The Polished Plate Meteoroid and Debris (PPMD) experiment was part of the MIR Environmental Effects Payload (MEEP) that was attached to a hand rail on the Shuttle-MIR Docking Adaptor from March 27, 1996 to October 1, 1997.

The aluminum alloy plate (anodized 6061-T6) on the wake-facing side of the experiment has been scanned with a 50X optical microscope. Thirty-eight hypervelocity impact craters were found ranging in size from 1.055 mm down to 23 microns in diameter. It is believed that all craters larger than 75 microns were found, and that perhaps all craters larger than 50 microns were found.

Figure 1.

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Hypervelocity craters in aluminum are nearly hemispherical below the plate surface and have rims (or lips) that rise above the plate surface. For each crater, measurements were made of the diameter at the top of the raised lip (lip diameter), the diameter at the surface of the plate, and the depth from the plate surface to the crater bottom. The results of the measurements are given in Table 1. The diameters were measured using the reticle in the microscope eyepiece and using the highest power objective lens for which the entire crater was within the range of the reticle. The maximum power used was 800X. The diameters are accurate to about ± 3 microns for craters larger than 125 microns and to about ± 1 micron for smaller craters. The craters were not exactly round and that contributes to the uncertainty. The depth measurements were accurate to about ± 2 microns for craters with diameters larger than about 180 microns and about ± 1 micron for the smaller craters, except that the surface roughness of the aluminum plate can add another ± 1 micron uncertainty to the depth measurements.

The location of the craters on the plate is shown in Figure 1. The distribution appears to be random.

Figure 2.

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The depth-to-diameter ratio for the 36 round craters is shown in Figure 2. The diameter here is the diameter at the plate surface. Differences in the shape of the craters is probably due to differences in the shape, density, relative speed, and impact angle of the particles, and for the smaller impactors, the local material properties of the plate.

The cumulative flux of craters (flux of a given threshold size and greater) is shown in Figure 3. The calculated flux is based on a plate area of 0.134 m² and an exposure time of 4.78 x 107 s (533 days). The uncertainty bars are the 90 percent confidence limits based on the number of craters in the sample. The upward-pointing arrow and the question mark for the 50 micron diameter crater flux indicates that it is uncertain whether all craters with a diameter of 50 microns and larger were found.

Figure 3.

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It was intended that the aluminum plate would always face in the wake direction. If this was well approximated (the attitude history of the MIR during this experiment is not known to the author at this time) then the impact flux on MIR (i = 51.6^o, 1996-1997) was several times greater than that on the Long Duration Exposure Facility (LDEF, i = 28.5^o, 1984-1990), which also had anodized aluminum alloy 6061-T6 plates. Models (ref. 1,2,3) do not predict that a wake-facing plate on the PPMD would have a greater flux of 50 micron to 1 mm diameter craters than that seen on the LDEF. The models predict a crater flux about 20 percent lower than on the LDEF. Either the aluminum plate had a significant exposure to other directions, or the models are in error. If the models are in error, it would be suspected that it is the orbital debris model, not the meteoroid model, that is in error, and that traffic to the space station is the cause of the increased debris flux. Analysis of impactor residue in the craters on the PPMD experiment, especially on the pure zinc and pure gold plates, should help determine the source of the particles.

1. Humes, D.H., Small Craters on the Meteoroid and Space Debris Impact Experiment. LDEF - 69 Months in Space, Third Post-Retrieval Symposium, NASA CP-3275, pp.287-322, 1993.
2. Anderson, B.J., editor, Natural Orbital Environment Guidelines for Use in Aerospace Vehicle Development. NASA TM-4527, Section VII, 1994.
3. Kessler, D.J., et al., A Computer-Based Orbital Debris Environment Model for Spacecraft Design and Observation in Low Earth Orbit. NASA TM-104825, 1996.