>From AstroBiology Magazine, 17 February 2003

Summary: The second part in our "Great Impact Debate" series brings
together a group of scientists who are experts on asteroids and comets.
Today's debate concerns the emphasis the media and others place on the
threat of asteroid and comet impacts. Given that large bodies hit the
Earth only very rarely, is the concern about impacts unjustified?

Much Ado About Nothing?
Great Impact Debate: Part II


Clark Chapman - scientist at the Southwest Research Institute's
Department of Space Studies, in Boulder, Colorado. Member of the MSI/NIS
(imaging/spectrometer) team of the Near Earth Asteroid Rendezvous (NEAR)
mission to Eros.

Alan Harris - senior research scientist at the Space Science Institute,
an affiliate of the University of Colorado at Boulder.

Benny Peiser - social anthropologist at Liverpool John Moores University
in the UK. He has written extensively about the influence of NEO impacts
on human and societal evolution.

Joe Veverka - professor of astronomy at Cornell University in Ithaca,
New York. Principle Investigator for NASA's Comet Nucleus Tour (Contour)

Don Yeomans - (debate moderator) - Senior Research Scientist at NASA's
Jet Propulsion Laboratory in Pasadena, California, and manager of NASA's
Near-Earth Object Program Office.

Don Yeomans: To sum up part of the discussion from last week, near-Earth
comets and asteroids are important in understanding the origin of our
solar system. The planets, including our own Earth, were formed from
collections of these near-Earth objects (NEOs). Life itself may have
been enabled by the water and carbon-based materials brought to the
early Earth as a result of comet and asteroid impacts. NEOs are among
the easiest objects within the solar system to visit with spacecraft, so
in the future NEOs may provide the raw materials (e.g., water, minerals,
metals) for human survival and for building structures in space.

Of course, most of the newspaper accounts reporting upon near-Earth
comets and asteroids have nothing to do with their scientific
importance. As last week's debate made clear, a hot topic is the effect
of past NEO impacts on Earth and the possibility of future impacts. In
fact, most of the press reports about NEOs have to do with those that
could impact Earth and possibly cause a significant number of deaths.
With regard to the possible threats that NEOs pose, scientists can point
to no person in recorded history who has been killed by an asteroid or
comet. On the other hand, we can all cite examples of people who have
been killed by auto accidents, floods, hurricanes, and airplane crashes.
So why all the fuss about comet and asteroid impacts with Earth?

Benny Peiser: I am not so sure about the accurateness of the claim that
"no one in recorded history has been killed by an asteroid or comet." As
a matter of fact, there are historical records that give accounts of
suspected meteorite impacts leading to fatalities. For example, a number
of historical reports exist about an alleged disaster in 1490 AD, said
to have occurred in the Chinese city of Qingyang (Shaanxi province).
According to these reports, over 10,000 people were killed when "stones
fell from the sky like rain." Perhaps, as John Lewis has suggested in
his book "Rain of Iron and Ice" (1996), it would be wiser to say, "No
one in recorded history has ever been killed by a meteorite in the
presence of a meteorite scientist and a medical doctor".

Clark Chapman: Still, the average American's chances of dying as a
result of an asteroid impact is about the same as an average American's
chances of dying in a tornado. The distinction is that many tornadoes
occur each year, and some of them kill up to dozens of people, whereas
big asteroid impacts occur very rarely, but might kill millions or even
billions of people.

Several U.S. citizens die every day, one at a time, by accidental
electrocution. Roughly the same number die in the very few jet airliner
crashes each year, where hundreds die at one time. Airline crashes are
relatively rare events: there were only two days during 2001 on which
people died in airliner crashes in the United States; one was Sept.
11th. Asteroid impacts are just a more extreme case of a rare but
extraordinarily deadly event.

Since the chances of death and destruction by cosmic impact are on the
same order for Americans as death by airliner crash, flood, tornado, and
other hazards society takes seriously, it is reasonable that the impact
hazard be taken seriously. In fact, an asteroid impact is a much more
serious hazard, statistically speaking, than many other hazards we have
experienced in the last few decades, including death by terrorism, by
nuclear power plant accident, by shark attacks, etc. And cosmic impacts
- if large enough - are nearly unique (along with nuclear war and
perhaps some 'Andromeda Strain' pandemic) of having the possibility of
sending civilization back into a Dark Age or even exterminating our
species -- although the chances of such a cataclysmic impact are
extremely tiny.

Historically, society has not spent equally to mitigate various hazards,
in terms of dollars per life saved or damage averted. The comparisons I
have cited do not necessarily require that society must spend the same
amount on NEO searches and mitigation measures as we do, for instance,
on tornado research, but policy-makers ought to at least seriously
examine the impact hazard.

Joe Veverka: Today asteroids and comets are largely irrelevant to life
on Earth on any reasonable human time scale. So why all the fuss?
Probably the most important reason is that most humans worry a lot and
are not equipped to evaluate certain risks properly. If there is
vigorous publicity concerning ANY potential danger to status quo and
well being, some people will worry and some will even become terrified -
no matter how trivial the alleged danger really is when viewed in
comparison with the other dangers that surround us. The fact that some
individuals feel threatened by asteroids and comets does not imply
anything about the reality or severity of the threat. Exactly the same
statement can be made about dozens of other "dangers'' peddled by
well-established advocacy groups to scare the public and thereby gain
support and funds. Thus, the reasons for the fuss are very clear and
fundamentally very human. They have very little to do with what is
really out there in space, or with what really are the dangers to life,
society, and environment posed by comets and asteroids.

