The International Space Station, photographed by astronauts in October 2018. Credit: NASA
Although it may be hard to believe, the International Space Station (ISS) is nearing the end of its life. Some readers may have been born after the first section of the ISS was launched in 1998 and have never lived in a world without it flying overhead. Construction of the ISS took years and required a total of 37 American Space Shuttle flights, as well as five Russian rocket launches, to bring the station to completion.
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Over the past quarter-century, more than 270 people have visited the ISS to help build the station, stay there, conduct scientific experiments and medical research, and, more recently, support various commercial space initiatives. Over its many years of service, the ISS has proven to be a boon to the five space agencies (the Canadian Space Agency, the European Space Agency, the Japan Aerospace Exploration Agency, Russia's Roscosmos, and NASA) that share operational responsibility for the station.
Limiting the options
Still, NASA plans to retire the ISS in 2030. Although the ISS is the largest space station built to date (it currently dwarfs China’s relatively new Tiangong space station), it is extremely expensive to operate and requires periodic boosts to maintain its orbit. Beyond that, much of the technology onboard the ISS is simply becoming obsolete. NASA and its partners never planned for the ISS to operate forever, and these organizations are now making plans for when operations aboard the ISS finally come to an end.
In contemplating the end of the ISS, NASA considered several options. One was to move the ISS to a higher (and more stable) “parking” orbit, where it could sit idle without a crew. But without humans to maintain the station, the hardware would likely begin to break down and progressively fail. Importantly, even a higher orbit would eventually cause the ISS to disintegrate and fall to Earth.
NASA also briefly considered decommissioning the station, but this would be extremely costly. It also raised the question of what to do with a used ISS module once it was back at the International Space Station. mainlandTo complicate matters further, without the space shuttle and its cargo bay to lower individual modules from the station, such a plan is, for all intents and purposes, impossible.
The only realistic option — and the one NASA ultimately chose — is to deorbit the station and let it descend through the atmosphere and crash into Earth.
Deorbiting can be a controlled or uncontrolled process. For example, a previous US space station, Sky Laboratorycrashed back to Earth after a largely uncontrolled deorbit in July 1979. An uncontrolled deorbit is cheaper (basically free), but risks hitting population centers, potentially injuring people or damaging buildings; some parts of Skylab landed in Australia, but fortunately, no one was hurt.
A controlled deorbit would allow the ISS to descend to Earth at a time and place of NASA's choosing, creating the least possible risk to people and infrastructure. This is the option NASA has chosen. And given its large size (about the size of a football field), the ISS is likely to descend over a wide area of the South Pacific Ocean, far from any inhabited areas.
Tearing down the station
So how exactly will NASA go about taking down a massive structure like the ISS?
Even at the station's current orbital altitude of about 400 kilometers (250 miles) above Earth's surface, it still experiences the drag of our planet's atmosphere as it orbits. Under the agency's current plan, when it's time to descend, the station's natural orbital decay will be enhanced by controlled maneuvers to lower its altitude even further.
Once the final crews leave the ISS, NASA plans to allow the station’s orbit to decay further for 12 to 18 months. Then, a modified version of SpaceX’s successful Dragon spacecraft will facilitate a final deorbit. This unique craft will be larger than a standard Dragon, have six times more propellant, and four times the propulsion power. The deorbited Dragon vehicle will use 46 Draco thrusters to perform multiple propellant burns to shift the orbit even further downward, as well as perform a final burn to precipitate the station’s final descent.
As the station descends through the atmosphere, it will heat up intensely due to friction with air molecules. This will cause the station to largely break apart and burn up. Many parts of the ISS will simply be consumed by the reentry fire, while some of the larger, more durable sections (such as the girders holding the station together) will likely survive the final fall to impact the ocean and sink to the seafloor.
SpaceX plans to have the deorbit vehicle completed by 2028 and NASA plans for the final deorbit event to occur in 2031.
The next big leap
NASA's next big space station project is the Moon GateIt will be smaller than the ISS, but much more complex, as it will not orbit the Earth, but the Moon. There is no direct replacement for the ISS on the drawing board.
It seems likely that at least one or two of the several smaller space stations currently being developed by private space companies will one day make it into space. These stations could offer further opportunities for scientific and medical research, although they may focus primarily on space tourism. Axiom Space, for example, has already published detailed plans for building a space station in several sections, each of which would be put into orbit by successive launches.
Whatever happens next, the International Space Station will always be remembered as a revolutionary piece of space technology, the construction of which required the combined efforts of many nations and countless people. Its legacy of achievement will be hard for its successors to surpass.
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