Many photographs of space agencies, whether NASA or Roscosmos, often feature parts wrapped in gold-colored foil when preparing spacecraft. Obviously, it serves as an isolation, but why is it gold? And how does it work?
As you know, there is solar radiation that affects the Earth’s atmosphere and is the main source of energy for all the natural processes of the planet. From the graph, where the intensity is graphically presented, it can be seen that the peak falls on the visible spectrum, although a considerable fraction of the radiation is in both the IR and UV ranges (for more details on the phenomenon, see Wikipedia, article " Solar radiation "):
Therefore, equipment that should work in space, it is necessary to protect as much as possible from the effects of infrared radiation. For this, some parts of spacecraft are wrapped in a metal heat-insulating “foil”. Conventional such “reflectors” are made of aluminum, silver, copper or gold. But in the space industry, gold is widely used. The graph below shows the change in reflection coefficient for each of the metals, depending on the wavelength:
All four metals cope well with the reflection of infrared radiation, the coefficient is close to 100% for waves longer than 700 nm. So why is gold used? Because, unlike copper or silver, gold is not susceptible to corrosion (for the same reason, it is widely used in the manufacture of printed circuit boards), and it is also easier to work with gold than with aluminum (Remember what happens with aluminum foil if it is wrinkled and then deploy again).
On the whole, the “gold foil” consists far from only gold. This is a multilayer metallized film, and different parts of spacecraft are wrapped in different types of films, depending on the needs. There is such a PDF from NASA, where it is described in detail, with pictures!
Often used as film materialkapton , which in turn is applied a thin layer of gold, aluminum, silver. The last two metals can give a golden color with very fine spraying.
A good example of the use of gold spraying is the technology of modern spacesuits: helmet filters are made of polycarbonate-based plastic (UV protection), which is also coated with a sufficiently thick layer of gold to protect the astronaut from solar radiation. As a result, the filter transmits no more than 35% of the light, which completely protects the eyes from the external influence of infrared radiation, which is outside the visible spectrum, which means that it can imperceptibly cause permanent damage to the astronaut's vision. The "Golden Filter" stops almost all UV and IR radiation and about 60% of the radiation in the visible spectrum.
PS Another application of gold foil: