Atomic oxigen effects and analysis
In low Earth orbits, satellites encounter the very low density residual atmosphere. At orbital altitudes, this is composed primarily of oxygen in an atomic state. (On ground, this is encountered pre-dominantly in the molecular (O2) state but at the top of the atmosphere solar UV breaks the moecular bonds). A satellite moves through the atomic oxygen at a velocity of about 7.5 km/sec. Although the density of atomic oxigen is relatively low, the flux (n v.s, where n is atomic oxigen density, v is relative velocity and s is the surface normal area vector) is high.
The large flux of atomic oxygen, which is in a highly reactive state, can produce serious erosion of surfaces through oxidation. Thermal cycling of surfaces, which go in and out of the earth's shadow frequently in this orbit, can remove the oxidised layer from the surface. Some surfaces respond differently by changing dramatically their surface structure and therefore properties, which are important for spacecraft thermal control.
The flux of atomic oxygen depends on the atomic oxygen density, the relative spacecraft velocity and the orientation of spacecraft surfaces. To a first approximation, the recession rate is proportional to the fluence (time-integrated flux). This rate is material dependent. Kapton recedes at about 3 µ m per 10**20 Atoms/cm² whereas a surface in LEO can easily accumulate 10**21 Atoms/cm² in a matter of months. Preliminary predictions for the International Space Station indicated that Kapton exposed on ram surfaces could recede at 360 micrometers per solar cycle (an eleven year period).
Atomic oxygen fluence prediction and analysis is based on:
- Orbit (position, velocity v) - an orbit generator is used and mission evolution treated;
- Surface orientation (s) with respect to velocity vector;
- Atomic oxygen density: the MSIS-86 (also known as the CIRA-86) model atmosphere of Hedin et al. is used to derive density, a function of altitude, time, solar and geomagnetic activity, latitude and longitude;
- Solar activity prediction (or observation).
The worst-case fluence is to a ram surface and is greatest at solar maximum when the atmosphere expands.
Clearly predictions made with solar activity estimates at the 2 sigma confidence level lead to pessimistic values and results will be doubly pessimistic if both atomic oxygen densities and orbital decay are predicted on the basis of this worst-case solar activity.
Since atomic oxygen density varies strongly with altitude, strong variations are also expected in atomic oxygen effects, and in a decaying satellite orbit such as Long Duration Exposure Facility (LDEF), most of the damage is caused towards the end of the mission.
Anti-sun-pointing surfaces in circular, low-inclination, low Earth orbits accumulate more atomic oxygen than sun-pointing ones because peak atomic oxygen density is after noon local time.
The above method has been adopted in the ATOXSAPRE program which computes fluxes, fluences and resulting erosion on simply-oriented surfaces (ram, sun-pointed, anti-sun-pointed). An ESABASE/Atomox application has also been developed which takes into account more complex geometries and orientations.
Ground-test facilities are needed to derive erosion rates and damage characteristics to be used with the fluence computations to make predictions about on-orbit damage. There is also a need for much more flight data to evaluate long-term effects. From this point of view the analysis of LDEF data will be crucial. Atomic oxygen is also implicated in the "ram glow" phenomenon.
Last update: 29 March 2007