PLEASE NOTE:


*

CCNet SPECIAL: THE XF11 DEBATE & A PROPOSAL 
-------------------------------------------

(1) "ACTION PLAN" SUBMITTED TO THE HOUSE SUBCOMMITTEE ON SPACE AND
    AERONAUTICS
    Clark Chapman <cchapman@boulder.swri.edu>

(2) CHAPMAN ON MARSDEN
    Wolfgang Kokott <W.Kokott@lrz.uni-muenchen.de> >

(3) 1997 XF11 WAS NOT SPECIAL
    Paolo Farinella <paolof@keplero.dm.unipi.it>

(4) CAN THERE BE PROGRESS IN THE CURRENT IMPASS IN THE XF11 DEBATE?
    A PROPOSAL FOR A MOCK NEO DISCOVERY
    Benny J Peiser <b.j.peiser@livjm.ac.uk>

========================
(1) "ACTION PLAN" SUBMITTED TO THE HOUSE SUBCOMMITTEE ON SPACE AND
    AERONAUTICS

From Clark Chapman <cchapman@boulder.swri.edu>


(1) "ACTION PLAN" SUBMITTED TO THE HOUSE SUBCOMMITTEE ON SPACE AND
    AERONAUTICS

From Clark Chapman <cchapman@boulder.swri.edu>

Dear Benny,

I would like to call your attention to the two-page "Action Plan" that
I submitted to the House Subcommittee on Space and Aeronautics
yesterday, in response to their request during the May 21st hearings. 
A Postscript file may be downloaded, using your browser, from:

www.boulder.swri.edu/clark/actnea.ps

Also, for those who haven't seen it, Arthur C. Clarke has written an
intriguing essay, "Presidents, Experts, and Asteroids" in the current
(June 5th) issue of SCIENCE, Vol. 280, pp. 1532-1533.

Clark Chapman

----
[Clark, I was unable to download your 'Action Plan' since my Netscape
browser did not recognise your postscript file. I (and I guess many
other list members) would be grateful it if you could be so kind and
post the text as an ordinary text file. Benny]

===================
(2) CHAPMAN ON MARSDEN

From Wolfgang Kokott <W.Kokott@lrz.uni-muenchen.de> >

Dear Dr Peiser,

I am developing a strong feeling of confusion. Marsden did make a
severe mistake when dealing with the re-discovery of P/Swift-Tuttle?

In my book, Brian was the last sceptic, refusing to accept the putative
identification by the Japanese re-discoverers before the facts became
confirmed. <Not so bad for someone who had waited for two decades to
get a confirmation of his prediction.>

So Marsden was wrong in not crying 'Swift-Tuttle', and again in crying
'evidence needed for PHA 1997 XF11'.

Confused regards,
Wolfgang Kokott

=================
(3) 1997 XF11 WAS NOT SPECIAL

From Paolo Farinella <paolof@keplero.dm.unipi.it>

Benny, I have some experience in `orbital calculations' and my opinion
you have missed an important point that Clark Chapman was trying to
stress in his rebuttal of Brian Marsden's claim that XF11 will become
dangerous AFTER the 2028 Earth encounter. This is a general point which
goes well beyond this specific case, so I think it worth to continue
the debate on it. You see, NEOs orbits are chaotic and cannot be
predicted determistically after very close planetary encounters. The
technical jargon is that NEOS have typically `Lyapounov times' of the
order of decades to centuries. Thus basically every PHA object which
has a close enough encounter with the Earth (and there are many in the
next century -- see the MPC list at
http://cfa-www.harvard.edu/iau/lists/PHACloseApp.html) COULD later hit
the Earth. XF11 is not special in this respect.

You appear to downplay, or misunderstand, the argument on the
`backround' or `statistical' hazard level, which in my opinion is an
essential one. This is due to the fact that we know so few of the
relevant objects and their orbits, but also to the fact that these
orbits are chaotic. Since we can predict deterministically the orbits
of NEOs only on a relatively short span of time (and anyway not after
very close planetary encounters), we need to resort to statistical
definitions of collision probability and hazard on a longer term. And I
agree with Chapman that in this case an object (such as XF11) is
particularly dangerous only if it exceeds the statistical background
level.

Also, I would like to say that in my opinion your note preceding
Chapman's text in the CCNet was a bit unfair. As an Editor perhaps you
should refrain from giving your personal opinion on an issue BEFORE one
side's statement is seen by the reader. Am I wrong?

Paolo Farinella

=====================
(4) CAN THERE BE PROGRESS IN THE CURRENT IMPASS IN THE XF11 DEBATE?
    A PROPOSAL FOR A MOCK NEO DISCOVERY

From Benny J Peiser <b.j.peiser@livjm.ac.uk>

Far from being resolved, the controversy about whether or not asteroid
1997 XF11 was, at any time prior to the discovery of the 1990 films, a
Potentially Hazardous Object with a very small, but real chance of
colliding with earth in the next century has been re-opened by
Brian Marsden's new calculations. The  m a i n  problem raised by his
hypothetical scenario is - due to the vital data provided by Helin and
Lawrence - not so much about who was right and who was wrong way back
in early March. Much more important, I believe, is the question
as to how the NEO community will react the next time an astronomer
discovers a similar PHO, i.e. an object which can be shown to have a
small but real chance of colliding with earth in the near future.

One possible way of overcoming the current disagreement and impass, I
hope, might be to stage a dry run of such a scenario. Such an
experiment could perhaps help us to assess what might happen and where
the main problems and differences among NEO experts currently are. What
I suggest is a mock discovery of asteroid 1997 XF11 and its risk
assessment by the NEO community. This, obviously, requires that we
completely ignore the 1990 data and just use the observational data
available since 6 December 1997.

I would also suggest that everyone taking part in such an excercise
should use Richard Binzel's NEO HAZARD INDEX as published in the
June issue of SKY & TELESCOPE, p. 29. After taking into account the
currently available data and calculating the orbital evolution of XF11
over the next 100 years, the participants should state into which of
the six hazard categories (ranging from 0=zero risk, to 5=certain on
specific date) they would place XF11. The results of this experiment,
it is hoped, might help to establish what the  u n d e r l y i n g
reasons for the hypothesised differences in the process of the hazard
categorising are.

I would much appreciate the views of list members about this proposal
(or further ideas for the refinement of the suggested procedures).

Benny J Peiser



*

CCNet DIGEST, 11 June 1998
--------------------------


(1) FIVE SHORT ITEMS OF INTEREST
    Ed Grondine <epgrondine@hotmail.com>

(2) ELECTROMAGNETIC INTERFERENCE FROM METEOR SHOWERS
    Luigi Foschini <L.Foschini@fisbat.bo.cnr.it>

(3) EVIDENCE FOR 6TH CENTURY CLIMATE CATASTROPHE AND CIVILISATION
    COLLAPSE IN THE SOUTHERN HEMISPHERE
    Steve Weintz <indy@hera.itg.uiuc.edu>

(4) SCEPTICISM ABOUT IMPACT HYPOTHESIS FOR THE AD 540 EVENT
    Timothy Bratton <bratton@ACC.JC.EDU>

(5) SPECIAL TUNGUSKA ISSUE
    PLANETARY AND SPACE SCIENCE Vol.46, Iss.2-3, FEB-MAR 1998 

(6) THE TUNGUSKA METEORITE PROBLEM TODAY
    N.V. Vasilyev, RUSSIAN ACADEDMY OF SCIENCE

(7) SEARCH FOR THE LOCAL CENTRES OF THE TUNGUSKA EXPLOSION
    V.D. Goldine, TOMSK VV KUIBYSHEV STATE UNIVERSITY

(8) CONSEQUNCES OF THE TUNGUSKA CATASTROPHE
    V.D. Nesvetajlo, TOMSK VV KUIBYSHEV STATE UNIVERSITY

(9) ISOTOPIC ANOMALY IN PEAT NITROGEN AS PROBABLE TRACE OF ACID RAINS
    E.M. Kolesnikov et al., MOSCOW MV LOMONOSOV STATE UNIVERSITY

