The International Space Station (ISS) is getting old. Construction started on the station in 1998 and it has been continuously occupied since November 2000. Construction took 10 years, 30 missions, 15 space agencies, and 15 countries to complete. The lifespan of the modules that make up the ISS was originally set at 15 years, but this has been extended to 30 years, with the ISS commissioned through 2028. It is unclear if it will be extended beyond that.

Throughout this time the station has been repaired and upgraded, but the basic infrastructure remains. There is a certain amount of unavoidable aging that happens to hull, maintaining pressure in the challenging conditions of low Earth orbit. Without environmental control, temperature variation on the ISS would range from 250 degrees F on the sun-facing side and -250 F on the sun-opposite side. Temperature variation like this tends to fatigue material. It is therefore unclear what will happen to the ISS, and to orbiting space stations, after 2028.

The ISS cost $150 billion to build, and $3.5 billion per year to maintain. The ROI has largely been research in microgravity, including researching the ability to maintain extended stays in space. NASA plans to deorbit the ISS after 2028, and has no plans for a replacement. Its vision is to largely cede low Earth orbit to private companies. There are at least two companies with plans for their own stations, Axiom and Bigelow Aerospace. Both companies are planning modules that will attach to the ISS, and then detach and become their own free-floating stations once the ISS is decommissioned.

One of the ISS partners, Russia, has commitments to 2025. It is increasingly looking like they plan on pulling out at that time, and some speculate this is because they wish to focus on their own station. They are starting to warn about the age and condition of the ISS, especially their own modules. In July the Nauka research module’s thrusters fired accidentally, temporarily throwing the ISS out of its usual orientation. There have also been several air leaks in the Russian Zvezda service module where some of the crew sleeps. Russia is now warning that 80% of the components on their modules are past the expiration date, and that small cracks are appearing and may spread catastrophically.

What does all this mean for the future of crewed space stations? We are in a transition phase right now and it’s unclear where the medium terms will lead us. The short term is fairly easy to predict because it’s happening now. The ISS is unsustainable. Russia is almost certainly out after 2024. It will likely be a challenge to keep the ISS running through 2028 as planned, and perhaps some modules will need to be closed. But NASA will probably keep the ISS going until 2028 and then decommission the ISS as planned, deorbiting it into the ocean.

Meanwhile China’s Tiangong station is just getting started. This is a large station in LEO, although not as large as the ISS (37 meters vs 109). It, of course, is a newer station just starting its lifespan, and can hold a crew of 3-6 (compared to the ISS of 6-9). There is already a queue of 1000 proposed experiments to take place aboard the station. It is unclear at this time if China’s station will motivate NASA to reconsider its plans for future government-funded space stations.

The big variable for the next few decades and beyond, however, is the private space industry. Will companies be able to make a profit from building, launching, and maintaining stations in Earth orbit? Again, this is NASA’s plan. It seems to be working out well in terms of launch capability, with multiple private companies now able to fulfill launch services and one (SpaceX) approved for crewed launches. NASA sees their role as exploration and blazing a trail, making it feasible for commercial interests to follow. The knowledge gained from the ISS will hopefully inform later endeavors.

Axiom appears to be the closest to launching a module to attach to the ISS. Their success or failure will probably tell us a lot. They will be able to make a return largely by renting space on their modules for research and for space tourism. We’ll see if this is a viable business plan.

There is also a company planning on building the Voyager station, which is designed primarily for space tourism. This is planned to be a series of modules in a rotating ring providing artificial gravity. They state:

The station has been designed from the start to accommodate business, manufacturing, national space agencies conducting low gravity research, and space tourists who want to experience life on a large space station with the comfort of low gravity and the feel of a luxury hotel.

Right now they are targeting 2027 for their open date, but that seems very close given that they have not started construction yet (planned for 2026).

We appear to be in the gray transition zone of space stations like this being feasible. The cost of getting into space has to be drastically reduced. This is happening with reusable parts, but another order of magnitude would be nice. The cost of manufacturing is also steadily decreasing, especially of unique parts or limited runs like the parts of a spacecraft. Three-D printing and CNC machines allow for programmable manufacturing. Material science is also steadily advancing. Make no mistake – building a rotating space station is a massive engineering project, and will cost tens of billions of dollars. But it is a different world technologically than 23 years ago when construction of the ISS began.

It is also interesting to consider – what will and should be the lifespan of any new stations we construct? That is a big investment for something that will last only 30 years. Or perhaps we should expect them to have a relatively short lifespan, because our technology is improving and whatever replaces it will be better. A good compromise is to take a modular approach – construct stations out of replaceable modules so that over time the entire thing can be slowly replaced, without loss of continuity.

The ISS is constructed of modules, but without the plan to leverage this feature to maintain it indefinitely. However, if the Axiom does become a module of the ISS, and then continues beyond the decommissioning of the ISS as its own station, with later modules added, then perhaps the ISS will be living on through Axiom. It’s a matter of semantics at this point, but the basic idea of using infinitely replaceable modules seems like a good one.