Fermion , 5 hours ago

planetary.org/…/why-international-space-station-c…

The ISS has gone through multiple reboosts to gain altitude because there is a small amount of atmospheric drag in its orbit. That’s not the limiting factor though.

The structure is aluminum. Aluminum accumulates fatigue damage every time it flexes. Every time the iss goes from sunlight to the earths shadow, there is significant thermal expansion/contraction. This fatigues the structure. The repeated docking maneuvers also stress the structure. Radiation and atomic oxygen also cause degredation. All those factors are relatively minor in any given year, but are always accumulating. The ISS is getting less safe and the risk of a structural failure is increasing.

On top of that all, a bunch of the systems on board were designed 30 years ago. There have been major changes in communications, power systems, etc. in the time since the modules were built. Even though new experiments are built all the time, they are still constrained by capabilities of the capsules they operate in. So there are also science advantages to moving to a newer platform.