Luna's (Earth's Moon) Thermal Environment

extracted from Thermal Environments JPL D-8160

The following table summarizes the range of Direct Solar, Reflected Solar (Albedo), and Planetary Infrared for the planet Earth's Moon.


                 Perihelion         Aphelion        Mean
----------------------------------------------------------------
Direct Solar     1414+/-7 W/sqM   1323+/-7 W/sqM  1367.5+/-7 W/sqM

Albedo              0.073            0.073            0.073
(subsolar peak)

Planetary IR.     
(subsolar peak)   1314 W/sqM      1226 W/sqM       1268 W/sqM
Minimum            5.2 W/sqM       5.2 W/sqM        5.2 W/sqM

The thermal environment in orbit around the moon is similar to that of Mercury's since there is no atmosphere. It is dominated by the planetary infrared term. As an approximation, the surface temperature can be described as falling off from the subsolar point as a cosine function

Temperatures on the dark side of the moon are on the order of 100K. The maximum temperature on the sunlit side of the moon peaks at 400K.

The temperatures are not equivalent blackbody levels (as used for most other planets), but are to be combined with an overall surface emittance of 0.92.

The average lunar albedo is about 0.073. Variations among surface features are shown below.


                   NORMAL ALBEDO*
  --------------------------------------------
   Copernican-type craters              0.126    
   Apennine Mountains                   0.123
   Mare Serenitatis                     0.093
   Mare Tranquillitatis                 0.092
   Mare Fecunditatis                    0.092
   Langrenus Crater                     0.129
  --------------------------------------------
  * approximate average values

The Apollo program missions provide some interesting facts pertinent to spacecraft thermal balance problems, for both orbiting and surface missions. The planetary infrared is of such a magnitude that the radiator surfaces are significantly affected in lunar orbit. In particular, the spacecraft attitude for "parking" or "sleep" periods should be picked to minimize the view to the lunar surface. Since most radiators surfaces have a relatively low solar absorptance, but a high infrared emittance, it can frequently be preferable to point the radiators toward the Sun to some extent in order to minimize its view to the lunar surface.

A similar effect occurs during lunar surface operations. The proximity of relatively low mountains near Hadley Rille (Apollo15) and Taurus Littrow (Apollo 17)affected thermal performance of lunar surface equipment. In particular, electronic packages having zenith pointing radiator actually had small view factors to nearby mountains of a few percent. The infrared load from the hot mountains influenced temperatures of the equipment by at least 10C which is the order of magnitude uncertainty in thermal predictions due to other uncertainties. Thus, the presence of mountains for lunar surface operations cannot be ignored.

Another factor in lunar surface operations is lunar dust. Such dust is easily thrown up during rover operations or just by a person walking near equipment. Since lunar dust has an inherent solar absorptance of about 0.93, it takes only a very small amount to significantly increase the absorptance of low absorptance radiator surfaces. This effect was so strong, that by the last Lunar Rover mission (Apollo 17), dust was brushed of radiator surfaces by crew at almost every stop for the rover vehicle.

The last factor is extremely low thermal diffusivity of the lunar soil. This results in the surface temperature of shadowed regions reaching almost the nightside value of 10K quickly. Thus, the presence of a shadow near a surface of interest will result in a much more reduced infrared load than thought. This was of particular concern for the Apollo 14 Modular Equipment Transporter which had rubber tires. The lower limit of the rubber was -70F (-57C) and the shadows created by the tires themselves required that the vehicle be parked such that one tire did not shadow the other tires.

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