As big as a football field and heavier than 200 elephants, de-orbiting the International Space Station represents a monumental challenge. But is there another future for the space station?
Drift into the wrong part of the Pacific Ocean in eight years, and you might be in for a shock. Tearing through the sky will be some 400 tonnes (880,000lbs) of metal, set aglow by its re-entry through the atmosphere. This raging inferno will crash into the ocean, across an area maybe thousands of kilometres in length, signalling the end of one of humanity's greatest projects – the International Space Station (ISS).
The ISS has been orbiting the Earth since construction on it began in 1998. It has hosted more than 250 visitors from 20 countries since its first crew arrived in November 2000. "The space station has been a huge success," says Josef Aschbacher, the head of the European Space Agency (Esa), one of the more than a dozen partners in the programme. It has been a boon for international collaboration, not least between the US and Russia, who partnered shortly after the fall of the Soviet Union. "It is really one of the big international victories," says Thomas Zurbuchen, Nasa's former head of science.
But much of its hardware is decades old, which could eventually see the station become dangerous or even uncontrollable in orbit – a fate that befell the Soviet Union's Salyut 7 space station in 1985, requiring two cosmonauts to revive the tumbling station. "We really don’t want to go through that again," says Cathy Lewis, a space historian from the National Air and Space Museum in the US.
To prevent such a catastrophe in space from happening once more, the space station will be deorbited in 2031, bringing it through the atmosphere to safely splash down in the Pacific Ocean. This will be the largest re-entry in history and, last month, Nasa asked Congress for funding to start development of a "space tug" that might be needed to perform the task – a spacecraft that can push the station back into the atmosphere. Kathy Leuders, head of Nasa's human spaceflight programme, later revealed it was estimated the tug vehicle would cost just shy of $1bn (£800m).
Working out how exactly to deorbit the station is a mammoth undertaking. Many large objects have burned up in Earth's atmosphere, most notably Russia's Mir space station in 2001 and Nasa's Skylab space station in 1979. The ISS represents a whole new problem, however, being more than three times the size of Mir. "It is a significant challenge," says Jonathan McDowell, an astronomer at the Harvard–Smithsonian Center for Astrophysics in the US. "A 400-tonne object falling out of the sky is not great."
Beginning as the single Russian-built Zarya module in 1998, the station today is enormous, boasting 16 modules, vast solar panels mounted on a metallic truss, and radiators to expel heat. At 109m (356ft) in length it is the size of a football field, the largest human structure ever assembled in space. "It's like the pyramids of Giza," says Laura Forczyk, a space analyst at the US consulting firm Astralytical. A rotating crew of seven inhabit the station today.
The lifetime of the ISS has been extended several times, but it's widely agreed that extending it beyond 2030 would be risky. Other alternatives, such as boosting it to a higher orbit, are inconceivable, according to Nasa, as they would require dozens of boosting spacecraft to push the station to a safe altitude. Instead, the plan outlined by Nasa in a report last year is to push the entire station back into the atmosphere.
Events will begin in 2026, when the orbit of the ISS will be allowed to naturally decay under atmospheric drag, dropping from 400km (250 miles) to about 320km (200 miles) in mid-2030. At this point a final crew will be sent to the station, likely ensuring any remaining equipment or items of historical significance that have yet to be removed are done so, also reducing the weight of the station. "That is still in discussion," says Aschbacher.
Once the final crew has left, the station's altitude will drop further to 280km (175 miles), deemed the point of no return – where the station could no longer be boosted back above the drag caused by our planet's thickening atmosphere – a process of several months from its initial orbit. Here, Russian Progress spacecraft are earmarked to then give the station a final push back into the planet's atmosphere.
Recent problems with some Progress vehicles, however, and the worsening political situation with Russia, has led to Nasa investigating its space tug alternative. "Nasa is hedging its bets on Russian participation," says Wendy Cobb, a space policy expert from the US Air Force's School of Advanced Air and Space Studies. Russia has even suggested it may pull out of the ISS as early as 2025.
Whatever spacecraft is used, after this final push, the station will reach an altitude of 120km (75 miles), where it will hit Earth's thicker atmosphere at some 29,000km/h (18,000 mph), beginning re-entry in earnest. First, the solar panels will be torn from the structure. "The headwind will be so much," says McDowell. Based on studies of the Mir re-entry, this might be expected to occur at an altitude of about 100km (62 miles) and take just minutes before they are all ripped away. Then at around 80km (50 miles) above the Earth's surface, the modules themselves start to be ripped apart from each other before they are set ablaze by the re-entry temperatures of thousands of degrees, causing them to melt and disintegrate. Several sonic booms will be heard as the wreckage streaks across the sky.
The deorbit of Mir captivated audiences from around the world.
The ISS, however, is nearly three times as massive as the 140 tonne (300,000lbs) Mir space station, and its re-entry is likely to be even more spectacular. "Now you have 400 tonnes of flaming chunks flying through the upper atmosphere at orbital speeds," says McDowell.
Presuming all goes to plan, however, this flaming debris should pose no risk to human life. (Learn more about what the risk is of being hit by falling space debris.)
Instead, any of it that survives re-entry will be targeted to fall in Point Nemo, an expanse of the Pacific Ocean between New Zealand and South America often used as a spacecraft graveyard. This area is deemed to be sufficiently far from human habitation to safely dump space equipment. Due to a quirk of ocean currents, it is also relatively devoid of nutrients so supports little marine life.
Even so, the debris path of the ISS will be huge and unlike anything seen before, stretching several kilometres wide and possibly up to 6,000km (3,700 miles) long.
As such, access to this portion of the Pacific Ocean will need to be restricted during re-entry to avoid any casualties. "How they're going to manage that with airplanes and shipping remains to be seen," says McDowell. Yet for anyone that does witness it, the death of the ISS is likely to be some spectacle. "If I were Nasa I would fly cameras and sensors and really detail the breakup," says McDowell. "There's definitely science to be done." The entire re-entry from the initial breakup of the solar panels to the splashdown in Point Nemo should last just 40 minutes.
While an impressive show, there are some who worry the deorbit of the ISS is a waste of materials. The ISS not only contains a lot of valuable equipment but also useful resources, such as the metal in its truss and its solar panels, that has been taken to space at great expense. "It's a sunk cost," says John Klein, a space policy expert at George Washington University in the US. "Let's reuse what we can."
In late 2022, a group of companies including CisLunar Industries and Astroscale in the US presented an idea to the White House to do just that. That could include melting some of the metal in the truss of the station to be re-used to build new structures or vehicles in space, or even detaching entire modules and repurposing them for other space stations. "We definitely think there's an opportunity here," says Gary Calnan, CisLunar's chief executive. "We want to build a salvage yard in space."
A Nasa spokesperson said the agency "welcomes proposals for new and innovative ideas" but at this time Nasa "has not called for nor received proposals to repurpose major structural parts of the International Space Station". Ron Lopez, president of Astroscale US, hopes the agency will reconsider. "I hope that we have an opportunity to think through all of these options," he says.
For the time being the plan remains to dispose of the entire ISS into the Pacific Ocean, a dramatic end to a decades-long display of human ingenuity and collaboration in space. If you happen to find yourself drifting across a seemingly uninhabited expanse of the Pacific in 2031, be wary. You might just spy a shower of molten hot debris raining down on Earth from space. "It's going to be a media extravaganza," says McDowell. "It'll be an irresistible fireworks display."
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