MadSci Network: Earth Sciences |
Greetings:
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
Adrian Popa
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