Pete Worden (Director, NASA Ames) opened the conference by stressing the importance of NEO studies (surveys and characterization), but he also cautioned that while we may have the technology to largely eliminate the impact threat, NASA at present has no budget to expand its NEO program beyond the current level of approximately $4M/yr.
Don Yeomans (JPL) and Al Harris (Planetary Science Institute) summarized the results of the current Spaceguard Survey in separate reports. Note that comparing their comments with previous survey status reports can be confusing, since the definition for 1-km asteroids in terms of the observable brightness has changed. With the new definition, there are fewer NEAs that meet the brightness criterion for 1 km diameter (absolute magnitude H=17.75). With this change, the current estimate for the population of >1km NEAs is 1000 +/- 50, and of PHAs (potentially hazardous asteroids) is 175 +/- 10. The current survey completeness for both NEAs and PHAs is approximately 75%.
Why has the Spaceguard survey slowed in the past 2 years? It was always known that the rate of discovery of new NEAs >1km would drop as the survey nears completeness, but the observed drop is greater than expected. Harris argues that this is a property of any asteroid survey, which initially finds NEAs at a higher than expected rate by preferentially discovering those in easy-to-find orbits, while later the rate drops below the expected curve because the easy ones are already mostly known. For this reason, it will always be difficult for a survey to meet a 90% goal in a timely way. It might be more realistic to set the metric at 80%, before the drop in efficiency reduces the expected rate of return on these surveys.
Harris also estimated the reduction in statistical risk from present and future surveys. He reported that the Spaceguard Survey has already reduced the exposure (retiring the risk of impact by unknown NEAs) by 90%. Applying the same methodology to future surveys, he concludes that the new ground-based surveys (Pan-STARRS-4 in Hawaii and LSST in Chile) will likely be able to reduce the risk from smaller sub-kilometer NEAs by 90% by 2020, even if they do not find 90% of all asteroids down to 140m.
Next Generation Surveys
Lindley Johnson (NASA HQ) summarized the conclusions of the current NASA NEO Study, developed in response to the congressional mandate. The study team considered various combinations of ground-based surveys (Pan-STARRS, LSST, and a separate NASA-dedicated LSST twin) and orbital surveys (optical and infrared, from both near-Earth space and near the orbit of Venus). To fully meet the Congressional mandate to find 90% of the NEAs >140m by 2020 requires several instruments and may cost of order $1 billion over the next 12 years, but finding 85%, or reducing the risk exposure by 90%, may be possible with the currently-planned ground-based systems at much lower cost to NASA. In any case, I note that uncertainty in the NEA population down to 140 m exceeds the differences between different survey strategies.
Paul Chodas and Steve Chesley (both of JPL) presented papers examining the way impact probabilities are computed and how our knowledge evolves over time as new observations (either optical or radar) are added to the calculations. This is a complicated problem, and for asteroids that come very close, we must expect a period of months, or even years, before we can say for certain whether a NEA will hit or miss. Chesley pointed out in particular that the calculated probability of impact for a NEA that is coming close to the Earth is likely to rise initially with the addition of new data, before it falls rather suddenly to zero. These are not errors or false alarms, but rather an aspect of calculating impact probabilities that we will need to live with.
Rick Binzel (MIT) reported on the classification and characterization of NEAs from remote (telescopic) observations, noting that this astronomical approach is the first line of defense against NEAs. In particular, the spectral matching to meteorites is an effective tool for physical and chemical characterization of NEAs.
Steve Ostro (JPL) showed several dramatic radar accomplishments. A major concern throughout the first two days of the conference was the pending closure by NSF of the unique Arecibo radar telescope. This facility has been an invaluable tool for understanding NEAs, and many speakers expressed dismay at its expected demise.
The results of the Hayabusa mission to Itokawa were enthusiastically received. These represent our best information on any sub-kilometer NEA, and attendees look forward to future Japanese NEA missions.
Erik Asphaug (UC Santa Cruz) described the challenge of probing the interior structure of small NEAs and discussed the value of radar tomography. David Morrison (NASA Ames) presented a paper discussing the role of future NEA characterization missions. The missions being planned or studied include NEAT (NASA Ames), an unnamed multiple-lander mission from Ball Aerospace, Deep Interior (UC Santa Cruz), OSIRIS (U Arizona), Don Quixote (ESA) and future Japanese missions. All of these spacecraft will either rendezvous or land on the NEA; flybys are not very useful for such small targets. Major issues surround the proper programmatic role of such missions. Obviously, if and when an asteroid is found on a collision course, there will be a major effort to characterize it in detail. But meanwhile what do we need to know, and with what urgency? Is it possible to plan for deflection technologies without learning more first about the physical and chemical and dynamical nature of the small NEAs?
This meeting included substantial advances in defining the options to deflect a small NEA if and when one is detected on a collision course with Earth, using ballistic or kinetic impact (KI), nuclear explosive, or gravity tractor.
Ed Luu (NASA JSC) discussed controlled deflection using the gravity tractor, a technique that allows precise measurement of the asteroid orbit before and during deflection. Even with small spacecraft (of order one ton mass), the gravity tractor works for many NEAs up to 200m, and it is most effective where there are close Earth flybys to amplify the applied orbit changes. The consensus was that a gravity tractor was the appropriate first defense mission approach, in part because it did not require a precursor mission.
