From: Ames Research Center
Posted: Friday, March 9, 2007
Below is a report on the second half of the Planetary Defense Conference held this week in Washington DC. These sessions dealt with a range of societal and strategy issues, and in this summary I am reporting on only a few of the papers. At the end of this edition of NEO News are more press comments from the meeting.
I am happy to note that the NASA Study Report to Congress, submitted in response to the mandate to present a program plan to find 90% of NEAs down to 140m, was submitted late this week. This 27-page report is titled "Near Earth Object Survey and Deflection Analysis of Alternatives". We will comment on this report in a later edition of NEO News.
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 the effects of 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 considerably 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 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).
Policy and Political 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 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 several nations, whereas the U.S. alone can 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 we pursue non-nuclear options to the extent possible. (This opinion is in contrast to the current NASA study, which gives considerable attention to nuclear options for asteroid threat mitigation.)
Preparing a White Paper
The final discussions at the meeting dealt with input to an AIAA White Paper. The strongest consensus concerned the critical importance of maintaining the Arecibo Planetary Radar; support for Arecibo was proposed as the first resolution in the White Paper. Other ideas included expansion of current surveys (taking advantage of the opportunities to include NEAs within the mission of the Pan-STARRS and LSST telescopes) and expanding the capability to process these data and generate NEA orbits and impact predictions. On the question of how to protect against an identified threat, the opinions seemed to fall toward using the gravity tractor or the ballistic impact in preference to nuclear explosives.
All three of these deflection technologies need to be developed, however, since they have very different capabilities, and we are dealing here with a huge range of NEA sizes and orbits. One of the complications is just getting to the asteroid, which is much harder when it chooses us, as opposed to us choosing it. The gravity tractor must match orbits (rendezvous) with the target. In contrast, the kinetic impactor does not need to slow down near the asteroid, but it does have to hit a small target at high speed. Nuclear charges would normally be used from a rendezvous spacecraft, but in extreme cases might be deployed like kinetic impact, with a high-speed intercept. There is no "one size fits all" solution.
Below are specific comments on these deflection strategies from Rusty Schweickart, as well as several press responses.
Comments from Rusty Schweickart on asteroid defense strategy:
In the last day of the conference it all came together nicely, integrating much of what we had heard both re deflection options, characterization and the anticipated discovery "demographics". My take on it is that we're now very close to the following:
There will be some cases where impact-threatening NEAs will experience close gravitational encounters (usually with Earth) prior to impact. In most of these cases, due to the multiplication effect of the associated keyhole, the gravity tractor (GT) will be adequate to the job.
If not (e.g. large object and/or "weak" keyhole) then a kinetic impactor (KI) will certainly be adequate. However, since the keyhole makes the uncertainty large, a transponder should be sent ahead to both collapse the line of variations on arrival and also be there for surveying the detailed results of the kinetic impact. Since a gravity tractor also has a transponder aboard, the transponder mission itself should be a GT, which also then has the advantage of not only surveying the final result of the KI but also "trimming" up the deflection with precision to assure (the world) that the deflection did not put the NEO into another keyhole.
Finally, when all else is inadequate, the world will have to make the tough decision of whether to take the chance of a hit or use a nuke. This should be an extremely small component of the overall threat, and a diminishing one over time since the major need for a nuke is the possibility of finding a NEA headed for a near-term impact during the next 15 years of the survey. After that it's only the small remaining component of the residual very large NEAs.
The GT is a bit wimpy, but precise and great for when the ball is on the green. KI is far more capable but more imprecise and should not be launched without a GT/transponder.
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