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5 Techniques to Obtain NEO Sizes
Pages 35-40

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From page 35...
... A very simple asteroid thermal model might be a spherical black body, which absorbs all radiation incident on it and has a uniform equilibrium temperature around its surface. In practice, however, surface elements facing the Sun will be warmer than those on the night side of the object, and a telescope might be observing a side that is only partially illuminated by the Sun.
From page 36...
... Over the past few decades, as more information on the physical properties of asteroids has been gathered, it has become clear that most objects have thermal properties somewhere between the extremes represented by the two simple models mentioned above. The rapid increase in computing power in recent years now allows detailed thermophysical models to be applied to observational data to provide very accurate sizes and reliable estimates of thermal inertia and surface roughness.
From page 37...
... For the estimation of sizes and albedos in the absence of information on physical properties, another option exists, the Near Earth Asteroid Thermal Model (NEATM) , which represents a compromise between the STM and FRM and removes the problem of not knowing which of the two alternatives to apply.
From page 38...
... TABLE 5.1  Representative Error Estimates in Asteroid Diameter Determinations Maximum Diameter Method Error Typical Errora Visible observations,b assuming albedo, pV, = 0.15 taking extreme range −80%, +100% 0.01 < pV < 0.5 Visible observationsb assuming albedo, pV = 0.15 taking typical range −70%, +75% 0.02 < pV < 0.35 IR/STM α = 20° −25% −10% α = 50° −25% −10% IR/FRM α = 20° +45% +30% α = 50° +25% +10% IR/NEATM, fixed η = f(α) , one purely thermal band ±40% ±20% IR/NEATM, fitted η, ≥ two thermal bands α = 20° +5% <5% α = 50° +15% +5% NOTE: IR, infrared; FRM, Fast Rotating Model; NEATM, Near Earth Asteroid Thermal Model; NEO, near Earth object; STM, Standard ­ hermal T Model.
From page 39...
... or can be calculated and subtracted from the measured total fluxes. Corrections for the reflected component in wavelength bands below 5 μm often result in thermal flux values with relatively large uncertainties.2 To optimize the accuracy of diameter determinations, the wavelength bands of filters should be chosen so as to minimize contamination by reflected solar radiation (see Figure 5.3)
From page 40...
... Thus, improved knowledge of the distribution of NEO physical properties will lead to improved models, which can be applied to existing and future infrared data sets to increase the accuracy of derived parameters, such as size. Furthermore, in the course of time, as more observational data become available for an object, thermophysical modeling based on an accurate shape model can significantly improve the initial results obtained from simpler models.


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