CCNet 139/2002 - 27 November 2002

"The problem with a search for big asteroids, Brian Marsden
explained, is that "we don't really know how many [big asteroids] there
are. We know how many we think we've found. But we don't know how
big they are; we can't even say how many are one kilometer," or 0.62 mile
wide. The difficulty in determining an asteroid's size stems from observing
methods, he explained. An asteroid's size is estimated according to its
brightness, "and we don't know how brightly they shine." Astronomers have
guessed that asteroids reflect about 4 percent of the light that hits them,
although some are far brighter and others appear almost black. "Surely
they're not all the same" in brightness, "so we don't really know how many
of 1- kilometer size have been found," Marsden said."
--Robert Cooke, Newsday, 26 November 2002


>From Andrew Yee <>

University Relations
University of Arkansas

David Stahle, professor, geosciences
(479) 575-3703,

Melissa Blouin, science and research communications manager
(479) 575-5555,



FAYETTEVILLE, Ark. -- A drought that lasted three times as long as the Dust
Bowl of the 1930s wreaked ecological havoc over much of the western United
States and Mexico, and occurred at about the same time as the fall of
Teotihuacan and classic Mayan civilization 13 centuries ago, say
University of Arkansas researchers.

David Stahle, professor of geosciences, presented some of his findings today
at the annual Council for the Advancement of Science Writing meeting in St.
Louis, Mo.

"The megadrought of the eighth century was three times longer than the Dust
Bowl," Stahle said. "The consequences, ecologically and socially, must have
been enormous."

Stahle and colleagues at the University of Arkansas (Matthew Therrell,
Malcolm Cleaveland, and Falko Fye), the National University in Mexico City
(Rodolfo Acuna-Soto), the University of Tennessee (Henri Grissino-Mayer),
and Columbia University's Lamont-Doherty Earth Observatory (Edward Cook)
have compiled tree ring and lake sediment records to study prolonged
multi-decadal drought over the past 2,000 years.

The tree-ring data indicate a severe drought in the eighth century that
lasted from AD 736 to 765, a period of 30 years. Rainfall over New Mexico
averaged only 90 percent of normal for this entire 30-year period and only
83 percent of normal during the worst decade of the eighth century
"megadrought." In Nebraska, during the heart of the Dust Bowl drought of the
1930s, annual rainfall averaged just 80 percent of normal. A single year
with only 80 percent of normal rainfall constitutes a serious deficit, but
decadal or multi-decadal dry spells of this magnitude can have catastrophic
environmental and social consequences, as witnessed during the Dust Bowl.

Widespread, intense droughts like the eighth century event can impact the
ecology of a region. Long term drought conditions affect a region's plant
life, animal life and hydrology and have been linked historically to
outbreaks of disease.

"They leave behind a big footprint," Stahle said. Indeed, Stahle and his
colleagues have shown evidence in the tree-ring record of another
continental-scale megadrought that occurred in the 16th century. The 16th
century drought conditions lasted 40 to 50 years, probably the most serious
drought in the last 2000 years over North America, a megadrought that
certainly affected the first European settlements in North America. The 16th
century drought seems to have contributed to the virulence of hemorrhagic
fever outbreaks that swept through the native populations in Mexico in 1545
and 1576 and caused the deaths of millions of people.

The tree-ring chronologies document eighth century megadrought over the
southwestern United States, but lake sediment records indicate that this
severe drought may have extended into the northern Great Plains and across
central Mexico, possibly including the Yucatan peninsula.
Severe drought has been well documented over central Mexico and the Yucatan
during the late first millennium AD, and the new tree-ring data suggest that
this period of prolonged drought may have begun in the middle of the eighth
century over the southwestern United States and northern Mexico.

The lake sediment records of epic drought in the Yucatan have been linked to
the collapse of classic Mayan culture. And the new research indicates that
the eighth century drought may have also contributed to the decline of
Teotihuacan in central Mexico, one of the six largest cities in the world at
the time.

"Any society would be stressed under the conditions of one of these
megadroughts," said Stahle. "And we now believe that prolonged drought may
have been a factor in the collapse of Teotihuacan."


>From Newsday, 26 November 2002,0,7912519.story?coll=ny%2Dhealth%2Dheadlines
By Robert Cooke

Like prisoners trying to identify snipers taking aim, astronomers are
spotting more and more chunks of rock in the sky that may yet whack us.

