Date sent: Wed, 19 Mar 1997 11:41:42 -0500 (EST)
Subject: IMPACT!
Priority: NORMAL


Notes on the implications and the reception of
Gerrit Verschuur's new book
(New York & Oxford: Oxford University Press, 1996)

by Benny J Peiser

The crash of comet P/Shoemaker-Levy 9 on Jupiter in the
summer of 1994 proved to be a watershed in the
development of neo-catastrophism. In retrospect, the
flood of publications on the crash and related issues
since this multiple impact suggest that we are in the
midst of the 2nd phase of a prolonged scientific

The dramatic changes in the understanding and perception
of our cosmic environment, initially triggered by the K/T
boundary controversy during the early 80s, appear to have
slowly shifted from the geological to the historical
time frame. Rather than limiting the debate to the giant
million-year events, the focus of many researchers has
switched to the smaller but more frequent punctuations in
the 1000-years range.

During the course of the last decade, most scientists
have accepted the idea of global disasters caused by
extra-terrestrial bodies. Until fairly recently, their
acceptance depended on the assumption that cosmic induced
cataclysms were restricted to primordial times, millions
of years before the origins of the human race.

This picture has changed significantly over the last
couple of years. As a direct result of the SL9 crash, an
almost endless stream of conference proceedings, research
papers and new books on NEOs and the impact hazard have
been published.

One of the most noticeable changes to the 1980s (which
focused primarily on dinosaurs and mass extinctions in
geological time), is the growing interest in and risk
assessment of the cosmic threat to civilisation.
Moreover, a number of archaeologists, climatologists and
planetary geologists have begun to search for possible
impact events during the historical and prehistorical
period of mankind.

This new recognition of historical catastrophism is the
result not only of the SL9 crash on Jupiter, but also the
consequence of new findings of Holocene impact craters
(like those in Argentina) and recent impact events (such
as the 1908 Tunguska blast and the 1930 Brasilian event).
If major impact catastrophes can happen in front of our
noses, they might just as well have punctuated the Earth
in ancient and prehistoric times.

This new appreciation, however, also stems from the
increasing awareness that the celestial hazard is not
limited to the odd asteroid which hits the Earth every
100,000 or 1,000,000 years. In contrast to the
traditional risk assessment (which is based on a
statistical analysis of the number of known impact
structures on Moon and Earth in addition to the currently
known asteroidal flux), it has become evident that there
are many other (and, in fact, many unknown) cosmic
sources which are capable of triggering ecological
disasters - often without causing impact craters on the
ground (i.e. 'dead' or disintegrating comets, cometary
dust veils, atmospheric impacts and oceanic impacts by
large meteor showers, etc.).

Traditionally, both the search for past cosmic
catastrophes and the extrapolation of future impacts has
been limited to crater producing events. "No crater, no
catastrophe". This is what could be called the motto of
this school of thought. If we were to accept this
restriction, Tunguska-like events or even
super-Tunguskas would be automatically taken out of the
equation. Due to their catastrophic detonation above
ground (or in the oceans), they often leave no obvious
fingerprints behind.

Yet, according to current knowledge, Tunguska-like
impacts occur every 100 years or so - with devastating
effects. It is, therefore, not far-fetched to hypothesise
that a super-Tunguska (i.e. massive high level
multimegaton explosions and widespread atmospheric and/or
oceanic impacts of fireballs over different locations of
the globe) is capable of triggering ecological crises on a
continental or even global scale and may occur every 2000,
3000 or 5000 years.