(10) NEW ASPECTS OF THE TUNGUSKA METEORITE PROBLEM
     V.A. Alekseev, TROITSK INST INNOVAT & FUS RES

(11) IRIDIUM AND OTHER ELEMENT ANOMALIES NEAR THE TUNGUSKA EXPLOSION
     SITE
     Q.L. Hou et al., CHINESE ACADEMY OF SCIENCE

(12) EVIDENCE FOR THE ASTEROIDAL ORIGIN OF THE TUNGUSKA OBJECT
     Z. Sekanina, CALTECH, JET PROP LAB,

(13) ON THE POSSIBLE RELATION BETWEEN THE TUNGUSKA BOLIDE AND COMET
     ENCKE
     D.J. Asher*) & D.I. Steel, COMM RESEARCH LABS, JAPAN

(14) THE COMETARY NATURE OF THE TUNGUSKA METEORITE
     S.S. Grigorian, MOSCOW MV LOMONOSOV STATE UNIVERSITY

(15) DAMAGE FROM THE IMPACTS OF SMALL ASTEROIDS
     J.G. Hills*) & M.P. Goda, LOS ALAMOS NATL LAB

(16) COMPLEX MODELLING OF THE TUNGUSKA CATASTROPHE
     V.P. Korobeinikov et al., RAS, INSTITUTE COMP AIDED DESIGN

(17) COMUTER MODEL OF THE ATMOSPHERIC ENTRY OF THE TUNGUSKA OBJECT
     J.E. Lyne et al., UNIVERSITY OF TENNESSEE

(18) GASDYNAMICAL MODEL OF THE TUNGUSKA FALL
     V. P. Stulov, MOSCOW MV LOMONOSOV STATE UNIVERSITY

(19) COULD THE TUNGUSKA DEBRIS SURVIVE THE TERMINAL FLARE?
     V.V. Svetsov, RUSSIAN ACADEMY OF SCIENCE

(20) EARTH CRATERING RECORD AND IMPACT ENERGY FLUX
     A. Montanari et al., OSSERVATORIO GEOL COLDIGIOCO

(21) GROUND-BASED OPTICAL SURVEYS FOR NEAS
     A.W. Harris, CALTECH, JET PROP LAB

(22) DISCOVERY AND FOLLOW-UP SIMULATIONS FOR EARTH CROSSING ASTEROIDS
     K. Muinonen, UNIVERSITY OF HELSINKI OBSERVATORY

(23) THE NEO CONFIRMATION PAGE
     B.G. Marsden & G.V. Williams, HARVARD SMITHSONIAN CTR ASTROPHYS,

(24) THE FLUX OF TUNGUSKA-SIZED FRAGMENTS FROM THE MAIN ASTEROID BELT
     P. Farinella et al.,  UNIVERSITY OF PISA

(25) PRODUCTION OF TUNGUSKA-SIZED BODIES BY EARTH'S TIDAL FORCES
     W.F. Bottke et al., CALTECH

(26) THE MACHA IMPACT CRATERS
     E.P. Gurov & E.P. Gurova, NATIONAL ACADEMY OF SCIENCE OF UKRAINE,

(27) SOME METALLIC SPHERULES
     A. Colombetti et al., UNIVERSITY OF MILAN

=================
(1) FIVE SHORT ITEMS OF INTEREST

From Ed Grondine <epgrondine@hotmail.com>

1) NO ONE HOME

Benny -

Last Tuesday I again rose at 3:30 in the morning to make the trip into
Washington. This time it was for a session held by the American
Astronautical Society  on "International Space Co-operation: Real World
Examples". There I learned something which may explain why NASA's
international co-operation on detecting potential Earth impactors has
been "less than optimal": there has been no one overseeing NASA's
international co-operative programs in space science for the last seven
months.

I spoke with Lynne Cline, who co-ordinates NASA's international
co-operation for manned space flight, and Lisa Shaffer, who
co-ordinates NASA's international co-operation in Earth observation. 
They told me that the position of co-ordinator for space science for
NASA's External Affairs (International Relations) Office has remained
unfilled for about 7 months, since the previous officer left. (During
that time it has been filled on a temporary acting basis by Jim
Gibbons.)

Hope is in sight, as NASA has completed accepting applications for the
position, and the selection of a new officer is expected shortly.

2) MORE MILLENIAL MADNESS

It was difficult to get up Tuesday morning for the drive, as Sunday
evening as I had been settling into sleep I had been startled by radio
show host Art Bell's interview with one Stefan Paulus about his book
"Nostradamus 1999".  According to Paulus, Nostradamus predicted that a
giant comet will hit the Earth in 1999, that the resulting crop failure
will lead to a nuclear war started by a starving China, and that this
will cause 4 billion deaths.

Needless to say the show kept me awake until 5 in the morning, as I had
to listen to the whole thing. If you're groaning by now, hold on for
second, because it gets worse.  The Art Bell Show is listened to by 10
to 15 million people here in the U.S., and I'm afraid that even our
international brethren aren't safe, as Paulus's book, while currently
only available in quality paperback form at $16, has been optioned into
8 languages besides English. And we can expect a cheaper English
version to hit U.S. newstands soon. Oh boy!

As  writer I have to admit that I am jealous, as this fellow is likely
to do at least as well as von Daniken, and will most likely end up rich
as Croessus.  As a researcher I feel frustrated: Here we've been
fooling with scientific analysis, struggling to locate historical
records of impact events, and fighting for funding for a telescope
search, when all we had to do was read Nostradamus!

If I am asked about Paulus's book, I intend to tell the questioner that
I am no expert on psychic phenomenon and that I think that I would
rather rely on a telescope search than on an interpretation of
Nostradamus to locate the next impactor. I suggest that the other
members of the list give some thought as to their responses, as
questions will most certainly be coming their way. I don't know about
Nostradamus, but I predict that Paulus's work is going to make it very
difficult to communicate to the public valid information on impactors. 

3) AF AND NASA AGREE: BOTH ARE TO PERFORM SAME TASK

Fatigue finally caught up with me Wednesday, and I slept through my
alarm clock for an hour and a half.  Thus I was sitting in traffic as
Lieutenant General Roger G. DeKok gave his presentation on the U.S.
military's space architecture at Wednesday's AAS session on "New Market
Forces in Space."

During the question and answer period, Lt. Gen. DeKok was asked what he
thought should be the military's role in defense against impacts by
asteroids and comets.  Lt. Gen. Dekok replied "There is always a giggle
factor, because you cross into science fiction, in the sense that in
our lifetimes we haven't had to worry about a comet or an asteroid
hitting a city. But on the basis of the historical record of the Earth
we have been hit many many times, and seriously, everyday, objects come
into our atmosphere and just get burned up.

In the DeKok's opinion, "Probably the military's role should be first
and foremost in surveillance, to know what's out there, so that we
have plenty of leadtime.  Or just be aware in our own solar system of
objects that potentially could become problems for the Earth."

DeKok did not go on to talk about mitigation efforts, and as I was
sitting in my car on the freeway at this time there was no follow up
question about this from me.  DeKok's statement explains the Air
Force's excellent co-operation with NASA in the search for potential
impactors: they watch small bodies burn up in the atmosphere everyday,
and as a result are definetly worried about a fairly large impact
setting off an accidental nuclear exchange.

It is also interesting to note that the AF and NASA are now claiming
the same role: that of finding potential impactors. While NASA is
stepping up its efforts, the AF is obviously and rightly reluctant to
entrust them completely at this time with such a serious task.

