PLEASE NOTE:
*
CCNet, 31/2003 - 18 March 2003
------------------------------
"Given all life's worries, new evidence that asteroids
smaller than a kilometer in diameter won't generate catastrophic
tsunamis is welcome news, and not only for coast dwellers. It
will save taxpayers the cost of financing searches for small
Earth-approaching asteroids, a savings of billions of dollars,
Jay Melosh said."
--Lori Stiles, University of Arizona, 17 March 2003
"In round numbers about 1% of the world population is at
risk [from impact tsunami], considerably less than had been
estimated a decade ago. The drop follows primarily from the
reduction in the run-in of impact tsunami as dictated by their
shorter wavelength. Chesley's initial results do not take into
account shielding of coastal populations by reefs, barrier
islands, seawalls, and harbor constructions. Harris and Chesley
suggest that at least half of all coastal population is protected
by these barriers from short- wavelength tsunami, thus further
reducing the at-risk population (and associated annual equivalent
fatality rate) by another factor of two (or perhaps even more).
The resulting hazard, while still greater than the land-impact
risk from sub-kilometer NEAs, is less than has been previously
estimated."
--David Morrison, NEO News, 17 March 2003
(1) WORRIED ABOUT ASTEROID-OCEAN IMPACTS? DON'T SWEAT THE SMALL
STUFF
Lori Stiles <lstiles@u.arizona.edu>
(2) TSUNAMI HAZARD
David Morrison <dmorrison@arc.nasa.gov>
(3) VESTA IN VIRGO: A NAKED-EYE ASTEROID
Sky & Telescope, 16 March 2003
(4) CCNet ESSAY: THE BULLET THEORY - A NEW LINK BETWEEN
MASS EXTINCTION EVENTS
Michael Martin-Smith <lagrangia@lagrangia.karoo.co.uk>
===========
(1) WORRIED ABOUT ASTEROID-OCEAN IMPACTS? DON'T SWEAT THE SMALL
STUFF
>From Lori Stiles <lstiles@u.arizona.edu>
>From Lori Stiles, UA News Services, 520-621-1877
March 17, 2003
The idea that even small asteroids can create hazardous tsunamis
may at last be pretty well washed up.
Small asteroids do not make great ocean waves that will devastate
coastal areas for miles inland, according to both a recently
released 1968 U.S. Naval Research report on explosion-generated
tsunamis and terrestrial evidence.
University of Arizona planetary scientist H. Jay Melosh is
talking about it today at the 34th annual Lunar and Planetary
Science Conference in League City, Texas. His talk,
"Impact-Generated Tsunamis: an Over-Rated Hazard," is
part of the session, "Poking Holes: Terrestrial
Impacts."
---------------------
Contact Information
H. Jay Melosh
520-621-2806
jmelosh@lpl.arizona.edu
----------------------
Given all life's worries, new evidence that asteroids smaller
than a kilometer in diameter wonąt generate catastrophic
tsunamis is welcome news, and not only for coast dwellers. It
will save taxpayers the cost of financing searches for small
Earth-approaching asteroids, a savings of
billions of dollars, Melosh said.
(The current NASA-funded effort to search and map truly hazardous
Earth-approaching asteroids those one kilometer or larger in
diameter is now half done and on track to be finished by the
end of the decade, Melosh noted. NASA funds NEAT, LINEAR and the
UA Spacewatch programs in this
effort.)
The idea that asteroids as small as 100 meters across pose a
serious threat to humanity because they create great, destructive
ocean waves, or tsunamis, every few hundred years was suggested
in 1993 at a UA-hosted asteroids hazards meeting in Tucson.
At that meeting, a distinguished Leiden Observatory
astrophysicist named J. Mayo Greenberg, who since has died,
countered that people living below sea level in the Netherlands
for the past millennium had not experienced such tsunamis every
250 years as the theory predicted, Melosh noted.
But scientists at the time either didn't follow up or they didn't
listen, Melosh added.
While on sabbatical in Amsterdam in 1996, Melosh checked with
Dutch geologists who had drilled to basement rock in the Rhine
River delta, a geologic record of the past 10,000 years. That
record shows only one large tsunami at 7,000 years ago, the Dutch
scientists said, but it coincides
perfectly in time to a giant landslide off the coast of Norway
and is not the result of an asteroid-ocean impact.