As of last year, according to a report in Sky & Telescope magazine,
observers had already found about 600 asteroids that are half a mile in
diameter, or bigger, capable of causing enormous damage on impact with
Earth. Many have a chance of hitting the Earth - sometime.

More recently, an estimate of how often smaller objects - asteroids only 50
yards in diameter - hit Earth's atmosphere was revised downward by
astronomers in Canada and the United States. Objects of that size probably
arrive, on average, once every 1,000 years, not every 200 to 300 years, in
light of eight years of observations.

Such asteroids are small enough so they tend to explode at high altitude,
but their shock waves sometimes reach the ground, as in the Tunguska event
in Siberia in 1908. That shock flattened a forest for miles around, even
though the asteroid never hit ground.

As for the really big objects, those half a mile or more in diameter, the
latest estimates, by J. Scott Stuart at the secretive Lincoln Laboratory in
Bedford, Mass., suggest 1,250 such asteroids exist. And some of them are in
orbits that may yet send them smashing into the Earth.

The 600 or so already identified near Earth asteroids have actually been
seen and cataloged, unlike many of the smaller asteroids, which are harder
to see.

Although impacts by the very large objects are rare, occurring roughly once
every 100,000 years, they are extreme events indeed, and one would be
capable now of erasing much of what civilization has created. Scientists
think an explosive impact would touch off massive fires, windstorms, seismic
sea waves and dust clouds that might last for weeks or months. The
disruption could be global, and long-lasting.

It was predicted about 20 years ago, by the late astronomer Eugene
Shoemaker, that thousands of asteroids exist in orbits that might make them
dangerous to life on Earth. Since then, a search of the skies has turned up
about 600 of the big ones, and some astronomers think there may be twice as
many, half of them yet unseen.

Also, research on the ground has identified some of the ancient craters
created by large asteroid impacts.

"NASA's aim is to find 90 percent of them in 10 years," a program that
officially began in 1998, said astronomer Brian Marsden, at the Harvard/
Smithsonian Center for Astrophysics, in Cambridge, Mass. Marsden is director
of the International Astronomical Union's central telegram bureau (sic),
which monitors such phenomena.

The problem with a search for big asteroids, he explained, is that "we don't
really know how many [big asteroids] there are. We know how many we think
we've found. But we don't know how big they are; we can't even say how many
are one kilometer," or 0.62 mile wide.

The difficulty in determining an asteroid's size stems from observing
methods, he explained. An asteroid's size is estimated according to its
brightness, "and we don't know how brightly they shine." Astronomers have
guessed that asteroids reflect about 4 percent of the light that hits them,
although some are far brighter and others appear almost black.

"Surely they're not all the same" in brightness, "so we don't really know
how many of 1-kilometer size have been found," Marsden said.

He added that the search for potentially dangerous asteroids is also
hampered by the lack of observations made in the Southern Hemisphere. The
Australian government cut off funding for sky searches in 1996.

While NASA is most concerned about 1-kilometer-wide objects, Marsden said
there should also be emphasis on asteroids half that size, or even smaller.
"One only 200 meters across would make a nasty tidal wave that could wash us
all away."

A report on the potential hazard was recently sent to the British
government, advocating a search for objects down to 300meters wide and
seeking a large new telescope to operate in the Southern Hemisphere.

As for expected close approaches by asteroids, Marsden said, "there's one
coming on April 6 that may be 300 meters wide." But that asteroid will pass
the Earth at a distance of more than 500,000 miles. At present, "it's about
8 million miles away," he added.

Ahead, in the year 2049, Marsden added, there is a big object - 750 meters
in diameter - that seems to have a one in 10,000 chance of colliding with
Earth. "The impact probability is getting bigger with time," but as more
observations come in, Marsden expects the possibility of impact may go away.
"But we'll have to observe it for a year" before anyone can be sure the
object, called 2002, will miss the Earth.

"It will be observable for the rest of the year, so we should be in good
shape in being able to follow it. And it's much more likely that it [the
risk of impact] will be zero rather than one." Still, if the risk grows,
"and it gets down to 1 in 10, then even I might get a little concerned."

The concern about potential asteroid impacts emerged during the space age as
scientists began assessing how the solar system was created, how it has
changed and what forces molded it. One obvious force - because of many huge
craters seen on the moon - were impacts by asteroids and comets.