Clark Chapman: Joe Veverka's answer implies that there are other
catastrophes that have been more deadly, at least in the last century.
That is illustrated in this chart, produced for an asteroid workshop by
John Pike. The statistical hazard from asteroids and comets would be a
slice of pie in this chart that is roughly the same size as the one for

Joe Veverka: Regarding the advocacy groups I mentioned, there are at
least three different, well-established advocacy groups whose future and
welfare depends on focusing the public's attention on the "threat from
space." (The term "advocacy group" is a polite one. A much more accurate
but cruder term can be found in Jean Giraudoux's play 'The Mad Women of
Chaillot.') The first advocacy group is the media. If all else fails,
stories about comets and asteroids destroying New York or Tokyo sell
newspapers and magazines and make for popular TV fodder.

Second, there is a strong advocacy group among astronomers. They want
more resources devoted to studying comets and asteroids. Publicizing the
"threat from space" has certainly proven an effective means for
generating government support for the study of NEOs.

Finally, there is an evolving engineering/industrial/military advocacy
group that promulgates the "threat from space" because members of this
group want public support to build and provide the defenses that will
shield us from this "peril."

Benny Peiser: I'm afraid I regard as misleading allegations that the NEO
community (or the missile defense community for that matter)
deliberately exaggerate the impact risk for selfish reasons. In reality,
most NEO researchers - in particular those in the U.S. - have underrated
the potential hazards from space. NASA only reluctantly began to address
the issue following the considerable 'wake-up call' caused by the impact
of comet Shoemaker-Levy 9 on Jupiter in 1994. Without these harmless
reminders, I doubt whether NASA would have established even a
rudimentary program to inventory the number of large asteroids out

Alan Harris: In response to Joe Veverka's point on advocacy groups, it
is often the case that those who are most qualified to offer advice also
have special interests in the matters of their expertise. You go to a
car salesman to buy a car, but if you are not equipped to evaluate what
he tells you, you risk being "taken for a ride." The same is true even
for consulting a doctor about an ailment. By Joe's argument, you should
never buy a car or see a doctor because any advice you receive may be
tainted by special interests!

Benny Peiser: Professional astronomers and planetary scientists study
comets and asteroids for a better insight into the evolution of our
solar system. In contrast to this outlook, the vast majority of people
around the world are not bothered with the specific aspects of NEO
research. What they want to know, first and foremost, is quite simple:
Do these objects pose any threat to me, my family, or to the stability
of our societies?

The information provided by scientists has not been very reassuring.
What do we expect the interested public to think about claims that the
K/T impact was "unique" and that it was the only impact that caused a
mass extinction? How credible are such sweeping statements, given how
incomplete our current understanding remains regarding the mechanisms,
effects, and rate of hypervelocity impacts?

Peter Ward: David Kring has suggested to me that another K/T type impact
could be devastating for the animal and plant world. New calculations
show that our planet would go into another "Snowball Earth event" like
the one that occurred 600 million years ago, when the oceans froze over.
While bacteria might readily survive such calamitous impacts, our new
understanding from the record of the Earth's mass extinctions clearly
shows that plants and animals are very susceptible to extinction in the
wake of an impact.

Anything that would increase the rate of large asteroid or comet impacts
on the Earth - or on any inhabited planet in this or any other galaxy -
would be detrimental to the evolution and survival of higher life forms.
But what determines impact rates? That depends on how many comets and
asteroids exist in a particular planetary system. It also depends on how
often those objects are perturbed from safe orbits that parallel the
Earth's orbit to new, Earth-crossing orbits that might, sooner or later,
result in a catastrophic K/T or Permian-type mass extinction.

Our understanding of those events now leads us to believe that any comet
or asteroid greater than 20 kilometers in diameter that strikes the
Earth will result in the complete annihilation of complex life - animals
and higher plants. How many times in our galaxy alone has life finally
evolved to the equivalent of our planets and animals on some far distant
planet, only to be utterly destroyed by an impact? Surely this has been
a commonplace event in the vast cosmos.

Yet it appears that Jupiter, with its stable circular orbit far from the
sun, assures the Earth a low number of impacts resulting in mass
extinctions. Jupiter sweeps up and scatters away most of the dangerous
Earth-orbit-crossing comets and asteroids. In 1995, astronomer George
Wetherill calculated that without Jupiter, the impact rate on Earth by
comets and asteroids would be 10,000 times higher. Under such
bombardment it is hard to conceive of how complex life would survive.
Jupiter acts as our cosmic guardian, making our neighborhood vastly
safer through a gravitational Planetary Protection plan.

Benny Peiser: Given the exceptionally low odds of large impacts,
astronomers are rightly staggered by the public's concern with and the
media's obsession about the impact hazard. I think one reason for this
bewilderment has to do with a genuine lack of sociological
understanding. After all, the irrational unease about the impact risk
needs to be seen in context of a global public that, to an overwhelming
extent, still adhere to religious beliefs that predict cosmic
catastrophes of apocalyptic proportions. For hundreds of millions of
devout Christians and Muslims, comets and asteroids are extremely
relevant to life on Earth - not for any scientific reasons, but as
traditional portents of doom.

In the past, one way to deal with such celestial dread was to deny that
there is any cosmic threat whatsoever. For most of the last 2,000 years,
astronomers have been extremely reluctant to acknowledge that there are
any risks from space. This approach, however, is no longer available to
modern science and any attempt to discount, minimize, or ridicule the
impact hazard will backfire. That's why I am uncomfortable with ongoing
efforts to underestimate or belittle the concerns people have about the
impact hazard.


Part III of the Great Impact Debate (02-24) series is entitled "The
Large and the Small", and features a discussion of whether we should
invest the resources to minimize the effects of smaller impacts.

CCCMENU CCC for 2002