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The chemistry and origin of micrometeoroid and space debris impacts on spacecraft surfaces
The chemistry and origin of micrometeoroid and space debris impacts on spacecraft surfaces. G.A.Graham a, A.T.Kearsley b, G.Drolshagen c, M.M.Grady d, I.P.Wright a and H.Yano e aplanetary 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 0BP, U.K. CTOS-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 eplanetary 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. 1. INTRODUCTION Our understanding of small particle populations has been aided by laboratory investigations of cosmic dust particles (approximately 1-400~tm 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. [4], 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 [5], but did not retum 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 - 372-
The chemistry and origin of micrometeoroid
diverse in nature, from paint fragments and human waste on a sub-millimetre scale, to spent rocket bodies on the metre scale [6]. 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 [7] and the COMET-1 experiment flown on the Salyut 7 spacecraft [8]. 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 [9]. 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. [10]. Detailed analysis was also carried out on non-dedicated surfaces that had experienced impact damage from micro-particle hypervelocity collisions, e.g. [11]. 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 [ 12] and [ 13]. Until recently [ 16], 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. 2. EXPERIMENTS 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. [ 14]. 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, [ 15]. 2.2. The Structure of Solar Cells and the MLI-Foils 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 [16]. The SFU MLI-foils consist of 12 layers of aluminised Kapton film interspersed with Dacron nets, a full description is given in [ 17]. 2.3. Laboratory Methodology 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 energydispersive 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
- 373 -
G.A. Graham et al.
description of the analytical protocol is given in [17]. 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. [ 19]. 3.
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 MLIfoils.
Figure 1. BEI of a typical small impact crater on the HST solar cells.
Figure 2. BEI of the top layer of an MLIfoil 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 1000~m 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~tm 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~tm 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
- 374-
The chemistry and origin of micrometeoroid
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.
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Figure 3. Secondary electron image of a residue fragment preserved on an MLI foil layer.
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Figure 4. EDS of the fragment, enriched in Mg, Fe and Si, suggesting a remnant of a mafic silicate mineral, or of glass.
4. CONCLUSIONS Important information about the chemistry of the micro-particles present in LEO can be obtained from analysis of non-dedicated surfaces. Returned spacecraft surfaces should continue to be examined for large numbers of residues, and will yield a valuable resource for interpretation of fluxes. Such studies complement micro-particle sampling by low-density capture-cell technologies such as aerogel [21]. Together, they offer the opportunity to sample large quantities of material, including some that may be relatively intact.
-375-
G.A. Graham et al.
REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.
D.E. Brownlee, B.A. Bates and M.M. Wheelock, Nature 309 (1984) 693. M.M. Maurette, C. Hammer, D.E. Brownlee, N. Reeh and H.H. Thomsen, Science 233 (1986) 869. J.P. Bradley, M.S. Germani and D.E. Brownlee, Earth Planet. Sci. Lett. 93 (1989) 1. M.J. Genge, M.M. Grady and R. Hutchison, Meteoritics Planet Sci. 31 (1996) 627. J.C. Zarnecki, J. Brit. Interplan. Soc. 41 (1988) 403. National Research Council, Orbital debris: A technical assessment, National Academy of Science, (1995). J.A.M. McDonnell, W.C. Carey and D.G. Dixon, Nature 309 (1984) 237. J. Borg, J.-P. Bibring, Y. Langevin, P.H. Salvetat and B. Vassent, Meteoritics 28 (1993) 641. M.E. Zolensky. T.H. See, R.P. Bernard, R. Barrett, F. Horz, J.L. Warren, C. Dardano, K.S. Leago, D. Kessler and T.R. Foster, Adv. Space Res. 16 (1995) (11)53. R.P. Bernhard, T.H. See and F. H~3rz, In LDEF 69 Months in Space - 2nd Post-Retrieval Symp., NASA CP-3194 (1993) 551. R.P. Bernhard, C. Durin and M.E. Zolensky, In LDEF 69 Months in Space - 2nd PostRetrieval Symp., NASA CP-3194 (1993) 541. J.L. Warren, H.A. Zook, J.H. Allton, U.S. Clanton, C.B. Dardano, J.A. Holder, J.A., et al. Proc. 19th Lunar and Planet. Sci. Conf. (1989) 641. F.J.M. Rietmeijer and G.E. Blandford, J. Geophys. Res. 93 (1988) 11943. G. Drolshagen, Proc. Hubble Space Telescope Array Workshop, ESA WPP-77 (1995) 295. H. Yano, S. Kibe, S.P. Deshpande and M.J. Neish, Adv. Space Res. 20 (1997) 1489. G.A. Graham, A.T. Kearsley, M.M. Grady and I.P. Wright, Proc. 2nd European Conf. On Space Debris, ESA SP-393 (1997) 183. H. Yano, K. Morishige, S.P. Deshpande, Y. Maekawa, S. Kibe, M.J. Neish and E.A. Taylor, Adv. Space Res. 25 (2000) 293. M.E. Zolensky, H.A. Zook, F. HSrz, D.R. Atkinson, C.R. Coombs, A.J. Watts et al. LDEF 69 Months in Space- 2nd Post-Retrieval Symposium, NASA CP-3194 (1993) 277. G.A. Graham, N. McBride, A.T. Kearsley, G. Drolshagen, S.F. Green, J.A.M. McDonnell, M.M. Grady and I.P. Wright, Int. J. Impact Engng. 26 (2001) 263. J.P. Bradley, Science 256 (1994) 925. P. Tsou, J. Non-Crystalline Solids 186 (1995) 415.