Shoemaker, working for the U.S. Geological Survey, calculated that the Earth
must have been severely bombarded by objects from space though the forces of
weathering and erosion have erased most of the evidence on Earth. Some
pockmarks, such as Meteor Crater in Arizona, however, do show that
bombardment is a continuing process.

Copyright © 2002, Newsday, Inc.


>From Sky & Telescope, 25 November 2002

By J. Kelly Beatty
November 25, 2002 | For two decades, planetary geologists have been painting
their dynamicist colleagues into an ever-tighter corner. Several dozen
meteorites with Marslike compositions argue that asteroidal impacts
occasionally blast rocks off the Red Planet, and that some of these rocks
eventually find their way to Earth. Yet dynamicists initially scoffed at the
notion that chunks of Mars could ever be accelerated to escape velocity (5
kilometers per second) without having them shocked to smithereens.

After much number crunching, however, impact modelers eventually deduced
that it could be done - if the impact event were powerful enough to leave
behind a crater at least 10 km across. The timing seemed plausible; the
youngest Martian meteorites are volcanic basalts only 180 million years old,
and collisions big enough to make 10-km craters occur on Mars about once
every 200 million years on average. But such an impact should have left a
sizable, fresh-looking scar on one of the planet's lava-covered plains, and
nothing so obvious has turned up. Moreover, the evidence in hand suggests
that at least six separate ejections have taken place.

Fortunately, computer impact simulations now suggest that such big bangs
aren't needed after all. In the November 7th edition of Science Express,
three researchers conclude that collisions yielding craters only 3 km across
are energetic enough to eject millions of small Martian rocks into
interplanetary space. Collisions of this size should happen on Mars every
200,000 years or so, and consequently chunks of the Red Planet should be
plunking down on Earth several times each year.

James N. Head (Raytheon Missile Systems), who performed the computer
modeling for his doctoral thesis at the University of Arizona, also managed
to solve another Martian-meteorite quandary. Most of these stones
crystallized within the last few hundred million years, yet roughly half of
the Red Planet's surface is a good 4 billion years old. So why haven't more
"old Martians" been found? The key, as Head and his colleagues explain, is
that the meteorites must have originally been buried in the layer of
regolith, or crushed rock, that covers the planet's exterior. Younger
regions, like the lava plains, have relatively thin crush zones, but the
most ancient terrains are covered to depths of hundreds of meters. Because
the presence of a thick regolith reduces ejection speeds, only very
energetic impacts can excavate material from these regions - and since big
impacts are infrequent on Mars, ancient samples of Mars should reach Earth
only rarely.

Copyright 2002 Sky Publishing Corp.

>From The Ottawa Citizen, 26 November 2002{388AC70F-74E2-47BC-BEA5-710331CD1911}

NASA is studying some of the most inhospitable places on Earth -- like the
crater in Sudbury -- in a bid to understand the red planet, writes Joanne
Joanne Laucius 

By studying the ancient crater in Sudbury, a team of scientists, including
Ottawa's Doreen Ames, is hoping to learn how life might thrive after a
planet gets smacked with a resounding blow from a comet or meteorite. Ms.
Ames, above, is seen with a chunk of sinter hydro-thermal rock from Sudbury.

NASA wants to know what the Sudbury crater has to tell scientists about life
on Mars.

Researchers from the U.S. space agency are probing the most inhospitable
places on Earth in the hopes they will find clues about how lifeforms could
survive another inhospitable place -- Mars.

They are studying how lifeforms adapted to survive in the Siberian
permafrost, the arid valleys of Antarctica and a dormant volcano in the
Chilean Andes. And they're hoping that Canada's ancient Sudbury crater also
has some secrets to reveal.

There are hundreds of "impact craters" on Mars, created by asteroids and
comets that struck the red planet millions or billions of years ago. By
studying Sudbury's own impact crater, a team of six scientists, including
Ottawa's Doreen Ames, are hoping they can learn how life might thrive after
a planet gets smacked with a resounding extraterrestrial blow.

The Sudbury crater was created 1.85 billion years ago, likely when a comet
slammed into the Earth. That impact started a chain of events that would
last for tens of thousands of years, said Ms. Ames, a geologist with Natural
Resources Canada.

The crater is 200 kilometres in diameter and about a kilometre deep. The
impact exposed the rich veins of ore that make Sudbury famous. It also
created hydrothermal systems much like those that still exist deep under the
sea and near volcanoes.

