CONFERENCE OUTLINE
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
NEA 2008 TC3 is the first asteroid to be discovered before impact (October 7, 2008). 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 threats" 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.
Boslough 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.
NEA 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.
PHYSICAL CHARACTERIZATION OF NEAS
A better understanding of the physical nature of NEAs is important for both science and planetary defense. Lance Benner (JPL) provided an update on the always-spectacular discoveries using the Arecibo and Goldstone radars, which are our most powerful tools for both orbital and physical characterization of NEAs (if they come with radar range). He showed an interesting comparison between the radar and spacecraft images of Itokawa; the radar got the size and shape right but could not resolve the fine detail, including the many boulders on the surface. Among the newly imaged objects were 1992 UY4, 1998 CS1, and the binary NEA 2000 DP107. Radar data indicate that about 10% of NEAs are actually contact binaries, and he stressed how different NEAs are, with "no such thing as a typical NEA."
Rick Binzel (MIT) reviewed telescopic data on the physical properties of NEAs. Large telescopes are being used to obtain visible and near-infrared colors, spectrophotometry, and polarimetry. He introduced a more accurate classification of 42-color spectrophotometry to create what he calls the Bus-DeMeo taxonomy, with more than 20 classes. Reminding the audience that we have thousands of direct samples of NEAs in the form of meteorites, Binzel noted that one of the objectives of telescopic studies is to link the remote measurements of NEA spectra with specific meteorite types. He used such an analysis to conclude that Apophis has the surface composition of a LL chondrite. If this is correct, we can use lab study of meteorites to estimate that Apophis is composed primarily of olivine and pyroxene with relatively low metal content. The bulk density of this material is 3.2 g/cc. Similar telescopic data for 2008 TC3 can be calibrated from the recovered meteorites. Binzel called this classification of NEAs from telescopic data "our first line of defense against NEOs."
The use of small spacecraft for detailed physical characterization of NEAs was the subject of several presentations and poster papers. The Japanese Hayabusa mission to Itokawa is an outstanding example of this approach. David Morrison stressed that flybys are of little use for small NEA targets, with a rendezvous required to determine such key properties as mass, density, and surface topography. He described a low-cost mission called MAAT that has been studied at NASA Ames Research Center. Another paper described the Didymos explorer mission designed to explore a binary NEA. Another proposed Apophis mission called Foresight is intended to tag Apophis with a radio transponder for very precise orbit determination, and PROBA-IP is an ESA technology demonstration mission targeted to Apophis. There was considerable discussion of the great value of a combined rendezvous and ballistic impact mission. Such a mission called Don Quijote was studied by ESA but now seems to have been dropped. However, ESA is working with Japan on an alternative NEA rendezvous mission without the impactor.
INTERNATIONAL ORGANIZATIONS AND STUDIES
This conference marked an acceleration of international interest in the protection of the Earth from asteroids. The International Academy of Astronautics (IAA), the primary sponsor of this meeting, has recently completed its own study of the impact threat, summarized at this meeting by Ivan Bekey, called "Dealing with the Threat to Earth from Asteroids and Comets" (2009, 140 pages). The abstract of this document says "The Earth has been struck by asteroids and comets many times throughout its history. This report of the International Academy of Astronautics addresses the nature of the threat, expected future impacts, and the consequences of impacts from various size NEOs. It reviews current programs to detect, track, and characterize NEOs, and the future improvements required in order to take responsible and timely action. It identifies a number of techniques that could alter an incoming NEO's orbit so as to avoid an impact. It addresses the organizational aspects that will have to be dealt with if a serious international capability is to be developed and employed to mitigate the threat. It then addresses behavioral factors and the sociological and psychological aspects of the threat and attempts at its mitigation before, during, and after an intercept attempt, whether successful or not. Lastly the report examines some of the principal international policy implications that must be dealt with if the world is to act in a timely, unified, and effective way with the very real threat due to NEOs." To access full text go to this website: http://iaaweb.org/content/view/229/356/. The report contains useful information, but since it was 4 years in preparation, parts of it are rather dated.
D. Koschny (ESA) reported on an initiative to include asteroid impacts within the European Space Situational Awareness Programme. Situational awareness is concerned with perception of the environment critical to decision-makers in complex, dynamic areas such as aviation, air traffic control, and military command and control. The objective of the European Space Situational Awareness initiative is to support the European independent utilization of and access to space for research or services, through providing timely and quality data, information, services and knowledge regarding the environment, the threats, and the sustainable exploitation of the outer space. The components of this initiative are space surveillance, space weather, and NEOs. For NEOs, they intend to study tracking, orbit determination, orbital databases, and identification of impact risks. The main objective is to issue impact warnings. They intend to provide information on the impact probability and/or miss distance of NEOs. To do this, they will assess impact analyses and perform their own risk assessments. As these are all tasks that are already being addressed by the NASA NEO Program, the initial ESA activity may be to transmit this information to European decision-makers. No independent European detection or orbital analysis plans were suggested.
Rusty Schweickart (B612 Foundation) provided an update to this meeting on the "Call for Global Response" of the international panel on Asteroid Threat Mitigation of the Association of Space Explorers (made up of astronauts and cosmonauts who have flown in space). Their report has been submitted to the UN committee on the Peaceful uses of Outer Space (COPUOS) and is currently entering a process of deliberation and potential action within the UN.