4) THE FIRST U.S. PLANETARY DEFENSE PROGRAM

Despite missing the opportunity of questioning DeKok myself, the drive
was well worth the effort, as I ran into a person with knowledge of the
first U.S. planetary defense effort. His recollections agree with those
of others who I have spoken with, so here goes:

After the first generation of U.S. reconnaisance satellites were
launched, their sensors detected the destruction in the atmosphere of
fairly large bodies. These observations led to the RAND Corporation
isuing an "SR" (Strategic Requirements) Report on the problem. 

According to my sources, the defense intended to use Atlas-Centaur and
Thor missiles to deliver a nuclear charge to an incoming impactor.
(A little optimistic, in my opinion, given 1960's computers, and the
Thor and Atlas's long preparation times, and their limited ranges, and
the approach speed of impactors. But anyhow...) According to my source,
the RAND Report also played a role in United States' nuclear testing in
space during that period.

According to my sources, the program wound down during the 1970's, as
the U.S. began to use the Titan instead of the Atlas to launch its
reconnaisance satellites, and as the Thors in the U.K. were taken off
operational status.

All in all, it almost makes one nostalgic for the old days, when these
problems were handled by trained individuals out of special funds, with
no need to go to the legislature to get funding. But we all know that
this defense was completely inadequate, and that the survey problem by
itself is so large that it is going to take an international program to
deal with it.

At least they tried.

5) FRENCH PROMISE JAIL TIME FOR SOCCER HOOLIGANS

Sorry to hear about your grandmother's health problems, and I certainly
wish her a speedy and complete recovery. But these things take time, so
don't become upset if  over the next several weeks she suffers
occasional relapses.

On the bright side, her health problems will most likely lead to your
not being able to get over to the continent during the soccer playoffs.
This definetly reduces the chance that you will get caught up after a
game with some over-enthusiastic celebrators and be thrown in Le Pokey
by les gendarmes!

                        Bon Chance - Ed

======================
(2) ELECTROMAGNETIC INTERFERENCE FROM METEOR SHOWERS

From Luigi Foschini <L.Foschini@fisbat.bo.cnr.it>

Andrew Yee <ayee@nova.astro.utoronto.ca> wrote:

> "It is possible," Ailor told the subcommittee, "that some satellites
> will be damaged, but the most likely source of damage will not be from
> a rock blasting a hole in a satellite, but rather, from the creation of
> a plasma, or free electric charge on the spacecraft. The charge could
> cause damage to computers and other sensitive electronic circuits on
> board the spacecraft, and ultimately cause the spacecraft to fail. For
> example," Ailor said, "during the 1993 Perseid meteor shower, it was
> determined that the Olympus communications satellite was damaged by a
> meteor strike and went off the air shortly thereafter as a result of an
> electrical failure."

I would like to point out a new possible source of damage: a plasma
cloud, generated during a hypervelocity impact of a meteoroid with an
artificial satellite, can radiate electromagnetic energy below the
microwave frequency range and, therefore, may disturb regular satellite
operations. During a storm, a high impact flux can produce a sequence
of interferences which can increase the noise level in electronic
devices.

More details on this argument will soon appear in: L. Foschini:
Electromagnetic interference from plasmas generated in meteoroids
impacts, EUROPHYSICS LETTERS (1998), in press.

Greetings,

Luigi Foschini
CNR - Institute FISBAT
Via Gobetti 101, I-40129 Bologna (Italy)
Tel. +39 51 639.9620/9622; Fax +39 51 639.9654
E-mail: L.Foschini@fisbat.bo.cnr.it
WEB: http://www.fisbat.bo.cnr.it/homepp/dinamica/foschini.html

=====================
(3) EVIDENCE FOR 6TH CENTURY CLIMATE CATASTROPHE AND CIVILISATION
    COLLAPSE IN THE SOUTHERN HEMISPHERE

From Steve Weintz <indy@hera.itg.uiuc.edu>

Hi Benny!

I hunted around for some references to Andean ice cores, and the one
that was handy is Mike Moseley's excellent survey "The Incas and Their
Ancestors" (Thames & Hudson, 1992).

"The second most dramatic event [after the Little Ice Age] that the ice
core measurements show is a great drought that began abruptly in AD 562
and continued until 594." p.209

While this is 20 years after the events of the 530's, there may be a
connection, either through unkown atmospheric coupling or perhaps a
periodic bombardment. The original papers on the Quelccaya ice cores
should be examined more closely in light of our discussion.

Best regards,

Steve Weintz

=======================
(4) SCEPTICISM ABOUT IMPACT HYPOTHESIS FOR THE AD 540 EVENT

From Timothy Bratton <bratton@ACC.JC.EDU>

[as posted on the History of Astronomy Discussion Group
at <HASTRO-L@WVNVM.WVNET.EDU>]

Dear Philip Burns and other list members:

In regard to Mike Baillie's efforts to dethrone Vesuvius as the cause
of the worldwide climatic cooling and darkening of 536 A.D., it is
interesting that not a single one of his quoted sources mentioned the
volcano.  If some contemporary chronicler had said that Vesuvius had
_not_ erupted in that year, or that it had shot out only a little ash
or gas, then it could be ruled out.  Does _anybody_ on this list have a
contemporary Italian account of Vesuvius erupting then?

Is Vesuvius a "minor volcano," as Baillie called it?  I beg to differ. 
From A.D. 380 through 395, it erupted every year, each major episode
rated at a volcanic explosivity index (VEI) of 2,[1] with a moderate
eruption of between 1 x 10^6 to 1 x 10^7 cubic meters of tephra tossed
into the sky, a cloud column height of 1-5 kilometers above the crater,
and a moderate injection of ash into the Earth's troposphere for each
eruption.  When it erupted on 6 November 472, according to Ammianus
Marcellinus, it "covered all Europe with fine ash,"[2] may have
attained a VEI of 4, and threw 1.6 x 10^8 cubic meters of ash into the
atmosphere.  Ash falls were recorded at Constantinople.  The next
eruption, on 8 July 512, killed the local crops with its dust.  From 79
A.D. until 1500, when the volcano became dormant for the next 130 or so
years, it had fairly large and explosive eruptions at long intervals. 
Regrettably, "documentation for that period (79-1631) is poor and
therefore very little is known about how Vesuvio behaved in that
period."[3]

As I mentioned in my original mailing, the 536 eruption has been
inferred largely from tephrochronology, not primary sources. If the
outburst of 536 had a VEI of 3 or 4, it would have injected material
into the Earth's stratosphere and have had widespread consequences. In
fact, the VEI value of 2 assigned to this outburst by Simkin and
Siebert, _Volcanoes of the World_, is a _conservative_ one, for as
those authors admit on page 23, "Eruptions that were definitely
explosive, but carry no other descriptive information in their record,
have been assigned a default value of 2, leading to a disproportionate
number of eruptions bearing this VEI."  Crank up Vesuvius' output
slightly, and there might well have been enough sulfur compounds and
ash output to cool the world's (or at least regional) weather for 18
months.  When Mount St. Helens in Washington state blew in 1990 with an
estimated VEI of 3, North Dakota had darkened skies, that summer and
winter were noticeably cooler, and it rained mud occasionally.

While malnutrition facilitates the spread of bubonic plague, flea
activity is inhibited by cold weather; the black rat flea, _Xenopsylla
cheopis_, is most active in moderately warm and moist weather,
preferring temperatures between 59 to 68 degrees Fahrenheit (15-20
degrees Celsius) and a relative humidity between 85% and 95%. Had a
hypothetical "nuclear (or asteroidal) winter" taken place in 536 and
been sustained until the outbreak of the plague at Pelusium in 542, it
is difficult to explain how the bacillus could have spread under
environmental conditions so unfavorable to fleas.  Pneumonic plague,
which is triggered by the coughing and sneezing of bubonic plague
victims with terminal lung involvement, would work, but the Plague of
Justinian was predominantly a bubonic plague epidemic.  Nor was it as
catastrophic as the "Black Death" of 1347.  The Plague of Justinian was
concentrated around the shores of the Mediterranean, but had difficulty
penetrating inward; I suspect that the smaller and less dense
population of early medieval Europe, the difficulty of travel, and
unsettled conditions made it less likely for travelers to carry the
disease from seaports to inland towns and cities.  Perhaps 20% of the
population died.  See my article, "The Identity of the Plague of
Justinian," _Transactions & Studies of the College of Physicians of
Philadelphia_, 3 (1981):113-124, 174-180.

