Date sent:        Tue, 03 Feb 1998 10:15:04 -0500 (EST)
From:             Benny J Peiser
Subject:          CC COMMENTS 03/02/98
Priority:         NORMAL


    Benny J Peiser

    Duncan Steel

    Clark Chapman

    Bob Kobres

    David Morrison

    Alan W. Harris"

    Victor D. Noto

    Jega A. Arulpragasam

    Timo Assmuth

Today's Cambridge Conference only includes personal statements and
comments by some of the world's leading NEO researchers. This is
the result of the Kobres/Chapman controversy about impact
probabilites and matters of planetary defense (which I have
attached again because some list members did not receive last
Friday's CC Digest).

As I have promised yesterday, I will try to minimise these (and
future) debates to essential and relevant arguments - otherwise we
would all get bored very quickly. This controversy, however,
does matter. It has major policy implications and people involved
or interested in NEO research should be aware of the different
views, research methods and conclusions within the scientific

In my introductory comment below, I have attempted to point out
some of the paradigmatic differences between the 'American School'
of NEO research on the one hand and the 'British School' on the
other. Nevertheless, I should emphasise that despite all these
differences, there is an overwhelming agreement among all
participants of this debate that we need to significantly enhance
NEO research in all related areas and start working on a global
system of planetary defense -   regardless of how individual
scholars may view human history or the impact hazard.


Benny J Peiser

"I am not arguing against the government of the Universe by law, but
merely protesting against measuring the possibilities and limits
of the universal law by our experience of it" (Sir Henry H
Howorth, 1887).

1.) During the last couple of decades we have witnessed the disintegration
and subsequent downfall of geological uniformitarianism. Charles Lyell’s
dictate that nature works uniformly and without significant punctuations has
commanded the scientific and astronomical agenda for more than 150 years.
Despite the recent demise of geological uniformitarianism, many NEO
researchers (the so-called American School) hold on to the belief that 400
years of telescopic astronomy have established enough evidence to conclude
that the currently observed flux of asteroids, comets and meteor streams has
always been the same. These scholars, consequently, base their impact risk
assessment (and their entire research agenda) on current observations of
asteroids and comets only and logically ignore any research into the more
distant, the ancient and prehistoric past of humankind.

In contrast to what Duncan Steel (1996) has called NEO-Uniformitarianism,
the ‘British School of Coherent Catastrophism’ (BSCC) questions the very
foundations of this assumption. Instead of blindly following a dubious
theory based mainly on contemporary or recent observations, the BSCC is
primarily interested in and researching about astronomical, archaeological,
climatological and historical evidence for major punctuations (and NEO
fluctuations) during the entire span of human evolution. It is thus not
surprising to notice that in recent years some of the best and most
interesting research on human and societal evolution has been conducted and
published by British astrophysicists and others linked to the BSCC.. At the
same time, the investigations of the BSCC are (in contrast to the American
School) not restricted to giant asteroids and comets but include the search
for periods of increased meteoric activity. Instead of limiting the
definition of what constitutes a "global catastrophe" to "Kill Curves" and
the odd giant impact (> 1 km), the research agenda of the BSCC includes
episodes of catastrophic meteor storms and atmospheric dust loading. Such
multiple impacts or Super-Tunguskas do not necessarily kill a large number
of people by means of impact but they can easily result in severe climatic
downturns, reduction of precipitation, mini ice ages, etc. and can thus lead
to the collapse of argicultural societies.

2.) One of David Morrison’s arguments (see further below) came
rather unexpected. David, who has been one of the most outspoken
critics of the British School, claims that "simple historical
arguments let you calculate the present probability of impact based
on past history". Yet historical and prehistorical reconstruction
of say the last 10,000 years are far from simple. Of course, if we
were to apply the same uniformitarian principle to historical and
prehistorical research, basing it on our experience during the
last 400 years, we would naturally conclude that (as Jega
Arulpragasam hence claims in his comment further below) no impacts
and natural catastrophes of global extent happened during the last
10,000 years. Yet this claim is based on uniformitarian
assumptions and not on hard facts.

3.) Among NEO researchers, only the advocates of the BSCC
have been actively engaged in historical and palaeoecological
research. The results of this research programme show that the
last 10,000 years (i.e. the Holocene period) has been repeatedly
punctuated by major natural catastrophes. Evidence from Greenland
ice cores, tree rings, lake levels and geological sediments reveal
beyond any doubt that during this period of time, human cultures
and civilisations were hit by at least 6 global climate disasters
(c. 8000 BP, 6500 BP, 5300 BP, 4300 BP, 3200 BP, 1400 BP) of which
the three last downturns coincided with, and most likely triggered
civilisation collapses. Admittedly, it is still undetermined what
might have caused these natural catastrophes. Yet they are clearly
detectable in the geological, climatological and archaeological
records. There is, moreover, growing circumstancial evidence that
some of these marked periods of natural catastrophe also coincided
with increased meteoric activity and impact events. List members
interested in reviewing the evidence for global Holocene
catastrophes are referred to the proceedings of the 2nd SIS
Cambridge Conference which will be published as a volume of the
British Archaeological Reports (BAR) this summer.

