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
--------------------------------------------------------------------
CCNet is a scholarly electronic network. To
subscribe/unsubscribe,
please contact the moderator Benny Peiser <b.j.peiser@livjm.ac.uk>.
Information circulated on this network is for scholarly and
educational
use only. The attached information may not be copied or
reproduced for
any other purposes without prior permission of the copyright
holders.
DISCLAIMER: The opinions, beliefs and viewpoints expressed in the
articles and texts and in other CCNet contributions do not
necessarily
reflect the opinions, beliefs and viewpoints of the moderator of
this
network.
--------------------------------------------------------------------
*
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