In addition, Melosh was highly skeptical of estimates that
project small asteroids will generate waves that grow to a
thousand meters or higher in a 4,000-meter deep ocean.
Concerned that such doubtful information was and is - being
used to justify proposed science projects, Melosh has argued that
the hazard of small asteroid-ocean impacts is greatly
exaggerated.
Melosh mentioned it at a seminar he gave at the Scripps
Institution of Oceanography a few years ago, which is where he
met tsunami expert William Van Dorn.
Van Dorn, who lives in San Diego, had been commissioned in 1968
by the U.S. Office of Naval Research to summarize several decades
of research into the hazard posed by waves generated by nuclear
explosions. The research included 1965-66 experiments that
measured wave run-up from blasts of up to 10,000 pounds of TNT in
Mono Lake, Calif.
The experiments indeed proved that wave run-up from explosion
waves produced either by bombs or bolides (meteors) is much
smaller relative to run-up of tsunami waves, Van Dorn said in the
report. "As most of the energy is dissipated before the
waves reach the shoreline, it is evident that no catastrophe of
damage by flooding can result from explosion waves as initially
feared," he concluded.
The discovery that explosion waves or large impact-generated
waves will break on the outer continental shelf and produce
little onshore damage is a phenomenon known in the defense
community as the "Van Dorn effect."
But Van Dorn was not authorized to release his 173-page report
when he and Melosh met in 1995.
Melosh, UA planetary sciences alumnus Bill Bottke of the
Southwest Research Institute and others agreed at a science
conference last September that they needed to find the report.
Bottke found the title - "Handbook of Explosion-Generated
Water Waves" - in a Google search.
Given a title, UA science librarian Lori Critz then discovered
that the report had been published and added to the University
California San Diego library collection in March 2002. Bottke
also tracked it down, and had the report by the time Melosh
requested it by interlibrary loan. Both made several photocopies.
Melosh said, "I since found out it was actually read into
the Congressional Record as part of the MX Missile
controversy."
BIOSKETCH: H. JAY MELOSH
Melosh, a professor in the UA planetary sciences department and
Lunar and Planetary Laboratory, is well known for his work in
theoretical geophysics and planetary surfaces. His principal
research interests are impact cratering, planetary tectonics, and
the physics of earthquakes and
landslides. His recent research has focused on studies of the
giant impact origin of the moon, the K/T boundary impact that
extinguished the dinosaurs, the ejection of rocks from their
parent bodies, and the breakup and collision of comet
Shoemaker-Levy 9 with Jupiter. Melosh also is active in
astrobiological studies that relate chiefly to the exchange of
microorganisms between the terrestrial planets. Melosh earned his
doctorate from Caltech in 1973 and joined the UA faculty in 1982.
He is on the 12-member science team for Deep Impact, a $279
million robotic mission that will become the first to penetrate
the surface of a comet when it smashes its camera-carrying copper
probe into Comet Tempel 1 on July 4, 2005.
============
(2) TSUNAMI HAZARD
>From David Morrison <dmorrison@arc.nasa.gov>
NEO News (03/17/03) Tsunami hazard
Dear friends and students of NEOs:
This edition of NEO news describes a workshop held on March 16 to
discuss the hazard due to tsunami from deep-water impacts by
sub-kilometer asteroids. The workshop developed a consensus on
the order of magnitude of this hazard, which is substantially
less than was estimated years ago. However, the group did not
support the position taken by Jay Melosh in a U Arizona Press
release today that tsunami from such small impacts do not pose
any hazard whatever.
David Morrison
============================================
SUMMARY OF TSUNAMI HAZARD WORKSHOP HOUSTON 03/16/03
by David Morrison
The tsunami hazard workshop was held to bring together impact
tsunami experts in an informal setting to resolve apparent
differences in their assessments of the magnitude of this hazard.
In particular, it provided the first opportunity for Jay Melosh
(U Arizona) and Steven Ward (UC Santa Cruz), the two main
proponents of these different estimates, to meet. A dozen others
attended also, with Bill Bottke (SW Research Institute) as
convener.
