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Introduction
Spaceborne observations enable a substantial portion of the research performed by the Earth and space science communities. Within the United States, most spaceborne observations intended for scientific research are funded by the National Aeronautics and Space Administration (NASA), although the ultimate beneficiaries include the greater scientific community, the broad spectrum of “users” employing applications derived from science, and the general public.
Traditionally, scientific satellite missions are planned and funded on the basis of a paradigm that assumes a nominal operational lifetime. It is common, however, for such missions to exceed their planned lifetime through a combination of good design and simple statistical luck.1 Although the intended scientific objectives may have been completed within this period, the opportunity for providing additional science or other benefits at a relatively low cost by extending the mission is often significant.
Today, NASA operates within very constrained resources, and the budget needed for extended mission operations is typically not included in the nominal mission plan. If a decision is made to extend a mission, NASA must either seek new funding or reallocate funding from other programs. Even the relatively low cost of additional mission operations must be weighed on a mission-by-mission basis against the use of those resources for developing new observational capabilities.
A formal, uniformly applied review process for mission extension, known as the Senior Review, has been employed in the space sciences for many years.2 In contrast, until recently, decisions at NASA on Earth science mission extensions have been made on a case-by-case basis without the benefit of a standardized process. While this approach has been adequate for many missions, the recent controversies over termination or extension of the Upper Atmosphere Research Satellite mission and the Tropical Rainfall Measuring Mission3 placed the need for a structured mission-extension process in the spotlight. Furthermore, the successful launch of multiple Earth-observing missions over the past decade4 has created a situation in which a large number of satellites will reach the end of their planned lifetimes in the near future. The establishment of a mission-extension process that is appropriate for the Earth sciences has thus become increasingly important to NASA.
