From: Ames Research Center
Posted: Friday, March 9, 2007
The second triennial Planetary Defense Conference is being held this week at George Washington University, Washington DC. The organizer is Bill Ailor of the Aerospace Corporation, and 150 people are registered. Following is a report of some highlights from the first two days (dealing with NEA surveys and population, characterization, and technologies for deflection), together with early press coverage.
Of special interest to the attendees is the long-awaited NASA NEO report to Congress. Although the report had not been delivered, both Pete Worden, Director of Ames Research Center, and Lindley Johnson of NASA Headquarters discussed the study findings. The lead of the AP story about the conference summarized: NASA officials say the space agency is capable of finding almost every asteroid that might pose a devastating threat to Earth, but because it lacks the money to do it, the job will not get done. On Monday we were told that the report would be submitted this week, but by Wednesday it was clear that it had been delayed for further editing at NASA Headquarters and/or OMB.
NEA Survey Status
Don Yeomans (JPL) and Al Harris (Planetary Science Institute) summarized the results of the current Spaceguard Survey in separate reports. In comparing their comments with previous "how are we doing" status reports, note that the definition for 1-km asteroids in terms of the observable brightness has changed, making direct comparison difficult. 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 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. The uncertainty in the NEA population down to 140 m exceeds the differences between different survey strategies. NEO News will report more on these issues when the NASA report is released.
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's paper on radar imaging showed several recent successes, some of which have been discussed recently in NEO News. 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. One attendee proposed that NASA should not contribute "one thin dime" toward the LSST until NSF reinstates funding for the Arecibo radar.
The results of the Hayabusa mission to Itokawa, also reported previously in NEO News, were enthusiastically received. These represent our best information on any sub-kilometer NEA, and we 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), a mission from Ball Aerospace, Deep Interior (UC Santa Cruz), OSIRIS (U Arizona), Don Quixote (ESA) and future Japanese missions. All of these spacecraft plan either rendezvous or landing 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 impact, 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 a 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 ballistic impacts are simple, cheap, fast, and require 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 asteroids, the challenge of hitting the NEA target was much greater than it had been for the Deep Impact comt mission because the targets are so much smaller.
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 resulting effects are 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 predicted for 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.
NEO News (now in its thirteenth 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 firstname.lastname@example.org. 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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