I guess this summarizes the 2024 take. They don’t really talk about reusing modules on that new Axiom commercial station, and I’d seen that proposed before. But it does describe why they can’t just keep it around, for example.
Why did NASA decide to deorbit the space station instead of alternative options?
NASA has examined several options for decommissioning of the International Space Station, including disassembly and return to Earth, boosting to a higher orbit, natural orbital decay with random re-entry, and controlled targeted re-entry to a remote ocean area.
Disassembly and Return to Earth
The space station is a unique artifact whose historical value cannot be overstated. NASA considered this when determining if any part of the station could be salvaged for historical preservation or technical analysis. The station’s modules and truss structure were not designed to be easily disassembled in space. The space station covers an area about the size of a football field, with the initial assembly of the complex requiring 27 space shuttle flights, using the since-retired shuttle’s large cargo bay, and multiple international partner missions, spanning 13 years and 161 extravehicular activities (EVAs), commonly known as spacewalks. Any disassembly effort to safely disconnect and return individual components (such as modules) would face significant logistical and financial challenges, requiring at least an equivalent number of EVAs by space station crew, extensive planning by ground support personnel, and a spacecraft with a capability similar to the space shuttle’s large cargo bay, which does not currently exist. Though large modules are not feasible for return, NASA has engaged with the Smithsonian National Air and Space Museum and other organizations to develop a preservation plan for some smaller items from the space station.
Boost to Higher Orbit
NASA evaluated moving the station from its present orbit to a higher orbital regime where its lifetime could be theoretically extended, thereby preserving the spacecraft for future generations. The space station flies at an altitude where Earth’s atmosphere still creates drag and requires regular reboosts to stay in orbit. The station operates in LEO around 257 miles (415 km) in altitude and as a mass of more than 945,000lbs (430,000kg). Depending on solar activity, the station’s orbital lifetime (the time before the station would naturally re-enter from atmospheric drag alone) at this altitude is roughly one-to-two years without reboosts. For this reason, the station cannot remain in orbit indefinitely, as it will naturally fall back to Earth, where an uncontrolled deorbit could pose a threat to people on the ground (see uncontrolled re-entry option).
Space station operations require a full-time crew, and as such, an inability to keep crews onboard would rule out operating at higher altitudes. The cargo and crew vehicles that service the space station are designed and optimized for its current 257 mile (415km) altitude and, while the ability of these vehicles varies, NASA’s ability to maintain crew on the space station at significantly higher altitudes would be severely impacted or even impossible with the current fleet. This includes the international crew and cargo fleet, as Roscosmos assets providing propulsion and attitude control need to remain operational through the boost phase.
Ignoring the requirement of keeping crew onboard, NASA evaluated orbits above the present orbital regime that could extend just the orbital lifetime of the space station. Boosting the International Space Station would require 120-140 m/s delta-V for a 100-year target orbit lifetime and 760 m/s delta-V for a greater than 10,000 year orbit lifetime, in comparison to 57 m/s for a controlled deorbit.
Additionally, ascending to these orbits would require the development of new propulsive and tanker vehicles that do not currently exist. While still currently in development, vehicles such as the SpaceX Starship are being designed to deliver significant amounts of cargo to these orbits; however, there are prohibitive engineering challenges with docking such a large vehicle to the space station and being able to use its thrusters while remaining within space station structural margins. Other vehicles would require both new certifications to fly at higher altitudes and multiple flights to deliver propellant.
The other major consideration when going to a higher altitude is the orbital debris regime at each specified locale. The risk of a penetrating or catastrophic impact to space station (i.e., that could fragment the vehicle) increases drastically above 257miles (415km). While higher altitudes provide a longer theoretical orbital life, the mean time between an impact event decreases from ~51 years at the current operational altitude to less than four years at a 497 mile (800km), ~700-year orbit. This means that the likelihood of an impact leaving station unable to maneuver or react to future threats, or even a significant impact resulting in complete fragmentation, is unacceptably high. NASA has estimated that such an impact could permanently degrade or even eliminate access to low Earth orbit for centuries.
Random v. Controlled Re-entry
The U.S. Government specifies that re-entering spacecraft must meet or exceed a 1-in-10,000 likelihood of public risk due to debris. An inability to meet this specification requires the spacecraft to conduct a controlled deorbit, which is a standard industry practice for spacecraft that exceed the U.S. Government’s safe re-entry requirements unless the spacecraft operates near a disposal orbit, such as a geosynchronous orbit. An uncontrolled deorbit occurs when a spacecraft enters the atmosphere without navigational or propulsive control and is only acceptable when the debris impact risk to the public is small (i.e., a small spacecraft or the structure breaks into small pieces and has a small debris footprint). The International Space Station requires a controlled re-entry because it is very large, and uncontrolled re-entry would result in very large pieces of debris with a large debris footprint, posing a significant risk to the public worldwide. Ensuring the space station is well maintained continues to be the safest operational approach while also planning for deorbit at the space station’s end of life.
The use of existing space station propulsion systems, such as the Roscosmos Progress vehicles, would provide an alternative to an uncontrolled re-entry prior to the arrival of the U.S. Deorbit Vehicle (USDV). However, these systems do not provide sufficient margin to lower the public risk to an acceptable level. The USDV will provide this margin to lower the public risk to U.S. Government standards.