PLEASE NOTE:


*

CCNet 57/2003 - 17 July 2003
----------------------------

"We show that the separated-fragments model predicts the total
atmospheric disruption of much larger stony bodies than
previously thought. In addition, our data set of >1,000 simulated
impacts, combined with the known pre-atmospheric flux of asteroids
with diameters less than 1 km, elucidates the flux of small bolides
at the Earth's surface. We estimate that bodies >220 m in diameter
will impact every 170,000 years."
    --P.A Bland and N.A Artemieva, Nature, 17 July 2003


"The model has implication not just for land-based impacts, but also
splashdowns in the ocean that can trigger devastating tsunamis. An
airburst is not likely to generate much of a tsunami, possibly lowering
that risk compared to what scientists had figured. The results suggest
rocks about 720 feet across (220 meters) are likely to actually hit
the surface every 170,000 years or so. Some previous research has
suggested a frequency of every 4,000 years or less."
    --Rob Britt, Space.com, 16 July 2003


(1) FEWER EARTHBOND ASTEROIDS WILL HIT HOME
 
(2) EFFICIENT DISRUPTION OF SMALL ASTEROIDS BY EARTH'S ATMOSPHERE

(3) STUDY: ASTEROIDS LESS LIKELY TO HIT EARTH

(4) FEWER ASTEROIDS THAN EXPECTED LIKELY TO HIT EARTH
 
(5) SMALL STONY ASTEROIDS WILL EXPLODE AND NOT HIT EARTH, STUDY SHOWS

(6) ASTEROID HUNTERS DISCOVER NEAR-EARTH OBJECT WITH NEW CAMERA
 
(7) TRANS-ALABAMA SUPERBOLIDE OF 5 DECEMBER 1999

(8) TUNGUSKA-CLASS IMPACTS

(9) REMEMBERING OUR WAKE-UP CALLS


=============
(1) FEWER EARTHBOND ASTEROIDS WILL HIT HOME
 
Ron Baalke - Near Earth Object Program <info@jpl.nasa.gov>

Contact: Judith H Moore
j.h.moore@imperial.ac.uk
44-0-20-7594 6702
Imperial College of Science, Technology and Medicine

July 16, 2003

Fewer Earthbound asteroids will hit home

Scientists say pancake model of asteroid impact won't stick

Scientists report in Nature today that significantly fewer asteroids
could hit the Earth's surface than previously reckoned.

Researchers from Imperial College London and the Russian Academy of
Sciences have built a computer simulation that predicts whether
asteroids with a diameter up to one kilometre (km) will explode in
the atmosphere or hit the surface.

The results indicate that asteroids with a diameter greater than 200
metres (the length of two football pitches) will hit the surface
approximately once every 160,000 years - way down on previous estimates
of impacts every 2,500 years.

The findings also predict that many more asteroids blow up in the
atmosphere than previous estimates, which means the hazard posed by
impact-generated tidal waves or tsunamis is lower than previous
predictions. The researchers suggest that proposals to extend monitoring
of Near Earth Objects (NEO) to include much smaller objects should be
reviewed.

Dr Phil Bland of Imperial's Department of Earth Science and Engineering
and a Royal Society University Research Fellow, said:

"There is overwhelming evidence that impacts from space have caused
catastrophes for life on Earth in the past, and will do so again.

"On the Moon it's easier to track the number, frequency and size of
collisions because there is no atmosphere, so everything hits the
surface. On Earth the atmosphere acts like a screen and geological
activity erodes many craters too.

"Massive impacts of the type thought to have wiped out the dinosaurs
leave an indelible print on the Earth but we have not been able to
accurately document the effect of smaller impacts. Now, we have a handle
on the size of 'rock' we really need to worry about and how well the
Earth's atmosphere protects us."

When small asteroids hit the atmosphere the two forces collide like two
objects smashing together, which often breaks the asteroid into
fragments. Until now, scientists have relied on the 'pancake' model of
asteroid impact to calculate whether the asteroid will explode in the
atmosphere. This treats the cascade of fragments as a single continuous
liquid that spreads out over a larger area - to form a 'pancake'. But a
new model known as the 'separate fragment' (SF) model, which was
developed by co-author of the study, Dr Natalya Artemieva of the Russian
Academy of Science, has challenged this approach.