4.) There are, then, among NEO researchers significant differences in the
assessment of both mankind’s catastrophic past and our uncertain future. Yet
despite all controversy, there is some good news too. The fact remains that
regardless of how many impact disasters might have occurred during the
period of human evolution, Homo Sapiens has survived all of these
punctuations – notwithstanding major evolutionary interruptions, regressions
and relapses. In view of this simple truth, I believe that we as a species
are capable to survive another minor or even medium size cosmic disaster
should it happen before a planetary civilisation has evolved and in a
position to avert such calamities.


Duncan Steel

Dear Benny,

I agree in broad scope with your comments. However, in your list of
uncertain parameters, I think that you left out the most significant
uncertainty: just what IS a 'civilization-ending impact.'

Most considerations on this have been done by physical scientists
using physical considerations. I believe that the normally-quoted
value is an object 1-2 km in size, which would cause a 'global
catastrophe,' the limiting effect being the production of oxides of
nitrogen in the blast and hence poisoning of the atmosphere (i.e.,
there are other deleterious effects, but this seems to be the major
one, the one which sets the limit on the impactor size); at least,
that was the impression I got from a talk by Brian Toon during the
meetings of the Spaceguard Workshop in 1991. Later publications I've
seen confirm this, I think, but I could be wrong.

My point, however, is that these are calculations based purely on
physical effects. When we're talking about 'civilization' I'm not
sure that many physicists have much useful to say (including me,
perhaps). But I think that the fallacy of the argument can be seen
if one just takes the 'global catastrophe' limit at 1-2 km and
assumes that that would cause the end of civilization, and have no
real concern for anything else; in that case one is assuming that a
0.5 km impactor would NOT cause a downfall of civilization (and of
course these arrive about ten times as frequently).

My own view (which as I've stated above may be worthless!) is that:
It all depends. I'm sure that the next Tunguska, no matter where it
occurs, will cause outrage amongst people (in the same way that the
Gulf War caused more outrage than World War I, because we saw it
live on CNN) and likely a significant effect on economic (and
military) affairs. In my view, though, the present western
civilization with its intricate economic links would be unlikely to
survive unscathed the aftermath of a 100-200 metre object arriving
either: (a) Over Central/Western Europe; (b) Over either the NE or
the West Coast of the USA; or (c) Over the Atlantic or Pacific but
close enough to populous coasts to cause megadeaths. I just think
that would unhinge people. On the other hand, such an event over
Australia, even if it took out a city of a million people, would be
shrugged off by the seats of civilization and economic power.

John Brockman runs a WWW site called The Edge ( ).
In it he asked members of the group what the questions were which we
are asking ourselves; some of the answers were carried in a feature
in the New York Times towards the end of December (Tuesday the 30th
I think). I thought that Jared Diamond's question was a good one:

"What do collapses of past societies teach us about our own future?"

If we knew how and why past civilizations had collapsed (and they
seem damn fragile) then we could take a step forward in answering
the question of what a 'civilization-ending impact' might be. But
until then, we have little idea. The way things are going (no
sensible funding of NEO work), it looks like the experiment may get
done so that we derive an empirical answer.

Best regards,



Clark Chapman

I appreciate your commentary on the recent interchange you have
published between Kobres and myself. I agree with you that there are
uncertainties in all the areas you mention. I would point out that
all of us engaged in estimating the "probabilities" have expressed
quite large error bars (typically plus-or-minus an order of
magnitude). And I know, from the history of science, that it is not
infrequent for responsible error bars to be shown to have been
wrong. Nevertheless, I assert that it is established fact that
certain human activities (I think of things like smoking cigarettes,
driving automobiles without seat-belts or while drunk, not to
mention nuclear proliferation) are far more certain causes of many
deaths in our lifetimes than are cosmic impacts. I also think it is
established that cosmic impacts have *not* caused many (or
practically any) deaths in our lifetimes, and that the case is very
questionable that they have done so in the last millennium. The
probability is "low" (in the colloquial sense) whether it is 1 in a
few-thousand per century (which I think is the best estimate) or 100
times different in either direction that a civilization-threatening
impact will happen. I still worry (because the consequences are so
extreme) but our best knowledge tells us that human lives will more
likely be saved (indeed civilization will more likely be saved) if
we pay attention to other arenas, like epidemics and potential
nuclear conflicts.

