From: Ames Research Center
Posted: Sunday, May 31, 2009
This edition of NEO News provides an overview of the Planetary Defense Conference held in Granada April 27-30, 2009, followed by a more detailed discussion of a few papers. Other reports from this conference will follow in later editions.
This meeting was officially named "Planetary Defense Conference (PDC): Protecting the Earth from Asteroids". It was the first planetary defense conference sponsored by the International Academy of Astronautics, although it follows the pattern of previous conferences sponsored by the American Institute of Aeronautics and Astronautics. As with the previous meetings, this one was organized by William Ailor of The Aerospace Corp, joined this time by co-organizer Richard Tremayne-Smith from the UK. Logistics were handled by ESA (European Space Agency). Approximately 40 oral papers were presented, in addition to a similar number of poster presentations plus several panel discussions.
The PDC sessions covered the following topics: (1) Discovery, Tracking and Characterization of NEAs (a full day session), (2) Mission and Campaign Design, (3) Deflection Technologies and Simulations, (4) Impacts and Consequences, (5) Policy, Preparedness and Deciding to Act, and (6) a concluding summary session. Among the new topics discussed were the experiences of the Japanese Hayabusa mission investigating the sub-km asteroid Itokawa, detailed discussion of the search capability of the proposed Pan-STARRS and LSST telescopes, reports on the Carancas impact event in the Altiplano (September 15, 2007), and reports on the discovery, impact (October 7, 2008), and subsequent recovery of fragments from the small asteroid 2008 TC3. This conference also followed a meeting of lawyers the previous week in Lincoln, Nebraska, considering the legal aspects of planetary protection, as reported by Frans van der Dunk of the University of Nebraska.
Keynote talks were presented by ESA astronaut Pedro Duque (who is from Spain) and Nature editor Oliver Morton (who is about to become an editor of The Economist). Morton provided an interesting journalist's perspective, noting that our interest in defending against celestial dangers marks a fundamental departure from the history of astronomical studies of the cosmos. Through history, astronomy has been the least practical of sciences, and most astronomers and space scientists still think of it that way. It is therefore perhaps no surprise that many traditional astronomers have not accepted or perhaps even grasped the significance of what we are doing toward protection of our planet. Morton also discussed lessons that the development of ideas about planetary protection might offer to other areas of human endeavor, such as global warming. Global warming is another field where technological capability, catastrophic potential, and planetary perspectives coincide.
The next Planetary Defense Conference is to be in Bucharest, Romania, May 9-14, 2011.
IMPLICATIONS OF 2008 TC3
TC3 is the first asteroid to be discovered before impact. The explosion point in northern Sudan was determined with sufficient accuracy to allow later recovery of meteorites. Clark Chapman and Rusty Schweickart of the B612 Foundation discussed some of the ways our new-found ability to detect very small NEAs close to the Earth can change our perspectives. They suggest the importance of coordination of NEA searches with disaster planning and response communities. It may be that even with the current Spaceguard system we are more likely to find a very small NEA just a few days before impact than to find a large one with decades of warning. Chapman noted that little work has been done to identify the smallest NEA that is dangerous, or the smallest that might be of interest to decision-makers. Recent models suggest that the threshold for significant ground damage is 30-40 m. But how would public officials react to a prediction of a 25 m impact, or a 15 m impact? There is about a 20% chance of an impact by a 15 m NEA in this decade. Decisions dealing with small impactors (including those that ultimately miss) may need to be made every few years, if we can predict them. Hyped or unreliable media stories might happen annually.
David Morrison also discussed some of these issues, looking at the historical progression of perspectives on the impact hazard.
1. Assessing the hazard. During the 1990s, the standard scientific tools of sampling and statistical analysis were essential to understand the impact hazard and communicate the risk to decision makers. Clark Chapman and David Morrison first identified the threshold for global damage and estimated the risk from impacts of different size. This early work compared the impact risk to other natural hazards, estimated the risk as a function of NEO size, and laid the foundation for establishing the Spaceguard Survey in 1998. The first Congressional language (in 1991) reflected this perspective when it noted, "the Committee believes it is only prudent to assess the nature of the threat" Note that while these statistical studies provide a tool to analyze various mitigation schemes, they do not in themselves reduce the hazard. The step of moving from a scientific risk-analysis perspective to actually mitigating the impact hazard required a major re-orientation of thinking.
2. Mitigating the Hazard. The public and decision-makers are not interested in a better statistical understanding of the impact threat; they want warning and protection. The public-policy goal is not to refine the estimate of the risk but to identify the next impactor and do something about it. That is the purpose of surveys, orbital calculations, and follow-up characterization. The Spaceguard goal of finding 90% of the NEAs larger than 1 km diameter focuses on NEAs that are large enough to risk a global catastrophe. These are also the impacts that might threaten the survival of civilization. While such impacts are very rare, happening less than once in a million years, they still dominate the risk over more frequent impacts. Future surveys (such as Pan-STARRS and LSST) are also designed to provide decades of warning. The objective is not the last-minute detection of incoming objects, and the surveys have not been optimized for such purposes.
