Low Earth Orbit Environment

Resistance of polymers to ozone attack is studied in space environments in actual applications. In the laboratory, glow discharge or plasma etching is the common method for laboratory study of ozone effect. Plasma and low earth-orbit environments are not equivalent. For instance, oxygen plasma contains a variety of other particles including electrons and free radicals in addition to atomic oxygen. In contrast, atomic oxygen is the dominant constituent of low earth-orbit. 

Measure the thermal radiative properties of spacecraft thermal protection materials Covington and Sawko, Stewart, and White have measured the reflectance and transmittance for a variety of spacecraft thermal protection materials and calculated the appropriate blackbody-weighted emittance and absorbance for the predicted conditions. In addition, spectroscopy has been used to detect changes in spacecraft surface materials resulting from long-term exposure to the low earth orbit environment. 

Spacecraft in low Earth orbit (LEO) must function in a harsh oxidizing environment. At LEO altitudes, ranging from 200 to 700 km, the residual atmosphere is predominantly neutral, with the dominant component being atomic oxygen. A typical 0-atom number density at space shuttle altitudes ( 300 km) is on the order of 10 cm . At orbital altitudes of 300-400 km, the LEO environment subjects materials on the ram side of a spacecraft to collisions with ambient oxygen atoms that have an average impact velocity of 7.4 km corresponding to a mean collision en-... 

Fig. 3. Scanning electron micrograph of a Kapton polyimide surface that was exposed to the low Earth orbital ram environment for 40 hours on the STS-46 space shuttle mission. The smooth portion of the Kapton surface was covered during exposure and therefore shows no erosion. The rougher portion of the surface shows the topography and extent of erosion (6 /rm deep) resulting from the exposure.

T. K. Minton, J. W. SeaJe, D. J. Garton amd A. K. Framdsen, Dynaunics of Atomic-Oxygen-Induced Degraidation of Materials, Proceedings of the 4th International Conference on the Protection of Materials and Structures from Low Earth Orbital Space Environment, held in Toronto, Ontario, 23-24 April 1998 (Kluwer Academic Pubhshers, Boston, 2000). 

Little data are available at this time on the tolerance of liquid lubricants to radiation exposure but assessment of this may be important for some future spacecraft. In low earth orbit applications the lubricant is normally within an enclosure and exposure to atomic oxygen, the main environment in this orbit, is not therefore considered to be a problem. 

Materials that are directly exposed to the space environment must also withstand or be protected from atomic oxygen. Atomic oxygen is prevalent in the low earth orbit (LEO), approximately 100 to 350 miles from Earth. At this distance, molecular oxygen reacts with ultraviolet light to produce monoatomic oxygen which is extremely chemically reactive and erosive. Most adhesives used in spacecraft, however, are sandwiched between two adherends, thus not directly exposed. 

The type of environment encountered in low earth orbit (LEO) (approximately 250 km above the Earth s surface) to which space systems would be exposed contains ... 

The gravity gradient and atmospheric drag account for the micro-g rather than zero-g time-averaged acceleration environment found in a q>acecraft. A q>acecraft in a low earth orbit is typically ut 300 kUometers above the earth s surface and about 6671 kilometers from ftie earth s center. It is int esting to note that r is onty about 5% greater in low earth orbit than on the earth s surfece. So the absolute value of A is only about 9% less in this orbit than on the ground... 

Spacecraft operating in low Earth orbit (LEO) are exposed to an extremely harsh environment. One component of this environment is atomic oxygen, which is produced by dissociation of molecular oxygen by UV radiation from the sun (cf section 7.2). Due to the high velocity required to maintain... 

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