When an asteroid or comet hits the Earth, it melts rock and produces a
red-hot sheet of lava. Over thousands of years, a muddy lake forms.

Like a pot of boiling water, a hydrothermal system has heat coming from
below -- the magma under the ground. The system also produces convection,
pulling water down through the rocks and generating nutrients.

It is a warm habitat -- even scalding. But some parts of the hydrothermal
system would be cool enough to support primitive life forms such as

Scientists are searching for fossil evidence of bacteria in rock samples. If
they find that evidence in the Sudbury crater, it would help point NASA in
the right direction when it sends a robot rover to the surface of Mars.

"It's a good place to look for potential life," said Ms. Ames, who has been
studying the Sudbury crater for about 10 years. "Mars went through an early
bombardment. There are hundreds if not thousands of craters on Mars. Some
are larger than the ones on Earth."

It is unlikely that hydrothermals provided the conditions for the genesis of
life, said project leader Kevin Pope. "We're not really looking for the
origin of life, although some people have suggested that hydrothermals might
have been candidates for that the happen," he said.

Impacts may not have created life, but they did change it. Mass extinctions
are associated with impacts, including the one that created Mexico's
Chicxulub crater 65 million years ago and wiped out the dinosaurs.

On the other hand, impacts may not have been a bad thing for microbes. There
was primitive life on Earth long before the comet hit Sudbury. The impact
may have affected its evolutionary path.

"You take a big right angle turn after an impact," said Ms. Ames.

The Sudbury crater is one of the largest craters on Earth. It is also the
best-preserved, although it has been eroded over millions of years, layers
of sediment have been deposited on top of it and tectonic forces have
moulded it like plasticine, distorting its original round shape until it now
appears to be an oval with one flattened side.

Still, the Sudbury crater is better exposed than Chicxulub, which is covered
by other rocks. It is also better preserved than the Vredefort crater in
South Africa, which is so badly eroded there's hardly anything left.

Craters on Earth have been buried and distorted by tectonic activity. On
Mars, the craters are still very clearly defined, said Mr. Pope.

The project is only about a year-and-a-half old. But so far, scientists have
been disappointed by what they've seen -- or not -- under the microscope. So
far, they have found no fossil evidence of ancient bacteria.

But researchers have to get a better idea of where to look, said Mr. Pope.
"First, you have to identify parts of the hydrothermal system that would be
cool enough to have have life in them."

© Copyright 2002 The Ottawa Citizen



>From Jonathan Tate <

Prior to giving the keynote speech at the MPAPW 2002 conference in Terre
Haute, Indiana in September I requested an update from the BNSC on the
government's progress on the implementation of the NEO Task Force report
recommendations. Their reply is as follows:
"While not committed to another formal update we are planning to pull
together one for around the end of the year (at least for internal use). In
the mean time you are probably as aware of what is ongoing as anyone.
However, the following is work in progress.
OECD Global Science Forum approved activity on NEOs last meeting and
worldwide teleconference in June during the GSF meeting in Paris. Paul
Murdin chairs the Steering Committee. Plans for the workshop proceeding,
Paul may be able to add something.  Expect to report back to next GSF
meeting at the end of January 2003.
In following up the recommendations of the UN world space conference
UNISPACE III and its Vienna Declaration a number of action teams were
formed. There is an action Team 14 on NEOs this is run by the UK and is
building up a following of developing as well as developed countries.  It is
early days for the group and it is working more at the level of bringing in
new people or countries who have little current understanding the IAU,
COSPAR, ESA, ECSS, Spaceguard Foundation (Carusi) and others are also
members of the AT and supporting the work.  I have been in touch with most
of the contacts for those organisations and following the GSF
recommendations next year we plan to then increase the level of activity of
the AT 14.  The group will meet again in February 2003 in Vienna.
The suitability of the INT and JKT telescopes has been successfully
demonstrated during recent tests, funding and related issues now need to be
The last sentence probably rings the death knell for this idea! It could
have developed into a "do" rather than a "talk".
The National Near Earth Object Information Centre website, on its
"Frequently Asked Questions" (FAQ) page currently contains the following
little "gem":
Question - Are all the Task Force's recommendations being implemented?
Answer - Half of the Task Force's recommendations have already been
implemented or are currently being implemented. Most of the other
recommendations are broad recommendations of principle, and further studies
are being undertaken to identify the best way to implement them in practice.
This is more than somewhat misleading, if not downright wrong. A query to
the Webmaster explaining this has elicited no response so far. 
In the absence of any other information from the BNSC my assessment of
current progress on the Task Force recommendations is as follows:

Description Politically Correct

Complete Triumph!
See - we're at least doing something.