The foundation of this astronomical hypothesis was first
developed by Victor Clube and Bill Napier back in 1978,
two years before Luis Alvarez and his team verified the
cosmic origin of the iridium found in the clay layer of
the K/T boundary. Since then, a growing number of young
astronomers (not to mention older ones such as Sir Fred
Hoyle) have come to similar conclusions and loosely form
what is known as the British school of "coherent

These scholars envisage trains of cometary debris which
repeatedly encounter the Earth, the largest and most
dangerous representatives of which may be between one and
several hundred metres in diametre. Such bodies,
depending on their cometary constitution and hence their
cohesive strength, can have catastrophic effects on the
ecological system in a variety of ways. The encounter
between cometary debris and the Earth are capable of i)
low or high level multimegaton explosions of fireballs
which destroy natural and cultural features on the
surface of the Earth by means of floods, blasts and
seismic damage, ii) massive high level influx of comix
dust high above the stratosphere which causes a dramatic
drop of global temperature and which can lead to the
suspension of agriculture, iii) massive high level influx
of cosmic chemicals (associated with dust) with, as yet,
incalculable biochemical potentils which may be harmful
to DNA and can trigger evolutionary mutations.

For almost 20 years, the astronomical mainstream has been
highly critical (to say the least) towards Clube and
Napier's giant comet hypothesis. In the fields of
archaeology and ancient history it was simply ignored.
The last couple of years, however, have signalled a
sudden shift in attitudes. This became apparent when a
number of new book publications by eminent astronomers
and science authors turned their attention to historical
catastrophism (see Duncan I. Steel: Rogue Asteroids and
Doomsday Comets: The Search for the Million Megaton Menace
that threatens Life on Earth, New York et al: John Wiley &
Sons, 1995; John & Mary Gribbin: Fire on Earth: Doomsday,
Dinosaurs and Humankind, New York: St Martin's Press,
1996; John S Lewis: Rain of Iron and Ice: The very real
threat of comet and asteroid bombardement, Reading, Mass:
Helix Books, 1996).

Perhaps even more telling is the generally positive
reception of these books in popular science journals and
the academic press. Here, I will restrict my comments to
the reception of the most recent publication, Gerrit
Verschuur's IMPACT! (which appeared last September in the
US and this January in the UK). Although my brief
discussion will only focus on the five reviews of this
book which I know of, they may be able to tell us more
about the general recation to the impact hazard and the
new appreciation of historical catastrophism.

Let us begin with a forecast review if IMPACT in the
American PUBLISHERS WEEKLY (12 August 1996). This short
review seems to place IMPACT in the context of PMT
(Pre-Millennium-Tention) and stresses that "not since
Spengler has the end of civilization been threatened so
often". Now, most neo-catastrophist researchers are used
to the unpleasent experience of being labelled (or even
denounced) as "doomsday prophets". Thus, it does not come
as a surprise that Verschuur's tone is described as "that
of a prophet in the desert, warning of doom with a
sometimes disturbing single-minded determination." What
is, however, remarkable, is the seriousness with which
the reviewer, nevertheless, considers recent research
findings - instead of trivialising them. "Recently", he
goes on to stress, "estimates of the size of the
impactor (or impactors) that could destroy much of the
world have been reduced as it has become clearer that
the real damage would not be so much to the land as to the
atmosphere." In view of these potential risks, the
reviewer concludes that "astronomer Verschuur may well be
right to be so alarmist."

Another early review appeared in the American KIRKUS
REVIEW (15 July 1996). The reviewer breifly summarises
the contents of IMPACT, underlining Verschuur's
discussion of current detection and future deflection
programmes and concludes that "occasionally overblown,
often jumpy in its organisation, this is nonetheless a
strong treatment of the key scientific discoveries of
our time."

The January issue of SKY & TELESCOPE reviewed IMPACT as
one in a collection of competing books on the impact
hazard. SKY & TELESCOPE has been traditionally hostile
to the whole idea of coherent catastrophism. In the
January issue, FIRE ON EARTH by British science authors
John and Mary Gribbin is even snubbed as a "tale" that
"smacks of Velikovskyian nonsense" - simply because
of their endorsement of some of Clube and Napier's ideas.
In contrast, IMPACT ("equally provocative title")
receives more even-handed treatment, although "it
explores much the same ground as the Gribbins' work."

Perhaps this contrasting reaction can be attributed to
the fact that the Gribbins belong to Britain's most
widely read popular science authors whereas Gerrit
Verschuur is an astronomer. Whatever the case, it is a
sad reflection of scholarly standards of this journal
that it tolerates derogatory statements against respected
members of the scientific community.

