This report has discussed the ISS and Mir and provided detailed information on these and other space stations and platforms. Some major differences between ISS and its predecessors stand out. Before describing some of these differences, it should be made clear that in comparing the capabilities of ISS to Mir and the Space Shuttle, we are largely comparing projections for the future with real data. For example, the ISS information describes what NASA and its international partners intend to do, while the other information describes what has been done and is being done in orbit.

The parameter of electric power is an enabling feature for space research, especially in the life and microgravity sciences, and it may be crucial to future efforts in technology development and commercial research. The low-user power levels on Mir have sometimes prevented the optimal use of onboard equipment for research. At the completion of ISS, the power level for research will be significantly greater than has existed on any previous space platform or than was projected for the SSF in its 1993 configuration. Once all sources of power are in place, ISS should be able to satisfy the large, intermittent demands of power-hungry equipment such as furnaces, as well as the lower but constant demands of components such as electric motors that will be used in refrigerators, freezers, and centrifuges.

The capacity of ISS to accommodate a permanent crew of six is double that of Mir and 50 percent greater than the previous plans for the assembly-complete configuration of SSF. The Space Shuttle routinely carries six or seven crew members but cannot stay in orbit permanently. The availability of crew members to conduct experiments remains contingent on the time that is needed to maintain and housekeep the station and will be difficult to project as accurately as many other parameters. However, it is clear that life sciences research is likely to benefit from the first-ever combination of long-duration missions, a large crew, and access to a wide variety of biomedical hardware in space—including a centrifuge facility to allow research on whole organisms in artificial gravity—as well as from relatively frequent opportunities to return samples to Earth.

The change in orbital inclination from that planned for SSF, 28.8 degrees, to 5l.6 degrees for the ISS, while advantageous for Earth sciences and remote sensing, has the disadvantage of limiting the payload weight capacity from non-Russian launch sites and shortening the launch windows for Space Shuttle launches. The high inclination has also necessitated the requirement for a number of complex on-orbit operations to provide enough electrical power to operate the space station and perform research during assembly. However, the high inclination allows Russian participation in the program, including multiple Russian launch systems, and this eliminates the complete dependence on the Space Shuttle for resupply and crew rotation that was part of the SSF program. ISS will be resupplied primarily by the Progress-M and -M2 spacecraft that will dock automatically to ISS. Therefore, a human crew will not be needed for routine missions to resupply ISS with propellant and other basic needs.

Last, the anticipated interior volume of ISS is almost double the pressurized volume that had been planned for SSF and is almost four times that of Mir. The capabilities for research on the SSF-derived portion of ISS will be approximately the same as were planned for SSF and additional capabilities—though not yet well-defined—are expected to be available from the Russian modules. The large size of ISS will require as many Space Shuttle flights as would have been necessary to assemble SSF as well as additional flights by Russian launch systems, but clearly promises to offer the opportunity for a large number of researchers from the partner nations to participate in the program. With the proposed U.S., Russian, Japanese, and European laboratory modules, as well as other facilities, and the robust system for logistical support of ISS, it will be possible to utilize the features of a long duration in space inherent to a space station without the power and other constraints faced currently on Mir and without the need to return to Earth after two or three weeks as is the case with the Space Shuttle.

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