I think that proposing Vesuvius, not an asteroid impact, as the most
probable cause of the cooling spell around 536 best meets the criterion
of Occam's Razor -- it's the simplest possible explanation that works,
and it requires the fewest presuppositions.  We know in fact that
Vesuvius _did_ erupt around 536, but that [unless somebody on this list
provides evidence to the contrary] this episode was not recorded by
contemporaries, although the tephra record indicates otherwise. 
Vesuvius has reached VEIs of 3 to 4, and it does not require much of a
"leap of faith" to intuit that it surpassed its usual VEI value of 2 on
this occasion.  That the noticeable darkening of the skies lasted only
18 months would be more compatible with a volcanic eruption than with a
major asteroid impact.  Was the cooling trend really worldwide, or only
restricted to the northern hemisphere? Baillie mentioned crop failures
in China and dendrochronological evidence from North America, but
Naples, the U.S., and China share the same line of latitude; why
couldn't the prevailing winds have carried Vesuvian ash or sulfur
particulates around the world?  As I mentioned in my original posting,
add some forest fires to Vesuvius, and you might have enough garbage in
the atmosphere to blot out a good portion of the sunlight.  Have you
considered a Maunder minimum as a contributing cause?  The
archeoastronomer John A. Eddy has argued that the Sun was entering a
period of decreased activity between A.D. 400 and 500, which bottomed
out around A.D. 700.[4]

Baillie's argument appears to be one from silence so far. To explain
his cooling trend, he has to posit a small asteroid striking the Earth
that somehow escaped detection and left no visible crater. One might as
well evoke the explosion of an UFO!  If an asteroid struck, there
should be residual traces of iridium in the appropriate rock or silt
strata; if Baillie could come up with these, I should be more
convinced.  Has anybody sent an expedition to the ice fields of
Antarctica to see if volcanic ash, soot from forest fires, or excess
levels of iridium are embedded in the ice layers for this period?  In
regard to another hypothesis advanced, the late Carl Sagan remarked,
"in the present epoch the solar system is only about 25 light-years
above (north of) the plane of the Galaxy, and there is no giant
interstellar cloud at our doorstep."[5]  Nothing Mike has provided yet
excludes a major volcanic eruption, for which the evidence seems
stronger.

Does anyone on this list have a full version of the software program,
_Dance of the Planets_?  As I understand it, the program extrapolates
gravitational encounters between major solar system bodies and
asteroids and comets back through time. Given that the Taurid meteor
shower, sometimes associated with periodic Comet Encke, has been
invoked, what would happen if the program were run back to A.D. 536? 
I'm curious.

I'm quite prepared to accept that many mass extinctions have been 
caused by asteroid impacts; I enjoyed _Deep Impact_, and look forward
to seeing _Armageddon_.  But I fear that comets and minor planets have
become such "sexy" topics that they are being trotted out, _deus ex
machina_, to explain climatic change (and even the Plague of
Justinian!) when there are still more plausible candidates around.

By the way, I sent an E-mail message to Roberto Scandone,
Professor of Vulcanology and Geoscience at the University of Rome (and
expert on Vesuvius) asking him for any light he can shed on the
eruption of 536.  I shall let you know his opinion when and if it
arrives.

1. Tom Simkin and Lee Siebert, Volcanoes of the World (Tucson, AZ.:
Geoscience Press, Inc., 1994), 2nd ed., page 186.

2. http://vulcan.fis.uniroma3.it/vesuvio/vesuvio.html

3. Posted on Boris Behncke's Volcano Pages at
http://www.geo.mtu.edu/~boris/VESUVIO_post79.html

4. John A. Eddy, "The Case of the Missing Sunspots," Scientific
American 236, no. 5 (May 1977):80-88, 92.

5. Carl Sagan and Ann Druyan, Comet (N.Y., N.Y.: Random House, 1985),
page 300.

Dr. Timothy L. Bratton                  bratton@acc.jc.edu
Department of History/Pol. Science      work: 1-701-252-3467, ext. 2022
6006 Jamestown College                  home: 1-701-252-8895
Jamestown, ND 58405                     home phone/fax: 1-701-252-7507

========================
(6) SPECIAL TUNGUSKA ISSUE
    PLANETARY AND SPACE SCIENCE Vol.46, Iss.2-3, FEB-MAR 1998 

TUNGUSKA EDITORIAL

M. DiMartino, P. Farinella, G. Longo: Tunguska 96 - Foreword of the
Tunguska issue. PLANETARY AND SPACE SCIENCE, 1998, Vol.46, No.2-3,
p.125

===============
(6) THE TUNGUSKA METEORITE PROBLEM TODAY

N.V. Vasilyev: The Tunguska Meteorite problem today
PLANETARY AND SPACE SCIENCE, 1998, Vol.46, No.2-3, pp.129-150

RUSSIAN ACADEDMY OF SCIENCE, COMMISSION FOR METEORITES & SPACE DUST,
MOSCOW 117901, RUSSIA

This paper contains basic data relating to the Tunguska Meteorite (TM)
as late as 1996, including flight and explosion of a giant bolide in
Siberia on 30 June 1908, related geophysical effects, the results of
search of the TM matter and the environmental after effects of the
event. The TNT equivalent of the Tunguska explosion is estimated
between 10 and 40 megatons (most probably 15 megatons), probable energy
4.2 x 10(23)-1.7 x 10(24) erg, altitude of the explosion 5-10 km. The
explosion devastated a 2150+/-25 km(2) forest area and produced a
radiant burn of flora at more than 100 km(2). The problem of optical
events in the summer of 1908 connected to the Tunguska explosion is
discussed, as well as that of the origin of element and isotropic
anomalies in the area, the most striking feature being peculiarities in
the C, H and Pb isotopic composition. A critical analysis is given of
hypotheses proposed to explain TM problem. The hypotheses proceeding
from concepts of the TM low and hyperlow (<0.01 g/cm(3)) density are
shown to be inconsistent. It is stated that nowadays the basic aspect
of the TM nature discussion is an alternative ''stony asteroid-comet''.
Though the final choice between them has not yet been made, the chances
of the stony asteroid version have recently grown substantially. The
first priority is to solve a set of questions in order that further
development of this problem can be defined as follows: (1) the fate of
the meteorite fragments in case the TM was really a stony asteroid; (2)
the possibility of a carbonaceous chondrite and mellow comet nucleus
penetration into the troposphere; (3) the nature of the element and
isotopic cosmochemical anomaly in the area of the Tunguska explosion
epicentre; (4) the mechanism of geomagnetic effect and optical
anomalies developed in the summer of 1908; (5) the possibility of a
quick (within hours) transfer of dusty matter from the area of the
Tunguska event to Central Asia, European Russia and Western Europe by
stratospheric and mesospheric winds and (6) the cause of the mutation
process in the area of the Tunguska catastrophe. All these questions
might be solved much faster within the frames of international
scientific cooperation as recently outlined and of continued fieldwork
in the area of the Tunguska explosion. In this way the necessary
conditions will be created to build up a synthetic theory of the
Tunguska event, in order to explain not only the explosion of a space
object at the Podkamennaya Tunguska, but the entire complex of
anomalous cosmophysical events in the summer of 1908. (C) 1998 Elsevier
Science Ltd. All rights reserved.