"While the pancake model can accurately predict the height from the
Earth's surface at which the asteroid will break up, it doesn't give an
accurate picture of how the asteroid will impact," explains Dr
Bland. "The SF model tracks the individual forces acting on each
fragment as it descends through the atmosphere."

To create a more accurate model of how asteroids interact with the
atmosphere the researchers ran more than 1,000 simulations using both
models. Objects made of either iron or stone, known as 'impactors', were
used to reflect the composition of asteroids and experiments were run
with varying diameters up to 1 km.

The researchers found the number of impacts for iron impactors were
comparable using both models. For stone the pancake model significantly
overestimated the survivability rate across the range used.

The SF simulations also allowed the researchers to define the different
styles of fragmentation and impact rates for iron and stone, which
correspond closely with crater records and meteorite data.

"Our data show that over most of the size range we investigated stony
asteroids need to be 1,000 times bigger than the iron ones to make a
similar sized crater. Much larger objects are disrupted in the
atmosphere than previously thought.

"But we are not out of the woods yet," added Dr Bland "asteroids that
fragment in the atmosphere still pose a significant threat to human
life."

Dr Phil Bland is a member of the Meteorite and Impact Group that
includes scientists from Imperial College London and the Natural History
Museum.

                                     ###

Notes to editors

Publication: Nature (17 July 2003)
Title: "Efficient disruption of small steroids by Earth's atmosphere"
Authors: P.A Bland (1) and N.A Artemieva (2)

(1) Department of Earth Science and Engineering, Exhibition Road,
Imperial College London, SW7 2AZ, Uk
(2) Institute for Dynamics of Geospheres, Russian Academy of Sciences,
Leninsky Prospect 38/6 Moscow, 117939 Russia.

==================
(2) EFFICIENT DISRUPTION OF SMALL ASTEROIDS BY EARTH'S ATMOSPHERE

Nature 424, 288 - 291 (17 July 2003); doi:10.1038/nature01757 
 
Efficient disruption of small asteroids by Earth's atmosphere
P. A. BLAND* AND N. A. ARTEMIEVA?

* Department of Earth Science and Engineering, Exhibition Road, Imperial
College London, South Kensington Campus, London SW7 2AZ, UK
? Institute for Dynamics of Geospheres, Russian Academy of Sciences,
Leninsky Prospect 38/6, Moscow, 117939 Russia

Correspondence and requests for materials should be addressed to P.A.B.
(p.a.bland@imperial.ac.uk).

Accurate modelling of the interaction between the atmosphere and an
incoming bolide is a complex task, but crucial to determining the
fraction of small asteroids that actually hit the Earth's surface. Most
semi-analytical approaches have simplified the problem by considering
the impactor as a strengthless liquid-like object ('pancake' models),
but recently a more realistic model has been developed that calculates
motion, aerodynamic loading and ablation for each separate particle or
fragment in a disrupted impactor. Here we report the results of a large
number of simulations in which we use both models to develop a
statistical picture of atmosphere-bolide interaction for iron and stony
objects with initial diameters up to 1 km. We show that the
separated-fragments model predicts the total atmospheric disruption of
much larger stony bodies than previously thought. In addition, our data
set of >1,000 simulated impacts, combined with the known pre-atmospheric
flux of asteroids with diameters less than 1 km, elucidates the flux of
small bolides at the Earth's surface. We estimate that bodies >220 m in
diameter will impact every 170,000 years.

2003 Nature Publishing Group

===============
(3) STUDY: ASTEROIDS LESS LIKELY TO HIT EARTH

Scripps Howard News Service, 16 July 2003
http://www.knoxstudio.com/shns/story.cfm?pk=ASTEROIDS-07-16-03&cat=AS

By LEE BOWMAN

- The odds of a city-block-sized asteroid hitting Earth may be much
smaller than once thought, according to a new computer simulation
developed by British and Russian scientists.

They calculate that asteroids with a diameter greater than 200 meters
will hit ground about once every 160,000 years - considerably less than
previous estimates that predicted such impacts occurring as often as
once every 2,500 years.

The researchers say the computer model shows that many more asteroids
will blow up in the atmosphere than had been thought. This means that
the threat from impact-generated tidal waves is also lower than
previously predicted, although there is ample evidence that such "air
bursts" can still release tremendous force toward the ground.

