The space environment has a strong degrading effect on some types of materials. Atomic oxygen, ultraviolet radiation, thermal cycling, hard vacuum, and induced contamination can all affect optical properties and mechanical integrity of spacecraft materials. Because these space environment effects are difficult to accurately simulate in the laboratory, on-orbit testing is needed to qualify materials for long-term use in space. The Mir space station provides a unique opportunity to study the natural space environment as well as the man-made environment, particularly contamination from orbiter dockings and venting activities around an active orbiting space station. POSA-I was proposed to study the Mir environment's effect on candidate materials for International Space Station (ISS) as well as state-of-the-art materials that might prove useful in future spacecraft design.

A. Description of the Experiment
The Passive Optical Sample Assembly (POSA-I) is one of four experiments of the Mir Environmental Effects Payload (MEEP). 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 (fig. 1) provided data on the performance of numerous spacecraft materials 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, such as thermal control coatings, polymeric films, optical materials, and multi-layer insulation blankets. In addition to the material samples, on each side of POSA-I were two atomic oxygen pinhole cameras and two solar ultraviolet fluence monitors.

POSA-I was attached to the Docking Module (fig. 2) and oriented so that one set of exposed samples faced the main core of the Mir space station while the opposite side faced space. Both sides had an 18 month exposure to the Docking Module and any attached hardware. In addition, the space facing side (-ZB direction using the Mir coordinate system) was periodically exposed to the Space Shuttle during dockings with Mir. During launch and retrieval the MEEP carrier was closed and sealed with a Viton O-ring. Each of the MEEP suitcase carriers could "breath" through a hepa filter vent, so they were not hermetically sealed. This design protected the exposed samples from the Shuttle environment during launch, return, and most of the EVA operations.

B. Background
The principal investigator of the POSA-I Experiment is James M. Zwiener, chief of the Physical Science and Environmental Effects Branch of the Materials and Processes Laboratory at the George C. Marshall Space Flight Center. Mr. Zwiener has 35 years experience in spacecraft design and contamination control. Co-investigators Rachel Kamenetzky, Jason Vaughn, and Miria Finckenor are members of the Physical Science and Environmental Effects Branch and are familiar with state-of-the-art polymer films and thermal control coatings used in spacecraft design, as well as simulating the space environment in world-class research facilities. Co-investigator Dr. Palmer Peters of the Space Sciences Laboratory at Marshall Space Flight Center designed and assisted in the manufacture of the atomic oxygen pinhole cameras. Guest investigators are industry leaders in spacecraft design and provided many flight and ground control samples for the POSA-I experiment.

C. Objectives
Risk mitigation for the ISS program includes an assessment of the magnitude of contamination expected for ISS critical surfaces and quantification of the performance of spacecraft materials in that environment. POSA-I also tested state-of-the-art materials for durability in the space environment. Data from POSA-I will be used in confirmation of models and ground facilities that simulate the space environment.


II. Mission Activities and Experiment Procedures

The MEEP experiments were deployed during the STS-76 mission EVA in March 1996 from the Space Shuttle Atlantis. American astronauts Linda Godwin and Rich Clifford attached the experiments to the exterior of the Mir Docking Module and opened the "suitcases" to expose the samples. 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.


III. Results

A. Contamination Observations
Contamination was detected on both sides of the POSA-I experiment, with the space-facing side receiving the most deposition. POSA-I contamination was fairly uniform as would occur from a photodeposition process over days or weeks. Fourier Transform Infrared Analysis indicated the presence of silicate, likely outgassed silicone deposits converted by atomic oxygen and ultraviolet radiation interactions. Electron spectroscopy for chemical analysis (ESCA) with depth profiling confirmed the presence of 26 to 31 nm of silicate on the Mir-facing side and 500 to 1000 nm of silicate on the space-facing side. Mass gain of thin polymer films on the space-facing side is also consistent with approximately 900 nm of contaminant deposition.

Also, the contamination on POSA-I appears to have directionality. The likely source of silicone outgassing is discussed in Section IV of this report. Some of the contaminant deposition was observed early in the mission from STS-79 Mir survey photos (figs. 3, 4). These photos were taken in September 1996, after POSA-I had been deployed for approximately six months.

B. ISS Candidate Material Performance
ISS candidate materials chosen for exposure on POSA-I included chromic acid anodized aluminum, sulfuric acid anodized aluminum, Z-93 white ceramic thermal control coating, and small 6" x 6" MLI blankets consistent with ISS design.

Table 1 consists of the percent change in solar absorptance and infrared emittance for ISS candidate materials, corrected with pre- and post-flight measurements of control samples. Z93P paint samples on 6" x 6" aluminum substrates were provided by guest investigators Hank Babel and Mark Hasagawa of Boeing, Huntington Beach. Paint suppliers IITRI and AZ Technology provided samples on 1" dia. aluminum buttons. AZ Technology provided samples with two different Kasil potassium silicate binders, as noted in the table.

Table 1. ChangeU in Optical Properties for ISS Candidate Materials. Solar Absorptance Infrared Emittance Material Mir-Facing Space-Facing Mir-Facing Space-Facing Boeing +4.4% +50.8% 0 0 Z93P IITRI -1.2% +43.4% 0 -1.0% AZ 2130 +3.6% +42.8% 0 0 AZ 2135 +4.1% +44.9% 0 0 Chromic Anodize +2.7% +6.7% +3.6% +19.8% Boric/Sulfuric Anodize -1.4% +22.7% +5.2% +21.8% MLI +3.4% +8.9% 0 0

Z93P on the Mir-facing side was very stable and remained nominally white. Z93P on the space-facing side was heavily contaminated with a yellow/tan appearance. Solar absorptance increased from about 0.16 to as much as 0.23 (fig. 5) after only 18 months of exposure with 571 equivalent sun-hours of solar UV radiation. This is a concern for ISS radiators, since a solar absorptance value of 0.28 is predicted for Z93 after 10 years exposure on ISS, and 0.30 is considered end-of-life. If similar contaminant deposition and thermal control coating degradation occurs on ISS, end-of-life for the radiators will be much sooner than 10 years.

