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Fig. 11 -15. Solar and Room Temperature Body Spectral Distribution. The solid lines identify the wavelength of peak emission for solar energy and a body at room temperature, but do not represent magnitude (no vertical scale). Note that solar energy wavelength is much shorter than that of a body at room temperature. The dashed line represents the absorptance or omittance of a quartz mirror radiator. The point is that we can select thermal control coatings that are highly reflective to high energy inputs like solar energy and highly emissive at room temperatures.

Albedo

Sunlight reflected off a planet or moon is known as albedo. A planet's albedo is usually expressed as the fraction of incident sunlight that is reflected back to space and is highly variable. Usually, reflectivity is greater over land as compared with oceans and generally increases with decreasing local solar-elevation angles and increasing cloud coverage. Because of greater snow and'ice coverage, decreasing solar elevation angle, and increasing cloud coverage, albedo also tends to increase with latitude. These variations make selection of the best albedo value for a particular thermal analysis rather uncertain, and it is not unusual to find variations throughout the industry. Representative values for orbits of different inclinations are shown in Table 11-45A.

Fig. 11 -15. Solar and Room Temperature Body Spectral Distribution. The solid lines identify the wavelength of peak emission for solar energy and a body at room temperature, but do not represent magnitude (no vertical scale). Note that solar energy wavelength is much shorter than that of a body at room temperature. The dashed line represents the absorptance or omittance of a quartz mirror radiator. The point is that we can select thermal control coatings that are highly reflective to high energy inputs like solar energy and highly emissive at room temperatures.

Wavelength (nm)

Wavelength (nm)

TABLE 11-45A. Typical Orbit-Average Earth IR and Albedo Values for Various Orbits (Data Courtesy NASA). This table shows the relative nature of Earth-emitted IR and percent Albedo as a function of orbit Inclination. Note that the highest values of Earth-emitted IR occur at lower inclinations associated with tropical and desert regions. The value of Earth-emitted IR decreases with increasing latitude. In general, albedo increases with Increasing latitude.

TABLE 11-45A. Typical Orbit-Average Earth IR and Albedo Values for Various Orbits (Data Courtesy NASA). This table shows the relative nature of Earth-emitted IR and percent Albedo as a function of orbit Inclination. Note that the highest values of Earth-emitted IR occur at lower inclinations associated with tropical and desert regions. The value of Earth-emitted IR decreases with increasing latitude. In general, albedo increases with Increasing latitude.

Orbit

Angle of Sun Out of

Emitted Radiation (W/m2)

Albedo (percent)

Inclination (deg)

Orbit Plane (deg)

Mln

Max

Mln

Max

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