Most of the comments below were subsumed into the pdf link somehow.

Regards,
Mark

"This edition of NEO News is devoted to the report of the NASA Science Definition Team (SDT), led by Grant Stokes, describing the situation with respect to NEAs less than 1 km in diameter: both the impact threat and proposals to deal with it. SDT members Don Yeomans and Steve Chesley will be presenting many of these results to a scientific audience at the American Geophysical Union Meeting in San Francisco on December 12, in a session organized by David Morrison and Alan Harris to mark ten years of progress on Spaceguard.

This NASA SDT was chartered to study what should be done to find near-Earth objects less than 1 kilometer in size. While impacts by these sub-km objects are not expected to cause global devastation, impacts on land and the tsunamis resulting from ocean impacts could still cause massive regional damage and pose a significant long-term hazard. Following this introduction are the Executive Summary of the SDT Report plus links to a website providing the entire 166-page report with figures and references.

This report is a successor to two other influential NASA studies of the NEO impact hazard. In 1992, the NASA Spaceguard Survey Report (David Morrison, chair) first identified the magnitude of the impact hazard and recommended a survey to find asteroids large enough to threaten global disaster if they struck (approximately those larger than 1 km in diameter). In 1995 a NASA follow-up report chaired by Gene Shoemaker developed a more detailed survey strategy and established the current Spaceguard goal to find 90% of the NEAs larger than 1 km diameter within 10 years. In 1998 NASA commenced its part of the Spaceguard effort, adopting the goal of 90% of the NEAs >1 km by the end of 2008. Approximately 60% of the estimated 1,000 to 1,200 NEAs in this size range have already been discovered.

To understand the next steps toward discovering the population of potentially hazardous asteroids and comets whose orbits can bring them into the Earth's neighborhood, NASA selected to this Science Definition Team of 12 scientists. The SDT, chaired by 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 NEOs whose diameters are less than one kilometer. (As an aside, just one scientist, Don Yeomans, participated on all three of these NASA studies).

Among the new results in this SDT Report are (1) a conclusion that comets account for less than 1% of the hazard from sub-km objects, so that they can be neglected, (2) a focus on reducing or retiring the risk as opposed to setting goals in terms of a percentage of NEAs to be discovered; (3) an analysis of the tsunami hazard that suggests the risk is less than had sometimes been asserted previously, but is still significant, (4) a discussion of the roles of long-term prediction through a survey vs. attempts to find asteroids shortly before their impacts, and (5) a comparison of ground-based and space-based survey strategies.

There are respects in which the SDT report differs from most previous discussions, so that some of the conclusions might at first seem contradictory. Most of the analysis in this report deals with potentially hazardous objects (PHOs), which are a subset of the NEOs or NEAs. Only about 20% of the NEAs are PHOs. Thus some numbers (on population and discovery rate for example) appear to be only 1/5 of those we usually see quoted, but they are in fact equivalent. The SDT also chose to assume no improvements in the current survey system, and they therefore conclude that the Spaceguard Survey will fall short of meeting its goal of 90% of NEAs >1 km by the end of 2008. In contrast, most past analyses have assumed continuing improvements. Finally, the SDT made very conservative assumptions concerning the operations of future ground-based surveys (by the proposed LSST, for example), thus concluding that such telescopes will not be as capable as others have suggested for finding the sub-km NEAs.

This detailed report with its careful quantitative hazard analysis establishes the new standard for evaluating the impact hazard, setting the stage for a broader policy discussion of the desirability of carrying out a survey of sub-km NEAs. "

David Morrison