|Session:||Session 4 Impact Consequences & Education (04)|
|Date:||Tuesday, May 10, 2011|
|Time:||14:30 - 18:20|
|Chair:||Mark Boslough, Alan Harris|
Consequences of the Tunguska Impact and their Interpretation
Longo, Giuseppe1; Gasperini, L2; Bonatti, E2; Stanghellini, C3; Serra, R4
1INFN and University of Bologna, ITALY;
2Istituto di Scienze Marine (ISMAR), CNR, Bologna, ITALY;
3Istituto di Radioastronomia (IRA), INAF, Bologna, ITALY;
4University of Bologna, ITALY
The best-known historical cosmic impact on our Planet took place in Siberia on June 30, 1908. A huge explosion, equivalent to 1,000 Hiroshima nuclear bombs, devastated 2,000 square km of Siberian taiga on an area sparsely populated by scattered groups of nomadic Evenk natives close to the river Podkamennaya Tunguska. An accurate analysis of the data collected by our 1999 expedition together with the results of field researches performed in 2002, 2008 and 2009 have lead to the formulation of the hypothesis that the Tunguska bolide underwent fragmentation or was one of the 20% of NEAs that have satellites or are double bodies. In this picture, the disintegration in the atmosphere of the main body was the cause of the known devastations, while a second body has originated a crater now filled by Lake Cheko. Some of the evidence in favour of this hypothesis is: 1) the unusual, funnel-like morphology of the lake, different from that of common termokarst Siberian lakes, but compatible with a "soft" low-velocity impact crater of a m-size projectile in a swampy ground with permafrost followed by rapid de-watering and de-gassing of the sediments and collapse of the crater; 2) Lake Cheko is not reported on any map up to 1928, including the 1883 map of Eastern Siberia compiled by the Central Headquarters of the Czarist army and the diagrammatic maps of the Tunguska site compiled in the twenties on the basis of Evenki testimonies; 3) the lake appears to be not older than 1908, as suggested by radiometric dating of the lacustrine sediments; 4) the seismic reflection profiles show a prominent reflector indicating the presence of a fragment of the body, or of a compacted sedimentary layer, ~10 m below the bottom of the lake; 5) sub-bottom acoustic reflection data show that of the ~10 m thick sediment pile only the top 1± 0.5 m is laminated, fine-grained, "normal" lacustrine sediments while the lower, chaotic material appears not to be deposited by normal lacustrine sedimentation; 6) whereas the upper sediment layers contain abundant evidence of aquatic plants, these signs are totally absent in the lower chaotic deposits, which hold plentiful quantities of pollen from forest trees; 7) some surviving tree on the northern lakeshore had in 1908 an inclination toward the lake that can be explained by collapse; 8) on the lakeshores, the surviving trees have thin rings before 1908 and larger ones from 1908 up to recent years as if living in a dense forest before 1908; 9) tree trunks and branches, that can testify the presence of a previous forest have been observed via underwater video; 10) the data on fallen tree directions are compatible with the hypothesis that the cosmic body was composed by at least two bodies, falling independently but very close one to the other; 11) a magnetic/density anomaly is present at the lake centre, ~10 m below the bottom. The Tunguska event is the only phenomenon of this kind that happened in relatively recent times, consequences of which can be studied directly. Many aspects of this impact are still unclear. Drilling the Lake Cheko bottom to search for markers of the "impactor" could clarify some of these aspects.
Creating Awareness - The Impact Hazard in Public Education Curricula Content, Students'Interests and Concepts and Educational Implementation
University of Eichstaett-Ingolstadt, GERMANY
Meteorite impacts have been an ongoing phenomenon in Eart's history. The impact hazard is truly a fascinating subject - covered in movies and TV documentaries.
Additional interviews with representatives of the MunichRe Group, the WBGU (German Advisory Council on Global Change) and the DKKV (German Committee for Disaster Reduction) show that meteorite impacts are dealt with as a high-risk potential also outside the scientific community. It was stated that the topic should be part of public education - with Geography being the subject able to cover both the natural science aspects as well as the area of social and human sciences.
