|MadSci Network: Earth Sciences|
Reference:The Photonics Handbook, Laurin Publishing Inc., Pittsfield, MA 1991
The answer to your question is that gold reflects the infrared energy (heat rays)
from the sun more efficiently than silver or aluminum.
Why is this important for spacecraft and astronauts?
One of the more difficult problems in designing a spacecraft is radiating the waste heat generated within
the spacecraft back into space to prevent over heating. The two greatest generators of heat on a typical
communications satellite are the solar cell panels which are about 22% efficient and the microwave
transmitters which are about 65% efficient. What the efficiency means is that only 22% of the solar energy
incident on the solar cell arrays is converted into electricity and 78% heats the spacecraft structure. The
microwave transmitters radiate about 65% of the battery power in microwave beams for communications
and 35% of the battery power used in the transmitters generates waste heating of the spacecraft structure.
Today communications satellites have solar cell arrays that generate 10,000 watts of electrical power
which is stored in the spacecraft batteries. This means that about 60,000 watts of waste heat are
generated by the solar cells. The microwave transmitters convert about 6,000 watts of battery power
into 4,000 watts of microwave power, generating 2000 watts of waste heat within the spacecraft.
Special black radiating panels are used to radiate the many thousands of watts of waste heat back into
space so that the spacecraft structure stays at a reasonable temperature and the electronics and solar cells
are not destroyed by overheating. These radiating panels are aimed away from the sun into cold deep
space so they do not gather solar heat.
So what can we do about the thousands of watts of solar energy that illuminate the spacecraft structure
and could generate even greater amounts of waste heat? We cover the exposed spacecraft structure with
very lightweight, thermally reflecting blankets of metalized mylar to reflect about 98% of the solar energy
back into space with only 2% heating the spacecraft. These metalized mylar blankets are similar to the
metalized balloons that are now for sale in flower and party shops. However, the very thin reflective mylar
coatings are gold rather that the aluminum used in the balloons. Aluminum is a very efficient reflector of
visible light generated by the sun; however, there is a great deal of invisible infrared energy also generated
by the sun. Aluminum is not a very efficient reflector at infrared wavelengths. Gold is an efficient
reflector of both visible wavelengths and infrared wavelengths. Infrared telescopes use gold plated mirrors
for the same reason. Gold is more expensive than aluminum and is softer and scratches more easily so
great care must be taken when placing the gold plated mylar blankets on the spacecraft before launch.
If you look closely at the visors used by the astronauts on the moon and during space walks, you will see
that they also have a very thin gold coating to reduce infrared solar heating within the helmet.
Best Regards, Your Mad Scientist
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