ISS Risk Mitigation

Passive Optical Sample Assembly (POSA-2)

Pre-Launch POSA-2 POSA-2 Installed


Risk mitigation for the ISS program included an assessment of the magnitude of contamination expected for ISS-critical surfaces and quantification of the performance of spacecraft materials in that environment. The POSA experiments was designed to assess the magnitude of potential molecular contamination on the ISS-critical surfaces and quantify the performance of candidate/baseline ISS external surface materials. Specifically, POSA-2 samples were:

· Polyimide Kapton H, 0.005" thick, for mass loss measurements, to calculate atomic oxygen fluence.
· SiO x coated Kapton H Solar Array Blanket, 0.001" thick, provided by Sheldahl, for atomic oxygen undercutting measurements.
· Front surface aluminized sapphire, to optically measure degradation as a result of contamination.
· Aluminum on spin-coated polyimide (protected by SiO x ) to measure specular reflectance degradation as a result of atomic oxygen attack at defect sites.
· Silver on spin-coated polyimide (protected by SiO x ) to measure specular reflectance degradation as a result of atomic oxygen attack at defect sites.
· CV1144 silicone, to look for embrittlement.
· Z-93-P thermal control paint to assess emittance and absorptance changes for comparison with LeRC ground qualification test results.

Shuttle-Mir Missions
STS-76, NASA-2 - NASA-5, STS-86

All four MEEP experiments delivered to Mir by STS-76. They were successfully deployed on March 26th and 27th, 1996 during STS-76. The MEEP was successfully retrieved on October 1, 1997 during STS-86 mission and returned to the ground. The MEEP was exposed to the Mir external space environment for 18 months.


Atomic Oxygen Fluence: Dehydrated Kapton mass was used to calculate atomic oxygen fluence: 8.2x1019 atoms/cm2 was measured for Mir-facing side and 2.1x1020 atoms/cm2 for the Shuttle-facing side. Essentially no change was observed in dehydrated mass of Sheldahl samples. Atomic oxygen fluence was too small for undercutting studies. The mass of organic contaminant was found to be negligibly small.

Sapphire Contamination Monitor, the CTM Series: Ellipsometry identified an inorganic contaminant as SiO2, at a thickness of 240 Angstroms for the Mir-facing side and 40 Angstroms for the Shuttle-facing side. Total reflectance decreased, which was consistent with the presence of contaminants on the surface. Solar absorptance increased. Diffuse reflectance also increased, probably due to the organic contaminant.

Aluminum and Silver Mirrors, the ALM and AGM Series: No significant change in optical properties was measured.

CV1144 Silicone, the SIM Series: Scanning Electron Microscope (SEM) photos of the Kapton used to protect the CV1144 show a variety of textures. Samples were submitted for Pico Indenter analysis, to look for silicone embrittlement, hardening, etc. These evaluations are still in progress.

Z-93-P, the ZP Series: There was no change in the solar absorptance, which was consistent with ground test results. There was also little change in infrared emittance, again consistent with ground test results.

Wilkes DR, Carruth MR, "In-Situ Materials Experiments on the Mir Station," SPIE International Symposium on Optical Science, Engineering, and Instrumentation, San Diego, CA, July 1998.

Principal Investigators
Gary Pippin
Boeing Defense & Space Group

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Page last updated: 07/16/1999

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