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The UN official had trained for this moment. She had run drills and table-top exercises at her offices in Vienna, housed inside a grey and unassuming 1970s concrete tower complex next to the Danube River.

Aarti Holla-Maini, a British lawyer with a background in the satellite business, needed to have at least played out the scenario step by step. As the director of the UN’s Office for Outer Space Affairs (Unoosa), she was required to know exactly what she was expected to do if – and it was a big if – she were informed that a significantly large asteroid was on a possible collision course with Earth. Or, as she says with a laugh: “Armageddon.”

Should that occur, Holla-Maini is the designated person who will promptly alert the UN secretary general. They will then disseminate her message to 193 member states – virtually every government in the world.

Sounding the alarm over a threat from an asteroid is not a regular part of the job. But a little over a year ago, Holla-Maini found herself doing just that. While between trips, she was pulled aside by a colleague at the office. “This wasn’t a simulation or a drill,” Holla-Maini recalls. “It was real.”

On 27 December 2024, a robotic telescope in Chile had spotted a distant rock hurtling through space, initially estimated to be the size of a small building. This by itself is not an unusual occurrence, and scientists normally then track the asteroid to reassess and refine its Earth impact probability (IP). Nor is it a secret, and news of the asteroid was spreading in public circles well before Holla-Maini became involved.

Yet in this case, the asteroid, emotionlessly named 2024 YR4, was seen as a growing threat. Its IP gradually increased over the next three weeks, as more observatories started to track it and further calculations were made. The initial estimate of a less than 0.05% chance of it hitting Earth had grown to more than a 1% chance of smashing down in 2032.

That, as well as its size, meant 2024 YR4 had met the criteria for Unoosa to issue its first global notification since the UN established a planetary defence collaboration in 2013. Although the chance was minuscule – in effect a 99% likelihood of a non-hit – it was considered noteworthy because the asteroid’s size and speed would mean an impact with Earth could release energy comparable to hundreds of times the power of the Hiroshima bomb. It could wipe out a city, or even a region.

By that point in late January 2025, Romana Kofler, a programme officer, had been up late. At Unoosa, she is the point of contact for planetary defence (an asteroid in the asteroid belt is named after her). Kofler had been in discussions with the International Asteroid Warning Network, a UN-backed body of astronomers, members of Nasa and the European equivalent, Esa, as well as experts who calculate the orbits and trajectories of distant rocks.

“We had trained for this with simulations, but this was the real thing,” says Kofler. “The adrenaline kicked in.”

After she had tapped Holla-Maini on the shoulder to break the news, they drafted the letter and sent it to António Guterres, the UN secretary general. Holla-Maini says: “We were very quick in preparing the drafts. So this was the first real-time test of an international response.”

Real-world threats

The threat posed by objects in outer space is far from theoretical. In 2013, a meteor exploded over Chelyabinsk, Russia. That event, caused by a rock measuring 20 metres (60ft) wide, released energy equivalent to 500 kilotonnes of TNT, generating a shock wave that shattered windows in thousands of apartment blocks. More than 1,200 people were injured by flying glass and debris. The fireball glowed 30 times brighter than the sun, causing immediate skin burns and proving that even smaller space rocks can inflict mass casualties without warning.

For a short period last year, 2024 YR4 was the most significant near-term threat since the discovery of Apophis in 2004, which made headlines after it was rated a four on the Torino Impact Hazard Scale, but was later downgraded as observations showed that it would pose no threat for at least a century. The Torino scale ranges from zero, when there is no risk, to 10, when a collision is certain and poses a threat to the future of civilisation as we know it.

On that scale, 2024 YR4 made it to level three, and its detection activated another UN-endorsed body, the Space Mission Planning Advisory Group, tasked with working on ways to save Earth. One option is deflecting the asteroid by smashing it with an intercepting spacecraft, a technique successfully tested on another asteroid in Nasa’s 2022 Dart mission.

The day-to-day reality of Unoosa does not always revolve around crisis. Pronounced “Younoosa”, it is not the most well-known UN agency. Nor is it very big: 35 employees work out of the modest Vienna International Centre, far from the more lively UN hubs in Geneva and New York.

It was established in the twilight of the 1950s, at the dawn of the space age, when the UN, itself still embryonic, decided to create a body with the “wish”, as it put it, to avoid the extension of political rivalries on Earth into space. Today the office’s small team manages a massive remit, as governments and businesses increasingly look to operate in space.

Having made the jump from 25 years in the commercial sector to the sprawling and bureaucratic UN, Holla-Maini spends much of her time shuttling around the globe to conferences and has many responsibilities, including promoting international law and regulation in an increasingly crowded and competitive space.

The agency runs a programme called Space for All, helping non-space-faring nations access the benefits of orbit. Its disaster and emergency response programme, UN-Spider, facilitates access to satellite imagery for countries facing natural disasters.

But it is Unoosa’s role as an official register of satellites launched into Earth’s orbit that has become the most critical. As the number of satellites climbs well above 10,000, and many times more that number are planned for launch, the vacuum of space nearest to Earth is becoming a congested and risky traffic zone.

The agency has found itself acting as an informal “hotline” for potential satellite collisions – a role that becomes terrifyingly complex when the satellites involved belong to nations with no diplomatic relations.

Holla-Maini recounts an incident last June when the Malaysian Space Agency called Unoosa on a weekend after one of its satellites, which was non-manoeuvrable, appeared to be on a collision path with a North Korean satellite. They were only 75 metres apart, says Holla-Maini – it was a “really hot” situation.

With no official two-way line to Pyongyang, Holla-Maini’s team had to send all the information to North Korean email addresses they knew would not reply. “The best you can do is send whatever notes and information you can to every official point of contact that you have,” says Holla-Maini. And then, without a response, the North Korean satellite suddenly moved. “They did move out of the way without us actually having a bilateral conversation.”

Whether it is sounding the alarm on an asteroid that could destroy a city, facilitating disaster relief imagery for floods in Morocco or preventing a collision in Earth’s orbit, the “tiny team” in Vienna is punching well above its weight, says Holla-Maini. “Because we have been in this straitjacket of not enough staff, not enough budget, it has forced the office to be extremely efficient,” she adds.

The 2024 YR4 asteroid scare served as a useful test for Unoosa’s relatively new role in planetary defence. For now, the asteroid is being monitored. Although its impact probability for a strike on Earth peaked at more than 3% in February last year, it has since dropped to a negligible level.

“All of a sudden,” says Holla-Maini, “it was gone.”