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The chemistry and origin of micrometeoroid and space debris impacts on spacecraft surfaces.
G.A.Graham", A.T.Kearsleyb, G.Drolshagen0, M.M.Gradyd, I.P.Wright3 and H.Yanoe
"Planetary and Space Science Research Institute, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK.
bSpace Science Research, School of BMS, Oxford Brookes University, Headington, Oxford OX3 OBP, U.K.
°TOS-EMA, ESTEC, European Space Agency, Keplerlaan 1, NL-2201 AZ Noordwijk, NL.
dMineralogy Department, The Natural History Museum, Cromwell Road, London SW7 5BD, U.K
'Planetary Science Division, The Institute of Space and Astronautical Science, Japan
Laboratory investigations of impact residues captured on the solar cells from the Hubble Space Telescope and on insulation foils from the Space Flyer Unit demonstrate preservation of abundant and diverse micrometeoroid and space debris remnants. Micrometeoroid residues often appear as complex melts of poly-mineralic origin derived from silicates, carbonates, metals and metal sulfides. The space debris includes paint-flakes, metal alloys and possible reactor coolant, but the most abundant components are aluminium and aluminium oxide remnants from solid rocket motor operation. The impactor origins have now been compared with the theoretical flux models for Low Earth Orbit.
Our understanding of small particle populations has been aided by laboratory investigations of cosmic dust particles (approximately 1 -400|im diameter). Such studies have focused mainly on material collected from 'terrestrial' locations, e.g. the ocean floor, polar ices and the stratosphere, e.g. [1-3]. However, these particles may have undergone selection and alteration during atmospheric entry, e.g. , and it is desirable to achieve some sampling outside of the Earth's atmosphere. The Giotto spacecraft investigated particles from Comet Halley using a dust-impact detection system , but did not return samples for further examination. Particle analysers have flown on several interplanetary and earth orbital missions. They have yielded important information, yet have not returned samples to Earth. Spacecraft deployed in near Earth orbits, e.g. low Earth orbit (LEO), do offer opportunities to retrieve material that can be analysed in the laboratory. LEO is an interesting environment to sample, it not only enjoys the passage of cosmic dust particles, but also contains an orbital population of artificial particles, space debris generated by human activity. Space debris is diverse in nature, from paint fragments and human waste on a sub-millimetre scale, to spent rocket bodies on the metre scale . Dedicated in-situ sampling techniques have been developed and successfully deployed in LEO, e.g. the microabrasion foil experiment flown on the STS-3 Space Shuttle mission  and the COMET-1 experiment flown on the Salyut 7 spacecraft . However, perhaps the most extensive sampling of particles in LEO was carried out by NASA's Long Duration Exposure Facility (LDEF), in orbit for 69 months . As part of LDEF's scientific payload, there were numerous experiments dedicated to the collection of micrometeoroids and space debris, analysed upon return to Earth, e.g. . Detailed analysis was also carried out on non-dedicated surfaces that had experienced impact damage from micro-particle hypervelocity collisions, e.g. , Subsequently, the utility of non-dedicated collector surfaces returned from LEO has been demonstrated by the success of post-flight investigation of thermal blankets and aluminium thermal control covers from the Solar Maximum satellite  and ,
Until recently , the full potential of residue analysis has not been apparent. Retrieval of one of the two solar array panels from the Hubble Space Telescope (HST) during the first service mission in 1993, and return of the Space Flyer Unit (SFU) have provided contrasting substrates for analysis of particle residues by analytical electron microscopy. Herein we discuss the chemistry of preserved micrometeoroid and space debris remnants on solar cells of the HST and on aluminised Kapton multi-layer insulation (MLI) foils from SFU.
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