While impacts by these smaller objects would not be expected to cause global devastation, impacts on land and the tsunamis resulting from ocean impacts could still cause massive regional damage and still pose a significant long-term hazard.

In 1998 NASA commenced its part of the "Spaceguard" effort, with the goal of discovering and tracking over 90% of the near-Earth objects larger than one kilometer by the end of 2008. An Earth impact by one of these relatively large objects would be expected to have global consequences and, over time scales of a few million years, they present the greatest impact hazard to Earth.

Approximately 60% of the estimated 1,000 to 1,200 large near-Earth objects have already been discovered, about 45% since NASA efforts started, and each of the five NASA-supported search facilities continue to improve their performance, so there has been good progress toward eliminating the risk of any large, undetected impactor.

To understand the next steps to discovering the population of potentially hazardous asteroids and comets whose orbits can bring them into the Earth's neighborhood, NASA turned to this Science Definition Team of 12 scientists. The Team, chaired by Dr. Grant Stokes of the MIT Lincoln Laboratory, was asked to study the feasibility of extending the search effort to the far more numerous, perhaps hundreds of thousands, of near-Earth objects whose diameters are less than one kilometer.

NASA considers the Science Definition Team's findings to be preliminary, and a much more in-depth program definition, refining objectives and estimating costs, would need to be conducted prior to any decision to continue Spaceguard projects beyond the current effort to 2008.

Science Definition Team Recommendations
The Team makes three specific recommendations to NASA as a result of the analysis effort:

Recommendation 1 - Future goals related to searching for potential Earth-impacting objects should be stated explicitly in terms of the statistical risk eliminated (or characterized) and should be firmly based on cost/benefit analyses.

This recommendation recognizes that searching for potential Earth impacting objects is of interest primarily to eliminate the statistical risk associated with the hazard of impacts. The "average" rate of destruction due to impacts is large enough to be of great concern; however, the event rate is low. Thus, a search to determine if there are potentially hazardous objects (PHOs) likely to impact the Earth within the next few hundred years is prudent. Such a search should be executed in a way that eliminates the maximum amount of statistical risk per dollar of investment.

Recommendation 2 - Develop and operate a NEO search program with the goal of discovering and cataloging the potentially hazardous population sufficiently well to eliminate 90% of the risk due to sub-kilometer objects.

The above goal is sufficient to reduce the average casualty rate from about 300 per year to less than 30 per year. Any such search would find essentially all of the larger objects remaining undiscovered after 2008, thus eliminating the global risk from these larger objects. Over a period of 7-20 years, there are a number of system approaches that are capable of meeting this search metric with quite good cost/benefit ratios.

Recommendation 3 - Release a NASA Announcement of Opportunity (AO) to allow system implementers to recommend a specific approach to satisfy the goal stated in Recommendation 2.

Based upon our analysis, the Team is convinced that there are a number of credible, current technology/system approaches that can satisfy the goal stated in Recommendation 2. The various approaches will have different characteristics with respect to the expense and time required to meet the goal. The Team relied on engineering judgment and system simulations to assess the expected capabilities of the various systems and approaches considered. While the Team considers the analysis results to be well-grounded by current operational experience, and thus, a reasonable estimate of expected performance, the Team did not conduct analysis at the detailed system design level for any of the systems considered. The next natural step in the process of considering a follow-on to the current Spaceguard program would be to issue a NASA Announcement of Opportunity (AO) as a vehicle for collecting search system estimates of cost, schedule and the most effective approaches for satisfying the recommended goal. The AO should be specific with respect to NASA's position on the trade between cost and time to completion of the goal.

The Science Definition Team members include: Dr. Grant H. Stokes (Chair) MIT Lincoln Laboratory Dr. Donald K. Yeomans (Vice-Chair) Jet Propulsion Laboratory/Caltech Dr. William F. Bottke, Jr. Southwest Research Institute Dr. Steven R. Chesley Jet Propulsion Laboratory/Caltech Jenifer B. Evans MIT Lincoln Laboratory Dr. Robert E. Gold Johns Hopkins University, Applied Physics Laboratory Dr. Alan W. Harris Space Science Institute Dr. David Jewitt University of Hawaii Col. T.S. Kelso USAF/AFSPC Dr. Robert S. McMillan Spacewatch, University of Arizona Dr. Timothy B. Spahr Smithsonian Astrophysical Observatory Dr./Brig. Gen. S. Peter Worden USAF/SMC

Related Links
Complete NEO Report from Science Definition Team
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