and selected by peer-reviewed competition is the better approach. This combination could complete the survey well before 2030, perhaps as early as 2022 if funding were appropriated quickly.
If cost conservation is deemed more important, the use of a large ground-based telescope is the better approach. Under this option, the survey could not be completed by the original 2020 deadline, but it could be completed before 2030. To achieve the intended cost-effectiveness, the funding to construct the telescope must come largely on the basis of non-NEO programs.
As noted above, neither Congress nor the administration has requested adequate funding to conduct the survey to identify ≥90 percent of the potentially hazardous NEOs by the year 2020. Multiple factors will drive the decision on how to approach this survey in the future. These include but are not limited to the perceived urgency for completing the survey of 140-meter-diameter NEOs as close to the original 2020 deadline as feasible and the availability of funds to provide for the successful completion of the survey. The combination of a space-based detection mission with a large ground-based telescope could complete the survey in the shortest time, that is, closest to the original 2020 deadline. A space-based mission alone could complete the survey only 2 to 4 years later than a survey conducted with both a space-based telescope and a large ground-based telescope. The cost of optimizing the LSST for NEO detection observations was estimated in 2007 to be an increment of approximately $125 million to the cost of the basic telescope system (Ivezič, 2009), becoming the most cost-effective means to complete the survey. (Note that the annual operating cost of a ground-based telescope is approximately 10 percent of the development and construction costs.) The completion date would be extended. The decision to extend this date requires the acceptance of the change in risk over that time.
Increasing concern with the possibility of smaller NEOs resulting in low-altitude airbursts has led the committee to raise the question of the identification of hazardous NEOs that have a diameter smaller than 140 meters. The ability to detect objects having diameters of greater than 50 meters and greater than 30 meters was therefore also compared among the modeled telescope systems.
Finding: It is highly probable that the next destructive NEO event will be an airburst from a smaller-than-50-meter object, not a crater-forming impact.
Recommendation: Because recent studies of meteor airbursts have suggested that near-Earth objects as small as 30 to 50 meters in diameter could be highly destructive, surveys should attempt to detect as many 30- to 50-meter-diameter objects as possible. This search for smaller-diameter objects should not be allowed to interfere with the survey for objects 140 meters in diameter or greater.
With the discovery of NEO 2008 TC3, found within 19 hours of impact into the Sudan desert, the committee discussed the question of an increasing capability to detect imminent impactors on their final approach to Earth. Optimizing the detection of imminent impactors requires a different observing strategy than the approaches discussed above designed to discover hazardous NEOs with long lead times before impact. The existing CSS (which found 2008 TC3) is configured such that with a change in observing sequence, it could discover up to 50 percent of the imminent impactors (i.e., bodies smaller than 1 kilometer in diameter that could impact in hours or weeks). Likewise, the Discovery Channel Telescope could make a significant contribution toward identifying imminent impactors. Other types of systems designed specifically to detect such objects could be built but were not considered by the committee. The imminent impactors represent the next level of survey and detection efforts, as their discoveries contribute to gains in the knowledge of NEO properties and their prompt discovery would allow for civil defense measures to be instituted in a timely manner.