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Defending Planet Earth: Near-Earth-Object Surveys and Hazard Mitigation Strategies
FIGURE 1.1 Current estimates of the average interval in years between collisions with Earth of near-Earth objects of various sizes, from about 3 meters to 9 kilometers in diameter. The uncertainty varies from point to point, but in each case is on the order of a factor of two; there is also a strong correlation of the values from point to point. SOURCE: Courtesy of Alan W. Harris, Space Science Institute.
65 million years ago in the Yucatan Peninsula. The asteroid or comet that caused this crater is estimated to have been about 10 kilometers in diameter; its impact wrought global devastation, likely snuffing out species, including dinosaurs, in huge numbers. Later, in the 1990s, the collision of comet Shoemaker-Levy 9 with Jupiter emphasized that impacts are currently possible.
To assess the current hazards, surveys were undertaken in the 1970s and were greatly augmented in the 1990s in order to discover and track all NEOs to determine the likelihood that one or more would collide with Earth. These surveys, involving relatively small telescopes whose primary mirrors ranged in diameter from 0.6 to 1.2 meters, were seeking objects with diameters greater than 1 kilometer; also detected were many smaller objects that approached Earth closely enough to be seen.2
Congress requested that the National Research Council (NRC) undertake a study, sponsored by NASA, to address two tasks:
Task 1: NEO Surveys
What is the optimal approach to completing the NEO census called for in the George E. Brown, Jr. Near-Earth Object Survey section of the 2005 NASA Authorization Act to detect, track, catalogue, and characterize the physical
Brightness is the key determinant of detectability; the apparent brightness of an object as seen from Earth varies with the inverse square of its distance from Earth (e.g., twice as close implies four times as bright).
National Aeronautics and Space Administration Authorization Act of 2005 (Public Law 109-155), January 4, 2005, Section 321, George E. Brown, Jr. Near-Earth Object Survey Act.
The committee notes that the statement of task includes the term “detect,” which includes spotting asteroids that have previously been discovered. The committee therefore uses the more appropriate term “discover” to refer to the locating of previously unknown objects.