Invited Paper VT2-ThA6
Recent Advances to Enhance Space Simulation

Thursday, October 18, 2007, 3:40 pm, Room 618a

Accurate ground-based simulation of low earth orbit (LEO) conditions experienced by a satellite has proven to be a challenge. The continuous progress that has been made toward this goal will be reviewed. A satellite in LEO has a speed relative to the atmosphere of approximately 8 km/s. The neutral atmospheric molecules exchange momentum upon collision with the surfaces of the satellite, leading to drag, lift, and moments, but ground facilities still have trouble simulating pure beams of this speed for the relevant atmospheric gases in their ground state. A facility that is making progress toward this goal will be described. The most important atmospheric molecule, atomic oxygen, collides with ram-direction satellite surfaces with a relative energy of 5 eV. Energetic atomic oxygen atoms plus solar UV radiation produce synergistic effects that result in many chemical reactions on or in the vicinity of the outer satellite surfaces. These can lead to structural or operational damage and the spacecraft glow phenomena. It is desirable to generate large beams of atomic oxygen in the ground state, with the atoms possessing energy of 5 eV. Several techniques for attempting this will be reviewed. Solar radiation has a wide spectrum. The UV spectrum is a composite of many emission lines and continuum, which must be simulated using special lamp systems. Satellite surfaces are exposed to high energy protons, electrons, and other particles. These are simulated in combined effects space simulation chambers for materials degradation studies. Thruster plumes, surface outgassing, and liquid dumps lead to surface contamination. Contamination can reduce the effectiveness of thermal control paints, the output of solar cells, and the effectiveness of optical lenses. Some electric thrusters exit directly to the vacuum of space, which must be simulated if the thruster plume is to be accurately simulated. Specially designed cryogenic pumps designed to simulate the conditions that these electric thrusters will experience in orbit will be described. All of the facilities to be reviewed require special diagnostic instrumentation, much of a specialized type. Some of this instrumentation will be described and the limitations of older techniques will be noted. Satellites in LEO also are immersed in a plasma but the effects of plasma charging, which has been well reviewed elsewhere, will not be covered. Links to inventories of space simulation chambers will be given.