I. Introduction/Background
The Passive Optical Sample Assembly (POSA-I) is one of four experiments of the Mir Environmental Effects Payload (MEEP). The other MEEP experiments include the Passive Optical Sample Assembly (POSA II) developed by Boeing Defense and Space Group in Seattle, the Orbital Debris Collector (ODC) assembled by the Johnson Space Center and the Polished Plate Meteoroid/Debris Experiment (PPMD) assembled by Langley Research Center.

The MEEP experiments were attached to the exterior of the Mir Docking Module during the STS-76 mission EVA in March 1996 from the Space Shuttle Atlantis. All of the MEEP experiments were retrieved on October 1, 1997 during the STS-86 mission EVA from the Space Shuttle Atlantis. Russian cosmonaut Vladimir Titov and American astronaut Scott Parazynski retrieved the four MEEP experiments. It is important to note that POSA I and POSA II were mounted on opposite sides of the Docking Module, but facing roughly the same direction. The two meteoroid experiments were mounted basically side by side, on the -XB side of the Docking Module, roughly pointing 90 degrees to POSA I & II.

Overall objective of the POSA-I experiment is the characterization of the Shuttle and Mir-induced external contamination space environment and the evaluation of the combined space environmental effects on space station baseline and candidate (future applications) materials.

POSA-I provided data on the performance of numerous external materials, such as thermal control coatings including various paints, exposed to solar ultraviolet radiation, atomic oxygen and contamination. Approximately 388 samples were exposed on POSA-I, which included over 100 different type of materials. In addition to the material samples, on each side of POSA-I were two atomic oxygen pin hole cameras and two solar ultraviolet fluence monitors.

POSA-I was orientated so that one set of exposed samples faced the main core of the Mir space station while the opposite side faced the space side. The space facing side was exposed to the docking of the Space Shuttle and had an 18 month exposure to the Docking Module and its attached hardware that faced in the -YB direction. During launch and retrieval the MEEP carrier was closed and sealed with an O-Ring made of Viton^®, but this is not a hermetic seal. Each of the MEEP suitcase carriers can "breath" through a hepa filter vent. This design protected the exposed samples to the Shuttle environment during launch, return, and most of the EVA operations.

II. Quicklook Results (preliminary)
Contamination was detected on the POSA-I, MEEP experiment as follows:

• POSA I had a visible contamination on the side facing space or in the -ZB direction using the Mir coordinate system.
• POSA I had no visible contamination on the side facing the main Mir core or in the +ZB direction in the Mir coordinate system.
• POSA II had a visible contamination on the side facing space or in the -ZB direction using the Mir coordinate system; note POSA II was facing roughly at 45 degrees to the POSA I viewing direction or approximately 45 degrees to the (+ZB , -ZB) axis.
• POSA II had no visible contamination on the side facing the main Mir core or in the +ZB direction in the Mir coordinate system.

Character of the type of visible contamination detected on POSA I versus POSA II is as follows:

• POSA I contamination was fairly uniform as would occur from a slow photodeposition process. A very definite film was deposited on the optical samples.
• In comparison POSA II had a mostly splattered or droplet type contamination spread across its surface on the space or -ZB exposed side. The character of the contamination would appear that it occurred from a single event rather than a slow buildup.

Also, the contamination deposition on POSA-I appears to be have a very directionality to it. Very definite shadowing effects can be observed. This shadowing may be a combination of solar ultraviolet photodeposition where shadowing can be very definite or marked if the spacecraft (Mir) orientation is stable for a considerable time period and contamination was present.

III. Conclusions and Recommendations POSA I was located approximately 40 feet from the main part of the Mir space station, but only approximately 4 feet from the Docking Module. The Docking Module was deployed on 11/95 only four months before the MEEP experiments were deployed. On the other hand the SPECTR module which the Mir facing side of POSA I had a very good exposure too, had been in space for ~1 yr or ~12 months; all other modules on Mir had in excess of six years in space prior to the MEEP experiment deployments. Since the Mir facing side of POSA I has shown no visible contamination (detailed optical measurements may change this result, slightly) it can be concluded that after ~12 months in space and at a distance of approximately 40 feet the Mir modules do not represent a contamination threat. On the other hand, new modules after only several months of on orbit deployment and at close distances may represent a very great contamination threat.

At this early stage in the post flight diagnostics of the POSA-I flight experiment, it is too early to make recommendations. Except, the experiment does demonstrate the importance of utilizing materials on spacecraft that have low offgasing properties and keeping a large distance between contamination sensitive surfaces and any potential contamination source.