In Progress
"Active discussions are underway"
Talk but no action A comfortable compromise for HMG
Under Discussion 
"Further studies are being undertaken to identify the best way to implement this in practice."
Nothing is happening, and  there are no plans to do anything.

Survey and discovery of Near Earth Objects
Recommendation 1 - We recommend that the Government should seek partners,
preferably in Europe, to build in the southern hemisphere an advanced new 3
metre-class survey telescope for surveying substantially smaller objects
than those now systematically observed by other telescopes.  The telescope
should be dedicated to work on Near Earth Objects and be located on an
appropriate site. 
Under Discussion
Recommendation 2 - We recommend that arrangements be made for observational
data obtained for other purposes by wide-field facilities, such as the new
British VISTA telescope, to be searched for Near Earth Objects on a nightly
Under Discussion
Recommendation 3 - We recommend that the Government draw the attention of
the European Space Agency to the particular role that GAIA, one of its
future missions, could play in surveying the sky for Near Earth Objects.
The potential in GAIA, and in other space missions such as NASA's SIRTF and
the European Space Agency's BepiColombo, for Near Earth Object research
should be considered as a factor in defining the missions and in scheduling
their completion. 
In Progress
Accurate orbit determination
Recommendation 4 - We recommend that the 1 metre Johannes Kapteyn Telescope
on La Palma, in which the United Kingdom is a partner, be dedicated to
follow-up observations of Near Earth Objects. 
Under Discussion
Composition and gross properties
Recommendation 5 - We recommend that negotiations take place with the
partners with whom the United Kingdom shares suitable telescopes to
establish an arrangement for small amounts of time to be provided under
appropriate financial terms for spectroscopic follow-up of Near Earth
In Progress
Recommendation 6 - We recommend that the Government explore, with
like-minded countries, the case for mounting a number of coordinated space
rendezvous missions based on relatively inexpensive microsatellites, each to
visit a different type of Near Earth Object to establish its detailed
characteristics.  Under Discussion
Coordination of astronomical observations
Recommendation 7 - We recommend that the Government - together with other
governments, the International Astronomical Union and other interested
parties seek ways of putting the governance and funding of the Minor Planet
Center on a robust international footing, including the Center's links to
executive agencies if a potential threat were found. 
Under Discussion
Studies of impacts and environmental and social effects
Recommendation 8 - We recommend that the Government should help promote
multi-disciplinary studies of the consequences of impacts from Near Earth
Objects on the Earth in British and European institutions concerned,
including the Research Councils, universities and the European Science
Under Discussion
Mitigation possibilities
Recommendation 9 - We recommend that the Government, with other governments,
set in hand studies to look into the practical possibilities of mitigating
the results of impact and deflecting incoming objects. 
Under Discussion
Organisation internationally
Recommendation 10 - We recommend that the Government urgently seek with
other governments and international bodies (in particular the International
Astronomical Union) to establish a forum for open discussion of the
scientific aspects of Near Earth Objects, and a forum for international
action.  Preferably these should be brought together in an international
body.  It might have some analogy with the Intergovernmental Panel on
Climate Change, thereby covering science, impacts, and mitigation. 
Under Discussion
Organisation in Europe
Recommendation 11 - We recommend that the Government discuss with
like-minded European governments how Europe could best contribute to
international efforts to cope with Near Earth Objects, coordinate activities
in Europe, and work towards becoming a partner with the United States, with
complementary roles in specific areas.  We recommend that the European Space
Agency and the European Southern Observatory, with the European Union and
the European Science Foundation, work out a strategy for this purpose in
time for discussion at the ministerial meeting of the European Space Agency
in 2001.  Under Discussion
Organisation in United Kingdom
Recommendation 12 - We recommend that the Government appoint a single
department to take the lead for coordination and conduct of policy on Near
Earth Objects, supported by the necessary inter-departmental machinery. 
British National Centre for Near Earth Objects
Recommendation 13 - We recommend that a British Centre for Near Earth
Objects be set up whose mission would be to promote and coordinate work on
the subject in Britain; to provide an advisory service to the Government,
other relevant authorities, the public and the media, and to facilitate
British involvement in international activities.  In doing so it would call
on the Research Councils involved, in particular the Particle Physics and
Astronomy Research Council and the Natural Environment Research Council, and
on universities, observatories and other bodies concerned in Britain. 
Under Discussion
Recommendation 14 - We recommend that one of the most important functions of
a British Centre for Near Earth Objects be to provide a public service which
would give balanced information in clear, direct and comprehensible language
as need might arise.  Such a service must respond to very different
audiences: on the one hand Parliament, the general public and the media; and
on the other the academic, scientific and environmental communities.  In all
of this, full use should be made of the Internet.  As a first step, the Task
Force recommends that a feasibility study be established to determine the
functions, terms of reference and funding for such a Centre. 
In Progress
Therefore, from an outsider's point of view, after two years the final score
reads something like this:
q       Out Tray (done)                      -  1
q       Pending Tray (still talking) -  3
q       Still undisturbed in the In Tray        - 10
To their credit, the BNSC has had success in persuading the OECD, and to
some extent ESA, to take the NEO issue to heart. I await the already delayed
report with interest. It is worthy of note though that a number of countries
have already stated that their response to the NEO hazard will be governed
by the OECD report (Australia and Sweden being the most notable). However
the UK seems to have lost the plot regarding the OECD GSF WG on NEOs.
Instead of being held in the UK the January conference is being organised in
Italy, which is the clear European leader on NEO studies.
However, the bottom line is that, when asked by taxpayers what the
government is actually doing, the honest answer has to be "next to nothing".
Statements from ministers and senior officials suggesting that the UK is
taking a world lead in the NEO issue simply are not true. Other countries,
such as the Czech Republic, Croatia, Uruguay, Italy and others have a much
better claim, as there are active programmes there. How many asteroids have
been discovered by BNSC or PPARC funded programmes in the past 5 years?
None. How many asteroid orbits have been calculated by BNSC or PPARC funded
astronomers in a similar period?  None.
To quote Duncan Steel, "the bottom line is that, as far as NEOs are
concerned, the ball has been dropped, and Britain is subject to scorn around
the world."
The Spaceguard Centre

