ABSTRACT
Anumberofspacecraftandsatellitesareoperatedinthealtituderangebetween 200and700kmabovesealevel,whichiscalledlowEarthorbit(LEO).InLEO, thedominantatmosphericspeciesisground-stateatomicoxygen(O{'P)),whichis producedbyphotodissociationof0 2 :
02+hv(<250nm)=>oCP)+oeP)(I) Although,thedensityofatomicoxygeninLEOislow(107-109atomsperem\
buttheorbitalvelocityofspacecraftisashighas7800m/s,resultinginahigh atomicoxygenflux(10 13-10 15atoms/cm2pers)fll.Sincetheorbitalvelocityof thespacecraft(7800m/s)ishigherthanthethermalvelocityofatomicoxygenin LEO,spacecraftsurfacesdirectedtothevelocityvectorofspacecraftsufferfrom bombardmentofatomicoxygenwithanaverageimpactvelocityof7800m/s.
This relative impact velocity corresponds to the impinging energy of atomic oxygen beam in a ground-based facility as high as 5 eV. Such a high-energy collision with chemically-active atomic oxygen induces surface reactions with exterior surfaces of the spacecraft. Satellites are usually covered by a multilayer thermal insulation (MLI) whose outermost surface is made of polyimide. The degradation of polymeric materials due to high-energy collisions with atomic oxygen is so severe that a few micrometers in thickness is lost within a few tens of hours of atomic oxygen exposure in LEO [2]. Figure la-c shows scanning electron microscopy (SEM) images of a polyimide surface exposed to atomic oxygen with a fluence of 3 x 1020 atoms/cm2• The exposed surface becomes rough and many sharp peaks are obvious in Fig. lc. This surface structure is called "shag-carpet-like structure" and is characteristic of a surface that has suffered high-energy atomic oxygen collisions. The surface was etched to a depth of 8.9 !J.m by atomic-oxygen-induced etching. It has been reported that after 25 years of operation, the International Space Station (ISS) will lose 2.5 mm thickness of polymers due to atomicoxygen-induced erosion [3].
