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 <firstname.lastname@example.org>
(2) TSUNAMI HAZARD
David Morrison <email@example.com>
(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 <firstname.lastname@example.org>
(1) WORRIED ABOUT ASTEROID-OCEAN IMPACTS? DON'T SWEAT THE SMALL STUFF
>From Lori Stiles <email@example.com>
>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."
H. Jay Melosh
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
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 <firstname.lastname@example.org>
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.
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 email@example.com. 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
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 <firstname.lastname@example.org>
THE BULLET THEORY: A NEW LINK BETWEEN MASS EXTINCTION EVENTS AND ITS IMPLICATIONS FOR HUMANKIND
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
CCNet is a scholarly electronic network. To subscribe/unsubscribe, please contact the moderator Benny J Peiser < email@example.com >. 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. The fully indexed archive of the CCNet, from February 1997 on, can be found at http://abob.libs.uga.edu/bobk/cccmenu.html. 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.
CCCMENU CCC for 2003
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.