What GAO Found
NASAs approach to determining, obtaining, and delivering necessary spare parts to the ISS is reasonable to ensure continued utilization of the station through 2020. The statistical process and methodology being used to determine the expected lifetimes of replacement units is a sound and commonly accepted approach within the risk assessment community that considers both manufacturers predictions and the systems actual performance. To date NASA has given equal weight to manufacturers predictions and actual performance, and currently has no plans to reassess this decision. However, as time goes on, the resulting estimates could prove to be overly conservative, given that NASA has found failure rates for replacement units to be lower than manufacturers predictions. Therefore, continuing to weigh the manufacturers predictions equally with actual performance could lead NASA to purchase an excess of spares. NASA also has a reasonable process for establishing performance goals for various functions necessary for utilization and determining whether available spares are sufficient to meet goals through 2020, but the rationale supporting these decisions has not been systematically documented.
NASA is also using reasonable analytical tools to assess structural health and determine whether ISS hardware can operate safely through 2020. On the basis of prior analysis of structural life usage through 2015 and the robust design of the ISS structures, NASA currently anticipates thatwith some mitigationthe ISS will remain structurally sound for continued operations through 2020. NASA also is using reasonable methodologies, governed by agency directives and informed by NASA program experts, to assess safe operations of the ISS as a whole as well as identify replacement units and other hardware, failure of which could result in an increased risk of loss of station or loss of crew through 2020. NASA plans to develop, through 2015, methods to mitigate issues identified and expects to begin implementing corrective actions as plans are put in place.
Why GAO Did This Study
In 2010 the National Aeronautics and Space Administration (NASA) was authorized to extend the life of the International Space Station (ISS) from 2015 through at least September 30, 2020. Gauging the feasibility of doing so is quite complex. Among the factors to be assessed are the reliability of key components, NASAs ability to deliver spares to the ISS, the projected life of structures that cannot be replaced, and the health of systems that affect safety. While some empirical data exist, the ISS is a unique facility in space and assessing its extended life requires the use of sophisticated analytical techniques and judgments. GAO provided a preliminary report on NASAs use of such techniques in April 2011. For this review, GAO assessed the extent to which NASA has ensured essential spare parts are available and ISS structures and hardware are sound for continued ISS utilization through 2020. GAO interviewed NASA officials and outside experts; assessed the methodology underlying NASAs findings; conducted a limited test of data supporting NASAs assessments; and analyzed documentation such as ongoing assessments, schedules and other relevant efforts.
Recommendations for Executive Action
|National Aeronautics and Space Administration||As the ISS program accumulates additional knowledge about the on-orbit performance of orbital replacement units (ORU), the NASA Administrator should direct the ISS program manager to revisit, as appropriate, the relative weight given to manufacturers' original reliability estimates and to the actual reliability of ORUs based on on-orbit experience by reexamining the program's choice of a value for the parameter that governs the variance of the original mean time between failure (MTBF)'s probability distribution.|
|National Aeronautics and Space Administration||To ensure decision makers have a full understanding of the rationale behind ISS functionality targets and confidence levels to better inform future decisions, the NASA administrator should direct the ISS program manager to ensure these rationales are documented, at a minimum, in the minutes from meetings of the Space Station Program Control Board.|