The Space Station is not just one structure, but is composed of four basic 

 components: the manned core, free flyers tended by the station, co-orbiting 

 platforms, and polar orbiting platforms. The core is by far the largest and most 

 complex element. It must provide the main structure to which the Shuttle will 

 dock and in which the astronauts will live. It will be constructed in a low Earth 

 orbit at an estimated altitude of 500 km with an orbital inclination of 28.5° to 

 the equator. Block I will consist of a 135-m horizontal boom with photovoltaic 

 arrays at each end to produce 50 kW of power. A single polar orbiting platform 

 for Earth remote sensing will also be part of the Block I configuration. At the 

 center of mass will be two interconnected pressurized modules. One of these 

 modules will be for the six crew members, providing living quarters, storage 

 areas, and communications, and one will be a laboratory. It is anticipated that 

 the modules will later be joined by two additional laboratory modules, one from 

 Japan and one from Germany. Block II will add two horizontal towers with an 

 upper cross boom for mounting attached payloads and telescopes and a lower 

 boom for Earth-looking instruments. The power will be increased to 75 kW. 

 Solar dynamic arrays and a satellite servicing facility for free flyers and 

 co-orbiting platforms will be added. 



The free-flying or attached platforms provide a shared environment for instru- 

 mentation modules, which can be periodically serviced and changed out. The 

 co-orbiting free-flying platforms will be tended by the station crew, but the 

 polar orbit platforms will be serviced by the Shuttle. The free flyers provide 

 their own power, and have generally been derived from the scientific satellites 

 and telescopes that have been planned in advance of the Space Station concept. 

 In addition, platforms can be attached to the Space Station boom, providing 

 greater ease in servicing and experiment retrieval, though in a more contami- 

 nated environment. These platforms generally provide their own power, but 

 experiment control and telemetry is possible through sharing of Space Station 

 facilities. The Space Station offers the possibility that the platforms can be 

 tended or retrieved by the crew using an orbital maneuvering vehicle (OMV) that 

 is being developed in parallel with the Space Station, but will not be functional 

 at Block I. Another parallel development is the orbital transfer vehicle (OTV), 

 required to place free flyers or platforms in higher orbits, up to geosynchronous 

 orbit. 



The scientific community has been quick to review the potential offered by 

 the Space Station. With the caveat that sufficient funds be provided to continue 

 the orderly development of the well-planned, long-term space missions, some of 

 which are already under way, they have endorsed the use of the Space Station 

 for many of the other missions and have conceived of new experiments that take 

 specific advantage of the Station environment. Competition for space in Block I 

 laboratory modules or on platforms is severe. The following list presents the 

 facilities of interest to the exobiology community. These items have high 

 priority, and should remain in the final version of Block I for 1993 to 1996. 



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