This labeling is, of course, wholly arbitrary since the
S&T reviewer in no way attempts to substantiate his
criticism against the concept of coherent catastrophism.
This inconsistency becomes even more apparent when, in
the following paragraph, Verschuur is suddenly credited
with putting the idea of "Clube and Napier's historical
impacts and periodic comets (...) into better
perspective." What was two paragraphs earlier denounced,
is now unexpectedly approved of: "The gist is that
perhaps we've been incredibly lucky and have grown
complacent in our seemingly calm solar system. The
turbulent times of the not-so-distant past may return
sooner than we might like."

Gareth William's rather patronising review, which
appeared in the NEW SCIENTIST (25 January 1997), features
a more pedantic criticism. After a positive
introduction, it then focuses on a number of alledged
factual errors ('the encounter distance of 1994 ES1 was
0.001 AU, rather closer than the given value of 0.1 AU')
before blowing them out of all proportion in the
concluding paragraph: "Despite the obvious skills of
popularisation, the book is spoilt by its errors of fact.
But a second edition, correcting the errors, would be
well worthwhile."

But the most interesting (and certainly the most
thoughtful) review of IMPACT which has come to my
attention is that by David Morrison. As one of the
world's leading astronomers and Director of Space at NASA
Ames Research Center, he has been perhaps the most
outspoken critic of the British school of
neo-catastrophism. It is therefore pleasantly surprising
that, on the whole, IMPACT receives a balanced assessment
in which the main criticism is voiced in a dispassionate
tone. "Verschuur has done a great deal of background
research, and his historical discussion of the saga of
Chicxulub is among the best written on these topics."
Yet, David Morrison only recommends "the first 94 pages
of this book (...). No one has covered this material
better." What, then, are the reasons for the half-hearted

Among other things, it is the ambiguous nature of the
Taurid cometary/astroidal/meteoric stream of debris which
the advocates of coherent catastrophism have linked to
the 'giant comet' theory. Morrison dislikes Verschuur's
sympathetic treatment of this concept despite the fact
that IMPACT clearly presents it as a legitimate
hypothesis which deserves further scrutiny.

Indeed, it is a conspicuous feature of most critics of
coherent catastrophism that they fail to consider (let
alone scutinise) whether the astronomical, archaeological
or climatological evidence might point to significant
punctuations during the last 5,000 years.

Admittedly, I sympathise with Morrison's complaint that
IMPACT "emphasizes data and interpretations that maximize
the impact influx as well as the damage that can be done
by impactors of given yield." In fact, I also agree that
there are still "substantial uncertainties in many of
these estimates." It is quite possible that Verschuur
selected "the worst case" and thereby exaggerated the
potential danger to civilisation.

In view of the apocalyptic undercurrent in Western
thought, Morrison's legitimate concern should not be
underestimated. Paradoxically, this anti-apocalyptic
attitude of the enlightenment can have the exact opposite
effect to scientific knowledge: The 'giggle factor' can
(and indeed has) put off vital research.

Whether or not Verschuur's book is 'fatally flawed' in
this respect and, more importantly, whether the overall
impact hazard is really 'exaggerated', ultimately depends
on a thorough and impartial investigation of the
archaeological, geological and climatological record of
our "not-so-distant" past. It is another curious paradox
that David Morrison's healthy scepticism and his
anti-apocalyptic stance (both of which I share) is likely
to deter scientists from further research (and tests) on
the historical and prehistorical record of mankind.
Gerrit Verschuur's "alarmist interpretation", on the
other hand, will be most likely to encourage such
investigations which are, after all, vital for any
realistic assessment of our cosmic environment and the
hazard from space.

For the last 150 years, it has been this notorious
conflict between 'irrational' alarmism and 'enlightened'
denial which has so often blocked scientific progress. I
cherish the hope that Gerrit Verschuur's new book might
help to overcome this outdated and obstructive
confrontation for the sake of a common good

CCCMENU CCC for 1997