- 376-
The chemistry and origin of micrometeoroid
diverse in nature, from paint fragments and human waste on a sub-millimetre scale, to spent rocket bodies on the metre scale [6]. 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 [7] and the COMET-1 experiment flown on the Salyut 7 spacecraft [8]. 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 [9]. 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. [10]. Detailed analysis was also carried out on non-dedicated surfaces that had experienced impact damage from micro-particle hypervelocity collisions, e.g. [11]. 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 [ 12] and [ 13]. Until recently [ 16], 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. 2. EXPERIMENTS 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. [ 14]. 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, [ 15]. 2.2. The Structure of Solar Cells and the MLI-Foils 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 [16]. The SFU MLI-foils consist of 12 layers of aluminised Kapton film interspersed with Dacron nets, a full description is given in [ 17]. 2.3. Laboratory Methodology 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 energydispersive 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
- 373 -
G.A. Graham et al.
description of the analytical protocol is given in [17]. 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. [ 19]. 3.
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 MLIfoils.
Figure 1. BEI of a typical small impact crater on the HST solar cells.
Figure 2. BEI of the top layer of an MLIfoil 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 1000~m 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~tm 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~tm 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
- 374-
The chemistry and origin of micrometeoroid
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.
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~OOS
-
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Prcset-I
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lOOs
RcmAi
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Os
D t'='d.
S i
C
Ml FM
S
, < -. FS-
1
-,
e
~., ~, ~, ,i~ A ~ , 5.
022 ch
OK
MEM 1 = 9 r
Figure 3. Secondary electron image of a residue fragment preserved on an MLI foil layer.
K
s2m2 n2 ?3 t
~
,i
kcU 512=,
M g- r i c h
9r~ i n2
,i 10 9 1 )' 61 c~s
/MEM2
Figure 4. EDS of the fragment, enriched in Mg, Fe and Si, suggesting a remnant of a mafic silicate mineral, or of glass.
4. CONCLUSIONS Important information about the chemistry of the micro-particles present in LEO can be obtained from analysis of non-dedicated surfaces. Returned spacecraft surfaces should continue to be examined for large numbers of residues, and will yield a valuable resource for interpretation of fluxes. Such studies complement micro-particle sampling by low-density capture-cell technologies such as aerogel [21]. Together, they offer the opportunity to sample large quantities of material, including some that may be relatively intact.
-375-
G.A. Graham et al.
REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.
D.E. Brownlee, B.A. Bates and M.M. Wheelock, Nature 309 (1984) 693. M.M. Maurette, C. Hammer, D.E. Brownlee, N. Reeh and H.H. Thomsen, Science 233 (1986) 869. J.P. Bradley, M.S. Germani and D.E. Brownlee, Earth Planet. Sci. Lett. 93 (1989) 1. M.J. Genge, M.M. Grady and R. Hutchison, Meteoritics Planet Sci. 31 (1996) 627. J.C. Zarnecki, J. Brit. Interplan. Soc. 41 (1988) 403. National Research Council, Orbital debris: A technical assessment, National Academy of Science, (1995). J.A.M. McDonnell, W.C. Carey and D.G. Dixon, Nature 309 (1984) 237. J. Borg, J.-P. Bibring, Y. Langevin, P.H. Salvetat and B. Vassent, Meteoritics 28 (1993) 641. M.E. Zolensky. T.H. See, R.P. Bernard, R. Barrett, F. Horz, J.L. Warren, C. Dardano, K.S. Leago, D. Kessler and T.R. Foster, Adv. Space Res. 16 (1995) (11)53. R.P. Bernhard, T.H. See and F. H~3rz, In LDEF 69 Months in Space - 2nd Post-Retrieval Symp., NASA CP-3194 (1993) 551. R.P. Bernhard, C. Durin and M.E. Zolensky, In LDEF 69 Months in Space - 2nd PostRetrieval Symp., NASA CP-3194 (1993) 541. J.L. Warren, H.A. Zook, J.H. Allton, U.S. Clanton, C.B. Dardano, J.A. Holder, J.A., et al. Proc. 19th Lunar and Planet. Sci. Conf. (1989) 641. F.J.M. Rietmeijer and G.E. Blandford, J. Geophys. Res. 93 (1988) 11943. G. Drolshagen, Proc. Hubble Space Telescope Array Workshop, ESA WPP-77 (1995) 295. H. Yano, S. Kibe, S.P. Deshpande and M.J. Neish, Adv. Space Res. 20 (1997) 1489. G.A. Graham, A.T. Kearsley, M.M. Grady and I.P. Wright, Proc. 2nd European Conf. On Space Debris, ESA SP-393 (1997) 183. H. Yano, K. Morishige, S.P. Deshpande, Y. Maekawa, S. Kibe, M.J. Neish and E.A. Taylor, Adv. Space Res. 25 (2000) 293. M.E. Zolensky, H.A. Zook, F. HSrz, D.R. Atkinson, C.R. Coombs, A.J. Watts et al. LDEF 69 Months in Space- 2nd Post-Retrieval Symposium, NASA CP-3194 (1993) 277. G.A. Graham, N. McBride, A.T. Kearsley, G. Drolshagen, S.F. Green, J.A.M. McDonnell, M.M. Grady and I.P. Wright, Int. J. Impact Engng. 26 (2001) 263. J.P. Bradley, Science 256 (1994) 925. P. Tsou, J. Non-Crystalline Solids 186 (1995) 415.
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