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A Graceful Exit, Part 1: How to Responsibly Retire ISS

The International Space Station, its solar arrays glowing gold as they are backlit by the sun.
Credit: NASA/STS-119

The International Space Station is a laboratory like no other. Built in space over the span of 13 years and more than 40 launches, the ISS is often counted among the most expensive endeavors in human history, and its capabilities are nothing short of exceptional. For the past 25 years, the United States, Russia, Japan, Canada, and Europe have operated a research facility in the most inhospitable environment known to humankind, kept it supplied with hundreds of tons of cargo, and enabled well over 250 people to live and work in low Earth orbit. It is impossible to overstate the benefit that has been brought down to Earth from our work in space.

Nevertheless, after many years and mission extensions, this testament to human ingenuity has aged, and every day it becomes more challenging to operate safely. Now, as the station approaches 30 years old, its parent agencies are faced with the difficult task of planning for its eventual decommissioning. Ultimately, NASA and its partners have chosen to deorbit the ISS into the Earth’s atmosphere, guiding its remains safely into the ocean. But responsibly disposing of the ISS will be a monumental undertaking all its own. As we prepare to lay to rest one of humanity’s greatest accomplishments, what will it take to give the ISS a graceful exit?

The crew of Expedition 65, representing the United States, Russia, France, and Japan, pose for a group photo aboard the ISS.
Credit: JAXA/NASA/Soichi Noguchi

How to Responsibly Retire the International Space Station

The International Space Station was not the first space station ever built, nor will it be the first to be destroyed by atmospheric entry. In fact, all previous space stations have ultimately met a similar fate, famously including the United States’ Skylab and the Soviet Union’s Mir. But the ISS is unique in being the largest, and longest-lived, station ever to fly. These attributes raise both practical and sentimental questions about its disposal: Can the ISS be safely and effectively deorbited? Shouldn’t it be preserved for its historical value?

NASA and the other ISS partner agencies have exhaustively studied many options for retiring the station, including both destruction and preservation. NASA has published the results of this analysis in a white paper available to the public, which details the rationale behind the decision to deorbit the ISS.

Simply put, the question boils down to public safety. Any object in low Earth orbit (LEO), such as the ISS, will eventually reenter the atmosphere if left unattended. As the station follows its ballistic loop around the Earth, it gradually loses energy to air resistance, even at more than 200 miles up. Without continuous boosts from visiting spacecraft, it would eventually slow down enough to fall irrecoverably into the atmosphere – potentially in just 1 to 2 years.

NASA is beholden to requirements from the U.S. Government regarding the safe reentry of space objects. Smaller objects might burn up completely, but larger objects often do not, allowing pieces of debris to reach the ground and damage people or property. Just this year, pieces of SpaceX’s Dragon spacecraft and a pallet ejected from the ISS survived reentry, striking people’s farms and homes. The International Space Station, the size of a football field, cannot be allowed to reenter uncontrolled. Inaction is not an option.

Furthermore, keeping the ISS under control is an endless marathon. Flight controllers, engineers, and countless other caretakers have fought to keep it running without catastrophe for 25 years. Astronauts are a crucial part of this battle; when not performing groundbreaking research, they are constantly fixing problems and conducting maintenance. Without human intervention, it’s a question of when, not if, a major failure will occur.

In summary, safe disposal of the ISS, regardless of the method chosen, requires it to remain controllable from the ground, and will involve a crew onboard until the last practical moment. Any failure to safely operate the ISS risks dooming it to an uncontrolled reentry, which in turn poses unacceptable risk to life and property on the ground. This consideration, it turns out, gets the final say in most situations.

Astronauts Robert L. Curbeam and Christer Fuglesang perform a spacewalk to help install an enormous ISS truss segment, with the Earth far below them.
Credit: NASA/STS-116

For example, the ISS was never meant to be disassembled, as is often suggested. Years of work by human and robotic hands has thoroughly tied its individual parts together, internally and externally, including critical wiring and ventilation pathways which cannot be easily untangled. Any false move could endanger the station’s structural integrity or livability – putting it at risk of an uncontrolled reentry. On the other hand, boosting the ISS into a higher orbit exposes it to a cluttered debris environment, increasing the risk of critical damage due to debris strikes. And, drag will still eventually pull the station back down, putting us in the same dilemma – only now with a derelict station, decades older, riddled with damage, and uncooperative to future handling.

Ultimately, the ISS is both our biggest asset and our biggest liability in space. Despite its value, the station must be managed as responsibly and ethically as any other space object. NASA recently codified its vision for Space Sustainability: balancing our current needs with those of future generations, and protecting the well-being of people on Earth. LEO will become a crowded environment over the next decade, and committing to leadership in sustainability means making hard choices with the benefit of all people in mind.

NASA and its partners are already developing the instruments of the station’s decommissioning. The cornerstone of this effort is the United States Deorbit Vehicle, or USDV, which will attach to the ISS and deorbit it using a series of high-thrust burns. The trajectory will be designed to bring the station down as quickly as is safely possible, ensuring its debris is confined to a narrow corridor in the ocean.

However, even at the end of its life, the ISS needs the active support of its own systems. Prior to the final set of deorbit burns, the station’s Russian orbital segment (ROS) will use its thrusters to gradually lower the station’s altitude. In order to maintain its orientation while fighting air resistance, the ROS will also use these thrusters to provide propulsive attitude control, keeping the station stable and on course. Each of these components – the ROS, the United States orbital segment, and the USDV – are interdependent, and they must work in concert to guide the ISS to a safe disposal.

A chart depicting the planned timeline for deorbiting the ISS, with key events labeled. Note the arrival of the USDV and the departure of the final crew.
Credit: NASA

NASA recently selected SpaceX to develop the USDV; according to a teleconference on June 28, the vehicle will be based on the company’s Dragon spacecraft. SpaceX will launch the USDV one year prior to NASA’s specified deorbit date, at which point the station’s orbit will already have decayed significantly. The final crew will remain aboard ISS to keep it running smoothly as long as they safely can, and depart shortly before final deorbit preparations begin.

It cannot be denied that the ISS is an incredibly historic artifact. The space station holds an untold wealth of anthropological information about our time living and working in space, with inestimable value for current and future generations alike – especially as we seek to take more people beyond Earth. The cultural legacy of this legendary piece of engineering would be greatly magnified if it could be preserved, in whole or in part. Unfortunately, keeping large pieces of the station intact poses too much risk to the people living on Earth right now. Still, NASA recognizes the need to remember the ISS, and is investigating what smaller artifacts from inside the station may still be returned and preserved, or even kept on orbit aboard future stations.

The unique cultural heritage of the ISS is precisely what makes it so difficult to talk about its end. The station created new opportunities and partnerships which transcended political barriers, uniting people across a multitude of diverse cultures and backgrounds. Although plans for future space stations are beginning to take shape, the true legacy of the International Space Station is much greater than the sum of its modules. To properly continue this legacy will take more than engineering expertise, and will push the frontiers of commercial and international policy. As new global powers rise and forge relationships with one another, the next generation of human spaceflight in LEO depends on decisions made today. In Part 2 of this article, Space Scout will examine the status, potential, and shortcomings of modern plans to replace the ISS.

A flag for every nation on Earth adorns the interior of the ISS.
Credit: ESA/NASA/Thomas Pesquet

Edited by Nik Alexander

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