Background: Since the time of the original NASA Spaceguard Survey
Report (Morrison, 1992) is has been clear that there is a major
qualitative and quantitative difference in the hazard posed by
impacts that have global environmental consequences, as compared
with smaller impacts that produce only local or regional effects
(see also Chapman & Morrison, 1994: Impacts on the Earth by
Asteroids and Comets, Assessing the Hazard, Nature 367: 33-39).
The total risk (measured by equivalent annual fatalities) for all
impacts below the threshold for global disaster is at least a
factor of 100 below that associated with global disasters,
assuming that the threshold for global disaster is between 1 and
2 km diameter (roughly 1 million megatons; Toon et al.,
1997: Environmental Perturbations Caused by the Impacts of
Asteroids and Comets, Rev Geophys 35, 41-78). Land impacts in
particular are a minor hazard for impactors less than 1 km
diameter. However, there has been a general concern that tsunami
from deep-water marine impacts could contribute substantially to
the hazard for people living near coasts -- perhaps amounting to
tens of percent of the global population. If the hazard from
ocean impacts of asteroids between 200 and 1000 m is substantial,
then these ocean impacts are the dominant hazard from
"small" asteroids, those below the global threshold.
The tsunami hazard is then the primary motive for extending the
Spaceguard Survey to smaller asteroids. It was partly in response
to concerns about ocean impacts in the sub-kilometer range that
two studies by the US National Research Council recommended that
a survey be carried out for NEAs smaller than 1 km.
Two science working groups are now studying the hazard associated
with sub-kilometer impacts: a NASA Science Definition Team that
is preparing recommendations to the NASA Office of Space Science
(due in June) and a Science Working Group organized by National
Optical Astronomy Observatory that is studying many of the same
issues for the NSF. Both groups have agreed that the risk from
land impacts of sub-kilometer NEAs is very low. Tunguska-class
impacts occur on the land only once every 2-3 millennia, and land
impacts that are expected to kill thousands are rarer still. The
more important challenge is to evaluate the hazard from tsunami
generated by sub-kilometer NEAs.
It has become apparent in recent years, and especially from the
work of Steven Ward and Erik Asphaug (2000: Asteroid Impact
Tsunami, A Probabilistic Hazard Assessment, Icarus 145, 64-78),
that impact-induced tsunami are significantly different from
earthquake tsunami. The impact tsunami have shorter wavelengths,
comparable to the size of the impact cavity (a few km in
diameter). This places them intermediate in other respects as
well between the more familiar long-wavelength earthquake tsunami
and ordinary storm waves. Their shorter wavelength influences
both the likelihood that they will break before reaching shore
and the degree of run-in, which is comparable to the wavelength
as the wave approaches the shore (typically about 1 km). Ward and
Asphaug have modeled all these effects using standard linear
seismic theory adjusted for the particular properties of impact
tsunami. Others, particularly Galen Gisler and Don Korycansky,
have modeled the impact process and initial wave generation using
more advanced hydrocodes. All of this work is in general
agreement, at the factor-of-two level.
Alan Harris (Space Science Institute) and Steven Chesley (JPL)
have used these results to calculate the hazard associated with
tsunami from sub-kilometer impacts. Harris has recently
recalibrated the NEA impact frequency, finding a reduction by a
factor of 5 in the frequency at all sizes relative to the
assumptions made by Ward and Asphaug. Thus the Ward and Asphaug
models can be transformed readily to agree with current impact
frequency estimates. Chesley has combined the Ward and Asphaug
results, so modified, with a new global population distribution
database. Using this known distribution of population as a
function of height above sea level and distance from the
shoreline, he can estimate the fraction of the population that is
at risk from impact tsunami and can calculate the frequency with
which coastal populations are subject to tsunami dangers. In
round numbers about 1% of the world population is at risk,
considerably less than had been estimated a decade ago. The drop
follows primarily from the reduction in the run-in of impact
tsunami as dictated by their shorter wavelength. Chesley's
initial results (presented at this workshop) do not take into
account shielding of coastal populations by reefs, barrier
islands, seawalls, and harbor constructions. Harris and Chesley
suggest that at least half of all coastal population is protected
by these barriers from short-wavelength tsunami, thus further
reducing the at-risk population (and associated annual equivalent
fatality rate) by another factor of two (or perhaps even more).