The June 1908 burst of a meteor or small comet over a remote part of
Siberia flattened and burned trees over a 4,000-square-mile area, and
released the energy equal to 2,000 Hiroshima atomic bombs. But the
impact left no crater and little material from whatever disintegrated
some five miles from the ground.

"There is overwhelming evidence that impacts from space have caused
catastrophes for life on Earth in the past and will do so again. But we
have not been able to accurately document the effects of smaller
impacts," said Phil Bland, a researcher in earth science and engineering
at the Imperial College in London and co-author of the study published
Thursday in the journal Nature.

While imprints of huge craters formed millions of years ago can still be
seen, evidence of smaller craters are harder to come by, Bland said.
"The atmosphere acts like a screen - and geological activity erodes many
craters, too.

"Now we have a handle on the size of 'rock' we really need to worry
about and how well the Earth's atmosphere protects us."

When a small asteroid hits the atmosphere, the collision often breaks it
into many fragments. Until now, scientists have relied on a "pancake"
model to predict what happens next. This treats the cascade of fragments
as a single continuous liquid that spreads out over a larger area, just
like pancake batter on a griddle.

But Natalya Artemieva of the Russian Academy of Science, the study's
other author, came up with the more complex "separate fragment" model to
consider what might happen to each individual fragment as it comes down.

"While the pancake model can accurately predict the height from the
Earth's surface at which the asteroid will break up, it doesn't give an
accurate picture of how the asteroid will impact," Bland said. "The new
model tracks the forces acting on each individual fragment."

To create a more accurate picture of what different types and sizes of
asteroids might do, the researchers ran more than 1,000 simulations of
both models, using impactors made up of either stone or iron, and
ranging in size up to 1 kilometer (six-tenths of a mile) across.

They found that the number of impacts from iron asteroids was about the
same using both models. But for stone asteroids, the pancake model
significantly overestimated how much material would hit the ground.
Simulations using the new model matched up much better with records from
known craters and meteorite samples.

"Over most of the size range we investigated, stony asteroids need to be
1,000 times bigger than the iron ones to make a similar-sized crater.
Much larger objects are disrupted in the atmosphere than previously
thought," Bland said.

"We are not out of the woods yet," Bland cautioned. "Asteroids that
fragment in the atmosphere still pose a significant threat to human
life."

On the Net: www.nature.com

Copyright 2003, Scripps Howard News Service

=============
(4) FEWER ASTEROIDS THAN EXPECTED LIKELY TO HIT EARTH
 
Reuters, 16 July 2003
http://www.reuters.com/newsArticle.jhtml;jsessionid=OZQADKFM0E31KCRBAEOCFFA?type=scienceNews&storyID=3101786

LONDON (Reuters) - The number of asteroids likely to collide with Earth
and cause huge damage is smaller than expected, scientists said on
Wednesday.

A computer simulation developed by scientists in Britain and Russia
shows that asteroids with a diameter of 200 meters (yards) will hit
the Earth's surface about once every 160,000 years, instead of every
2,500 years.

"Fewer asteroids (than expected) will make it to the surface of the
Earth," said Dr Phil Bland, of Imperial College in London.

If a massive near-Earth object measuring more than a kilometer (0.6
mile) in diameter slammed into the planet it would cause global
devastation and kill an estimated quarter of the world's population.

But scientists believe an event of that size would only occur about
every 700,000 years.

Hollywood films such as "Deep Impact" and "Armageddon" raised public
awareness of the threat of near-Earth objects and have prompted calls
for early warning systems.

Bland and Natalia Artemieva of the Russian Academy of Sciences used the
computer simulation and existing data on impacts to reach their
estimates which are reported in the science journal Nature.

A collision with the Earth 65 million years ago is thought to have wiped
out the dinosaurs by changing the global climate. A smaller asteroid is
credited with leveling 1,000 sq km (386 sq miles) of Siberian forest in
1908.

Copyright 2003, Reuters

===================
(5) SMALL STONY ASTEROIDS WILL EXPLODE AND NOT HIT EARTH, STUDY SHOWS

Space.com, 16 July 2003
http://www.space.com/scienceastronomy/asteroid_breakup_030716.html

By Robert Roy Britt

When asteroids fall through Earth's atmosphere, a variety of things can
happen. Large iron-heavy space rocks are almost sure to slam into the
planet. Their stony cousins, however, can't take the pressure and are
more likely to explode above the surface.

