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Atomic oxygen

The effects of atomic oxygen (monatomic oxygen) bombardment were originally highlighted with the early shuttle flights, by a visible effect on exposed polymer surfaces such as Kapton, where changes in characteristics due to atomic oxygen were found to cause undesirable temperature excursions in low Earth orbit and shorten the useful lifetime of many spacecraft components. It has also been proposed that atmospheric atomic oxygen plays a role in the production of a visible shuttle glow upon re-entry into Earth's atmosphere. Post-flight analysis of painted surfaces on the shuttle were also noted to have been returned with a brighter surface than prior to launch.

Atomic oxygen bombardment contributes significantly to surface degradation, erosion, and contamination of materials with which it collides due to its high speed of 1.15 km/sec compared to an average speed of a spacecraft relative to the atmosphere of 7.24 km/sec, and high collision energies (4-5eV). At this energy, atomic oxygen initiates a number of chemical and physical reactions with the materials and penetrate surfaces, substituting oxygen to form oxide compounds, more stable than those originally present. The element exists in monatomic form in the upper regions of the atmosphere (>400km altitude). The effects of atomic oxygen bombardment on a polymer film produces a heavily etched and eroded surface. Polymer films that have been coated with thin metallic layers have suffered atomic oxygen attack through pre-existing pinholes in the metal film, this leads to underlying cavities which eventually produces complete loss of the polymer, leaving a free-standing metal film.

As the orbit of the LDEF was inclined to the equator (28.5°), its 12-sided geometry caused the atomic oxygen fluence to vary from experiment to experiment. The angle of each experiment surface relative to the ram direction was used to determine the atomic oxygen fluence by the fixed structural geometry of the vehicle and its constant flight attitude in orbit. The extent to which on-board materials were exposed to atomic oxygen, the total atomic oxygen fluence, is of primary interest. This fluence is the flux (atoms/cm2/sec) times the exposure period (seconds), with the flux defined as the number density of atomic oxygen (atoms/cm3) times the orbital velocity (cm/s). The altitude of the flight, orientation of the surfaces, and the extent of solar activity determine the amount of atomic oxygen exposure.

Figure 8, Atomic oxygen flux vs time for LDEF rows B8 and G12

Figure 9, Atomic oxygen fluence vs time for LDEF rows B8 and G12

Atomic oxygen flux was not constant during the orbital lifetime of the LDEF as decreasing solar activity caused atomic oxygen flux to decrease during the first three years of the flight. Following this, the combination of increasing solar activity and decreasing altitude caused the atomic oxygen flux to increase rapidly. The flux during the latter months of the mission was almost two orders of magnitude greater than the flux encountered early in the mission. Figure 10 shows the accumulated atomic oxygen fluence expressed as a percentage of the total fluence exposed for the mission. This highlights the combined effect on atomic oxygen fluence caused by the varying solar activity and the loss in altitude. Approximately 75% of the total atomic oxygen exposure accumulated during the last year of the flight with approximately 50% accounted for in the last six months.

Figure 10, Cumulative Atomic Oxygen fluence as a percentage of total exposure

Comparing the atomic oxygen fluence experienced on the LDEF with that required for the EOS HIRDLS instrument shows the expected atomic oxygen bombardment to be equivalent to that experienced by the optical components on the leading edge tray (B8). Figure 11 shows the expected atomic oxygen fluence profile for a five year polar orbit at an altitude of 705km with values ranging from approximately 8x1020 to 1x1020 , this compares to a range of 1x1020 to 5x1021 experienced by the LDEF leading edge tray B8. It should be noted that the optical elements on the LDEF experiment were under direct exposure to the flux of atomic oxygen, whilst in HIRDLS the filters are internal to instrument, providing far greater protection for survival.

Figure 11, Atomic oxygen fluence requirement for the HIRDLS instrument after five years in orbit
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Effects of the space environment on infrared filters and materials