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NEO News (03/10/05) Impact Hazard Summary for UN

Status Report From: Ames Research Center
Posted: Thursday, March 10, 2005

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The following summary of the status of international programs dealing with the NEO impact hazard was presented at a recent meeting in Vienna of the UN Committee of the Peaceful Uses of Outer Space. The author is Hans Rickman, representing the International Astronomical Union and ICSU, the International Council of Science.


Comet/Asteroid impacts have played a major role in shaping the planets and satellites in the Solar System. Moreover, it has become clear that such events are not confined to the distant past, but are an ongoing phenomenon. They represent a natural hazard that is unique in that, with existing techniques, the comets and asteroids can be detected and their possible impact can in many cases be predicted decades in advance. Thus, measures can be designed and implemented to mitigate the consequences of a impact -- perhaps even prevent it altogether. Governments are therefore being faced with the issue of assigning priorities to this task among the many that claim a share of society's limited resources.

Compared with other natural hazards, the one of comet/asteroid impacts has very peculiar features that challenge a rational response by citizens and policy-makers alike. The impacts span an enormous range of size and energy, from entirely harmless to extremely calamitous on a global scale. The ones that expose humanity to the largest risk are caused by relatively large objects that are, in general, easy to observe. Thus their orbits may be established early on, and timely mitigation efforts are facilitated, in case a future impact is found likely. However, before charting the population, the a priori statistical likelihood of such an impact during the 21st century is very small. The expected death toll, in the case of an actual impact, is large enough to make the impact hazard rank highly among natural hazards in general, but one faces the problem of comparing hazards posed by different events, where some -- like earthquakes - happen frequently, and others -- like major impacts -- are unlikely to occur at all during the next century.

Planning ahead for natural disasters, setting up communication channels and other facilities that aid in dealing with the consequences, and thus creating a reasonable level of preparedness is an important issue, already being dealt with regarding geophysical events like earthquakes, volcanic eruptions or tsunami waves thus triggered. It may be essential to consider comet/asteroid impacts as well in the same portfolio, but it is also worth emphasizing that those impacts can in principle be foreseen ahead of time, and it is even possible to prevent them from happening, unlike most geophysical hazards.

Clearly, in order to judge the impact hazard and make correct decisions on assigning its priorities -- not only internationally but, more importantly, within individual countries -- one must have a reasonable understanding of the risks involved. Broadly speaking, one may characterize the current state of knowledge as very good, when it comes to estimating the annual likelihood of an impact as a function of the size or energy involved. But less confidence can currently be placed in predictions of the indirect physical or environmental effects, like dust loading of the stratosphere, the destructive power of impact-induced tsunamis, or ozone depletion arising from the generation of large amounts of chemically active gases. And even less can be said about the response of human psychology, sociology, political systems or economies to a catastrophe caused by such effects.

However, concerns over the comet/asteroid threat are not limited to the physical effects of an actual impact. Search programs have led to the detection of asteroids that, in the foreseeable future, will pass so close to Earth that an impact cannot be excluded with total certainty. The recent case of asteroid 2004 MN4 is instructive. The orbit is known well enough to exclude the possibility of an impact with the Earth during the close encounter foreseen in April 2029 -- in fact the asteroid will pass at a minimum distance of only several Earth radii -- but in the absence of some critical observations we might have been faced with a lingering, important impact risk associated with this encounter for quite a long time. How are scientists, the media, and authorities to handle such issues in a manner that is both open and responsive to the need to avoid undue public alarm? What are the psychological effects on individuals of a perceived apocalypse? What are the moral implications of warning versus not warning society of a pending global or local disaster?

All governments should require a comprehensive, independent, authoritative evaluation of the consequences to human life and society from comet/asteroid impacts, and from the uncertainty that may exist around foreseen, close encounters. This has been realized since some time, and as a result the OECD Global Science Forum organized an international workshop that was held in Frascati, Italy, in January 2003. Conclusions were reached on several different aspects of the general problem of risks, policies, and actions relating to the impact hazard. One of them dealt with strengthening risk assessment through research and development, and in particular it was noted that "The scientific community could provide the information and advice that government officials require to carry out the national risk assessments. This scientific work should extend beyond the traditional NEO community (principally astronomers) to include experts in areas related to the consequences of NEO impacts on the Earth, on society, and on the biosphere in general." ICSU, the International Council for Science, was identified as an appropriate organizing body for this work.