===================
(7) SEARCH FOR THE LOCAL CENTRES OF THE TUNGUSKA EXPLOSION

V.D. Goldine: Search for the local centres of the Tunguska explosions.
PLANETARY AND SPACE SCIENCE, 1998, Vol.46, No.2-3, pp.151-154

TOMSK VV KUIBYSHEV STATE UNIVERSITY, NII PMM, LENIN STR 36, TOMSK
634010, RUSSIA

The structure of forest destruction caused by the Tunguska meteorite
explosion is studied by a special method. This method consist in
constructing a set of points of intersection of the directions of
fallen trees and an analysis of the distribution density of this set.
It is found that in addition to main epicentre of the forest
destruction region, previously determined, the method indicates another
critical point located about 4-6 km to the west of the main epicentre.
This feature can be interpreted as a consequence of the flight and
destruction of small piece of the Tunguska meteorite. (C) 1998 Elsevier
Science Ltd. All rights reserved.

=================
(8) CONSEQUNCES OF THE TUNGUSKA CATASTROPHE

V.D. Nesvetajlo: Consequences of the Tunguska catastrophe:
dendrochronoindication inferences. PLANETARY AND SPACE SCIENCE, 1998,
Vol.46, No.2-3, pp.155-161

TOMSK VV KUIBYSHEV STATE UNIVERSITY, BIOLOGY & BIOPHYSICS
INSTITUTE, TOMSK 634050,RUSSIA

Reviewed are the results of the dendrochronological analyses (DCA)
relative to the Tunguska catastrophe. The main segments of the DCA
research are presented. They reflect research on natural processes and
phenomena in the annual rings of the trees in the Tunguska catastrophe
area. The morphometric DCA was applied to identify the exact dates of
tree death in the area of the ''telegraph pole'' forest in the
epicentral zone of the Tunguska catastrophe. More than 91% of dead
standing trees died in 1908. The morphometric DCA was used for the
study of a particular type of burns in trees which perished as a result
of the 1908 Tunguska catastrophe, as well as in trees that survived it
and had accelerated growth after 1908. A conclusion is arrived at about
the lack of correlation between the impact of the Tunguska meteor
matter and this phenomenon is made. Outlined are the results of a thin
structure analysis of the 1908 tree rings, and briefly the results of a
biogeochemical DCA are presented, based on the atomic-abundance and
neutron-activation analyses of the wood in the annual rings of trees
that survived in the epicentral area of the Tunguska event. Also
considered are results of the isotopic DCA based on the analysis of the
radiocarbon content of the wood from 1898-1930 annual tree rings. In
the conclusion it is underlined that there exist a vital necessity to
create a data bank (and a storage place) of the tree samples from the
epicentral area that would enable future recovery of information about
the Tunguska event. (C) 1998 Elsevier Science Ltd. All rights reserved.

===========
(9) ISOTOPIC ANOMALY IN PEAT NITROGEN AS PROBABLE TRACE OF ACID RAINS

E.M. Kolesnikov, N.V. Kolesnikova, T. Boettger: Isotopic anomaly in
peat nitrogen is a probable trace of acid rains caused by 1908 Tunguska
bolide. PLANETARY AND SPACE SCIENCE, 1998, Vol.46, No.2-3, pp.163-167

MOSCOW MV LOMONOSOV STATE UNIVERSITY, FACULTY OF GEOLOGY, MOSCOW
119899, RUSSIA

ln peat sampled at the Tunguska Cosmic: Body (TCB) explosion area, the
sharp increase of the N concentration (about three-fold) and the
positive N topic anomaly (delta(15)N = +3.5 parts per thousand, see
eqn) have for the first time been revealed. In contrast with the C and
H effects observed earlier which were clearly limited to the epicentre
area (Kolesnikov et al., 1997 in press), the same N effect has also
been shown in peat sampled near the Vanavara settlement, 65 km south of
the explosion epicentre. A clear connection of the observed anomalies
in peat to the 1908 permafrost boundary, synchronism of the changes of
delta(15)N and the N concentration and also good agreement with data on
the K/T boundary deposits allow us to connect the observed effects to
acid rain fall-out after passage and an explosion of the TCB. (C) 1998
Elsevier Science Ltd. All rights reserved.

=================
(10) NEW ASPECTS OF THE TUNGUSKA METEORITE PROBLEM

V.A. Alekseev: New aspects of the Tunguska meteorite problem
PLANETARY AND SPACE SCIENCE, 1998, Vol.46, No.2-3, p.169

TROITSK INST INNOVAT & FUS RES, TROITSK 142092, MOSCOW OBLAST, RUSSIA

Numerous solid particles of size up to about 5O mu m were found in the
1908 growth ring or deeper in a tree that has survived the Tunguska
catastrophe near its epicentre. We consider these particles to be
remnants of the Tunguska Cosmic Body. Most of them have jagged edges.
An attempt to classify these particles is made. Based on the
information obtained, we suggest a hypothesis according to which the
final explosion in the Tunguska event could be caused by differences in
flowability inherent in solid material under high pressures. A
similarity between the features of the Tunguska catastrophe and of the
phenomena studied in the laboratory is discussed. (C) 1998 Elsevier
Science Ltd. All rights reserved.

==================
(11) IRIDIUM AND OTHER ELEMENT ANOMALIES NEAR THE TUNGUSKA EXPLOSION
     SITE

Q.L. Hou*), P.X. Ma, E.M. Kolesnikov: Discovery of iridium and other
element anomalies near the 1908 Tunguska explosion site
PLANETARY AND SPACE SCIENCE, 1998, Vol.46, No.2-3, pp.179-188

*) CHINESE ACADEMY OF SCIENCE, INST GEOL, LAB LITHOSPHERE TECTON
   EVOLUT,POB 9825,BEIJING 100029,PEOPLES R CHINA

Fifteen peat samples collected from a core including the layer affected
by the 1908 Tunguska explosion in Tunguska area of central Siberia,
Russia, were analyzed by NAA to determine the contents of Ir, REE,
Ni,Co, Fe and Sb. The analytical results indicate that Ir concentration
in and under the event layer were 0.54-0.24 ppb, about 10-20 times
higher than those reported in the work of Korina et al. (1987) and more
than 2 times higher than in layers above, and other elements were
enriched by factors of 2 to 5. fn addition, the variation of Ni
concentration is closely related with Ir in the explosive layer, and
the patterns of chondrite-normalized REE are almost hat, which are
different om the traps of Siberian plateform. Hence, it can be inferred
from the characteristics of elemental geo-chemistry that the explosion
likely was associated with an extraterrestrial object. Because it
exploded before reaching the ground, the density of the projectile must
have been quite low, and its chemical composition was probably similar
to carbonaceous chondrites (Cl). According to the Ir flux in the
explosion area, it can be estimated that the celestial body weighed
more than 3.5 x 10(4) tons, corresponding to >60 m in diameter, and
might have released energy equivalent to more than 10(6) t of TNT. If
the projective was a comet, the total mass was more than 7 x 10(6)
tons, and the diameter of core was more than 160 m. (C) 1998 Elsevier
Science Ltd. All rights reserved.

===================
(12) EVIDENCE FOR THE ASTEROIDAL ORIGIN OF THE TUNGUSKA OBJECT

Z. Sekanina: Evidence for asteroidal origin of the Tunguska object
PLANETARY AND SPACE SCIENCE, 1998, Vol.46, No.2-3, pp.191-204

CALTECH,JET PROP LAB,4800 OAK GROVE DR,PASADENA,CA,91109

The progress in the understanding of the Tunguska object is reviewed in
the light of evidence presented in numerous recent investigations,
which appeared following the publication of my 1983 paper on the
object's proposed asteroidal nature. The issues addressed extensively
in the present review involve: (i) the fundamental characteristics of
the event, such as the object's energy, altitude and velocity at the
time of: its terminal explosion and the dynamic pressure involved; (ii)
the problem of atmospheric fragmentation of very massive impactors and
the implications for their ablation and deceleration; (iii) new
analysis of the orientation of the Tunguska object's heliocentric orbit
based on the best data on the apparent radiant of the fireball's
atmospheric trajectory; and (iv) comparison with the findings of other
recent investigations, including compositional studies. Also employed
in the arguments are the results now available on the impacts of comet
Shoemaker-Levy 9's fragments into Jupiter and the findings of a recent
comparative study of two huge fireballs (one cometary, one stony, both
involving impactors several meters across) observed-with the cameras of
the European Network of fireball monitoring. It is concluded that
hypotheses based on presumed cometary origin of the Tunguska object
encounter unsurmountable difficulties and that the interpretation of
the event as a fall of a small stony or carbonaceous asteroid is not
only plausible, but virtually certain. (C) 1998 Elsevier Science
Ltd. All rights reserved.