Clark Chapman


Bob Kobres

I find it interesting that Clark Chapman views as he does my
position on our contemporary situation.

First let me assure all of you who might be concerned:  I do not
lose sleep worrying that an impact event will occur in the near
future. My position all along (and this is documented)is that we
presently have the capacity to develop a defense system for the
biosphere and that it would show considerably more foresight if we
chose to do that rather than devoting such a large percentage of
our global resources to the development of ever more sophisticated
weapon systems. My original proposal-Nuclear Reaimament-is on my Web
site at:

With regard to my 'green slime' euphemism--I have never sought or
received any funding for what has become an unexpected but
interesting avocation for me. My suggestion, to state the facts in a
more to the point fashion, was for the benefit of individuals who
are having trouble staying in business. As I tried to convey, from
my experience many people do not understand that probability is
a static expression of likelihood during a defined period of time.

It is important, I think, to ensure that people really do understand
that by electing not to develop a defense as soon as possible we
are, in fact, choosing to gamble when we do not need to. There is
nothing dishonest or deceptive in stating that we do not yet know
when the next impact event might happen, nor is it misleading
to profess ignorance with regard to predicting all the possible
consequences of such an occurrence. I do not think that maintaining
credibility requires using flat terminology and language that could
be confusing to some individuals.

As for the notion that it might be better to spend resources to
mitigate more frequently occurring disasters than to develop an
Earth defense system soon--I think it depends ultimately on just
what it is we plan to do. The idea I've hawked all along is that we
could allow the threat of cosmic collision to serve as a focal point
for a global commitment to protect our living environment now
and into the future basically by learning as much as we possibly can
about what can influence our world, and how, from the superior
vantage point of outer-space. Conceptually at least, it seems
possible that we as a species might adopt the role of an
autoimmune-system with respect to the biosphere. In other words, a
desirable biosphere protection system would not be solely for the
purpose of deflecting objects that could impact Earth but would also
seek to reduce the impact of other factors that could be injurious
to Life--rapidly mitigating human ignorance with regard to the
actual global influence of applying a new technology for example.

In my mind, I've done what I could to encourage research on and draw
attention to what has obviously been an under-appreciated natural
phenomenon. My own investigation into the likelihood of recent
impact events has convinced me that there is still much to learn and
that we have the tools to find out what the influence of this
natural phenomenon has been. This, to me, is all largely a social
issue--we live in a time period of profound and rapid change in which
former beliefs and customs are being modified by a torrent of novel
information and possibilities. Choices we make over the next few
decades are apt to apply considerable bias to the course of cultural
evolution among all peoples. The goal, I suggest, needs to be
prudent social behavior.

Is it actually wise to retard development of a globally agreed upon
defense implementation and rely on luck as we gather data which,
though needed, can only refine our knowledge? It seems to me that we
should be particularly cautious when we already know that impacts
have produced serious problems for Life in the past and that,
regardless of the currently calculated probability, such an event
could occur as you read this. The cost of becoming prepared rapidly
(within a decade or so--I've never  advocated a reckless program) to
protect ourselves is an accelerated space development program. Is
that really an expenditure or might it better be seen as an
investment for ourselves and other living creatures on Earth?

Subjectively optimistic...;^)
Bob Kobres


From: David Morrison


While I agree with the final paragraph in your essay on disaster
probabilities (see below) that there are substantial uncertainties
in the probability of an impact catastrophe happening in any given
year, I believe you have made several errors in your discussion.

1. If one assumes your first point, that the asteroidal and cometary
flux is more or less constant over time, then ONLY ONE OF THE FIVE
historical arguments let you calculate the present probability of
impact based on past history. This requires no knowledge whatever of
orbital dynamics of comets, the nature of meteor streams, the
numbers or dynamics of giant asteroids (whatever they are), or the
absolute dates of the last 4 or 5 cosmic catastrophes of global
extent (something that most of us doubt is known for even one). It
is only if you discard the uniformitarian hypothesis that you need
to consider these other factors.

2.  The one critical factor that produces most of the uncertainly
about the current impact hazard is what you call the "effects" of
impacts. Determining the KILL CURVE as a function of impact energy
or other variables is very difficult. Thus (for example) you and
some others have asserted that many Tunguska-type impacts occurring
within a few decades might have some sort of synergistic effect that
multiplies their danger, but I know of no support for this assertion
in calculation or history.

