Space stations

A space station is distinguished from a spacecraft by one requirement: indefinite habitation. That single change makes almost every subsystem harder, because nothing can be sized for a fixed mission duration.

What permanence requires

The maintainability requirement is the deepest departure from ordinary spacecraft practice. A satellite is built to be as light as possible and never touched. A station is built to be opened, so it carries connectors, access panels, standardised fasteners and spares, all of which cost mass that a satellite designer would never spend.

The ISS

The station has been continuously crewed since November 2000, which is the longest uninterrupted human presence off Earth by a wide margin.

The 51.6° inclination is not a scientific choice. It is the lowest inclination reachable from Baikonur given overflight constraints, and the station is in that orbit because Soyuz had to be able to reach it. Every other vehicle pays a plane change penalty as a result. See Plane changes and inclination.

Two subsystems illustrate the difference from satellite practice.

Power is distributed at 160 V primary, far higher than typical spacecraft, because harness mass scales inversely with voltage and the station’s runs are long. That brings the plasma interaction problems described in The space environment, managed by plasma contactor units that hold the structure near ambient potential.

Thermal rejection uses pumped ammonia loops feeding deployable radiators, because roughly 70 kW of waste heat cannot be moved by heat pipes alone. This is the only crewed spacecraft with an active external fluid loop at that scale, and it is a recurring maintenance item.

Logistics

Two constraints in that diagram are easy to miss.

Downmass is scarce. Most cargo vehicles are destroyed on re-entry, so returning samples, failed hardware for analysis, or anything else is limited to the few vehicles designed for recovery. For a research platform this is a real limitation on what science can be done.

A lifeboat is always docked. Crew size is bounded by return seats, not by volume or consumables. If a docked vehicle is suspected of damage, the station’s effective crew capacity falls immediately.

Reboost is continuous rather than occasional. At 400 km the station loses roughly 2 km of altitude per month, and the propellant to counter it is delivered by visiting vehicles. See Station keeping.

What microgravity is for

The research case is that removing gravity removes buoyancy-driven convection, sedimentation and hydrostatic pressure, which are ordinarily unavoidable confounds.

The physiology work is the most defensible, because there is no substitute. Bone density loss of 1 to 2% per month, the fluid shift that causes spaceflight-associated neuro-ocular syndrome, and immune dysregulation are all prerequisites for any long-duration mission, and the ISS is the only place to study them. See Mars habitats.

The materials and pharmaceutical claims deserve more scepticism than they usually receive. Microgravity effects are real, but so is the cost per kilogram and per crew hour, and few products have demonstrated value exceeding that cost. The honest position is that the platform is scientifically valuable and that its commercial output has consistently underperformed projections.

Commercial successors

The ISS is expected to be deorbited around 2030, and NASA’s stated intent is to become a customer of commercial stations rather than an operator.

Two structural questions hang over all of them.

The demand is uncertain. NASA is the anchor customer and everyone knows it. Whether a market exists beyond agency purchases, sovereign astronaut programmes and a small amount of manufacturing has not been demonstrated, and the ISS’s own commercial record is not encouraging.

The gap risk is real. If the ISS is retired before a successor is operational, continuous human presence in orbit ends, and the skills, ground infrastructure and supplier base built over three decades are difficult to reconstitute.

China’s Tiangong provides the counterexample of a state-operated successor. Assembled from 2021 to 2022, with three modules and a crew of three, it is smaller than the ISS and considerably more capable than Mir, and it is the only other continuously crewed station currently in orbit.

References

• Ley, W., Wittmann, K. and Hallmann, W. (eds.) Handbook of Space Technology, Wiley, 2009, chapters on crewed systems.
• NASA, Reference Guide to the International Space Station, current edition.
• Chaikin, A. A Passion for Mars, and NASA historical series volumes on ISS assembly, for the programmatic history.
• Sibonga, J. D. “Spaceflight-induced bone loss: is there an osteoporosis risk?”, Current Osteoporosis Reports, 11:92, 2013.
• Lee, A. G. et al. “Spaceflight associated neuro-ocular syndrome”, npj Microgravity, 6:7, 2020.