Either outcome can be dismal. But the consequences vary.

So scientists who study the potential threat of asteroids would like to
know more about which types and sizes of asteroids break apart and which
hold together. A new computer model helps to quantify whether an
asteroid composed mostly of stone will survive to create a crater or
not.

A stony space rock must be about the size of two football fields, or 720
feet (220 meters) in diameter, to endure the thickening atmosphere and
slam into the planet, according to the study, led by Philip Bland of the
Department of Earth Science and Engineering at Imperial College London.

"Stones of that size are just at the border where they're going to reach
the surface -- a bit lower density and strength and it'll be a low-level
air burst, a bit higher and it'll hit as a load of fragments and you'll
get a crater," said Bland, who is also a Royal Society Research Fellow.

The distinction would mean little to a person on the ground.

Two ways to destroy a city

"An airburst would be a blast somewhere in the region of 500-600
megatons," Bland said in an e-mail interview. "As a comparison, the
biggest-ever nuclear test was about 50 megatons."

A presumed airburst in 1908, over a remote region of Siberia called
Tunguska, flattened some 800 square miles (2,000 square kilometers) of
forest. The object is estimated to have been just 260 feet wide (80
meters). Bland said the event was probably equal to about 10 megatons.

"If most of it made it to the ground you might actually be a bit better
off, because the damage would be a little more localized," he said. "A
lot of energy would still get dumped in the atmosphere, but you'd
probably also have a ragged crater, or crater field, extending over
several kilometers, with the surrounding region flattened by the blast."

Smaller stony asteroids, say those the size of the car, enter the
atmosphere more frequently but typically disintegrate higher up and
cause no damage. In fact, as many as two or three dozen objects ranging
from the size of a television to a studio apartment explode in the
atmosphere every year, according to data from U.S. military satellites.

Separate research in recent years has shown that stony asteroids are
often mere rubble piles, somewhat loose agglomerations of material that
may have been shattered in previous collisions but remain
gravitationally bound.

Pieces and parts

The new computer model is detailed in the July 17 issue of the journal
Nature. It was created with the help of Natalia Artemieva at the Russian
Academy of Sciences.

Previous models treated the cascade of fragments from a disintegrating
asteroid as a continuous liquid "pancake." The new model tracks
individual forces acting on each fragment as the bunch descends.

The researchers can plug in asteroid size, density, strength, speed and
entry angle at the top of the atmosphere. With "reasonable confidence" a
computer program then details how that rock should behave in the air and
what will happen at the surface.

The model has implication not just for land-based impacts, but also
splashdowns in the ocean that can trigger devastating tsunamis. An
airburst is not likely to generate much of a tsunami, possibly lowering
that risk compared to what scientists had figured.

The results suggest rocks about 720 feet across (220 meters) are likely
to actually hit the surface every 170,000 years or so. Some previous
research has suggested a frequency of every 4,000 years or less.

Looking back

The model can also "hindcast" what sort of rock might have generated a
certain known crater.

"You see a crater field on Mars, we can tell you what sort of object
caused it," Bland said.

In fact, he and Artemieva have done just that. In their most recent
tests, which are not discussed in the Nature paper, they plugged in the
atmospheric details of Mars, as well as Venus, and hurled some
hypothetical space rocks at those planets.

"The simulated crater fields that the model produces look almost exactly
like the real thing," Bland said.

For now, the model does not handle very large asteroids, those that
could cause widespread regional or even global damage, though Bland said
the flaw may be fixable. He is careful to point out that computer models
do not provide solid proof for what might happen.

"There are still a lot of unknowns in this," he said.

Copyright 2003, Space.com

================
(6) ASTEROID HUNTERS DISCOVER NEAR-EARTH OBJECT WITH NEW CAMERA
 
NASA Jet Propulsion Laboratory <info@jpl.nasa.gov>

<http://www.jpl.nasa.gov> NASA News

DC Agle (818) 393-9011

NEWS RELEASE: 2003-099                            July 15, 2003

Asteroid Hunters Discover Near-Earth Object with New Camera

NASA astronomers in pursuit of near-Earth asteroids have already made a
discovery with the newly installed Quasar Equatorial Survey, or 'Quest,'
camera mounted in mid-April on Palomar Mountain's 1.2-meter (48-inch)
Oschin telescope.

