GLENAIR
environment ever so slightly. If the charge hits an insulating material, it will not be free to move around, and that insulator will build up charge with every electron that hits it in this area. When charges have nowhere to go, they accumulate, and before long, a voltage builds up and may be discharged, typically with an arc to a nearby conductor at a lower voltage. For this reason the outermost layer of a spacecraft needs to be conductive, or at least able to ‘bleed’ off charge fast enough so as to prevent a voltage build-up. This is particularly important for wires, since their outermost layer is non-conductive and they are often in close proximity to grounded metallic structures. Aluminum foil shields are a popular means to prevent electrons from building up charge on wire insulation. It is important to make sure that those foils are grounded to the spacecraft structure. You may wonder, what happens if a spacecraft keeps accumulating electrons? Does that impact the electrical networks on board? How does one control the voltage when charge keeps accumulating on a ground structure? The answer is that unlike on earth, Satellites do not have an absolute ground. If we could tie a wire between earth ground and a satellite, we might measure a huge voltage difference. But since nothing on the spacecraft is tied to earth ground, the electronics ‘don’t know’ that they are operating with a different zero volt reference than their cousins on earth. Think of it like the tide of an ocean; when boats float in the open waters, they don’t feel the tides, and as long as they don’t come close to shore, the height of the tide does not matter. When two spacecraft come in contact however, they do need to level out their ground structures in order to avoid an un-controlled discharge. The surface of the spacecraft is also where most of the electromagnetic radiation hits. Most organic molecules suffer long-term damage from this radiation. They become brittle, shrink, and lose adhesion. In electrical systems, this can impact impedance. As mentioned earlier, in low earth orbit we also have a large amount of ionized oxygen, ready to oxidize and add resistance to certain metallic surfaces like aluminum. For this reason, gold is the preferred choice for surface coverings. It is very resistant to corrosion, reflects a broad spectrum of electromagnetic radiation, is immune to absorbed radiation, and is an extremely good conductor. At NASA, the tongue-in-cheek answer to the question, “why is everything plated with gold on your satellites?” is, “because we didn’t have enough budget to use solid gold!”
Satellite plating with gold is used when insulation alone is inadequate to protect the satellite from radiation from heat, light, and impact. Gold is effective in reflecting radiation away from the satellite, is a good heat and electrical conductor, and does not react to atomic oxygen.
Material
Acceptable Dose (Mrad)
Bipolar Power Transistors MOSFET Transistors (on SiC) Schottky Diodes (on SiC)
0.2 Mrad 1 Mrad 1 1 Mrad 1 100 Mrad 2 400 Mrad 3 10 Mrad 2 1 Mrad 4 0.1 Mrad 2
Epoxy Resin
Kapton (polyimide)
Kynar (PVDF)—mild damage
Silicone rubber
A table of acceptable radiation levels for a few popular materials used on spacecraft. Small semiconductor components will certainly have an additional aluminum shield around them.
Teflon (FEP)
Epoxy-Glass Laminates 10 Mrad 2 Acceptable radiation doses for typical materials used in electronics. Measurements were performed
using 60 Co Gamma radiation. 1 Steffens et.al. RADECS 2017 2 Hanks et.al. NASA-CR-1781, 1971
3 Golliher et.al. NASA/TM-2001-210245 4 NASA Langley SP-8053, June 1970
QwikConnect • January 2021
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