The goal of combining scientific concepts and students' prerequisites in an educational context is achieved by setting up a Geography course module on the impact topic. Core component of the module are the CRATER simulation and online information materials. With the help of the simulation (http://www.geosim.org) it is possible to calculate the size of a crater, formation time and the energy released during impact. The learning materials provide information on impacts in Earth's history, probability and consequences of an impact today and current NEO search activities.
The central aim should be the implementation of the topic in geographic curricula - clearly supported by the conducted research on scientific and students' concepts.
New Classification Scale for Impact Consequences
Gritsevich, Maria1; Stulov, V. P.2
1MSU, University of Helsinki, RUSSIAN FEDERATION;
2Institute of Mechanics, MSU, RUSSIAN FEDERATION
In this study, we analyze consequences that can accompany collisions of cosmic bodies with planetary atmosphere and planetary surface. Using the basic differential equations  one can introduce dimensionless parameters describing a problem. Then we distinguish two key dimensionless parameters with the following physical meaning: (1) the ballistic coefficient, which shows the ratio between the mass of the atmospheric column along the trajectory and the body’s pre-entry mass; (2) the mass loss parameter, which depends on the ratio between the fraction of the kinetic energy arriving at the unit body's mass as heat and the effective destruction enthalpy. These parameters explicitly characterize the ability of entering body to survive during atmospheric entry and to reach the ground . The processes accompanying atmospheric entry essentially depend on values of these two parameters as well. Thus different events could be associated with different groups with similar predictable consequences. The ballistic coefficient and mass loss parameter can be derived by several existing techniques, see e.g. . Therefore we analyze consequences for different types of events, for example, formation of a huge single crater (Barringer, Lonar Lake), dispersion of craters and meteorites over a large area (Sikhote-Alin), Tunguska. The proposed scale helps to summarize data on existing impact data and to formulate recommendations for further studies valuable for planetary defense.
Airburst Warning and Response
Sandia National Laboratories, UNITED STATES
It is virtually certain (probability > 99%) that the next destructive NEO event will be an airburst. Planetary defense is usually assumed to have the primary goal of maximizing the number of lives saved, but it can be argued that more emphasis should be placed on maximizing the probability of saving lives. For the latter goal, it is far more effective to create an early warning and civil defense plan than a mitigation plan that involves deflecting a large NEO. Because early warning and civil defense will almost certainly be needed long before the first deflection is ever required, the credibility of the planetary defense community and its recommendations will be put to its first serious test by an airburst. Successful response to an airburst event will make it much more likely that recommendations for mitigation by deflection will be accepted by decision makers and the public. Focusing more attention on the second goal will, as a side effect, benefit the primary goal.
Airbursts are local events and unlikely to create international conflict if they have been predicted in advance, so early warning can generate goodwill and trust leading to cooperation for large NEO mitigation. If airburst recommendations save lives, those lives are very likely to be citizens of nations other than those responsible for the warnings. Conversely, if we ignore the airburst threat and there is an event with casualties, future mitigation recommendations are much more likely to be ignored. Moreover, if an imminent Tunguska-class impactor were observed but not recognized because of the insufficient observational resources or inadequate period between observations, the resulting criticisms and conspiracy theories could irreparably damage our credibility. Airburst "mitigation" by early warning and civil defense should be taken more seriously for that reason, if no other.
A reasonable approach would be to use computational models to generate "lookup tables", reduced-order models, or scaling laws to generate maps of damage on the surface and convolve with uncertainty footprints based on astronomical observations and orbital dynamics projections. This method would be used only to issue warnings associated with airbursts that are virtually certain to happen ("8" on the Torino Scale or undefined if smaller than 20 m). Such an alert would provide the time, coordinates, and a scale number indicating maximum possible damage at the epicenter. Such a system could be implemented to provide maps showing contours based on the convolution. The system would need to be very fast and automatic, and therefore based on simulation output that is linked to orbital output. The threat maps would be analogous to the National Hurricane Center's operational hurricane maps, which explicitly include uncertainty. Local authorities would then issue instructions based on the alert. Civil defense would be the responsibility of the target nation (just as foreign nations use the NHC alerts without requiring further US help).