3. You also confuse the PROBABILITY of impact with a PREDICTION of
impact. Even if we knew the probability perfectly, we would not be
able to predict the next global catastrophe. Prediction is an
entirely different matter, and it depends on searching for and
identifying threatening objects. Historical research and probability
calculations are irrelevant to prediction.

For technical discussions of these matters I refer you and your
readers to two papers:  Chapman and Morrison (1994) Impacts on the
Earth by Asteroids and Comets: Assessing the Hazard, Nature 367,
33-40; and Morrison, Chapman, and Slovic (1994) The Impact Hazard,
in Hazards Due to Comets and Asteroids, T. Gehrels editor, U.
Arizona Press.

David Morrison


From: Alan W. Harris"

Dear Benny,

I must respond to your "brief comment on the Kobres/Chapman
controversy," which I found interesting if not brief.  Most of your
points have relevance for the question of whether the impact threat
today, that is, say this century, is the same as it has been
throughout the age of the Earth, but have little relevance to the
question of what is the current threat. My point is, the time scale
for the impact flux to change is very long, so to borrow a computer
term for word processors, "WYSIWYG," or "What You See Is What You
Get."  We have observed the frequency of 1 km and larger objects
"crossing our bow" to within a factor of 2 or so, thus we can simply
calculate the frequency of how often such an object hits the Earth,
again within a factor of 2 or so. There really is not much more
uncertainty than that in the present impact rate, or the rate for
the past and next century or even more.

It seems to me that you have completely missed the real uncertainty
that exists in the "impact hazard."  It is not in the flux of stuff
coming in, it's in the consequence vs. size of impacts. We really
don't know if the threshold for global effects is 1 km (or even
smaller), 2 km, or maybe even as large as 4 km. A factor of 2 in
diameter is almost a factor of 10 in mass, and more to the point, a
factor of 10 or more in frequency of impacts. So we are uncertain of
the frequency of globally devastating impacts by an order of
magnitude in either direction, a full factor of 100 from the lower
limit to the upper limit. This factor utterly overwhelms our
uncertainty due to the flux of bodies out there. You will find this
range, more or less, reflected in Morrison's and Chapman's charts of
the frequency of globally catastrophic events.

Best regards,

Alan Harris


From: Victor D. Noto

The short but true answer is yes and no? At this point in our
knowledge of near Earth objects survey the truthful and simple
answer is NO we can not defend the planet from impacts. YES if
certain guidelines are met. The most important item we need to have
in order to defend the planet is lead time and lots of it. First we
need to detect an object heading toward impact with Earth in enough
lead time to address the problem which is not so simple to do
either. At this time it is unlikely with all our telescopes,
CCD cameras, radars and instruments on and off the planet that we
would be able to detect an incoming large or small object in time to
do something about it. Preventing impact would take a tremendous
amount of hard work and luck. There is just too many objects, even
in near Earth space that the scientist have not detected and
recorded yet. If Earth is impacted the most likely source of impact
will be from the 100’s of thousands (some say millions) of near
Earth Asteroids and Comets. But Scientist have another problem,
large and small objects can come out of nowhere from the billions
and billions of objects beyond near Earth space to impact us from
such known areas as the Oort cloud or Kuiper belt in the far reaches
of our solar system. This would be a very daunting and impossible
task to survey every object in the solar system. What I am driving
at is that Earth can never ever be 100% safe from impact even after
all our surveys are done, all our detection systems are in place and
defenses are ready, armed and in place. Lets get real. That being
said, how safe are we now. Today we are pretty much at the mercy of
mathematical probability laws and random impacts can hit Earth at
will. Earth is a target of opportunity so to speak.

In fact, there are small objects hitting the planet every day and
larger objects like large bolides every week which we can not detect
except as they explode in our atmosphere. Recent bright meteors like
El Paso, Texas, USA a few months ago, Greenland last month and
Denver, Colorado USA recently should tell us something about Earth’s
ability to detect early warning of objects hitting Earth. El Paso
and Denver are in the backyard of the USA’s space defense and ICBM
early warning systems main nerve center. Make no mistake about it,
any of these bright meteors could have been the size of Mount
Everest and we would have received the same amount of early warning
- none.

The move from at the mercy of impact to 100% safe is an infinite
task and impossible but within a few decades of survey of Near Earth
objects we can go a long way toward the 100% since near Earth
asteroids is the most probable source of very large impacts with
Earth. We must also realize that finding an object during a survey
which will impact Earth at sometime in the future is a long way from
declaring victory. Can we divert such an object from impact?  Here
too the answer is yes and no.  If the asteroid or comet is very
large say 20 kilometers or better with a few months lead time before
impact chance of a successful diversion is zero. This is of cause
the realistic answer but mankind ability at self deception is
infinite and most likely every effort would go into trying to divert
such an object or any object should Earth get any lead time at all
of impending impact.  God speed John Glenn.