RECENT UNITED STATES STUDIES AND PLANS
A new study of the NEO Hazard being carried out by the National Research Council of the U.S. National Academy of Sciences was described by Mike A'Hearn (University of Maryland). This study, chaired by Irwin Shapiro, the Director emeritus of the Harvard College Observatory, is primarily addressing requests from Congress. They are considering the challenge of surveying potentially hazardous NEAs at smaller sizes, to reach 90% completeness at a diameter of 140 m. They are also considering a wide variety of techniques for characterization and mitigation. Congressional staff members have told the NAS-NRC steering committee that the Congress is interested in understanding how international collaboration should work in this area. This NRC study and recommendations should be completed at the end of 2009.
The capabilities of two large ground-based survey projects were discussed: Pan-STARRS presented by M. Granvik (with the first of four survey telescopes near completion in Hawaii, funded by the U.S. Air Force), and LSST presented by Z. Ivezic (a single wide-field 8-m telescope to be constructed in Chile, with detailed studies underway supported by the U.S. National Science Foundation and the U.S. Department of Energy). Space-based asteroid detection in the thermal infrared was discussed by Amy Mainzer of JPL, who described the capabilities of WISE (the NASA Widefield Infrared Survey Explorer), which is to be launched in November 2009, and of a possible dedicated infrared NEA survey called NEO-CAM to follow.
Peter Garretson (Lt. Col., US Air Force) reported the results of a scripted multi-agency deflection and disaster exercise. Participants included middle-level representatives of U.S. Air Force, NASA, National Security Council, Department of Defense, Department of State, Department of Homeland Security, Navy, Coast Guard, Federal Emergency Management Agency, and Defense Threat Reduction Agency. The objective was to better understand how responsible government organs would respond in the absence of clear policy on roles and missions. The table-top simulation involved two specific threats: a binary asteroid the size of Apophis targeted in the Atlantic Ocean off the coast of Nigeria, and a 50-m metallic asteroid targeted on the Eastern U.S. near Washington DC. They studied possible mitigation of the ocean impact given 7 years warning, and disaster management for the U.S. impact given 72 hours warning. The participants were aware of lack of previous planning. While a number of useful analogs exist, as well as procedures that could be used or adapted, attempts or do so in the moment are likely to be much less successful than advance preparation. Participants recommended that the NEO scenario should be elevated to higher levels with more senior players. They also concluded that proper planning and response to a NEO emergency requires delineation of organizational responsibilities including lead agency and notification standards, but they were not able to agree on which should be the lead agency. This was the first time such a multi-agency exercise had taken place in the U.S. Dealing with similar issues on an international scale would be even more problematic.
SUMMARY COMMENTS BY DAVID MORRISON
This was an excellent meeting: well attended and with some fine technical and policy papers presented. Most of the leaders in the NEO defense arena were present, as well as many new participants, especially many young scientists and engineers from Europe.
Several speakers assumed that the current estimates of the impact rate are greater than previously thought, adding urgency to this threat. Judging by some of the presentations at this meeting, these assessments were based on faulty or misinterpreted data. As Al Harris (Space Science Institute) showed (discussed above), at least 90% of the risk associated with impact by an unknown asteroid has been eliminated by the Spaceguard Survey, and in addition the NEO population data summarized by Harris indicate fewer sub-km asteroids by at least a factor of 2, relative to previous power-law estimates. It is curious to see claims by policy analysts that the impact threat is increasing while the scientific community comes to the opposite conclusion.
There was thus a certain unreality about some of the discussions. Many international and European organizations are becoming concerned about the impact hazard just at the time it is being rapidly reduced by the Spaceguard Survey. While there have been some excellent paper studies (for example, the U.K. impact hazard study in 2001), so far only two nations have made significant contributions to actually mitigating the impact hazard: the United States and Japan. The U.S. has financed the Spaceguard survey, the Minor Planet Center, the NEO Program Office at JPL, the planetary radar systems at Arecibo and Goldstone, studies of the next generation of survey telescopes (including construction of the Pan-STARRS-1, casting the 8-m mirror for the LSST, building the WISE infrared survey satellite, and sending the NEAR-Shoemaker mission which orbited and landed on Eros). Japan has carried out the only spacecraft study of a sub-km NEA, Itokawa, including brief landing and collection of a sample now en route back to Earth.
Three surprising new results presented at this meeting stand out in my mind. (1) The strange case of the Carancas meteorite fall on the Altiplano at the Peru-Bolivia border on September 15, 2007, which was described by Tancredi. A stony (ordinary chondrite) meteorite with an original mass of a few tons hit at a few km/s speed and formed a 15-m wide crater. This should not happen according to conventional models. (2) Boslough's supercomputer simulations of the Tunguska impact indicate that stony objects <40m in diameter can produce highly destructive airbursts, and somewhat larger airbursts can also generate substantial melting of silicates at the surface when their fireball reaches the ground. (3) The unique case of 2008 TC3 showed that even with the current survey systems it is possible to detect a small NEA very close to impact, accurately predict the impact time and place, and recover meteorites. This is also our first chance to directly compare remote sensing observations of the parent object in space with "ground truth" from recovered samples. TC3 has greatly increased interest in detection of small asteroids within a few days of impact. |