3. Dealing with Public Concerns. Issues that worry the public (and many decision makers) are not necessarily the greatest threats. The very rare large impacts pose the greatest hazards, but most people are more concerned about the next impact (which is likely to be small). 2008 TC3 is an example; too small to pose any danger, but something that would be of substantial public interest if it fell over a populated region. From this perspective, it is the number of "threat warnings" that matters, not the size of the threat. A 20 m object detected this week with one-week warning time will get much more attention than a 2 km object that won't actually threaten an impact for the next century. Ideally we should design a survey system that detects both distant large NEAs and close small ones. But (Morrison argued) if a choice must be made between optimizing for the deep surveys and searching for small impactors near the Earth, then it is more important to maintain the capability of detecting larger NEAs at great distances in deep surveys. The larger asteroids still dominate the hazard. Faced with both a cloud of mosquitoes and a venomous snake, we may be tempted just to go after the numerous mosquitoes, but we ignore the snake at our peril.
DESTRUCTIVE POTENTIAL OF IMPACTS BY SMALL NEAS
As the focus of new searches moves toward smaller (sub-km) NEAs, we are also learning more about the destructive potential of impacts at the smaller end of the NEA size spectrum. Mark Boslough (Sandia National Laboratories) and Galen Gisler (University of Oslo) both have used supercomputer models to investigate airbursts and tsunami formation, respectively.
Boslaugh has been simulating low-altitude airbursts of hypervelocity impacts from NEAs <100 m in diameter, finding an increased damage potential relative to earlier models that did not include the downward momentum of the exploding mass. Fireballs from nuclear explosions rise, but those from an asteroid initially continue downward from the point of disintegration. Because of this downward flow, larger blast waves and stronger thermal radiation pulses are experienced at the surface than would be produced for a nuclear airburst of the same yield. The 1908 Tunguska explosion is an example of an airburst in which the hot jet of vaporized projectile material continued downward but lost momentum before it made contact with the surface. The models suggest that the total energy released in the Tunguska event was not more than 5 megatons, in contrast to earlier analyses that suggested an energy of 10-15 megatons. For somewhat larger impacts, the fireball descends all the way to the ground, where it can melt silicate materials. The mysterious Libyan glass may have been produced by such a fireball.
Gisler (with co-author R. Weaver) did 2-D and 3-D computational analyses of the effects of ocean impacts of NEAs <500 m in diameter. They concluded that the near-field effects (that is, within 100 km of the impact) are the dominant danger, with central jets rising several km into the atmosphere and generating highly non-linear breaking waves that could devastate shorelines. However, the impact does not generate long-distance tsunami-like waves, so the area of damage remains local.
NEO POPULATION AND IMPACT RISK
With the nominal completion of the 10-year Spaceguard Survey focused on NEAs >1 km diameter, it is important to assess where we stand. Alan Harris (Space Science Institute) provided for this conference a re-evaluation of the population of NEAs and an estimate of the remaining risk from impacts. As of January 19, 2009, the present surveys have discovered 765 NEAs larger than 1 km (as estimated from their brightness) out of an estimated total population of 940. This is 81% completeness. Note that these numbers reflect a re-evaluation made a few years ago to the asteroid magnitude scale and the conversion factor from observed magnitudes to diameters, resulting in a fewer NEAs >1 km. Since the survey is more nearly complete at 2 km diameter, which is close to the probable threshold for globally catastrophic impacts, the Spaceguard Survey has actually "retired" more than 90% of the total impact risk. Almost half of all NEAs as large as Apophis have already been discovered, but only a negligible fraction of Tunguska-size NEAs.
As noted above, it now appears that ground damage from airbursts extends to considerably smaller impactor sizes than was previously inferred. The main risk in the size range from 150 m to 1000 m is from tsunamis, but with adequate warning the actual fatalities from tsunamis can be small. Harris re-evaluates the likely casualties using known population distributions and improved estimates of the damage from impacts. With the current level of survey completeness (which includes many sub-km objects as well as larger ones), Harris estimates that the remaining risk from the undiscovered population (expressed as average annual fatalities) is roughly 20/yr from local/regional land impacts, 4/yr from impact tsunamis, and 54/yr from globally catastrophic events (the undiscovered big ones). The next generation surveys, aimed at finding 90% of NEAs >140 m, will further reduce impact risk. Using these models of population, completion, and impact damage, Harris estimates a residual risk of roughly 6/yr from local/regional impacts, <1/yr from impact tsunamis, and 11/yr from globally catastrophic events, plus a continuing background risk of 10/yr from long-period comets.
Harris concluded that within a few years, if not already, we will have found essentially all dangerous asteroids large enough to be a risk of global climatic effects. We will be left with some fractional probability that even one such object remains undiscovered. Mid-size impacts, presenting mainly tsunami risk, are less frequent and probably less damaging than previously estimated. In the smallest size range capable of causing ground damage, the next generation survey may find ~25%, providing long-term warning. Ground-based optical surveys can also be designed with about 25-35% chance of detecting a "death plunge" object, down to the smallest size capable of producing ground damage, providing days to weeks' warning. Thus in a very short time on the scale of civilizations, and even quite short in terms of a human lifetime, the impact hazard should be reduced to a negligible risk. The one exception to this is the risk from long-period comets, for which present technology can offer no protection beyond short-term warning. Fortunately this risk is estimated to be quite small. Harris also noted that it is obvious that doing something about the global catastrophic events is worthwhile by almost any accounting. It is in the smaller size range where more careful cost-benefit accounting is in order to evaluate programs and policies.
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