=============
(13) ON THE POSSIBLE RELATION BETWEEN THE TUNGUSKA BOLIDE AND COMET
     ENCKE

D.J. Asher & D.I. Steel: On the possible relation between the Tunguska
bolide and comet Encke. PLANETARY AND SPACE SCIENCE, 1998, Vol.46,
No.2-3, pp.205-211

COMM RESEARCH LABS, 893-1 HIRAI, IBARAKI, OSAKA 314, JAPAN

Almost two decades ago L. Kresak (Bull. Astron. Inst. Czechoslov. 29,
129-134, 1978) suggested that the Tunguska bolide might be a fragment
of comet Encke, a hypothesis that Z. Sekanina criticized in a
publication a few years later (Astron. J. 88, 1382-1414, 1983). One
aspect of the proposed genetic relationship is investigated, namely the
required differential orbital precession of the two objects so as to
make an impact upon our planet possible for the Tunguska projectile,
even though the comet's orbit in the current epoch is far from the
condition of Earth intersection. This work was foreshadowed in a
previous paper in which it was shown how theoretical meteor radiants
may be calculated for objects with orbits similar to 2P/Encke (Earth,
Moon and Planets 68, 155-164, 1995). By applying appropriate secular
perturbation theory and numerical integration techniques, it is shown
that the necessary dispersion can be attained within 10 kyr if the
semi,major axes of the orbits differ by similar to 0.05 AU, an amount
easily achieved even under the presently observed non-gravitational
forces of 2P/Encke. (C) 1998 Published by Elsevier Science Ltd.
All rights reserved.

==================
(14) THE COMETARY NATURE OF THE TUNGUSKA METEORITE

S.S. Grigorian: The cometary nature of the Tunguska meteorite: on the
predictive possibilities of mathematical models
PLANETARY AND SPACE SCIENCE, 1998, Vol.46, No.2-3, pp.213-217

MOSCOW MV LOMONOSOV STATE UNIVERSITY, INST MECH, MICHURINSKY PROSPEKT
1, MOSCOW 119899, RUSSIA

Critical discussion of modern state of mathematical modeling of the
process of celestial bodies penetration in planetary atmospheres is
presented, as well as a new development in theoretical description of
disintegration of these bodies in the course of their movement in
atmosphere. (C) 1998 Elsevier Science Ltd. All rights reserved.

==============
(15) DAMAGE FROM THE IMPACTS OF SMALL ASTEROIDS

J.G. Hills*) & M.P. Goda: Damage from the impacts of small asteroids
PLANETARY AND SPACE SCIENCE, 1998, Vol.46, No.2-3, pp.219-229

*) LOS ALAMOS NATL LAB,DIV THEORET,THEORET ASTROPHYS GRP,T-6,MS
   B288,LOS ALAMOS,NM,87545

Previous work is extended by using a model spherical atmosphere with a
fitted density,profile to find the damage done by an asteroid entering
it at various zenith angles. At zenith angle 0 degrees and a typical
impact velocity at the top of the atmosphere of V= 17.5 km s(-1), the
atmosphere absorbs more than half the kinetic energy of stony
meteoroids with diameters, D-M < 230 m and iron meteoroids with D-M <
50 m. At zenith angle 45 degrees the corresponding figures are 360 and
70m while at 60 degrees they are 500 and 100m. Far comets with V = 50
km s(-1) the Values are D-M<1900 and 3000 m for 45 and 60 degrees,
respectively, using typical values of ablation, but they are much
smaller if ablation is reduced. Only impactors with D-M above these
critical values are effective in producing ground impact damage.
craters, earthquakes, and tsunami. Smaller impactors can still produce
atmospheric blast waves. It is found that the area of destruction
around the impact point in which the overpressure in the blast wave
exceeds 4 p.s.i. = 2.8 x 10(5) dyn cm(-2), which is enough to knock
over trees and destroy buildings. It is found that for chondritic
asteroids entering at zenith angle 45 degrees and an impact velocity at
the top of the atmosphere of 17.5 km s(-1) that it increases rapidly
from zero for those less than 50m in diameter (13.5 megatons) to about
2000 km(2) for those 76m in diameter (31 megatons). If we assume that a
stony asteroid 100m in diameter hits land about every 1000 years, we
find that a 50 m diameter one (causing some blast damage) hits land
every 125 years while a Tunguska size impactor occurs about every 400
years. If iron asteroids are about 3.5 per cent of the frequency of
stony ones of the same size, they constitute most of the impactors that
produce areas of blast damage of less than 300 km(2). While the optical
flux from a small asteroid such as Tunguska is enough to ignite pine
forests, the blast from it goes beyond the radius within which the fire
starts. The blast tends to blow out the fire, so it is likely that the
impact will char the forest las at Tunguska), but it will not produce a
sustained fire. (C) 1998 Published by Elsevier Science Ltd. All rights
reserved.

=================
(16) COMPLEX MODELLING OF THE TUNGUSKA CATASTROPHE

V.P. Korobeinikov, L.V. Shurshalov, V.I. Vlasov, I.V. Semenov: Complex
modelling of the Tunguska catastrophe
PLANETARY AND SPACE SCIENCE, 1998, Vol.46, No.2-3, pp.231-244

RAS, INSTITUTE COMP AIDED DESIGN, 2ND BRESTSKAYA 19-18,MOSCOW
123056, RUSSIA

The main results are related to the creation of a complex model for the
flight, stress-strain state and fracture of a meteoroid and the
determination of the action of the shock waves system and radiation on
the Earth's surface during the Tunguska cosmic (celestial) body event.
The body's motion is accompanied by its strong deformations, ablation,
radiation and fracture. The whole complex model of the flight and blast
of TCB now includes several units. Numerical modelling of the body's
flight, a stress- strain state inside the body, the time and place of
the body fracture and the parameters of the crushed volume, an
explosion-like expansion during the final stage of the flight as well
as calculation of the action of the shock waves system and the
radiation on the Earth surface are mainly considered. A comparison of
the results with data observed is also presented and conjectures
concerning a plausible nature of TCB are made. (C) 1998 Elsevier
Science Ltd. All rights reserved.

=======================
(17) COMUTER MODEL OF THE ATMOSPHERIC ENTRY OF THE TUNGUSKA OBJECT

J.E. Lyne, M.E. Tauber, R.M. Fought: A computer model of the
atmospheric entry of the Tunguska object. PLANETARY AND SPACE SCIENCE,
1998, Vol.46, No.2-3, pp.245-252

*) UNIVERSITY OF TENNESSEE, DEPARTMENT OF MECHANICAL ENGINEERING, 414
   DOUGHERTY BLDG, KNOXVILLE, TN, 37996

Mathematical models of the entry trajectory for various types of
meteors have frequently been applied in an effort to determine the
nature of the Tunguska object. This approach has been used to support
both a stony asteroid and a cometary object as the most probable cause
of the event. An accurate trajectory model must include an evaluation
of both the mechanical fragmentation and the aerothermal ablation and
must couple these two processes. Inaccuracies in the calculated
ablation rate can lead to substantial errors in the predicted terminal
altitude of a given entry body; this is particularly true for
relatively weak, icy objects such as comets. The present study uses an
analytical approximation of the mechanical fragmentation and radial
spreading of the bolide and examines aerothermal ablation in some
detail, including an evaluation of radiative cooling of the shock layer
gases and the effect of radiation blockage by ablation products coming
off the meteor's surface. Such calculations can be performed only in an
approximate manner since the properties of high temperature gases are
not Well established at the extreme pressures and temperatures
involved. It is found that the sudden release of energy approximately 8
km above the surface which was associated with the Tunguska event could
have been produced by the disruption of either a comet or an asteroid,
although a cometary origin would have required a very steep atmospheric
entry angle. Therefore, although an asteroidal origin seems more
likely, it is concluded that a trajectory analysis of this type cannot
be: used at the present time to exclude either type of object with
absolute certainty. (C) 1998 Elsevier Science Ltd. All rights reserved.

