Four-hundred tonnes of flaming metal blaze through the atmosphere at hundreds of kilometres per hour. A shooting star like no other, it’s one we’re likely to see in 2031 as the International Space Station (ISS) comes crashing back to Earth after spending three decades circling the planet.
The largest object ever constructed in orbit, the ISS has long been a stalwart of human space exploration. Construction began in 1998, at an eventual total cost of $150 billion (£117bn), and the first long-duration stays began in November 2000.
Since then, the ISS has never been empty, with a constant stream of astronauts from a total of 20 countries having come and gone. A generation of young adults today can say – for the first time in human history – that there has been someone living off-planet every single day of their lives.
But all good things must come to an end and the ISS is starting to show its age.
Russia has warned that at least 80 per cent of its section of the ISS has in-flight systems that are past their expiry date. Cracks have started to appear in the Zarya cargo module and there have been a series of air leaks in the crew’s living quarters.
Russia is committed until 2028, but the fallout from the war in Ukraine has affected the willingness of the international space community to continue to engage with Russia beyond then.
The politically fragile consortium of space agencies behind the ISS agreed a couple of years ago to end the mission by 2030 at the latest and we now know a little more about how this final act will unfold.
In June this year, NASA announced that it had awarded a $1bn (approx. £778m) contract to Elon Musk’s company SpaceX to help destroy the ISS. SpaceX will attach a tow to the space station and haul it down into the atmosphere to meet its demise.
Clean-up operation
Sadly, the ISS will need to be removed from orbit and destroyed – if we just left it up there, it would become a sitting duck for collisions with our space junk or other, natural debris. The station already has to undertake the occasional evasive manoeuvre to dodge these dangers.
“The ISS could break apart and create a lot of fragments and debris,” says Prof John Crassidis, an expert on space debris from University at Buffalo, New York. “That debris could hit other debris and we end up with something called Kessler Syndrome,” he says.
Kessler Syndrome is a chain reaction whereby a series of collisions debris create more debris and more collisions until our critical space infrastructure, which includes weather and communication satellites, is threatened. At its worst, low Earth orbit – the altitude range within which the ISS and satellites orbit – would become unusable for generations. For this reason, simply blowing the ISS up isn’t an option.
Another reason we can’t just leave the ISS where it is, says Crassidis, is that “it’ll come down by itself anyway.” The ISS naturally loses around 100m (328ft) in altitude every day, due to the drag caused by what little remains of Earth’s atmosphere at its lofty height.
Mission controllers have to give the ISS regular boosts to keep it up there. Once the final crew abandons ship, this natural drag will continue to rob the ISS of orbital energy until it re-enters the atmosphere.
According to Prof Hugh Lewis, from the University of Southampton, this will happen “within a few months to a year.” Lewis also points out that some satellites in higher orbits undergo a ‘death burn’, a boost that rockets them into a more distant ‘graveyard orbit’, where they’re safer. The ISS orbits too low, however, and is far too big for this to work, he says. So come down it must.
Controlled descent
There have been recent warnings of the consequences of leaving objects to re-enter the atmosphere in an unsupervised, uncontrolled manner, Crassidis says.
In March this year, a 1kg (2.2lb) piece of space debris crashed through the roof of a house in Florida, ripping through two storeys and leaving a hole in the floor, although, thankfully, nobody was injured.
The object was part of a 2.9 tonne (3.2 ton) pallet of used batteries jettisoned from the ISS in 2021. The ISS is over 100 times heavier than that pallet and, when it eventually comes down, roughly the equivalent of two blue whales falling out of the sky.
Crassidis also points out that in recent years the Chinese space agency has been letting its Long March rockets fall back to Earth in uncontrolled re-entries, with little idea where the debris will end up.
In 2022, one crashed into the sea just off the coast of the Philippines, with some parts making landfall in Indonesia and Malaysia. But then, earlier this year, chilling footage emerged of villagers in Xianqiao, China frantically running away as a trail of yellow smoke dropped out of the sky.
Understandably, it’s far safer to have a good idea of where the pieces will end up. “With a controlled re-entry there’s less than a 1 in 10,000 chance of someone being hurt on the ground,” says Crassidis. “There’s significantly more chance of you being hit by lightning than being injured by falling space debris.”
Finding Nemo
To maximise the chances of success, mission controllers will aim to bring the ISS down as far from people as possible. The prime spot is a remote part of the Pacific Ocean called Point Nemo, also known as ‘the spacecraft cemetery’.
The nearest land is the tiny volcanic island of Motu Nui, close to Easter Island, over 2,700km (1,678 miles) away. This watery grave is the final resting place of hundreds of spacecraft, including the Mir space station, which came down in 2001. It’s Mir that offers the best blueprint for the end of the ISS.
The year 2026 will see the beginning of the end, as mission controllers let the ISS sink naturally from its current orbit of 400km (248 miles) to just 320km (199 miles). The final astronauts will leave about six months before the slated re-entry date.
Whether the last crew will be asked to rescue any artefacts and return them to Earth for use in museums and other educational programmes is yet to be decided.
Once empty, the ISS will drop to 280km (174 miles), before SpaceX’s tug heaves it down to 220km (137 miles). The tug’s design is an adaptation of SpaceX’s Dragon capsule, which has been delivering cargo to the ISS for over a decade – although things don’t always go to plan, as the recent saga surrounding NASA astronauts Butch Wilmore and Suni Williams shows.