"The Quest camera is still undergoing commissioning trials," said Dr.
Steven Pravdo, project manager for the Near-Earth Asteroid Tracking
Project at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "But
that doesn't mean we can't do some real science in the meantime. What we
found was a near-Earth asteroid, estimated to be about 250 meters (820
feet) in size."

The detection of the near-Earth object, 2003 NL7, occurred on the
evening of July 8. It has been confirmed by follow-up measurements from
three other observatories and subsequently certified by the official
clearinghouse of the solar system's smaller inhabitants, the Minor
Planet Center. While 2003 NL7 has been labeled a near-Earth asteroid, it
is considered non-hazardous, with a 2.97-year orbit of the Sun in which
its closest approach to Earth's orbit is about 25.1 million kilometers
(15.6 million miles).

The Quest camera is being developed as a multi-purpose instrument by
Yale and Indiana universities with Dr. Charles Baltay, chairman of
Yale's physics department, as the principal investigator. It is
designed for use in detecting and characterizing quasars, near-Earth
asteroids, trans-Neptunian objects, supernovas, and a large variety of
other astrophysical phenomena, by scientists from Yale, JPL and the
California Institute of Technology in Pasadena. The complex camera
consists of 112 electronic chips known as charged coupled devices (CCDs)
arranged over the Oschin telescope's focal plane. This gives the Quest
camera 161-megapixel capability. By comparison, a good store-bought
digital camera would probably be in the four-megapixel range.

"When Quest becomes operational, it will be a significant advancement
for the Near-Earth Asteroid Tracking team," said Dr. Raymond Bambery,
the Near-Earth Asteroid Tracking Project's principal investigator. "We
expect the new camera to increase the efficiency of detection of
near-Earth asteroids by some 3 to 4 times that of the camera it
replaced. This will make a major contribution to NASA's goal of
discovering more than 90 percent of near-Earth objects that are greater
that 1 kilometer (.62 mile) in diameter by 2008."

The Near-Earth Asteroid Tracking System is managed by JPL for NASA's
Office of Space Science, Washington, D.C. JPL is a division of Caltech.
More information on the Near-Earth Asteroid Tracking Program is
available at http://neat.jpl.nasa.gov/ <http://neat.jpl.nasa.gov/>  .

============LETTERS==============

(7) TRANS-ALABAMA SUPERBOLIDE OF 5 DECEMBER 1999

David T. King, Jr. <kingdat@auburn.edu>

Dear Benny

The CCNet of 7 Dec 1999 included an item (#3) about a bright
fireball over Alabama USA.  Since this event, we have been collecting
and studying eyewitness accounts and other information and have
published in the 8 July 2003 issue of EOS, Transactions of the American
Geophysical Union a detailed account of this event titled "Trans-Alabama
Superbolide of 5 December 1999" by David T. King, Jr. and Lucille W.
Petruny. Our investigation shows that eyewitness accounts contradict the
Department of Defense (DoD) information about flight path and trajectory
contained in the DoD news release on this event (dated 16 March 2000).
Further, we establish the simultaneous timing of ignition of ground
fires and superbolide passage. On the internet, there are numerous
credible reports of simultaneous fires like the ones in Alabama on 5
December 1999, which defy easy explanation (e.g., the central
Pennsylvania daylight superbolide of 23 July 2001;
http://www.southpole.com/headlines/y2001/ast27jul_1.htm;
http://www.cnn.com/2001/TECH/space/07/24/fire.ball/index.html), the Bayt
Eides, Jordan, superbolide of 18 April 2001
(http://www.jas.org.jo/mett.html), and the
England, Arkansas, superbolide of 9 March 2000
(http://www.ipa.net/~historyhides/fireball/page01.html). [Be warned this
latter web page has a government conspiracy/UFO bent and one must sift
through that type material for pertinent pictures and accounts of the
fires.]  If any reader is a member of the AGU, you can see our article
at http://www.agu.org/cgi-bin/membership_services/login.cgi by logging
in.
--
________________________________________
David T. King, Jr., Professor, Dept. of Geology
Auburn University, Auburn, AL 36849-5305 USA
VOICE  334 844-4882, -4282 FAX  334 844-4486

