2.1. Flight Details of the HST and the SFU
In 1993, during the first service mission of the Hubble Space Telescope, the "V2" solar panel was successfully replaced and returned to Earth. Prior to the retrieval, the array had been in LEO for 1320 days at an operation orbit of ~600km. Individual solar cells from the returned array were investigated extensively in a detailed post-flight investigation program, e.g. , The SFU was retrieved from LEO after 301 days of exposure in an operational orbit of ~480km. As with the solar cells from the HST, the surfaces from the SFU were extensively examined and were subject to detailed post-flight investigations, .
The HST solar cells are multi-layered structures. The top layer of borosilicate glass rests upon silicone resin, which is underlain by the silicon layer. On the back of the silicon is another resin layer that also contains metallic silver connectors. The complex assembly is supported by a backing-tape of resin-bonded glass-fibre mesh. A full, detailed description of the solar cell composition is given in . The SFU MLI-foils consist of 12 layers of aluminised Kapton film interspersed with Dacron nets, a full description is given in .
The analytical work was carried out on a Jeol 840 scanning electron microscope (SEM) with 2nA beam current and at 20kV accelerating voltage. The samples were carbon-coated to reduce the effects of electrical charging during SEM investigations. Most of the electron imaging was carried out using a solid-state back-scattered electron detector. The X-ray elemental maps and X-ray spectra were acquired using an Oxford Instruments eXL energy-dispersive spectrometer (EDS) microanalyser with a Pentafet detector, fitted with an ultra-thin window (this allows the detection of X-rays from light elements such as carbon). A full description of the analytical protocol is given in . The interpretation of samples of material from LEO, whether micrometeoroids or space debris, is a complex task, the particles will have had encounter velocities of > 5km/s relative to the collector surface. The particles experience extensive shock deformation, melting, and often even complete vapourisation during the impact process. Hypervelocity impact events may modify the original chemical composition of an impactor, fractionating volatile from refractory elements. Micrometeoroid residues may therefore not necessarily retain the stoichiometric chemical signature of a parent mineral, and quantitative analysis of residue is usually impractical. The classification of micrometeoroid and space debris residues employed was based on similar chemical and mineralogical criteria to those used for LDEF, e.g. ,
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