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The viability and long term success of the International Space Station (ISS) is at least partially dependent on the effective use of the six attached payload sites provided on its truss structure. These sites, two on the port end and four on the starboard end of the truss, provide locations for scientific and commercial payloads that require direct access to the space environment. These sites have been designed to accommodate external payloads that are nominally attached using the Space Station Robotic Manipulator System (SSRMS). Although certain redundancies have been designed into the berthing system used at each site, a potentially catastrophic hazard has been identified in the unlikely event that the berthing system and its redundancies fail when a payload is partially captured but not fully berthed to the site. Under this scenario, it is imperative that the payload be fully disengaged from the site before posing a catastrophic risk to the crew or the space station.

The process of developing a Payload Attach System (PAS) which will support a wide range of experimental and commercial payloads on the International Space Station (ISS) has experienced an interesting evolution during its design, development, test and evaluation (DDT&E) phase. This evolution has been caused in large measure by requirements intended to insure compatibility of the PAS with the extravehicular activity (EVA) crewmember during nominal and contingency operations in and around the PAS sites. As the design of the ISS transitioned from its Freedom predecessor, the effort to keep costs down by preserving as much of the original Freedom design as possible led to design decisions that challenged engineering thinking.