An artificial orbital station is an isolated man-made habitat for humans to exist in the inhospitable and hostile environment of space. Figure 6.9 shows MIR in orbit near the end of its 15 years in space and Figure 6.10 shows the International Space Station (ISS) in orbit early in its lifetime. MIR was not as elaborate as ISS, but is was the longest-lived functional orbital station. Its modular design allowed different
functional modules to be added as needed. Note that, in the absence of a United States supply and rescue vehicle, for both orbital stations the Soyuz capsule is the supply and rescue vehicle. There is a Soyuz attached to the ISS (bottom of Figure 6.10) but there is no Soyuz attached to MIR because the picture was taken by the last crew departing MIR before its entry into the atmosphere. Since both stations had their origins in the Russian station modules, there is a similarity of structure. The characteristics of such a station require its sustained and continual support to sustain a human crew over the operational life of the station, as given below.
The defining characteristics of an artificial Earth satellite/orbital station are:
(1) The station is without any self-sustaining resources, and must be continuously resupplied.
(2) The orbital station is the only inhabitable facility; survival outside the orbital station can be by space-suit only and is limited by life-support resources of the space-suit.
(3) The micro-gravity environment begins to induce significant physiological changes in the human crew for orbital stay times that exceed roughly 6 months.
(4) Solar and space radiation are serious hazards, especially over long orbital stays. A "safe house'' is required for the crew to wait out hazardous solar events (e.g. unpredictable solar flares).
(5) Solar wind and atmospheric drag requires propulsion burns to sustain orbital altitude. Failure to re-boost to operational orbital altitudes can result in atmospheric entry and destruction of the orbital station.
(6) The orbital station must be attitude-controlled to maintain solar panel and antenna orientation.
(7) Solar radiation is currently the sole, sustained, renewable power source via solar cells. Solar driven heat engines (Stirling or Rankine cycles driving generators) and nuclear power systems are yet to be considered or designed, much less tested or implemented.
(8) With human inhabitants, there is a critical requirement for means of rapid evacuation to Earth. This was one of the overriding considerations of the support systems for the 1964 USAF Manned Orbiting Laboratory. Only Russia has implemented a rescue system, sized for the station crew, which is attached to the orbital station whenever the crew is on board the station. Had the former Soviet Union not collapsed, the Lozino-Lozinski BOR 5 hypersonic gliders would be that crew re-supply/escape system, rather than the Soyuz ballistic capsule.
If the orbital station is to be more than a crewed pressurized container, then a sustained support and transportation system must be an integral part of the orbital station system. In terms of ISS that is not the case, even with the Space Shuttle in operational status. As discussed in Chapter 5, a LEO infrastructure is a demanding operation because nothing associated with the infrastructure is self-sustaining. Everything must be supplied from the Earth's surface. Secondly, unless some type of gravitational acceleration (of magnitude required to overcome physiological changes, yet to be determined) is generated, long-term human habitation will have serious health risks. Considering these challenges, General Stafford and his synthesis group determined that there is an approach that avoids most of these complications.
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