C. State-of-the-Art Material Performance
In addition to ISS candidate materials, POSA-I flew a number of innovative materials. Among these were samples of colored anodizes suitable for markers and astronaut visual aids. Post-flight evaluation of red, yellow, blue, and black colored anodizes provided by AaChron showed them all to be space-stable. The heavy contamination apparently had little effect on the colored anodizes, with no more than a 3% increase in solar absorptance and negligible change in infrared emittance.

AZ Technology provided samples of an innovative inorganic yellow marker coating. This coating is an alternative to colored anodizing. Mir-facing samples showed negligible changes in optical properties, while the space-facing side increased 6.7% in solar absorptance due to contamination.

POSA-I contained a number of optical witness samples, including gold, platinum, iridium, and magnesium fluoride/aluminum mirrors. These samples were useful in ellipsometry and ESCA analysis of the space-side contamination. Transmission measurements of magnesium fluoride windows showed little change for the Mir-facing samples and a marked decrease in the UV and visible wavelengths for space-facing samples (fig. 6).

Triton Systems, Inc. of Chelmsford, MA has developed several atomic-oxygen resistant polymer films under the Small Business Innovative Research (SBIR) program. TOR (Triton Oxygen Resistant) film has shown promise as a replacement for Kapton polyimide film. Triton has also developed COR (Clear Oxygen Resistant) film, similar to TOR in AO reactivity but clear rather than yellow translucent. Samples of TOR and COR with and without silver/Inconel backing were flown on POSA-I.

Atomic oxygen fluence on the Mir-facing side was determined to be 7 x 10¹⁹ atoms/cm² by mass and thickness loss of well-known polymer films such as Kapton, Lexan, and Teflon. Atomic oxygen fluence was not determined for the space-facing side due to the contamination. Atomic oxygen reaction efficiency for the TOR films was calculated to be 9 x 10^-25 cm³/atom. Reaction efficiency for the COR films was approximately 2 x 10^-25 cm³/atom, for an erosion rate fifteen times less than that of Kapton.

Triton's oxygen-resistant films do have an initial reaction to atomic oxygen, which forms an oxide layer theoretically impervious to further erosion. As a test of this oxide layer, some samples of TOR and COR were exposed to atomic oxygen in the lab before flight. Reaction efficiency of TOR dropped to 1 to 2 x 10^-25 cm³/atom for previously AO-exposed samples. It was not clear whether previous exposure to AO had an effect on COR's reaction efficiency.


IV. Conclusions and Recommendations
Contamination on POSA-I was determined to be silicone offgassing converted to a silicate during exposure to atomic oxygen and ultraviolet radiation. Photos from the STS-79 mission show that visible contaminant deposition had already occurred. On-orbit activities, possible contamination sources, and likelihood of deposition on POSA-I are discussed in Table 2.

Mir's orbit has a significant impact on induced environmental effects. Because of the high inclination of 51.6^o, there are times when the Mir sees zero shadow in orbit ¹ for several days. During these zero-time-in-shadow periods, which occur about twice a year, temperatures on the Mir rise dramatically. TQCMs from other Mir experiments ^2,3,4 show a marked increase in outgassed contamination, even when the line-of-sight is older Mir modules. Computer models indicate that Mir spent zero time in shadow June 1996, January and June 1997. ISS will also have to withstand zero-shadow periods with higher than normal temperatures. During times of high contamination deposition rates, which also includes dockings and deliveries of new modules, contamination-sensitive systems or surfaces on ISS may need to be shuttered or shielded temporarily. Monitoring of the space environment is invaluable for these conditions.

The POSA-I experiment does demonstrate the importance of utilizing materials on spacecraft that have low outgassing properties and keeping a large distance between contamination sensitive surfaces and any potential contamination source. It is imperative that contamination control be practiced in the form of good materials selection, thermal vacuum bake-out, clean room assembly, and attention to line-of-sight for contamination-sensitive materials, such as optics. Materials are generally regarded as acceptable for spacecraft use with 1.00% or less total mass loss (TML) and 0.10% or less of collected volatile condensable materials (CVCM), as tested using the ASTM-E595 method. For line-of-sight to sensitive optics, more stringent criteria may be required by MSFC Contamination Control, such as testing with an optical witness sample using MSFC-SPEC-1443. Good materials selection is also important for ground support equipment (GSE), as cross-contamination can occur in ambient conditions. MSFC Contamination Control requires that GSE materials meet MSFC-SPEC-2223, "Ambient Outgassing Testing of Clean Room Materials."

1. Carlos Soares, Boeing/Houston, briefing at Mir Contamination TIM, Houston, TX, April, 1998.
2. Andrei Krylov, RSC-Energia, briefing at Mir Contamination TIM, Houston, TX, April, 1998.
3. 3. D. R. Wilkes, Optical Properties Monitor (OPM) Preliminary Science Status, briefing to the MSFC Projects Office, Huntsville, AL, May, 1998.
4. D. R. Wilkes and M. R. Carruth, In-situ Materials Experiments on the Mir station, SPIE International Symposium on Optical Science, Engineering, and Instrumentation, San Diego, CA, July, 1998.