Realistically though, today we would need years of lead time to
engineer a successful diversion of even a  kilometer and smaller
asteroid or comet heading at us. These objects  are not slow movers
usually going 37,000 mph to 112,000 mph and better. I am confident
in such a situation a way will be found.

Maybe actor Bruce Willis will show us the way to divert impacts in
the new movie coming out this week call "Armageddon".

Victor D. Noto

Victor Noto - Kissimmee, Florida USA Website theme quote: "Life really
is a Rock and the Big Rock giveth and taketh away all life!!"

From: Jega A. Arulpragasam

While Dr. Peiser is clearly right, in his broad claim that "IMPACT
estimate lower bound to the AVERAGE interval between such impacts,
and furthermore, set a confidence level for this estimate.

Since civilisation can be traced back only about 10,000 years, the
fact that civilization has not been destroyed doesn't tell us enough
to make a reasonable estimate of the average time between impacts. A
period with no impacts (of the required size) doesn't give us any
information on which to make an estimate of the average interval.
However, it does allow us to set a lower bound on the average
interval between impacts.

If there have been no impacts in 10,000 years, we have a high degree
of confidence that the average interval is NOT less than a
millennium, a very much higher level of confidence that it is not
less than a century, etc.

While civilisation, in its early years, might well have been wiped
out by a localized event, I think it is reasonable to assume that it
is now sufficiently widespread in its geographical distribution that
it will not be destroyed, except by an impact that causes a truly
global catastrophe.

The geological record does give us a count of global catastrophes
(as far as life-forms are concerned) over a period of at least 500
million years. While it is by no means established that all these
catastrophes were caused by cosmic impacts, we can assume that all
of them were, since we are attempting to establish a LOWER bound to
the mean time between impacts.

It appears that the highest level of confidence for an estimate of
the mean time between impacts is of the order of 100 million years.
However the actual confidence level for such an estimate is not
particularly high, since we still have only a very small sample.
Nevertheless, we can have an extremely high level of confidence
that, given ANY instant, the probability that an impact will occur
in the next 200 years, say, is very low.

I would claim that these conclusions "are based on sound scientific
evidence and reasoning," although of course, these conclusions do
NOT necessarily imply that the next week will be free of impacts

I would add that there is evidence that the rate of cosmic
bombardment within the inner solar system was much higher in the
past. While one explanation might be that the solar system "is
running out of ammunition," we do not know whether such periods of
intense bombardment will recur due to disturbances in the Oort
cloud, for example. But assuming that they will, particularly within
a short period of cosmic time such as 200 years, seems rather

As a final note, I might mention that the view that global
extinctions might have been caused by neutron-star mergers within
3,000 light years of the solar system, seems to be gaining currency
in some circles. I have seen an estimate that the mean time between
such events within this particular volume of space is about 100
million years. While the method of estimating a lower bound for the
mean time between global catastrophes is equally sound for the
composite effect of catastrophes from multiple causes, preparing a
defense "within the time allowed" (apparently from subjective
considerations) becomes somewhat more difficult.

Jega A. Arulpragasam
Lunenburg, MA
71 deg 43'W    42 deg 37'N


From: Timo Assmuth

Dear Dr. Peiser,

as a response to your very welcome recent message and digest on
extraterrestrial impacts, I wish to make the following points:

1. with regard to the list of phenomena and variables to be considered
in an impact risk/probability calculation, and the highly interesting
and crucial debate (also among our U.S. colleagues) on risk assessment
and risk communication:

- some of the variables mentioned may be estimable (and extrapolatable
from those known) to SOME extent (relieving your concerns a BIT);
this extent, and thus the relative contribution to uncertainty, differ
among the variables, however, and may in themselves be rather
uncertain  - other (even independent) variables may play a role, as
you rightly stressed (cf. also your criteria of the ESSENTIALITY of
variable selection, i.e. identification of uncertainty)

In summary, regardless of the selection and exact formulation of the
mechanisms and variables, the crucial GENERAL methodological and
ethical/strategic evaluation you put forth seems quite justified:
we simply do not know enough to assure the probability of an 'big'
impact (even in a 'short' time span) is 'small'.

Of course, apart from facts (and factual notions of uncertainties),
VALUATIONS of risks and uncertainties and their (policy) significance
as well as 'tactics' of risk communication/management affect choices
of what (if anything) to say/do. Being acquainted with risk and

CCCMENU CCC for 1998