Material Ease

Space is an unforgiving environment – one of harshest known to mankind. Yet, space is far less understood than other service environments in which DOD systems operate. It has been known to limit the useful life of satellites, with 20 to 25% of all mission failures being attributed to the effects of the space environment.[1] Since access to space is so costly, materials and components must function throughout the life of a mission without repair or replacement. Yet, overdesign is not an option, as the cost of launch (driven by payload weight and volume) severely limits the redundancy and size of components. Adding to these challenges, future spacecraft will be designed to last longer, their functionality will become ever more complex, and their power requirements will continue to increase. The natural space environment (often referred to as space weather) depends heavily on the satellite orbit, the time of year, and the solar activity. The environment is largely determined by the type of orbit. Low-EarthOrbits (LEO) provide maximum sensor resolution for weather and reconnaissance missions. Satellites and spacecraft in LEO are subjected to atomic oxygen and can experience greater thermal cycling than in other orbits. Moving farther away from Earth, satellites in geosynchronous orbits (GEO) around the equator have the same angular velocity as the earth, thus GEO satellites remain over a fixed point on the Earth. Many communication and weather satellites are parked in GEO. Mid-Earth Orbits (MEO) are between LEO and GEO. Systems in MEO are subjected to the trapped radiation in the Van Allen Belts. A polar orbit is a special LEO to MEO orbit where the satellite travels over the poles. Since the Earth turns underneath the satellite, it can cover the entire globe in a short time period. Polar orbiting satellites typically experience higher radiation environments. A sun-synchronous orbit is a special polar orbit where the satellite passes over the same part of the Earth at roughly the same local time each day (for more information, please refer to the ‘Orbits’ section of the article, ‘Materials and the Final Frontier’ in this issue). This can make communication and various forms of data collection convenient. Space weather causes a range of problems to materials on spacecraft. Thermal/vacuum effects, atomic oxygen, micrometeorites, ultraviolet radiation, and electron and proton radiation can all adversely impact space materials. The following sections describe these environmental factors and the materials problems they create.