This, in fact, was exactly the aim of a project run by the International Astronomical Union. Accordingly, the IAU together with several other Unions and Committees of the ICSU family, and the USA as a national member, prepared an application for ICSU sponsorship of an international, multidisciplinary, scientific assessment of a broad range of issues around comet/asteroid impacts and human society. This was successful, and funding was allocated to the project within the 2004 ICSU/UNESCO Grant Programme. The activity focussed on bringing together a fairly large but manageable number of world experts on essentially all aspects of the problem to a workshop, whose primary aim was to establish the necessary contacts for a long-term research effort across all academic subject borders, and to produce a first, albeit preliminary version of the assessment. The main scientific organizers were Peter Bobrowsky, representing the International Union of Geological Sciences, and myself representing the IAU. The workshop was held on November 29 -- December 1, 2004, in the town of La Laguna, Tenerife, and the local organizer was Mark Kidger of the Astrophysical Institute of the Canaries.

Several topics of general discussion in splinter groups had been identified, and the results can be briefly summarized as follows. Society's vulnerability to impacts was found to have increased, both in developed countries that are increasingly dependent on complex information networks and economic linkages, and in developing countries where people often live under marginal conditions and disaster preparedness tends to be poor. Concerning methods for reduction of the consequences of impacts, it was noted that efficiency in this respect will greatly benefit from the availability of disaster response organizations in different countries, and national NEO policies should be developed. It is wise to extend the inventory of NEOs down to smaller size levels, and to increase the geographic accuracy of impact calculations. The tools for communicating scientific assessments of impact risks at future close encounters need to be further improved and generalized to include contributions from other fields than astronomy, as appropriate. Of great importance is public education about impacts and their consequences, as well as better reporting from scientists via journalists to the citizens. Protocols for the flow of information, including society's political leaders, are also needed. Finally, a better understanding of the consequences of impacts was perceived to be one of the most important goals for the scientific work.

In scientific terms, the risk posed by comet/asteroid impacts can be broken down into a product of the exposure to impacts, the vulnerability expressed by the likelihood of various physical effects of impacts, and the cost in terms of a measure of the negative consequences of those effects, like mortality rates or economic losses. Concerning the exposure part, there is a frequency distribution of impacts vs energy arising from astronomical observations, which is in broad agreement with the cratering record of the Earth that geologists provide for relatively large impacts. But some problems seem to remain at the high-frequency, low-energy end of the spectrum, thus requiring further investigation. As the charting of the impactor population proceeds, we also need to learn more about the detailed physical properties of the objects in order to support possible mitigation strategies.

The size and energy level where global effects of an impact set in also needs to be more precisely defined. For the time being, the amount of atmospheric dust loading after very large impacts is judged to be very uncertain, and the main killing agent may instead be the global ejecta fallout and the ensuing thermal pulse, popularly described as the "broiling effect". Another effect that needs further study is ozone depletion resulting from NO, H2O, and sea salt injected high into the atmosphere as a result of the impact, where global health concerns might arise for impacts smaller than the "global threshold" hitherto estimated. Current models are, however, very preliminary.

For smaller impacts that cause only regional damage, the main subject of controversy is the destructive effects of impact-induced tsunami waves, for which different models have yielded very different results. One of the main issues is whether these waves break at the edge of the continental shelf.

The negative consequences for individuals and society raise a very complex problem, where it is too early to draw any conclusions of the same generality as those already discussed. Here is where most of the effort needs to be spent in the near future in order to reach a safe ground for risk assessment. There are, nonetheless, some interesting attempts to explore economic consequences like the loss of a major city, or a serious financial crisis following disruption of the capital market. From the side of psychology there has also been significant advances in understanding the ways humans react to statements about the impact hazard, and thus valuable advice exists as to what should be done to assist a rational response.

A preliminary assessment in the form of a white paper will be issued under the aegis of ICSU. But as already mentioned, the effort just described should be seen as an initial kick-off into a multidisciplinary research program that will, eventually, lead to the assessment that society needs to tackle the impact hazard. The meeting was the first, where experts from such a wide range of disciplines came together, and it was very successful in promoting lively discussion and establishing numerous contacts across the subject borders. There is clearly a need for continued support in the future, in order to monitor the progress of the work and help guiding the activities for maximum efficiency.


NEO News (now in its tenth year of distribution) 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.

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