Wilmore and Williams were due to stay in space for just eight days after testing out a new Boeing Starliner capsule. Due to issues with the capsule, however, they may now have to stay up there until February 2025.
In 2023, Frank Rubio inadvertently broke the American record for the longest stay in space when the Soyuz capsule that was supposed to bring him home earlier was damaged.
Should everything go according to plan, bringing the ISS down to 220km (137 miles) will allow the thicker atmosphere to do the rest of the work and rob the space station of so much orbital energy that its fate is sealed.
Within an hour, any remaining parts of it will lie in ruins on the seafloor, having hit the ocean at ‘just’ a few hundred miles an hour after slowing from 28,970km/h (18,000mph) at the top of the atmosphere.
Intense friction with the atmosphere will spark a destructive inferno. “The solar panels will be the first to separate and break up,” says Crassidis.
Not every part of the ISS will be incinerated, though. The truss segments, which make up the ISS’s skeleton, are one of the parts most likely to survive re-entry and make it to the ocean. The whole saga should be visible from some of the Pacific islands (back when Mir came down, Russian rocket scientists watched as it tore through the skies above Fiji).
Not that mission controllers can be entirely certain about where all the bits end up, even in a so-called controlled re-entry. The debris from Mir ended up scattered over a huge stretch of ocean measuring 1,500 x 100km (932 x 62 miles). New Zealand and Japan both warned seafarers of the small – but not negligible – risk of getting caught in the deluge of raining space metal.
In 1979, NASA brought down the Skylab space station, aiming for the Southern Atlantic or Indian Ocean. They missed their target, however, and the pieces ended up making landfall in Western Australia.
There were no injuries, but the local Esperance Shire Council did issue NASA with a tongue-in-cheek fine of AU$400 (about £200/US$260) for littering, which still hasn’t been paid. At least aiming the ISS at Point Nemo gives its mission controllers a sizeable margin for error.
Perils and pitfalls
One potential stumbling block with the above plan is that some of the variables are still poorly understood. “There are things we don’t know as much about as we’d like to,” says Crassidis, “like exactly how many air molecules there are in the upper atmosphere.” This controls the amount of natural drag the ISS will feel. It’s also changeable.
When Mir deorbited, it was losing 200–650m (approx 650–2,100ft) in altitude per day due to atmospheric drag. The exact amount was unpredictable due to variations in the way the Sun was heating the upper atmosphere.
Solar storms can also mess with the upper atmosphere. Explosive eruptions from the Sun called coronal mass ejections can temporarily thicken the outer layers of the atmosphere and increase drag. With the ISS, mission controllers will have to keep a close eye on all these factors and play the resulting odds carefully.
Even if everything goes according to plan and the ISS reaches the waters around Point Nemo safely, there have been concerns in some quarters about the effects of dumping so much space hardware into the ocean.
NASA has conceded that some toxic and/or radioactive materials could survive re-entry and may even leak into the waters of the Pacific. One obvious example is the hydrazine that’s used as rocket fuel.
Dumping the ISS in the ocean may even be illegal. According to Article 192 of the United Nations Convention on the Law of the Sea, nation states “have the obligation to protect and preserve the marine environment.” There’s really no other option, however, with any alternatives likely to have far worse outcomes.
The plan may also go against the new Global Ocean Treaty that was agreed at the United Nations in 2023 after a decade of intense negotiation. It aims to protect the marine environment in international waters from human activity, but so far it has only been ratified by a handful of nations – it needs to be ratified by a total of 60 before it becomes part of international law.
What comes next?
Once the ISS is no longer in service, what will the future hold for long-duration human spaceflight? Well, there’s already the Chinese Tiangong space station, which has been in orbit since 2021.
But there’s also a big shift on the horizon. Low Earth orbit will become less the domain of professional astronauts and more a place of recreation. A host of planned commercial space stations are already being built, at least in part, to accommodate paying customers. They’ll also be used to experiment with manufacturing and other industrial processes in zero gravity.
Examples of planned US commercial projects include the Axiom space station, Orbital Reef and Starlab. Then there’s NASA’s own Gateway – a space station in orbit around the Moon. Gateway will serve as a base from which astronauts can control robots on the lunar surface in real-time or drop down to study and sample the surface themselves. In-orbit assembly is currently scheduled to begin in 2028.
India is also making great strides in space exploration and has pulled off a series of coups in recent years, including being the first to find water on the Moon. India is planning to build its own space station in the 2030s, but stays will be restricted to a maximum of 20 days. That’s significantly shorter than some ISS missions, which can last for more than a year.
None of these projects would be possible without the multitude of lessons learnt from the three decades the ISS has spent hurtling around Earth. If we eventually spread out to establish outposts on the Moon, Mars or beyond, future historians will surely look back at the International Space Station as the landmark project that kickstarted it all.
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About our experts
John Crassidis is a professor of Innovation at the University at Buffalo, where he teaches spacecraft dynamics and engineering. His research has been published in journals including the Journal of Guidance, Control, and Dynamics, the Journal of Spacecraft and Rockets and the Journal of Computational and Applied Mathematics.
Hugh Lewis is a professor of astronautics at the University of Southampton. He has served as an expert on space debris, space operations, and space situational awareness, representing the UK Space Agency at meetings of the Scientific and Technical Sub-Committee of the United Nations Committee on the Peaceful Uses of Outer Space (COPUOS).
