Environmental exposure conditions for Teflon (R) fluorinated ethylene propylene on the Hubble Space Telescope

The outer layer of Teflon(R) fluorinated ethylene propylene (FEP) multi-lay er insulation (MLI) on the Hubble Space Telescope (HST) was observed to be significantly cracked at the time of the Second HST Servicing Mission (SM2) , 6.8 years after HST was launched into low Earth orbit (LEO). Comparativel y minor embrittlement and cracking were also observed in the FEP materials retrieved from solar-facing surfaces on the HST at the time of the First Se rvicing Mission (3.6 years exposure). After SM2, a failure review board was convened to address the problem of degradation of MLI on the HST. In order for this board to determine possible degradation mechanisms, it was necess ary to consider all environmental constituents to which the FEP MLI surface s were exposed. Based on measurements and various models, the environmental exposure conditions for the FEP surfaces on the HST were estimated, includ ing: the number and temperature ranges of thermal cycles; equivalent sun ho urs; fluence and absorbed radiation dose of x-rays, trapped protons and ele ctrons and plasma electrons and protons: and atomic oxygen (AO) fluence. Th is paper presents the environmental exposure conditions for FEP on the HST, briefly describing the possible roles of the environmental factors in the observed FEP embrittlement and providing references to the published works which describe in detail testing and analysis related to FEP degradation on the HST.