The resulting hazard, while still greater than the land-impact
risk from sub-kilometer NEAs, is less than has been previously
estimated. These models are not yet complete, but Harris and
Chesley seem to be converging on a reasonable answer.
The prime reason for this workshop was not to review the progress
of this modeling, interesting though that was. It was to deal
with a challenge from Jay Melosh suggesting that impact tsunami
approaching the shore from deep ocean break near the continental
shelf and dissipate their energy offshore, yielding no coastal
inundation. Melosh is also presenting a paper at this Lunar and
Planetary Science Conference (on 17 March), and the University of
Arizona is issuing a press release to this effect, stating that
the removal of the alleged hazard due to tsunami "will save
taxpayers the cost of financing searches for small NEAs, at a
savings of billions of dollars". Melosh had first
presented these results to the NASA team, but most of us
attending this workshop had not yet had the opportunity to hear
his analysis, and he and Ward had not previously met.
Melosh began by saying that he was "only the
messenger", and that his purpose was to call attention to
the 1968 report by W.G. Van Dorn of the Scripps Institution of
Oceanography, who had studied explosion-generated waves for the
U.S. Navy (TTR Report TC-130, Handbook of Explosion-Generated
Water Waves, Volume 1 - State of the Art). While never formally
classified, this report has been generally unavailable, and as
recently at 1996 Van Dorn himself had asserted that it did not
exist. However, a handful of copies had been distributed to
academic libraries long ago, and these were eventually located
and distributed to the attendees at this workshop. Van Dorn
carried out an extensive analysis of the entire subject of
"small" tsunami based on both theory and experimental
results from nuclear explosions (both on and under the ocean, and
up to 10 megatons yield), and also on a series of smaller-scale
chemical-explosion tests carried out in Mono Lake. Most important
for our purposes is the so-called "Van Dorn Effect",
which asserts that small (short-wave) tsunami break when they
cross the continental shelf, generating large-scale turbulence
there but relieving the coast of any wave run-in. The Van Dorn
Effect apparently has had important implications in nuclear
strategy, for example in the basing of ballistic missile
submarines.
Much of the workshop was devoted to discussing the Van Dorn
Effect and comparing his report with the modern work of Ward,
Gisler, and Korycansky. While the report was generally well
received by the workshop attendees, parts of it eluded our
comprehension. Especially cryptic is the genesis of the Van Dorn
Effect, which is not derived or explained in any detail in this
report. It is simply asserted in the text and summary. Thus the
most critical part of this argument for our purposes is not
justified.
Aside from this major mystery, there was a consensus (shared by
Melosh) in support of the kind of analysis presented by Ward and
Korycansky, and used by Chesley, recognizing the substantial
uncertainties embedded in the computations. This consensus
extended to each element: (1) estimating the size and shape of
the original explosion cavity (as long as it did not extend to
the ocean floor; e.g., for NEAs up to about 500 m); (2) modeling
the expanding wave front (which can be approximately dealt with
by linear theory and which indicates that the maximum wave
amplitude varies as about 1/r; (3) shoaling, which results in
reduced wavelength and modestly increasing wave amplitude as the
wave approaches the shore; and (4) the most uncertain part, the
run-in on the shore. It was agreed that a run-in equal to the
near-shore wavelength (and associated run-up no higher than about
2 times the height of the deep ocean wave) provides a reasonable
and probably conservative approach to estimating the population
at risk.
Other effects not yet modeled (such as sheltering of coasts or
the mysterious Van Dorn Effect itself, if real) will tend to
reduce the estimated hazard. Thus the current Harris-Chesley-Ward
approach probably yields an upper limit for the impact tsunami
risk. Undoubtedly they will refine their results for the NASA
team report to be written in the next two months. Meanwhile, it
is reasonable to take their upper limits to estimate the tsunami
risk.
