found in the Nilpena ureilite. The
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
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
between CI and CM2 chondritic meteorites (Zolensky et al., 1998 GCA 61,
5099). The latter group are relatively abundant meteorites and are
undoubtedly derived from asteroids.

(5) The Antarctic CI-chondrite Yamato-82162 contains heated
phyllosilicates that suggest metamorphism on the parent body to
temperatures of 700-800C that would not be possible on a cometary body.

An asteroidal origin for CI chondrites does not, however, imply that
the Tunguska impactor was a fragment of an asteroid since there are
likely to be strong chemical affinities between CI chondrites and
comets since both are thought to be highly primitive materials. It does
however suggest that primitive C-type asteroids can have material
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.ac.uk
Staff internet page http://www.nhm.ac.uk/mineralogy/genge/genge.htm

========================
(4) THE NATURE OF THE TUNGUSKA IMPACTOR

V.A. Bronshten: The nature of the Tunguska meteorite. METEORITICS &
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
(sic!) are adduced along with reasons against the asteroidal
hypothesis. A critical analysis is given for the hypotheses by Sekanina
(1983) and Chyba et al. (1993). On the basis of the azimuth and
inclination of the trajectory of the Tunguska body with plausible
values of the geocentric velocity, the semimajor axis of the orbit and
its inclination to the ecliptic plane are calculated for this body. It
is noted that the theory of the disintegration of large bodies in the
atmosphere put forward by Chyba et al. (1993) is crude. Applying more
accurate theories (Grigoryan, 1979; Hills and Goda, 1993) as well as
taking into account the realistic shape of the body yield for the
cometary body lower disruption heights than obtained by Chyba et al.
Numerical simulations carried out by Svettsov et al. agree well with
the cometary hypothesis and the analytical calculations based on
Grigoryan's theory. The asteroidal hypothesis is shown not to be
tenable: the complete lack of stony fragments in the region of the
catastrophe, cosmochemical data (in particular, the results of an
isotope analysis), and some other information contradict this
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
the Earth's surface. Copyright 1999, Institute for Scientific
Information Inc.

=====================
(5) TRAJECTORY & ORBIT OF THE TUNGUSKA IMPACTOR

V.A. Bronshten: Trajectory and orbit of the Tunguska meteorite
revisited. METEORITICS & PLANETARY SCIENCE, 1999, Vol.34, No.SS,
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
on eyewitness testimonies or on the mathematical treatment of the
forest-leveling field in the area of the catastrophe. The eyewitness
testimonies collected in the neighborhood of the Nizhnyaya Tunguska
River indicate the most probable azimuth of the trajectory projection
to be 104 degrees from the north to the east, which is close to the
most recent azimuth estimate from the forest-leveling field, 99
degrees. For the most part of the trajectory, its inclination could not
exceed 15 degrees. However, it is seen from aerodynamic calculations
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
trajectory was approached. Meteoroid orbits are calculated for a broad
family of trajectories with azimuths ranging from 99 degrees (Fast et
al., 1976) to 137 degrees (Krinov, 1949) and geocentric velocities
ranging from 25 to 40 km/s. Orbits with large azimuth values (120
degrees and larger) are shown to belong to the asteroidal type. They
are succeeded by the orbits of short-period and long-period comets,
whereas very small azimuth values and large geocentric velocities
correspond to the region of hyperbolic orbits. Certain restrictions on
the possible trajectory azimuths and geocentric velocities of the
Tunguska body are imposed by this study. Copyright 1999, Institute for
Scientific Information Inc.

==================
(6) SEARCHING FOR TUNGUSKA COSMIC BODY MATERIAL

E.M. Kolesnikov*), T. Boettger, N.V. Kolesnikova: Finding of probable
Tunguska Cosmic Body material: isotopic anomalies of carbon and
hydrogen in peat. PLANETARY AND SPACE SCIENCE, 1999, Vol.47, No.6-7,
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
using layer-by-layer analysis of the isotopic composition of light
elements in peat has been offered. Four peat columns sampled at the
explosion epicentre indicated significant carbon and hydrogen isotopic
effects in its 'near catastrophic' layers. The shifts, opposite in
direction, for carbon (Delta(13)C reaches +4.3 parts per thousand) and
hydrogen (Delta D reaches -22 parts per thousand) cannot be attributed
to any known terrestrial reasons (fall-out of terrestrial dust and fire
soot; emission from the Earth of oil-gas streams; climate changes,
humification of peat, and so on). Moreover, the isotopic effects are
clearly associated with the  area and with the time of the 1908 event.
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
peat with material similar to ordinary chondrites or achondrites, too.
Such heavy carbon occurs in the most primitive CI and CM types of
carbonaceous chondrites. However, C/Ir ratio in a cosmic admixture is
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
the Ir content observed in peat, but, at the same time, small content
of Ir points to the low content of dust in the Tunguska comet that
sharply differs it from Halley's comet. (C) 1999 Elsevier Science Ltd.
All rights reserved.

================
(7) IMPACT CRATERS OF NORTHEASTERN EURASIA

V.L. Masaitis: Impact structures of northeastern Eurasia: The
territories of Russia and adjacent countries. METEORITICS & PLANETARY
SCIENCE, 1999, Vol.34, No.5, pp.691-711

KARPINSKY GEOL INST,SREDNY PROSPEKT 74,ST PETERSBURG 199106,RUSSIA

More than 30 impact structures have been discovered in the last three
decades in northeastern Eurasia, which includes Russia, Ukraine,
Belarus, Lithuania, Latvia, Estonia, Kazakhstan, and Mongolia. The
largest impact events of the twentieth century also occurred here: the
Tunguska and Sikhote Alin events in Siberia and Primorye, respectively.
Many of these impact structures have been studied in detail. This is
particularly the case for the largest examples: Popigai (100 km),
Puchezh-Katunki (80 km), and Kara (65 km). Detailed data and
descriptions of the geological characteristics, morphology, and the
nature of the impact breccias and impactites (impact-melt rocks) are to
be found in numerous publications published mostly in Russian. As these
data and descriptions are not generally available to non-Russian
scientists, this review summarizes the current state of knowledge on
these impact structures. It also provides references to the principal
publications detailing them. Copyright 1999, Institute for Scientific
Information Inc.

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