The warning could also contain maps showing locations from which one could safely view the event, and where it would appear in the sky--even if over the horizon. This would create the opportunity to obtain images, video, and other useful data from smaller events, helping validate models and improving our understanding of the airburst process. It is highly likely that the first events with such warnings would not be threats at all, but opportunities for science and amateur astronomy. The optimal and most likely outcome would be a series of harmless 2008-TC3-class events, a few of which would be observable and documented. This would lead to positive media coverage, increased awareness of the threat, more confidence and respect for the predictive capabilities of our community, better response to subsequent serious civil defense warnings, more support for planetary defense activities, and ultimately--more lives saved.
Calculation of the Impact of a Small Asteroid on a Continental Shelf
University of Oslo, NORWAY
An impact of a 200-meter-diameter body with the earth is far more likely within the next thousand years than the cataclysmic impacts that could cause extinction events. Such a "small" impact in the deep ocean is not likely to cause great damage to coastlines. Possibly more serious is the impact of such an object on a broad continental shelf, where a sediment-laden splash wave could damage structures and populations on the nearby shore. Building communities that are resilient to tsunamis will mitigate the damage, but there are also concomitant effects associated with the ground shock, the blast wave, and the high temperatures that are generated during impact that will be harder to defend against. I have performed a detailed simulation of such an impact, including atmospheric thermal effects, wave heights, sediment transport, and inundation. The impactor, a 200-meter-diameter stony sphere, strikes a shelf of 0.5 degree slope at a point where its depth to bedrock is 200 meters, overlain with 100 meters of sediment, with the shoreline 12 km away. A crater of depth 450 meters and diameter 3 km forms in the bedrock, and all the sediment within that region is either lofted into the atmosphere or transported shoreward with the splash wave. The wave is roiling and turbulent, and achieves a maximum envelope of 50-70 meters. The leading wave is accompanied by a hot wind with a temperature of 60-65 Celsius, and preceded by pulses of 20-40% overpressure. The ground shock that precedes both of these may be sufficient to topple non-reinforced buildings. The total energy dissipated in such an event is comparable to the total by a severe tropical storm over its lifetime, but because the duration is measured in minutes rather than days, the local effects can be considerably more severe.
Dynamics of Tsunamis Generated by Asteroid Impact in the Black Sea
Isvoranu, Dragos1; Danaila, S2; Badescu, V3
1University Politehnica of Bucharest, ROMANIA;
2University Politehnica of Bucharest, Fac. of Aerospace Eng, Dept. of Aerospace Sciences "Elie Carafo, ROMANIA;
3University Politehnica of Bucharest, Candida Oancea Institute, ROMANIA
Two-dimensional and one-dimensional models are used to evaluate the seashore effects of the tsunami generated by an asteroid impacting the deep water in the Eastern region of the Black Sea. The distance between the impact point and the coast is about 150 km. The effects on the coastal regions depend on many factors among which the most important is asteroid size. The tsunami generated by a 250 m asteroid reaches the nearest dry land location in 20 minutes and needs about two hours to hit all over the Black Sea coast. The run-up value is more than 10 m high on Eastern and Crimean coasts. In the Western Black Sea regions the wave height is two or three times smaller. The run-up and run-in values strongly depend on bathymetry and topography peculiarities. The run-up values in case of the tsunami generated by a 1000 m size asteroid are three to ten times larger than in case of the 250 m impactor, depending on location. The tsunami effects in case of an asteroid of diameter 70 m are small in the Eastern Black Sea basin (wave height of about 0.15 m on the shore) and even smaller in the Eastern basin. Possible social consequences and prevention are briefly discussed.
The Protective Role of the Earth’s Atmosphere against the Threat of Asteroids
VERANT, Jean-Luc1; MOSCHETTA, J.-M.2; FERRIER, L.1
Usual accepted approach to characterize impact energy of asteroids or equivalent bodies confuses Earth impact kinetic energy to the entry kinetic energy. However, neglecting the atmosphere role may lead to inaccurate estimate of actual impact or risk conditions. Furthermore, in some situation, unphysical impacts could be predicted if atmosphere is not accounted for.