Max KingJun 14, 2022

Why can't the ISS operate forever?

Since November 2000, the International Space Station (ISS) has been a permanently crewed laboratory.

Assembled in stages over several years, the data collected on board the ISS have helped advance everything from climate change studies to dark matter research.

In January 2022, NASA released a plan to begin deorbiting the ISS in 2030, culminating with the space station plunging into the Pacific Ocean by 2031. But after an eventual 30 years in orbit, why does NASA want to deorbit the ISS at all? Why not continue the mission well beyond 2030?

As much as NASA and its partners may want the ISS to last beyond 2030, it just might not be possible.

The International Space Station
The International Space Station The International Space Station as seen from the space shuttle Endeavour in STS-134.Image: NASA

The ISS’s "check engine" light is on

Long-term installations like the ISS require regular maintenance. But just like the maintenance for a car or an old house, that maintenance continually grows more expensive.

The systems the ISS needs to use for power, communication with Earth, and life support for the crew are all designed to be repairable in orbit by astronauts or robotic operations. While maintenance and upgrades to these systems happen all the time, the degradation of the station’s structure will limit its time in orbit.

The structure of any spacecraft is exposed directly to the harsh environment of outer space can cause damage, but additionally — in the case of the ISS — stressful docking and undocking maneuvers from other spacecraft lead to wear.

Too hot or too cold with no in-between

Without an atmosphere protecting it, the ISS is subjected to enormous temperature variations, along with intense radiation.

The exterior surface of the ISS regularly experiences temperature shifts from -120 degrees Celsius (-184 degrees Fahrenheit) to 120 degrees Celsius (248 degrees Fahrenheit) as it travels around the Earth passing from sunlight to shadow. These fluctuations cause expansion and contraction unevenly over the whole structure, weakening the hull needed to keep the crew safe.

Engineers plan for thermal fluctuations and design exposed surfaces with a variety of coatings to try and minimize these temperature swings. But over time in orbit, the direct exposure to UV radiation along with atomic oxygen found at high altitudes degrades these coatings to the point where they no longer reflect or absorb sunlight as expected. This results in even more uneven expansion and contraction with every 90-minute orbit.

SpaceX Crew Dragon approaches the International Space Station
SpaceX Crew Dragon approaches the International Space Station In the foreground, as the SpaceX Crew Dragon (right center) approaches the International Space Station, is JAXA's (Japan Aerospace Exploration Agency) robotic arm attached to Japan's Kibo laboratory module. Astronauts Doug Hurley and Bob Behnken of NASA's Commercial Crew Program were aboard the Crew Dragon before it docked to the Harmony module's forward International Docking Adapter.Image: NASA

Does docking damage the ISS?

The ISS structure has to withstand the docking and undocking of spacecraft continually. The relative velocities between docking spacecraft and the ISS are on the order of 0.1 ft/s (0.07 miles per hour, 0.11 kilometers per hour, 0.00003 kilometers per second), which, while small, still results in dynamic stresses on the ISS structure. (You can try docking with the ISS for yourself using SpaceX’s docking simulator).

The ISS’s docking mechanisms and structures are robust enough to survive the necessary impacts, but they are also designed to be as mass minimal as possible. When constructing anything off Earth, the cost of the mission goes up significantly with any additional mass, so these mechanisms have an upper limit on how much wear they can take.

Spacecraft dock with the ISS roughly every three months, which comes out to a ballpark of 120 docking events and 120 undocking events between 2000 and 2030. Some dockings are assisted by the station’s robotic arm, with others more recently being able to berth without the station’s robotic arm. However, all still strain the structure of the ISS and weaken the hull incrementally.

While internal electronics and life support equipment can be repaired by astronauts or robotics, the structure of the ISS itself cannot be so readily maintained as the decades go by.

If the ISS is not expected to be operable past 2030, what is next for a peaceful human presence in space?

In the updated International Space Station Transition Report, NASA outlines several goals for avoiding disruption to a human presence in space, which take the form of enabling Commercial Low earth orbit Destinations (CLDs) by 2030.

NASA is encouraging the development of commercially operated space stations or habitats that can take over some of the functionality the ISS has provided for several decades. Axiom, Blue Origin, Nanoracks, and Northrop Grumman have all stepped forward as companies wanting to provide the international community with access to a CLD, supported by NASA funding. This progression from commercial resupply to commercial crew to a commercial space station demonstrates the trust NASA has gained in private companies being able to provide the reliability and cost-effectiveness to improve access to space.

The ISS needs to maintain its health until CLDs can be operational. If NASA's estimates are correct, then the CLDs will be operating in time for the ISS to pass the torch before deorbiting in 2030, continuing a human presence in space.

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