Lesson learned on Satellite Thermal.
For years, we have been trying to demonstrate to students the extreme differences in Temperature of a satellite based simply on its color. In space, far from earth, here is what you should get for three identical satellites:
Black will be about +55 deg F White will be about -60 deg F Aluminum will be about +225 deg F
(from memory anyway)... But we have never been able to see the extreme temperature of the aluminum in our vacuum chamber. Of course, we are not using a SUN, but an incandescent lamp which has 95% of its radiation as heat and only 5% as light, so we attribtuted our wishy-washy results as due to the lack of real solar spectrum.
This year, we finally have a Tvac chamber that has a liguid nitrogen cold plate so we can actually better simulate the blackness (and cold) of space.. Again, same result. Aluminum was about the same as black. This was a frustrating result from this new chamber.
Then I noticed the fingerprints on the aluminum. I cleaned the aluminum with a swab of alcohol to eliminate all the surface contaminants, and fingerprints and re-did the test...
BINGO. Now the temperature of the aluminum goes up and off scale HOT, way way different from the black or White. And now the WHITE also goes colder..
So just the thinneest innvisible layer of surface contamination completely changed the thermal emissivity of the Aluminum. And we all know this anyway, since "low-E" coatings of glass and other materials is now so common. It is only the outer surface of molecules that set the absorbtivity and emissivity... And the differnce between Black and Aluminum is 30-to-1 (if the aluminum is clean)... That's why we wrap baked potatos in Aluminum foil! (don't use greasy hands)...
Of course we always clean our actual spacecraft to clean-room conditions before flight, so this does not impact our on-orbit results, but it sure does make a difference in the lab when we are demonstrating absorbtivity and emissivity in the chamber with samples handled by the students!
When I get a chance, Ill post the results...
Bob, WB4APR