Discussion

3.1. Impact Morphology

The impact damage suffered by solar cells and the MLI-foils is very different. The glass and silicon layers of solar cells are brittle, and therefore generate complex crater structures that contain radial and conchoidal fractures (figure 1). Extensive breakage results in spallation, and most of the central melt pit may be lost. The MLI-foils, in contrast, show a simple perforation, sometimes with overturned collar, on the top or bottom of the foil layer (figure 2). Dispersed impactor remnants may be preserved on one or more layers of the MLI-

Figure 1. BEI of a typical small impact Figure 2. BEI of the top layer of an MLI-crater on the HST solar cells. foil containing an impact feature.

3.2. Micrometeoroid and Space Debris Chemistries on HST Solar Cells

Our initial survey of HST craters with conchoidal diameter (Dco) between 100 and lOOOjim identified that micrometeoroid remnants were dominant [19], Residues were composed of remnants from silicate minerals, calcite, metal sulfides and metals. Residues often appeared as complex poly-mineralic melts within the melt pit. The second survey, of 10-100(im Dco craters, identified the most common impactor as space debris. Aluminium and Aluminium Oxide residues (from solid rocket motor operation) were dominant, particularly in craters below 30(im diameter. The micrometeoroid residues identified were again remnants of silicates and metal sulfides. The most interesting discovery was a residue composed of Si and C, this may represent interstellar material. Rarer space debris remnants included steels and paint flakes. One small impact crater appeared enriched in K, perhaps from alkali-metal reactor coolant. During the 1320 days that the HST solar array was in orbit, a Russian 'RORSAT' nuclear-powered satellite in a higher orbit was reported to have had leakage from the liquid Na and K coolant system. The attributions as to impactor origin from these chemical studies have now been compared with predictions derived from Long Duration Exposure Facility flux data, and a recent meteoroid flux model [19]. Interestingly, there is close agreement between predictions and observations.

3.3. Micrometeoroid and Space Debris Chemistries on SFU MLI-Foils

The micrometeoroid material captured on MLI foils shows residue to be in greater abundance and larger in size (figure 3) than is preserved on HST solar cells (with discrete micron-size rather than probable nanometer-sized grains). As on HST solar cells, micrometeoroid remnants are dominated by Mg-Fe residues (figure 4). It could be assumed that these are melted or condensed remnants of stoichiometric silicate minerals, but it is possible that they are relatively unaltered remnants of Glass-Embedded-Metal-Sulfide particles (GEMS), as have been identified in IDPs [20]. Other identified micrometeoroid residues include Fe-Ni sulfide and plagioclase feldspar. The space debris chemistries have included components of steels and other metallic alloys. The investigation of the MLI-foils is still on-going.

Figure 3. Secondary electron image of a residue fragment preserved on an MLI foil layer.

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