=================
(18) GASDYNAMICAL MODEL OF THE TUNGUSKA FALL

V.P. Stulov: Gasdynamical model of the Tunguska fall
PLANETARY AND SPACE SCIENCE, 1998, Vol.46, No.2-3, pp.253-260

MOSCOW MV LOMONOSOV STATE UNIVERSITY, INST MECH, MICHURINSKY AVE
1, MOSCOW 119899, RUSSIA

The following are reliably established consequences of the Tunguska
fall: a forest fall with an area of about 2000 km(2) and the absence of
a meteoric crater. Thus far, no substances have been found which are
believed to belong to the Tunguska object. It is shown that the
question about the source of the Tunguska explosion can be eliminated
by modelling the events as a rapid evaporation of the meteor with
subsequent movement, deformation and braking of a gas volume which
consists of the products of evaporation in a mixture with air. Using
parameters of the Tunguska space body from the work of Korobeinikov et
al. (Astron. Bull. 25, 327-343, 1991) an ablation parameter of 25.5 is
determined. Using the asymptotic form of the solution for trajectories
at large ablation parameters, it lis found that the evaporation of a
snow-ice sphere occurs at an altitude between 20 and 37 km. The gas
products; continue moving, being mixed with air and rapidly
decelerating. The last phase of the Tunguska phenomenon-the fall on
Earth of a shock wave and E-the gas driven after it-explains the forest
fall and burning of trees and the absence of an impact crater. (C) 1998
Elsevier Science Ltd. All rights reserved.

===================
(19) COULD THE TUNGUSKA DEBRIS SURVIVE THE TERMINAL FLARE?

V.V. Svetsov: Could the Tunguska debris survive the terminal flare?
PLANETARY AND SPACE SCIENCE, 1998, Vol.46, No.2-3, pp.261-268

RUSSIAN ACADEMY OF SCIENCE, INST DYNAM GEOSPHERES, 38 LENINSKY
PROSP, BLD 6, MOSCOW 117979, RUSSIA

The purpose of this paper is to show that lack of residual meteorites
is typical for a fall of a stony or carbonaceous bolide tens of meters
in size. A Tunguska-sized body penetrates deep into the atmosphere and
is broken into a great deal of fragments the maximum size of which is
smaller than 10 cm. Numerical cal simulations of analogous problems
show that the fragments are separated from each other at a stage of
dramatic deceleration of the bolide. Computations made here give that
3-10cm stony fragments fully ablate either inside or outside the
fireball due to high radiation flux. Only if the fragments accidentally
gain significant lateral velocities at altitudes above 15 km, could
their ponderable remnants reach the ground at 5-10km from the explosion
epicenter. Vaporized material of the impactor does not touch the ground
and moves upward along the wake. (C) 1998 Elsevier Science Ltd. All
rights reserved.

=================
(20) EARTH CRATERING RECORD AND IMPACT ENERGY FLUX

A. Montanari*), A.C. Bagatin, P. Farinella: Earth cratering record and
impact energy flux in the last 150 Ma. PLANETARY AND SPACE SCIENCE,
1998, Vol.46, No.2-3, pp.271-281

*) OSSERVATORIO GEOL COLDIGIOCO,I-62020 FRONTALE DI APIRO,ITALY

Although the Earth's cratering record is subject to strong bias (i.e.
unknown craters yet to be discovered or obliterated by geological
processes, geo-chronologic uncertainties of impact events) a
compilation of the 33 best dated large impact craters on Earth with
diameters larger than 5 km, and younger than 150 Ma, their diameters,
geochronologic ages, and the corresponding uncertainties can be used to
construct a diagram summarizing our current knowledge on the influx of
impact energy onto the Earth versus time. From the crater diameters, we
estimated the corresponding impact energies through suitable scaling
laws. Then, we associated with each crater a gaussian (bell) function
of time centered at its age, with a half-width consistent with the age
uncertainty and a total area proportional to the impact energy.
Finally, all the bell functions corresponding to different craters were
summed up and the resulting curve (smoothed out by computing running
averages over 4 Ma) was plotted on a semilogarithmic scale. From this
curve, it is apparent that the recently discovered 144.7 Ma old
Morokweng crater in South Africa, which is associated with the
Jurassic/Cretaceous boundary and related mass extinction, corresponds
to the highest energy influx peak, almost an order of magnitude larger
than the Chicxulub crater in Yucatan which, on the other hand, is
associated with the Cretaceous/Tertiary; boundary mass extinction, the
most severe in the marine record of the last 150 Ma. The third largest
impact energy flux peak corresponds to the Late Eocene Popigai
(Siberia) and Chesapeake Bay (U.S.A.) giant impact craters, which are
also associated with a global biotic crisis. From the overall record
examined herein it emerges that there is probably a threshold size of
approximately 3 km for the smallest projectile capable of triggering
large-scale extinctions. There is no convincing evidence for
periodicities in the distribution of crater ages. A few groups of
several craters appear to be more closely spaced in time than in a
purely random distribution. (C) 1998 Published by Elsevier Science Ltd.
All rights reserved.

=======================
(21) GROUND-BASED OPTICAL SURVEYS FOR NEAS

A.W. Harris: Evaluation of ground-based optical surveys for near-Earth
asteroids. PLANETARY AND SPACE SCIENCE, 1998, Vol.46, No.2-3,
pp.283-290

CALTECH, JET PROP LAB, MS 183-501, PASADENA, CA, 91109

Near-Earth asteroids (NEAs) pose a long-term hazard to the Earth and
its inhabitants as a result of impacts, such as the Tunguska event of
1908, and especially from still larger, although even less frequent
events. An evaluation is presented of the ability of ground-based
telescope systems to inventory the population of NEAs, as a function of
telescope parameters, NEA size and albedo, and duration of the survey.
The optimum strategy for surveying appears to be to cover the entire
visible sky every month, rather than covering a smaller area of sky to
a fainter threshold of detection. An optimally designed and operated
system consisting of two or three 1 m telescopes should be able to
achieve similar to 75% completeness of survey down to a size of 1 km
diameter, in 10 years of operation. To achieve a completeness >90% in
the same time would require several telescopes of 2-3m aperture.
Achieving near-completeness down to the size of the smallest NEAs
capable of causing significant ground damage (similar to 70m diameter)
on a timescale of 10 years appears impractical with present technology,
however because of the long timescale of events, even present or
soon-to-start surveys are likely to discover the next Earth-impacting
body before it hits. (C) 1998 Elsevier Science Ltd. All rights
reserved.

======================
(22) DISCOVERY AND FOLLOW-UP SIMULATIONS FOR EARTH CROSSING ASTEROIDS

K. Muinonen: Discovery and follow-up simulations for small
Earth-crossing asteroids. PLANETARY AND SPACE SCIENCE, 1998, Vol.46,
No.2-3, pp.291-297

UNIVERSITY OF HELSINKI OBSERVATORY, POB 14, FIN-00014 HELSINKI, FINLAND

Discovery and follow-up strategies are simulated for a model population
of Earth-crossing aster;; olds (ECAs), in line with the 1908 Tunguska
explosion being used by a stony asteroidal body. ECAs with minimum
diameters varying from 10m to 1 km are concentrated on, and attention
is paid to the discovery completeness as a function of the semimajor
axis, eccentricity, and inclination to understand the discovery biases
in the proposed Spaceguard Survey for near-Earth objects. It is noted
that the so-called standard survey strategy does not favor the
discovery of large ECAs with small orbital intersection distances om
the Earth's orbit. Orbital covariances and quality metrics are
introduced to simulate the follow-up prospects for ECAs discovered at
different locations on the sky plane. The quality metrics confirm an
intuitively clear east-west asymmetry: ECAs discovered in the east are
easier to follow up than those discovered in the west. For ECAs larger
than 1 km, opposition searches field the best orbits, whereas for small
ECAs, the optimum search window shifts to the east. (C) 1998 Elsevier
Science Ltd. All rights reserved.

