Subject: CC DIGEST, 30/01/98
Date: Fri, 30 Jan 1998 14:31:59 -0500 (EST)
From: Benny J Peiser






From: 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,



From: 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


From: 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
immune-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

CCCMENU CCC for 1998