The space shuttle is a unique national resource. One of only two operating vehicles that carries humans into space, the space shuttle functions as a scientific laboratory and as a base for construction, repair, and salvage missions in low Earth orbit. It is also a heavy-lift launch vehicle (able to deliver more than 18,000 kg of payload to low Earth orbit) and the only current means of returning large payloads to Earth. Designed in the 1970s, the shuttle has frequently been upgraded to improve safety, cut operational costs, and add capability. Additional upgrades have been proposed—and some are under way—to combat obsolescence, further reduce operational costs, improve safety, and increase the ability of the National Aeronautics and Space Administration (NASA) to support the space station and other missions.

In May 1998, NASA asked the National Research Council (NRC) to examine the agency's plans for further upgrades to the space shuttle system. The NRC was asked to assess NASA's method for evaluating and selecting upgrades and to conduct a top-level technical assessment of proposed upgrades. The complete statement of task is reprinted in Appendix A.

In June 1998, the NRC, under the auspices of the Aeronautics and Space Engineering Board, formed the Committee on Space Shuttle Upgrades to carry out this task. (Short biographies of the committee members appear in Appendix B.) In July, the committee met with shuttle program managers and received briefings on current and proposed upgrades to the space shuttle and the process for selecting upgrades for implementation. Additional teleconferences and site visits were held in August and September to gather more detailed information

The National Academy of Sciences is a private, nonprofit, selfperpetuating society of distinguished scholars engaged in scientific and engineering research, dedicated to the furtherance of science and technology and to their use for the general welfare. Upon the authority of the charter granted to it by the Congress in 1863, the Academy has a mandate that requires it to advise the federal government on scientific and technical matters. Dr. Bruce M. Alberts is president of the National Academy of Sciences.

The National Academy of Engineering was established in 1964, under the charter of the National Academy of Sciences, as a parallel organization of outstanding engineers. It is autonomous in its administration and in the selection of its members, sharing with the National Academy of Sciences the responsibility for advising the federal government. The National Academy of Engineering also sponsors engineering programs aimed at meeting national needs, encourages education and research, and recognizes the superior achievements of engineers. Dr. William A. Wulf is president of the National Academy of Engineering.

The Institute of Medicine was established in 1970 by the National Academy of Sciences to secure the services of eminent members of appropriate professions in the examination of policy matters pertaining to the health of the public. The Institute acts under the responsibility given to the National Academy of Sciences by its congressional charter to be an adviser to the federal government and, upon its own initiative, to identify issues of medical care, research, and education. Dr. Kenneth I. Shine is president of the Institute of Medicine.

The National Research Council was organized by the National Academy of Sciences in 1916 to associate the broad community of science and technology with the Academy's purposes of furthering knowledge and advising the federal government. Functioning in accordance with general policies determined by the Academy, the Council has become the principal operating agency of both the National Academy of Sciences and the National Academy of Engineering in providing services to the government, the public, and the scientific and engineering communities. The Council is administered jointly by both Academies and the Institute of Medicine. Dr. Bruce M. Alberts and Dr. William A. Wulf are chairman and vicechairman, respectively, of the National Research Council.