==============
(23) THE NEO CONFIRMATION PAGE

B.G. Marsden & G.V. Williams: The NEO confirmation page. PLANETARY AND
SPACE SCIENCE, 1998, Vol.46, No.2-3, pp.299-302

HARVARD SMITHSONIAN CTR ASTROPHYS, 60 GARDEN ST, CAMBRIDGE, MA, 02138

The use of a facility in the World Wide Web for the confirmation of
candidate near-earth objects (NEOs) is described. This facility allows
users to derive topocentric ephemerides of objects recorded on only a
single night. The facility is operated by the Minor Planet Center,
which subsequently distributes more detailed information by e-mail to
its subscribers if and when an object is confirmed as an NEO. (C) 1998
Published by Elsevier Science Ltd. All rights reserved.

=================
(24) THE FLUX OF TUNGUSKA-SIZED FRAGMENTS FROM THE MAIN ASTEROID BELT

P. Farinella*) & M. Menichella: The flux of Tunguska-sized fragments
from the main asteroid belt. PLANETARY AND SPACE SCIENCE, 1998, Vol.46,
No.2-3, pp.303-309

*) UNIVERSITY OF PISA, DIPARTIMENTO MATEMAT, GRP MECCAN SPAZIALE, VIA
   BUONARROTI 2,I-56127 PISA,ITALY

The numerical model described in Menichella et al. (Earth, Moon and
Planets 72, 133-149, 1996) is used to investigate the flux of
Tunguska-sized asteroid fragments into chaotic resonant orbits leading
them to attain an Earth-crossing status. The assumed main-belt size
distribution is derived from that of known asteroids, extrapolated down
to sizes approximate to 1 m and modified in such a way as to yield a
quasi-stationary fragment production rate over times approximate to 100
Myr. Collisional physics consistent with the results of laboratory
hypervelocity impact experiments and evidence from asteroid families
are used, and the sensitivity of the results to-the most critical
poorly known parameters is analysed. The results of simulations show
that the main asteroid belt on average can inject into the resonant
escape hatches about one Tunguska-sized fragment per year, with an
uncertainty of about a factor of -/+3. Owing to their limited dynamical
and collisional lifetimes (as inferred from the better-known behaviour
of km-sized:near-Earth asteroids), only a fraction approximate to 1% of
the Tunguska-sized near-Earth fragments are likely to hit the;;Earth,
yielding an average flux of the order of one impact per century,
consistent with observations (within the existing uncertainties).
Large-scale stochastic collisions in the main belt can enhance this
fragment flux by a factor of up to 6 over intervals approximate to 1
Myr, assuming that this corresponds to the typical dynamical timescale
in the resonances. Such enhanced-flux episodes are expected to occur
every several tens of Myr. (C) 1998 Elsevier Science Ltd. All rights
reserved.

==================
(25) PRODUCTION OF TUNGUSKA-SIZED BODIES BY EARTH'S TIDAL FORCES

W.F. Bottke*), D.C. Richardson, S.G. Love: Production of Tunguska-sized
bodies by Earth's tidal forces. PLANETARY AND SPACE SCIENCE, 1998,
Vol.46, No.2-3, p.311

*) CALTECH, DIVISION OF GEOLOGY & PLANETARY SCIENCE, MAIL CODE 150-
21, PASADENA,CA,91125

Tidal disruption of rubble-pile bodies (stony or icy aggregates held
together by self-gravity) during close Earth encounters may produce
significant numbers of Tunguska-sized (50 m) fragments. Using an N-body
simulation to model encounters between strengthless, elongated,
rotating, particulate bodies and the Earth, two disruption categories
were found which produce small bodies: (a) ''Shoemaker-Levy-9 type''
catastrophic disruptions, where the progenitor is pulled into a line of
similarly sized bodies, and (b) rotational disruptions, where the
progenitor is distorted and spun-up by tidal torque such that particles
are ejected along the equator. These events occur frequently at low
encounter velocities (i.e. low e and i);it is predicted that Earth's
tidal forces should be effective at disrupting larger bodies into
Tunguska-sized fragments in this region of phase space. By creating a
map of tidal disruption outcomes for the progenitor's encounter
parameters and integrating over all possible values of those
parameters, it is found that the tidal production rate of
Tunguska-sized bodies (upper limit) was; comparable with the main-belt
injection rate of Tunguska-sized bodies into resonant orbits. It is
concluded that tidal disruption plays an important role in maintaining
the steady-state fraction of small Earth-crossing asteroids. (C) 1998
Elsevier Science Ltd. All rights reserved.

=================
(26) THE MACHA IMPACT CRATERS

E.P. Gurov & E.P. Gurova: The group of Macha craters in western Yakutia
PLANETARY AND SPACE SCIENCE, 1998, Vol.46, No.2-3, pp.323-328

NATIONAL ACADEMY OF SCIENCE OF UKRAINE, INST GEOL SCI, CHKALOV STR
55-B, UA-252054 KIEV, UKRAINE

The group of Macha impact craters in western is represented by five
crateriform structures from 60 to 300 m in diameter. The craters were
formed in sandy strata of the Quaternary period and in underlying
sedimentary rocks of Late Proterozoic ages. Shock metamorphic effects
including planar features in quartz were established in the rocks from
the craters. The age of the craters is 7315 +/- 80 yr. The nature of
the projectiles is not totally clear, although they might be iron
meteoritic. (C) 1998 Elsevier Science Ltd. All rights reserved.

=================
(27) SOME METALLIC SPHERULES

A. Colombetti, G. Ferrari, F. Nicolodi, F. Panini: Some metallic
spherules in calcareous-marly sediments of the Romanoro Flysch,
Sestola-Vidiciatico tectonic unit (Modena district, Northern Apennines,
Italy). PLANETARY AND SPACE SCIENCE, 1998, Vol.46, No.2-3, pp.329-340

UNIVERSITY OF MILAN, DIPARTIMENTO SCI TERRA, VIA MANGIAGALLI 34,
I-20133 MILAN,ITALY

Some metallic microspherules were found in a calcareous-marly sample
from the Romanoro Flysch (Modena district, Northern Apennines, Italy).
The outcrop is a tectonic scale, part of the Sestola-Vidiciatico Unit,
without stratigraphic contacts with other formations. The age of the
setting stratum was attributed at the middle Miocene, but this datum
contradicts the previous authors, who dated the outcrop at the Eocene
(Zanotti, 1988), or Cretaceous (Reutter, 1969; Rentz, 1971; Daniele et
al. 1996). Morphological, mineralogical and chemical studies were
carried out on the microspherules and on some grains. The crust of
microspherules is essentially made of iron (from 66% to 48%, like FeO),
with other minor elements (Al, Ti, Mn, Ni). The core is made of an
Fe-Ni alloy. There are other grains of : quartz, calcite, pyrite, mica
and black minerals, with edges. The black ones are polymetallic
aggregates of : Fe, Mn, Ti, Al, Cu. About the origin of these
microspherules four hypotheses : cosmic origin (exceptional
extraterrestrial event), volcanic origin, diagenetic origin, wartime
origin (Extrema Ratio). The first  one has been confirmed. (C) 1998
Elsevier Science Ltd. All rights reserved.

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