Defending Planet Earth Near-Earth-Object Surveys and Hazard Mitigation Strategies (2010) / Chapter Skim
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4 Characterization
4 Characterization
Pages 51-65
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From page 51... ...
Such information is important for assessing the hazard potential of individual NEOs that may threaten Earth and the viability of proposed mitigation strategies. A majority of the work supported under NASA's NEO Observations Program to date has focused primarily on the detection and orbit determination of NEOs.
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From page 52... ...
Radar investigations are exploring the physical properties of individual NEOs, including their sizes, shapes, surface roughness, rotation periods, and rotation pole orientations, as well as whether they have satellites. In addi tion, time variations of brightness as NEOs spin ("light curves")
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From page 53... ...
1 It is a powerful tool for rapidly improving the knowledge of the orbit of a newly found object and thus for characterizing its potential hazard to Earth. In addition to orbit improvement, the interaction of radar signals with the surface of the NEO yields information about its physical characteristics.
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From page 54... ...
Opportunities for radar imaging with a caliber comparable to those shown here occur several times annually. Within its latitude coverage, Arecibo can detect objects at twice the distance as can Goldstone for similarly sized objects and has contributed two-thirds of all radar range and radial velocity measurements on NEOs obtained in the last decade.
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From page 55... ...
radar images of near-Earth asteroid 1999 JM 8. Illumination is from the top; range increases downward, and the wavelength of the echoes of the radio waves increases to the left.
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From page 56... ...
However, the Goldstone radar competes for telescope time with numerous NASA spacecraft missions that have higher priority and that often limit the time available for radar observations. The antenna is also available for short-notice target-of-opportunity observations if the flight projects have sufficient scheduling flexibility to accom modate changes, and if radiation clearance can be obtained in time from the numerous military and other government organizations whose airspace surrounds Goldstone.
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From page 57... ...
A recent JPL internal study found that, despite its restricted pointing capabilities, Arecibo is capable of observing up to two-thirds of newly discovered potentially hazardous nearby NEOs because these nearby objects move so rapidly across the sky that many pass through Arecibo's latitude "window" before they exceed detectable range. The corresponding figure for Goldstone (whose detectable range on a given object is about one-half of Arecibo's)
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From page 58... ...
Finding: The number of NEOs observed by radar per year could be increased about fivefold by obtaining sufficient observing time. Arecibo and Goldstone radar observations of more than 20 NEOs have revealed that surface roughness depends on composition and that very rough surfaces are common.
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From page 59... ...
TABLE 4.2 Number of Near-Earth Objects in Different Size Intervals That Are Detectable by the Arecibo and Goldstone Radar Observatories Diameter (D) Number Detectable D > 1 km 46 0.14 km < D < 1 km 110 D < 0.14 km 252 Total 408 them (see Figure 4.3)
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From page 60... ...
, but neither is funded for dedicated observations of NEOs. The annual cost for Arecibo to carry out up to 300 hours of radar observations plus adequate maintenance is estimated at $2 million (approximately $1 million for the cost of purely radar operation [fuel, salaries, and so on]
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From page 61... ...
Finding: The Arecibo and Goldstone radar systems play a unique role in the characterization of NEOs, providing unmatched accuracy in orbit determination and offering insight into size, shape, surface structure, and other properties for objects within their latitude coverage and detection range. Finding: Congress has directed NASA to ensure that Arecibo is available for radar observations but has not appropriated funds for this work.
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From page 62... ...
Ground-based studies of NEOs using data on both rotation rates and satellites suggest that most NEOs larger than about 150 meters in diameter are rubble piles, while most smaller ones are monolithic, with enough long-term tensile strength to prevent them from flying apart. The larger objects that are weak rubble piles easily disintegrate during atmospheric entry and create airbursts that somewhat resemble high-altitude nuclear explosions.
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From page 63... ...
IN SITU CHARACTERIZATION RELEVANT FOR MITIGATION Detailed knowledge of the physical characteristics of several representative NEOs would improve understand ing of the overall NEO population and help the design and implementation of the mitigation techniques that may be employed should an NEO threaten Earth (but that understanding may well not improve the knowledge of a specific object on an impact trajectory)
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From page 64... ...
, dedicated spacecraft missions to NEOs providing extended periods for observations and investigation close to NEOs obtain detailed characterizations of their rotational motions, masses, sizes, shapes, surface morphology, internal structure, mineral composition, and collisional history. The data collected from NEO characterization missions would also help to calibrate the ground- and space-based remote sensing data and may permit increased confidence in the remote classification of NEOs and their associated physical characteristics, which could inform future mitigation decisions.
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From page 65... ...
, Proceedings of the 236th Symposium of the International Astronomical Union, Prague, Czech Republic, August 14-18, Cambridge University Press, Cambridge, U.K.
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