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1 Introduction This report documents the work of the National Research Council Committee on Engineering Challenges to the Long- Term Operation of the International Space Station. The report deals only with the postassembly phase of the Inter- national Space Station (ISS) Program (so-called "Assembly Complete") and not with the requirements for the assembly process. The substance of this report is based largely on information obtained by the committee from the National Aeronautics and Space Administration (NASA) and associ- ated contractors between September 1998 and August 1999. At the first meeting in September 1998, the committee reviewed the report of the Cost Assessment and Validation (CAY) Task Force of the NASA Advisory Council, which had been released several months earlier (CAV,1998~. The Terms of Reference for the CAV Task Force were compiled by NASA on October 14,1997, concurrent with, andin direct response to, the congressional interest expressed in the Appropriations Act of October 17, 1997, that chartered this study. The CAV report was published in April 1998, just one month after the official start date of this study. The CAV report included a detailed review of the cost and sched- ule risks in the ISS Program. The report, which focused on the assembly phase of the program, predicted a one to three year schedule slip and a most likely date for Assembly Com- plete of December 2005. For the operational phase of the ISS, the CAV report rec- ommended a budget level of $1.5 to $1.6 billion per year, compared with NASA's goal of $1.3 billion per year. The task force concluded that the major budget risk factors dur- ing the operational phase would be replacing components due to equipment failures and obsolescence. The committee found the CAV report to be a comprehen- sive and timely study of the ISS budget risks and decided that it need not be duplicated and that the study on the long- term operation of the ISS should be complementary to it. Therefore, the committee decided to focus on the engineer- ing challenges of long-term ISS operations and delve into budget issues only if they were not covered in the CAV 6 report. However, the CAV report covered the budget issues thoroughly and the recommendations in this report fit the CAY's budget assumptions for the operational phase. NASA's goal of $1.3 billion per year for the operational period, beginning in fiscal year (FY) 2005, is a top-level esti- mate. The budget for that time period has not yet been devel- oped in detail. Hence specific budget impacts could not be addressed in this study as they were in the CAV study, which included the initial five-year assembly phase. Throughout this study, various committee members noted that the long-term operation of the ISS will constitute a major departure for NASA from its traditional mode of operation for human space flight operations. Until the Phase 1 Mir program, and with the exception of the three crew visits to Skylab in the mid-1970s, all of NASA's space flight experi- ence has ranged from missions of several days to a few weeks. A major purpose of the ISS is to provide a platform for long duration microgravity experiments in the physical and life sciences. The ISS offers unique capabilities for research in the following areas: biomedical research and countermeasures system development (preventive measures for cardiovascular and musculoskeletal deconditioning) gravitational biology and ecology (under variable gravity) materials science biotechnology fluids and combustion human-machine interfaces and advanced life support systems low-temperature physics earth observation and space science The following areas will be investigated on the ISS: the technologies best suited for long-duration human space exploration (NRC, 1996)

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INTRODUCTION the role of gravity in the evolution, development, struc- ture, and function of life forms, and as a result of gravity, how life forms interact with their environment (NRC, 1998) the requirements for ensuring the health, safety, and productivity of humans living and working in space (NRC, 1998) the controlling mechanisms in cellular aggregation and differentiation for the in vitro growth of cells, organ- isms, organs, and other biologically interesting struc- tures (NRC, 1995) the optimum relationship between the process used to form a material and its resultant properties, how this relationship can be achieved in space and on the ground, and how the space environment can help us obtain highly accurate fundamental physical measure- ments (NRC, 1998) the most effective energy conversion process involving combustion (NASA, 1998) the unique characteristics of fluid flow and heat and mass transfer in reduced gravity (NASA, 1998) the formation and evolution of the universe, galaxies, stars, and planets (NASA, 1998) 7 the causes of change in the Earth environment over time (NASA, 1998) The long-duration operation of the ISS will provide a new environment for research to answer these questions. REFERENCES CAV (Cost Assessment and Validation Task Force). 1998. Report of the Cost Assessment and Validation Task Force on the International Space Station. Washington, D.C.: National Aeronautics and Space Adminis- tration. NASA (National Aeronautics and Space Administration). 1998. NASA 1998 Strategic Plan. Washington, D.C.: National Aeronautics and Space Administration. NRC (National Research Council). 1995. Microgravity Research Opportu- nities for the 1990s. Space Studies Board. Washington, D.C.: National Academy Press. NRC. 1996. Engineering Research and Technology Development on the Space Station. Aeronautics and Space Engineering Board. Washington, D.C.: National Academy Press. NRC. 1998. A Strategy for Research in Space Biology and Medicine in the New Century. Space Studies Board. Washington, D.C.: National Acad- emy Press.