![]() Both the impact energy and the high reactivity of the atomic oxygen result in oxidation and erosion of most hydrocarbon polymers. The spacecraft in LEO runs into the atomic oxygen with an impact energy of about 5 eV. For LEO missions longer than a week, surface materials that are non-reactive with high-energy atomic oxygen must be used. However, not all materials are reactive to atomic oxygen. Indeed, only a few weeks of time in LEO is required to destroy 1-mil layer of a highly reactive material like Kapton. Thus, instead of a time scale of 10 years for sputtering, we might expect a degradation time scale of a few days under atomic oxygen. In contrast, reaction rates for atomic oxygen may be 1.0 and the ram flux 1014 per square centimeter per second. However, such a sputtering rate per incident particle may be 0.1 and the flux on the order of 1012 per square centimeter per second. Sputtering by ions in LEO is also possible. It is desirable to place surfaces such that they have no direct line of sight to surfaces that might undergo sputtering. Optical, thermal or other specialized coatings in the line of sight to the sputtering pinhole will become coated with the sputtered material over a long period of time. Materials of low sputtering rate also may be used, provided that they satisfy thermal, atomic oxygen, and other requirements. Underlying conductors must be of sufficient thickness to withstand puncture at accelerated sputtering rates. Earlier in the ISS designs, engineers showed that the sputtering rate greatly increases by ions focusing onto the pinhole where micrometeoroids, debris or manufacturing defects produce small pinholes in insulators. For long life missions in a high-density plasma environment, surfaces should be insulated to prevent direct contact of high potential conductors with the plasma. Sputtering may also be a long-term problem in the lunar environment if the locally produced environmental plasma has a high enough density. Because the spacecraft voltage is usually well under 1 kV, sputtering is only important for mission durations in years in high-density plasma, such as that in LEO or low Mars orbit. The sputtering rate is a strong function of the surface voltage. One distinguishing characteristic of the model is that there is no absolute threshold for sputtering, but the rate at low energy is determined by the thermal tail of the surface material atom velocity distribution. Measurements of low-energy sputtering by atomic oxygen support a theoretical model. However, some work has been done for developing a sputtering model to help evaluate the surface material loss rates in LEO. A complicating factor is that little information is available on the nature of chemical effects on sputtering rates under AO. Sputtering may also be a concern for long duration LEO missions carrying surfaces at high negative voltage. Thus, shielding with metal oxides, such as oxidized aluminum, can offer adequate protection against AO damage, provided that the shielding is strong enough to resist micrometeoroid damage. ![]() Materials in order of high to low degradation resistance are metal oxides, metals, inorganic polymers, and organic plastics. The reaction rate on a material depends on the AO density, velocity vector, and the material reactivity. Mechanism of material reaction is not well understood.Fluence (atoms/cm2) is dependent on solar activity, velocity vector, and altitude.Average velocity is approximately 8km/s.Most abundant species in LEO atmosphere (80% AO, 20% nitrogen).The following are significant characteristics of AO in LEOs: The AO has significant impact on the design and durability of the materials. Such considerations are important mainly for low planetary orbits, such as in LEO, and also in low Mars's orbit. Moreover, chemically active ions may be attracted by charged spacecraft surfaces, and the reaction rates increase with energy. Volatile oxidation products may be lost, leaving a surface without its protective coverings and this can lead to electrostatic discharge to or from exposed conductors. The AO can oxidize and damage surfaces, especially in the ram direction with the spacecraft ram velocity simulating a high-energy beam. ![]() It reacts with many commonly used spacecraft materials resulting in degradation and mass erosion of exposed spacecraft surfaces. It is formed by photo-dissociation of diatomic oxygen by ultraviolet radiation from the sun, more in LEO, where recombination or the formation of ozone is improbable. Atomic oxygen (AO) is a significant environmental constituent for spacecraft in a low Earth orbit. ![]()
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