Kinetic impactors were presented by Jesse Koenig (SpaceDev). He used simulations involving all known PHAs to show that kinetic impacts using current large launch vehicles can deflect most sub-km NEAs through combination of direct momentum transfer and cratering, given several decades of warning. In this approach, we need only a high-speed intercept, not a rendezvous. Koenig argued that KI is simple, cheap, fast, and requires no new technology; however, the orbital change is uncontrolled. Primary questions concern the response of the asteroid including the possibility of catastrophic disruption. For smaller asteroids, the energy of even a modest kinetic impact exceeds the gravitational binding energy of the asteroid. Others noted that for these very small asteroids, the challenge of hitting the target was much greater than it had been for the Deep Impact Mission.
Deflections by nuclear explosions were discussed by David Dearborn (Livermore Lab). Deflection is based either on surface heating by absorption of neutrons from a stand-off explosion, or by cratering with a surface explosion. Nuclear blasts might also be used to disrupt and disperse sub-km NEAs if warning time is short. The nuclear option delivers the greatest energy per unit mass, but the coupling of the nuclear energy into the required asteroid momentum change is not well understood, and the resulting deflection is thus not well controlled. While it is technically attractive, several speakers on the final day of the meeting noted the strong opposition to be expected from other nations and therefore recommended that any nuclear option should be a defense of last resort.
Keith Holsapple (U Washington) discussed the results to be expected of kinetic impacts. He expects most small NEAs to be rubble piles, based on frequency of impacts of their parent bodies in the main asteroid belt. The effectiveness of all the deflection techniques except the gravity tractor depends on asteroid interior structure. One major issue is momentum multiplication: because of cratering, the transmitted momentum is larger than the impacting momentum by factors of several (up to ten). Even when impact energy is greater than gravitational binding energy, the target will not disperse unless there is some efficient way of distributing the energy throughout the target. Normally, we expect only a small fraction of the mass to achieve escape velocity.
Impacts and Effects
Several papers were presented on the effects of impacts of various sizes. These included talks by both Galen Gisler (U Oslo) and Steven Ward (UC Santa Cruz) on impact tsunamis. Both emphasized the uncertainty that is caused by the fact that the wavelength and frequency of impact tsunamis is intermediate between that of ordinary storm waves and seismic tsunamis. Gisler presented detailed computer simulations of impacts, creating extremely high waves that also have high turbulent dissipation. However, his computer runs trace the waves only to 100 km from the point of impact, so they do not address the important question of how far these waves propagate. Ward's presentation noted one new effect, the build-up of water when a series of short-wavelength waves hit the shore. If the waves are coming in once every few minutes, they do not have time to retreat before the next in the series hits, so that the run-up and run-in of the impact tsunamis is greater than had been assumed previously.
Mark Boslough (Sandia) discussed interesting simulations of atmospheric explosions such as Tunguska. Noting that the fireball from a meteor explosion has considerable downward momentum (unlike the classic mushroom cloud from a nuclear explosion), he concludes that the Tunguska impactor exploded higher and was smaller (energy of order 5 megatons) than usually inferred.
Preparing the Public
Richard Davies (Western Disaster Center) challenged us to consider the political ramifications if a small (few megaton) impact took place without warning. Comparing with the government investigation following Hurricane Katrina, he noted that difficult questions would be raised concerning lack of preparation. This scenario highlights the absence of communication today between the NEO community and the Department of Homeland Defense, let alone the many state and local disaster agencies.
Several other speakers made similar points -- we are not connecting with the communities that exist to deal with natural hazards. They have an infrastructure that could include planning for a Tunguska-class impact, but they don't know we exist. We have thought about response to an impact warning, but not much to the consequences of an impact without warning, which remains the most likely (at least until completion of the next generation survey).
Social and Policy Issues
Several distinguished speakers from outside the normal NEO community spoke to the conference about these questions. One major issue was the role of international activities. Rusty Schweickart (Association of Space Explorers) made a strong case that this is a global problem, and it is likely that nearly every nation will at some point in the next few years have to think about its own vulnerability to an impact. Other speakers agreed that this is a topic that needs to be on the agenda of the UN and other international agencies, and that decisions to intercept a threatening NEA must be made internationally. Yet the fact is that there is very little international contribution today to NEA studies. Japan and the European Space Agency have scientific and technology demonstrations missions planned to NEAs, but the United States is the only country contributing to the Spaceguard Survey or the Arecibo Radar, two key elements in planetary protection that are happening now.
Geoff Sommer (Homeland Security Institute) noted that increasing quantification of the impact risk as a result of NEA surveys might work to our disadvantage in seeking resources, relative to other societal risks that are not quantifiable. People are generally more concerned about unknown risks than those that are well understood (e.g., terrorism or pandemics rather than auto accidents or malaria).
Chris Chyba (Princeton U) also addressed the impact hazard in the context of global threats, comparing it to issues such as global warming, nuclear proliferation, pandemics, and terrorism. He concluded that dealing with most global issues requires a response involving many nations, whereas any one spacefaring nation like the U.S. could mitigate the impact risk. He noted that the impact risk is static, while most other global threats are increasing -- which may decrease the motivation to allocate resources to protecting against impacts. Chyba also emphasized how strongly most of the international community mistrusts any mitigation strategy that involves nuclear explosions, and he recommended that our NEO community not take any action that might undermine other important security objectives such slowing nuclear proliferation.