>From Bob Kobres <>

Don't worry Ed, everyone is not forgetting about comets. Just last weekend
Mike Baillie gave a well argued paper at a Celtic studies conference (Tionól
2002) to hopefully enlist these scholars in the consideration of social
aspects possibly caused by a close encounter with a comet.  An abstract of
Mike's paper can be read here:
The actual paper: "Tree-ring chronologies, and stories from myth" is here:

Deep Impact did not get nixed and there is an increasing understanding of
just how much dust and debris these objects shed due to improving detection
equipment and cooperating comets.  See:

It's looking like we will know a whole lot more of what to expect when the
next encounter occurs!

BTW: The odds of an impact event have not changed at all! They are precisely
the same as they were around forty years ago when it began to dawn on some
individuals that we were in possession of tools that might be used to
mitigate such odds. There have been considerably more individuals involved
in attempts to divine this mystical number however--it's pretty important,
sort of like knowing the number of angels that can dance on the head of a
pin. ;^)   (3)

Might find some interesting books here:

Hot money:

More-oil war?:


MODERATOR'S NOTE: I'm afraid I have to disagree with Bob's belittling of
impact rate estimates. Decades of astronomical progress and an advanced
understanding of the lunar and terrestrial crater records have given us a
much improved understanding of the rate of impacts (despite our limted
konwledge about the frequency of cometary impacts). The attempt to calculate
the terrestrial impact rate - on the basis of the current NEO flux and an
analysis of terrestrial and lunar impact craters - is not an endeavour to
"divine a mystical number" but a vital instrument to base our response to
the impact hazard on a reliable basis - and to give us peace of mind.
Without the possibility to put the risk of impact into a realistic
perspective, we would be faced with terrifying apocalyptic fear-mongering.

ARCHIVER'S NOTE: Perhaps I should have said 'determine this ambiguous number'
but my point is that the actual odds of being affected by an impact event cannot change
without our physically altering the conditions that would allow an impact to occur.
The only thing that such reckonings do is vary the perception of risk for the public and
policy makers, as the recent spate of headlines suggest.

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