Interconnect Technology for Manned Space Flight

Solar array power degradation due to displacement damage High-energy protons

Solar array arc from surface charging Low-energy electrons

Materials degradation Photons, ions, atoms

Microelectronics, bit flips, latch-ups High-energy protons and ions

Discharge from internal charge accumulation High-energy electrons

Electromagnetic pulse from vehicle discharge Low-energy electrons

Total dose effects High-energy particles

Information source: Air Force Research Laboratory

earth’s atmosphere in a similar manner. The charged ions and free electrons create a layer of conductive gas (plasma) in our upper atmosphere, called the ionosphere. LEO satellites at altitudes between 100 and 300 kilometers are in that zone. For these satellites, it’s important to consider the exact orbit and their orientation with respect to the sun in order to truly assess the potential impact of charged particles on their surfaces. For example, higher concentrations of charged particles are present in the Van Allen belts closest to earth’s magnetic poles, while lower concentrations exist in equatorial regions. This has a very practical impact on orbit the trajectories of LEO satellites, as they are relatively safe from charged particles except and if their orbits traverse the Van Allen belts. Avoiding long transits through dense belts is an important consideration when engineers select the optimal orbit of a satellite. Sometimes there is no choice, a geosynchronous satellite above Central America will necessarily feel the impact of the outer belt and will have to be designed accordingly. Material degradation due to radiation In terms of what damage particles can do, scientists employ a unit of measurement called the Rad, which measures how much energy was deposited by radiation into a unit of mass. It is the preferred unit when evaluating a material (how many Rads can it handle before degrading), or when comparing orbits, or calculating the lifespan of a spacecraft. Laboratories use electron beams and gamma rays from Cobalt 60 to simulate radiation exposure, again measured in Rads. In some materials (optical fibers, semiconductors, dielectric insulators) it is also important to understand how fast the radiation is implanted into the matter. In these cases, we use either the Fluence (power per surface area—important for solar cells for instance), or the flux (particle count per unit time into an angle area). We will consider the impacts starting from the outermost layers of the spacecraft. The most prevalent particle for those areas are low energy electrons. Low energy means they don’t have enough speed to penetrate deep into the structures. When they get absorbed, they add a unit of negative charge to the material they collided with. If it’s a metallic (or conductive) material, the additional charge is free to move around and will change the voltage balance of the spacecraft with its

This picture illustrates some of the impact ionizing radiation and low- energy charged particles can have on a satellite.

Materials under test to determine potential damage from absorbed radiation, such as silicone-based microelectronics, are exposed to radiation doses measured in “Rads”. Doses in excess of 100 mRads can result in material hardening and embrittlement, making them unsuitable for use in satellite applications.