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Particle Compositions

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The initial inspections of the impact craters on LDEF surfaces revealed that there were no, obvious physical crater characteristic that could be used to distinguish between craters formed by natural meteoroids from those formed by man-made debris. The large majority of the craters appeared to be the result of a hypervelocity impacts ( >6 or 7 km/s) and were generally symmetric and hemispheric in shape, exhibiting the typical lip or rim around the crater. The crater interior surfaces generally displayed evidence of being molten during the crater formation process. Only in a very few rare craters could chunks or fragments of the impacting particle be found.

With the lack of any distinguishing physical features in the craters, searching for and analyzing the chemical composition of impactor residues became necessary in order to establish the impactor's origin. A number of techniques were used to search for the remaining traces of the impactors and to identify their compositions - Energy Dispersive X-ray spectrometry (EDX), Scanning Electron Microscopes (SEM), and Secondary Ion Mass Spectrometry (SIMS), to name a few.

EDX techniques could identify the composition of remaining visible chunks and/or to analyze impact melt found within craters. Utilizing this technique, approximately 50% of examined craters could not be classified. The more sensitive SIMS technique can locate and analyze remaining traces of the impactor. In some craters, residue of the impactors were found condensed on exterior surfaces surrounding the crater. In fact, there were a number of capture cells flown on LDEF that were designed specifically to trap impactor condensate and fragments that are expelled during the crater formation process. There were also a number of specially selected target materials (e.g., gold, germanium, and tantalum coated thin films or sheets) flown on LDEF to facilitate the identification of impactor residues. If the impactor was composed of materials also present in the tarrget/collector composition, they could not be identified. For example an aluminum particle could not be identified on an aluminum collector surface. Surface contaminants arriving after the crater was formed also complicates the analysis of impactor residues.

Inspite of all of these difficulties, impactor residues were identified in approximately one-third (1/3) of the LDEF craters that have been chemically analyzed to date, and in the majority of those craters the impactor could beclassified as either a natural meteoroid or a man-made debris particle.

The majority of the natural meteoroid impactors can be categorized into one of the following groups: (1) Chondritic - largely made up of relatively well-mixed and homogenized fine-grained matrices, (2) Monomineralic silicates - characterized by high concentrations of Si, Mg, and Fe and generally found in molten form, and (3) Fe-Ni-sulfide rich particles - also generally found as melts. Man-made debris impactors can be categorized into one of the following groups: (1) Fe-Ni-Cr rich particles - representing stainless steel, (2) Zn-Ti-Cl rich particles - characteristic of thermal control paints used on spacecraft, (3) Ag, Cu, or Pb-Sn rich particles - originate from arrays of solar cells, and (4) Aluminum particles (metallic or oxidized) - originate from spacecraft structures or solid rocket motor exhaust.

The Chemistry of Meteoroid Experiment exposed gold surfaces on the trailing edge of the LDEF and Aluminum surfaces on the forward facing surfaces. A summary of the analysis of the impactor residues on these two surfaces is presented in Figure 1 (Ref. 1). Kessler's orbital debris model did not predict the substantial number of orbital debris impacts that were found on the this experiment, which resided on LDEF's trailing edge . Thus, these data proved that there is a substantial population of orbital debris residing in elliptic orbits.

Many craters on LDEF experiment tray clamps have also been analyzed, the results of which are presented by Bernhard and Zolensky (Ref 2&3). These reports also include SEM images of the crater and spectra of those craters that could be clasified.

References

(1) Hörz F., et. al. (1993) Natural and Orbital Debris Particles on LDEF's Trailing and Forward-Facing Surfaces, LDEF - 69 Months in Space. Third Post-Retieval Sysmposium, NASA Conference Publication 3275, p. 420-429.

(2) Bernhard R.P. and Zolensky , M.E. (1993) Analysis of Impactor Residues in Tray Clamps from the Long Duration Exposure Facility; Part 1: Clamps from Bay "A" of the Satellite, NASA TM 104759.

(3) Bernhard Ronald P. and Michael E. Zolensky (1994) Analysis of Impactor Residues in Tray Clamps from the Long Duration Exposure Facility; Part 1: Clamps from Bay "B" of the Satellite, NASA TM 104784.

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