Initial Observations-POSA I & II, Post-Flight
Harold G. Pippin, Boeing - Seattle, Washington
Specimens, sample holders and the MEEP container on "space-facing" side show evidence of a discrete contamination event. The contamination event is composed of numerous similarly-colored streaks and drops that discolored areas of the samples, and their directionality suggested a common origin. The MIR-facing side appeared visibly free of the streaks and droplet contamination.
The 0.3 mil thick Kapton specimens were not completely eroded. Using the previously known Kapton reaction efficiency of 3.0 x 10-24 cm3/atom, the results indicate that the side oriented nominally 45 degrees from ram received less than 2.5 x 10 20 atoms/cm2 of atomic oxygen. Many specimens removed from the tray holders showed clear differences between exposed and unexposed surfaces. Typically the exposed areas appeared darker than the unexposed areas. These results will be quantified as the optical properties are measured. Very few impacts were visible on either POSA I or II. The VUV photodiode coulometer on the Mir-facing side read over 20 megohms resistance, possibly indicating saturation. The coulometer on the other side was around 80% of full scale. One of the photodeposition cell baseplates had to be pried loose from the UV-hardened "varnish" attached to its edges. Residue of a formerly liquid material appeared at the fill ports and around the base seals of some of the photodeposition cells, often indicating that the liquid had all run to the same side of the POSA II as if under gravitational influence.
Break-away and free-running bolt torques were measured for many on the fasteners. The overwhelming majority of torques were quite low, break-away torques ranged typically from 5-15 in-lbs and many free running torques were near or essentially 0 in-lbs. The shaft used to hold POSA II to the mounting clamps had two bolt heads sheared off. The shaft appears slightly rotated from its proper configuration. A 3-4" length of the o-ring gasket around the edge of POSA II is missing from the side of the MEEP container to which the shaft is mounted. Traces of grease on the O-ring were quite evident on the facing surface.
Certain areas on the "space-facing" side appeared darkened, corresponding areas on the "MIR-facing" side did not show the same darkening. On the specular optical samples, interference patterns indicated the thinnest coating was toward the mounting shaft, evidently due to shadowing by the cover plate on that side. The "shuttle-facing" side received about twice the solar exposure as the "MIR-facing" side. On-orbit photographs from STS-79 show that certain optical specimens had contaminant present at that time. Post-flight observations of these contaminants indicated there was a three-dimensional colorless structure extending above the sample surface by approximately 3-4 mm. Over several days the material appeared to disintegrate, leaving interference patterns on the mirror surfaces. Long fibers in the contaminant wavered in the cleanroom airflow and periodically jumped off the surface. Over several days the material disintegrated, leaving interference patterns on the mirror surfaces. This suggests that perhaps the contaminant was deliquescent. The o-ring around the outside of the POSA I MEEP container is contiguous and shows little evidence of grease residue; however, it had been rubbed very hard in one corner, producing quite a few small black particles.