===================
(8) TUNGUSKA-CLASS IMPACTS

Jens Kieffer-Olsen <dstdba@post4.tele.dk>

Dear Benny Peiser,

Further to my previous comment on Andrea Milani's article I have perused
the article he referenced (1) Harris, A. Bull. Amer. Astron. Soc. 34,
(2002) which is available on the Internet
http://www.aas.org/publications/baas/v34n3/dps2002/6.htm
 
The article states:

"The population of Near-Earth asteroids down to ~ 1 km diameter
(absolute magnitude H = 18) is reasonably well determined. However,
in the size range of ``Tunguska event" NEAs (diameter ~ 50-75 m),
estimates of population, or equivalently of impact frequency, range
from once per couple hundred years to once per 10,000 years. The fact
that one such event occurred just a century ago argues for a population
closer to the former value."

This is indeed my major objection to the low frequency estimate,
since the probability of mankind actually detecting a Tunguska-class
impact on Earth over the past two centuries was likely as low as 1 in
three. Were the impact frequency only once per millennium, the fact
that an event happened and was detected took place at odds 1 to 15 or
less. For have the annals been systematically searched for reports of
atmospheric phenomena like those that accompanied the 1908 impact? 

Alan Harris arrives at his low frequency estimate in the following
manner:

"The LINEAR survey has now discovered ~ 30 NEAs in the "Tunguska"
size range (H ~ 24.0-24.5), thus a better estimate is possible. In
order to make such an estimate, one can calculate the expected fraction
of a synthetic population that should be detected given the known survey
parameters, and then inflate the detected number by dividing by that
fraction. However for small NEAs, the completion factor is a very
small number and a very large simulation is needed in order to obtain
even a few ``detections" in the model. I report here a new simulation
from which I have obtained the relative detection efficiency in the
size range 21.5 < H < 25.5. Taking the actual number of NEAs discovered
by LINEAR in 0.5 mag. bins of H, and dividing by the relative
completion factors, I obtain relative populations in each size bin. By
normalizing the populations to agree with the absolute population
estimates of Stuart (Science 294, 1691-1693, 2001) in his two smallest
size bins, I extend his curve from H = 22.5 to H = 25.5, spanning the
size range of "Tunguska" objects. I find a population of the order of
half a million objects in this size range (H ~ 24.0-24.5), corresponding
to an expected impact interval of the order of once per thousand years.
This estimate is uncertain by a factor of about 3, largely due to
uncertainty in the actual size of the Tunguska event and uncertanty in
relating absolute magnitude to size of object."

Whether this simulation based on observations of 30 NEAs in the
Tunguska-class is more meaningful than a consideration based on the
fact that an actual impact took place in 1908 is doubtful in my view.

Furthermore, as Harris mentions, uncertainty exists whether the 1908
impactor was actually a Tunguska-class object, as defined by the size
range 50-75 m.  Halving the diameter of an NEA almost increases its
frequency by one order of magnitude, so if the real Tunguska impactor
was only 30-40 m across, a similar event could soon occur again, even
if the results of Harris' simulation were confirmed based on
observations of a more substantial number of NEAs.

Yours sincerely
Jens Kieffer-Olsen, M.Sc.(Elec.Eng.)
Slagelse, Denmark

=================
(9) REMEMBERING OUR WAKE-UP CALLS
 
Andy Smith <astrosafe22000@yahoo.com>

Hello Benny and CCNet,

Today (16 July) marks the 9th anniversary of our big wake-up call...the
impacts of Shoemaker-Levy 9 on Jupiter. The whole World watched, that
week in 1994, as more than 20 giant rock bombs struck Jupiter...and we
had front-row seats because the Shoemakers and David Levy (with help
from Jim Scotti) had been given our invitation...many months earlier.

It was what we needed, to underscore the significance of our first
wake-up call...the near-miss of our planet by asteroid 1989 FC. These
two important events or signs caused us to embark on a decade of real
progress...thanks to the efforts of many volunteer and dedicated
specialists....and we have made a lot of progress.

We have increased our Near-Earth Object (NEO) discovery rate by more
than two orders-of-magnitude....and we are now developing the next
generation of asteroid hunting telescopes. We have completed many
successful research and development missions to asteroids and
comets...and we now feel some confidence in our ability to intercept
an in-bound NEO. We are not yet able to respond quickly, but we have
the needed technology...and we have tested it.