In this upper-limit case, the tsunami hazard still dominates over
that of land impacts by factors of several for the sub-kilometer
NEAs, but both are quite small relative to the risk from NEAs
larger than 1 or 2 km. Another way to look at the situation is
that the population at risk from tsunami is small, only a few
tens of millions, consisting of those who live very close to
unprotected coasts (for example in Los Angeles and the coast of
Bangladesh, as well as many low islands without barrier reefs.)
For those people, the risk from tsunami is comparable to that
from a global-scale catastrophe. That is, such a person (a
resident of Venice Beach, California, for example) is roughly
equally at risk (at a level of about one in a million per year)
from impact tsunami as from a global ecological catastrophe. (Of
course, this person is even more at risk from earthquakes or
seismic tsunami).
The workshop concluded with general agreement, as well as some
frustration concerning our inability to fathom the Van Dorn
Effect. We will all look forward to the completion of the Chesley
et al. hazard analysis, which will add some quantitative meat to
this unoficial and rather skeletal summary from the workshop.
+++++++++++++++++++++++++++++++++++++++++++
NEO News is an informal compilation of news and opinion dealing
with Near Earth Objects (NEOs) and their impacts. These
opinions are the responsibility of the individual authors and do
not represent the positions of NASA, the International
Astronomical Union, or any other organization. To subscribe
(or unsubscribe) contact dmorrison@arc.nasa.gov.
For additional information, please see the website: http://impact.arc.nasa.gov.
If anyone wishes to copy or redistribute original material from
these notes, fully or in part, please include this disclaimer.
===========
(3) VESTA IN VIRGO: A NAKED-EYE ASTEROID
>From Sky & Telescope, 16 March 2003
http://skyandtelescope.com/observing/objects/asteroids/article_895_1.asp
By Greg Bryant
During the spring of 2003 Vesta is an easy target for binoculars,
being noticeably brighter than the dimmest stars on this chart.
The asteroid's passage close to numerous galaxies adds to the
excitement of viewing or imaging it with a backyard telescope.
Only the brightest galaxies are plotted here; see page 3 for
previews of Vesta's many encounters with fainter ones. Sky &
Telescope illustration.
During the first half of 2003, observers with binoculars and
small telescopes will be able to watch the asteroid 4 Vesta loop
gracefully through the constellation Virgo, making its most
favorable return in several years. On March 26th Vesta stands at
opposition to the Sun and can be observed throughout the night.
For several weeks before and after that date, with the help of
the chart (left), it may even be glimpsed with the unaided eye.
Vesta's favorable apparition continues through June and July, the
period when it becomes best placed (highest in the sky) for
viewing during early-evening hours. The constellation Virgo is
famous for its galaxies, and Vesta's journey takes it past many
such deep-sky denizens near the heart of the Virgo Galaxy
Cluster. By the start of March Vesta is already magnitude 6.4; it
peaks at 5.9 on March 27th, then fades to 6.4 by end of April. It
continues a gradual decline to 7.0 on June 1st, 7.4 on July 1st,
and 7.6 at the beginning of August.
For some observers the mention of Vesta brings back memories of
moonlight when it last reached opposition in our night skies, in
July 2000. That year the asteroid became even brighter, peaking
at magnitude 5.4, but on a night that coincided very nearly with
the full Moon - and a total lunar eclipse! This year the Moon is
well out of the way for observers wishing to catch sight of
Vesta.
In the Footsteps of Vesta's Discovery
Among the tens of thousands of numbered asteroids in our solar
system, Vesta is the only one that ever becomes so easy to spot.
It actually brightens past magnitude 6.0 every few years. Such
was the night of March 29, 1807, when German physician Heinrich
Wilhelm Olbers (1758-1840) discovered the "unknown
star" that would soon be hailed as the fourth known
asteroid.
Olbers is best known today for the paradox he investigated: why
the sky appears so dark at night (S&T: December 2001, page
44). But he was no stranger to discoveries. He had earlier found
2 Pallas, the second known asteroid, in 1802. Three comets bear
his name, and one of them returns every 70 years. He also devised
an extremely efficient method for calculating a comet's orbit
that is still in use today (for an initial solution, when the
orbit can be assumed to be parabolic).
