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--> 1 Introduction Most space science missions initiated in the late 1970s and during the 1980s relied on large, complex spacecraft that carried a broad array of scientific instruments. These large spacecraft required long lead times for development, and cost and schedule overruns were common, often due to changing or poorly understood requirements (NRC, 1994). As the 1990s progressed, space science budgets were increasingly constrained due to pressures on the federal budget and within the National Aeronautics and Space Administration (NASA). In response, NASA has initiated several new programs, such as the Small Explorer and the Earth System Science Pathfinder programs, with limited, clearly defined scientific goals that can be met by instrumentation flown on less complex, lighter weight spacecraft (Baker et al., 1991; NRC, 1995b). A strategy to build and launch scientific missions within three years after program initiation has improved NASA's ability to respond to research opportunities. To conform to current budget constraints, space science proposals now are evaluated almost equally on cost, scientific merit, and technical requirements. Unless there appears to be an overriding scientific justification, lighter-weight, less complex (i.e., "small") spacecraft are generally preferred because they are individually less expensive. Several efforts have been made to determine whether the cumulative results of small spacecraft missions can provide the quality of science that is offered by the larger missions of the past, to identify the primary sources of savings, and to assess whether additional cost savings can be made. Three National Research Council (NRC) reports, Technology for Small Spacecraft (NRC, 1994), The Role of Small Missions in Planetary and Lunar Exploration (NRC, 1995b), and Managing the Space Sciences (NRC, 1995a), have addressed some of the issues in question.
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