Several NEO offices have been opened, around the World....thanks to the
efforts of NASA, the Spaceguard Foundation, the Space Shield Foundation,
the Planetary Society, the Space Frontier organization, ProSpace, and
many other groups. We are still underfunded and we need a lot of
additional governmental support, but we are making progress and much of
that progress is due to the efforts of the people, themselves.

It was especially gratifying to note the recent open-letter to members
of the U.S. Congress, the United Nations and others, asking for more
help. This letter and the many others, sent by volunteers, to the
governments of many nations, underscore the important and continuing
roles of the public and of the friendly media. It has been our good
fortune to witness the beginning of a quest which will continue as long
as humans inhabit Earth...and we can take some pride in our performance,
thusfar. 

On this day, on the behalf of the millions of people who are at risk,
we thank all of you for your efforts and we wish you the best. We will
toast you all and our many pioneers and we will continue to pray that
we will be given the time we need to prepare to protect ourselves and
our beautiful planet.....and again, we will enjoy the Beethoven First.

Cheers

Andy Smith, International Planetary Protection Alliance
astrosafe22000@yahoo.com

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*

SPIN ON NEO IMPACT RATES

Duncan Steel <D.I.Steel@salford.ac.uk>

Dear Benny,

The various media reports carried in CCNet today concerning the Bland
and Artemieva letter in Nature provide for an interesting insight into
the attitude now beginning to prevail. Instead of the alarmist media
accounts a while back, the emphasis now is on playing down the impact
hazard. Maybe a middle ground will be reached eventually.

For example, two of the headlines you carry:
STUDY: ASTEROIDS LESS LIKELY TO HIT EARTH
FEWER ASTEROIDS THAN EXPECTED LIKELY TO HIT EARTH
 
Both are misleading. The paper in question deals only with what happens
to asteroids after they have entered the atmosphere (i.e. hit the Earth).
From the perspective of asteroid interaction with the atmosphere, almost
all the interaction occurs within an altitude above the solid ground that
is less than one percent of the radius of the planet. All such events are
dangerous in one way or another.

To show what I mean in terms of the spin content, try considering
the results of the paper in question from two other aspects:

(a) Let the asteroid entry be above a continent rather than the
ocean. An airburst causes more damage than a surface strike (one
reason why nuclear weapons have been targetted to detonate at altitude),
and asteroids in the size range in question tend to release their
energy at around the height that would cause destruction over the
maximum area. The fact that large tsunamis would not be produced by
small [50-200m] asteroids was, I think, already well-known, even if
they do reach the ocean surface intact. Thus one could argue that
this paper leads to an enhancement of the previously estimated
human risk, rather than a reduction. It all depends on the spin
one wants to apply.

(b) If these small asteroids are so unlikely to reach the ground
intact, then the terrestrial cratering rate, if naively interpreted,
would lead to a gross underestimate of the frequency of impacts in
the (say) 1 to 1000 Mton range. (In fact, all bar one of the known
terrestrial craters smaller than a mile across are the result
of metallic asteroids, as it states in the paper. This means that
there have been perhaps 30-40 times as many stony asteroids of similar
energy that have arrived over the same time period.)

This new paper by Bland and Artemieva is therefore a useful piece of
research work, although one should note that the results do not seem
markedly different to those derived by Hills and Goda ten years ago
(and I'd have to say that I differ from some of the extrapolations
made). In the end, although the arguments can be perennial over the
different models employed for investigating how an idealised asteroid
behaves as it enters the atmosphere, the limiting factor is knowledge
of the nature of the incoming object. We simply do not know very much
about the strength and other physical parameters of the bodies in
question, and that limits what is computable about how they will behave.

I also note that the next one to hit will be a singular object; that is,
statistical models are all very well, but it is the specifics of the
object next to arrive that we would like to know.

Finally, the later discussion in today's CCNet about the probability
of Tunguska-type events is somewhat misguided. The 1908 event occurred,
so that a posteriori its probability was unity. On its own, it tells
us nothing about the a priori probability of such occurrences. The
statement that Al Harris made about it - "The fact that one such event
occurred just a century ago argues for a population closer to the
former value" - really is about the most that can be made of it.

Regards,

Duncan Steel



CCCMENU CCC for 2003

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The content and opinions expressed on this Web page do not necessarily reflect the views of nor are they endorsed by the University of Georgia or the University System of Georgia.