CCNet, 24 February 2000


From DISCOVERY.COM, 23 February 2000

The role of the multi-megaton impacts of comets and asteroids could be
very significant, says Benny Peiser, a social scientist from Liverpool
John Moores University in England.

"The focus over the years has been the big events," Peiser says,
referring to the alleged global catastrophe that followed the impact of
an asteroid with the Earth 65 million years ago -- wiping out the

But smaller events similar in size to the 1908 Tunguska blast that is
believed to have leveled 2000 square miles of forest could play a big
role in history, Peiser explained to scientists on Saturday at the
meeting of American Association for the Advancement of Science in
Washington, D.C.

The Tunguska event left no crater because the lightweight comet or
asteroid exploded in the atmosphere before reaching the ground,
scientists believe.

Similar extraterrestrial blasts in history might have triggered the
demise of Bronze Age civilizations in the Mediterranean, account for the
mysterious disappearance of certain cultures in the Americas, and
explain the rise of apocalyptic religions and biblical references to
localized disasters, says Peiser.

It also would mean that we are more vulnerable to such disasters than
currently thought.

"We haven't got any indication of when another asteroid might come
close," says Peiser.

Astronomers, however, are skeptical of Peiser's danger assessment.

"I can't say it's never happened," says asteroid impact specialist
Richard Binzel of the Massachusetts Institute of Technology, regarding
the possibility of a Tunguska-sized event hitting a populated area. "But
on the entire surface of the Earth the area of destruction is very

In other words, the chances are tiny, and were downright miniscule when
there were fewer people and fewer cities.

As for the odds of another global catastrophe like the one that
supposedly killed the dinosaurs, says Binzel, there's a one in 10,000
chance of it happening in any given century.

"We're just more conservative" in danger estimates, Binzel says.

Copyright 2000,


10,000 YEARS
In a presentation to the Annual Meeting of the American Association
for the Advancement of Science (AAAS), Dr Benny Peiser of Liverpool
John Moores University, presented new evidence suggesting that more
than 500 impact events of extraterrestrial orgin have punctuated
Earth during the last 10,000 years.
The great majority (70%) of these events have been of the Tunguska-type
class of atmospheric impacts with an average energy yield of between
20 and 100 Mt TNT. However, more than 100 surface impacts, including
more than a dozen oceanic impacts, are believed to have repeatedly
devasted whole regions, small countries and early civilisations around
the globe. In a worst case estimate, Dr Peiser said that up to eight
climatic downturns detectable in the geological and climatological
records of the Holocene may be directly associated with multiple impact
"Episodes of increased cometary or meteoric activity punctuating
societal evolution should be looked upon as potential agencies
determining the rise and fall of ancient civilisations. Both the
emergence and the collapse of human cultures, the Pleistocene-Holocene
transition and the Neolithic Revolution, the onset and collapse of the
Bronze Age civilisations, and even the collapse of the Roman Empire
may be associated with episodes of increased cosmic activity and
multiple impacts that may well have included incidents of cosmic dust
While most of these impacts occurred over unpopulated areas of the
globe, there are historical accounts about devastating cosmic
catastrophes. According to a number of Chinese records, about 10,000
people were killed in the city of Chi'ing-yang in 1490 AD due to the
break-up of a small asteroid.

About a dozen hypervelocity impact craters that date from the Holocene
period (i.e. since the end of the ice age) have been discovered to
date. The majority of impacts, however, that occurred during this
crucial period of societal evolution have not been detected yet.
According to Dr Peiser, "the 14 known Holocene impact craters most
certainly paint a rather deceptive picture of our past. The fact that
no massive impact crater dating to the Holocene has been detected, has
led to the belief that no hemispheric or global impact disaster can
possibly have happened. However, this is a widespread delusion."
Yet there still is a regretable lack of interest by the scientific
community to scrutinise the Holocene for major impact events.
"The widespread ignorance of such cosmic disasters in historical time
is due to the limited research focus on crater producing events. Yet
only 3% of fatal impacts produce a hypervelocity surface crater on
land", Dr Peiser points out. Tunguska-like impacts or
"Super-Tunguskas" are thus taken out of the equation. Due to their
catastrophic detonation above ground (or in the oceans), they often
leave no obvious fingerprints behind.
Dr Peiser also presented new impact simulations that estimate expected
fatalities of cosmic impacts for the next 10,000 years. Without the
establishment of effective strategies of planetary defense in the
future, more than 13 million people are expected to die as a direct
result of impact catastrophes in the next ten millennia. "We can, and
indeed have to live with smaller, Tunguska-type impacts. There is
little we can do about them. But we need to prevent the impact of
larger objects which threaten the stability of our civilisation. Unless
we start planning ahead and develop the technology for the deflection
of this threat, cosmic impacts will inevitably lead to major disasters
in the future", Dr Peiser stressed. 
Based on computer simulations that take into account the current flux
of near-Earth objects, a typical 10,000 years period with a constant
human population of 5 billion can expect to experience:
*110 fatal impacts resulting in a total of 13 million fatalities (an
average of 120,000 fatalities per event).
*300 "Tunguska" style airbursts over land, with 80 of these producing
fatalities (roughly 1 fatal event per century).
*12 ocean impacts that produce tsunami, with an average of 500,000
fatalities per event.
* 4 land impacts, with an average of 500,000 fatalities per event.
"These estimates are based on the assumption that the current
asteroidal and cometary flux will be constant in time and quantity over
the next 10,000 years. However, there is growing evidence to suggest
that there have been peak levels of meteoritic activity in the past
that differed significantly from the cosmic calm of the last 300
years", Dr. Peiser pointed out.
In January, a UK Task Force was set up by Lord Sainsbury, the Science
Minister, to look into the way in which Britain should respond to the
impact hazard and how it can contribute to the international efforts to
prevent major impacts from happening in the future.

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