Virgo was a charmed part of the sky for Olbers. In 1796 he found
his second comet south of Virgo's brightest star, Spica. In 1802
he made the first recovery of 1 Ceres in Virgo (thanks to
calculations by mathematician Carl F. Gauss), a year to the day
after its discovery. A few months later he discovered Pallas in
the northern part of Virgo. Five years and one day after that, he
likewise spotted Vesta in Virgo, close to its current location.
Vesta's track among the stars this year is rather similar to the
course it took 196 years ago.
Why So Bright?
Vesta is not unlike other asteroids, and indeed the major
planets, in that its brightness is not the same from one
opposition to the next. For example, if we look at the period
1990-2020, Vesta's peak brightness ranges between 5.3 (June 2018)
and 6.5 (November 1990). Over the next few years Vesta will shine
as brightly as 6.1 in September 2004, 6.2 in January 2006, and
5.4 in May 2007 - when, during the bicentennial year of its
discovery, it will again be brighter than Uranus.
Obviously, the position of Vesta along its elliptical orbit is a
contributing factor to the variation in visibility. The closer
Vesta is to the Sun in its 3.6-year circuit, the closer it is
likely to be to Earth. In 2003 Vesta doesn't reach perihelion,
the point nearest the Sun, until October 28th, seven months after
opposition. Then it will be just 2.15 astronomical units
(Earth-Sun distances) from the Sun, compared to 2.57 a.u. at
aphelion, the far point.
Size is also a factor in Vesta's prominence, though not as much
as you might think. According to the 2003 Astronomical Almanac,
Vesta is believed to be 530 kilometers in diameter (though not a
perfect sphere), edging out Pallas's width of 524 km. These
values stand well in the shadow of Ceres' 933 km. In the main
belt of asteroids between Mars and Jupiter, 10 Hygiea ranks
fourth place in size at 429 km. But no longer is it correct to
say that these are the four largest asteroids. Several newly
discovered objects in the far-flung Kuiper Belt - 20000 Varuna,
28978 Ixion, and 50000 Quaoar - may outrank even Ceres. Needless
to say, those in the main belt are much easier to hunt down than
those lying far beyond Neptune.
Given that Vesta is neither the largest nor the closest of the
asteroids, you might wonder what distinctive characteristic makes
it so readily seen. The answer lies in its albedo, or surface
reflectivity. Asteroids like Ceres reflect around 11 percent of
the sunlight reaching their surfaces, and others like 18
Melpomene have albedos as high as 22 percent. But Vesta is
geologically quite different and reflects a substantial 42
percent, much like a pink grapefruit! That's why it can become so
bright in our night sky.
- - - - - - -
Contributing editor Greg Bryant writes a monthly column for Sky
& Telescope about the sky seen from south of the equator.
Copyright 2003, Sky & Telescope
===========
(4) CCNet ESSAY: THE BULLET THEORY - A NEW LINK BETWEEN
MASS EXTINCTIONS
>From Michael Martin-Smith <lagrangia@lagrangia.karoo.co.uk>
THE BULLET THEORY: A NEW LINK BETWEEN MASS EXTINCTION EVENTS AND
ITS IMPLICATIONS FOR HUMANKIND
Benny,
About 2 years ago, I wondered, in my rather amateurish way,
whether Impacts and Volcanism could be tied together as causes of
mass extinctions, and, inspired by "The Day of the
Jackal", proposed a linking theory, which I dubbed the
"Bullet Theory". I had a letter published in the UK
Journal "Geology Today" in the summer of that year, and
emboldened by this, submitted an article - see below - to
Scientific American, both its US and Chinese (Ke Xue) versions.
The concept was perhaps rightly considered too speculative at the
time - but maybe the time has now come for it to be put before a
learned if eclectic audience? Perhaps it would now interest
members of our Network?
The Bullet Theory proposes a link between two widely held
explanations for the demise of the dinosaurs, and, possibly,
other Mass Extinction Events. Briefly, these are the Asteroid
Impact theory, and the Volcanism theory.
FULL PAPER at http://abob.libs.uga.edu/bobk/ccc/ce031303.html
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