You're woken in the middle of the night by an urgent phone call. An asteroid is headed straight for Earth and it's your job to find out if it'll strike the planet and rain destruction down upon us all.
It sounds like an overly-dramatic movie trailer, but that’s the real-life task of the people in the European Space Agency’s (ESA) Near-Earth Objects Coordination Centre, or NEOCC – a multidisciplinary centre staffed by astronomers, mathematicians and engineers who work on predicting asteroid impacts.
People like Richard Moissl, Marco Fenucci and Dora Föhring (see ‘Meet the asteroid hunters’, at the end of the page). They’re part of the dedicated team monitoring our skies to protect the planet from potential threats from above.
On the lookout for asteroids
When it comes to defending a planet, the first step is finding out where the largest asteroids are.
“You can only do something to address a threat if you know about it,” Moissl says. “So the very first step in the whole chain is observations. You need to find the asteroids.”
The aim here is to identify any potential threats with as much advance warning as possible, so that there’s time to take action to protect us and the planet.
For example, we could launch an intervention like NASA’s DART (Double Asteroid Redirection Test) mission in 2022, which saw a spacecraft deliberately crash into an asteroid to alter its course.
With enough time, even a relatively small intervention such as this would be enough to deflect a very large, dangerous asteroid.
But if it helps to set your mind at ease, know that the vast majority of very large asteroids have already been located.
“The kind of objects that we’re really worried about are the ones that could destroy a city,” Föhring says.
The advantage of searching for these objects, which are over 100m (almost 330ft) across, is that they’re big enough to spot easily. “These really dangerous ones, we think we’ve discovered 99 per cent of them.”
The main focus for asteroid hunters now is medium-sized objects, which aren’t big enough to destroy the entire planet, but could cause terrible harm if they landed in an inhabited area.
Systems like ESA’s upcoming Flyeye telescopes and a whole host of other ground-based observatories scan the sky, searching for the slowly moving dots that mark an asteroid. But a single observation isn’t enough to know about their size or path, so follow-up observations are required.
That’s where the NEOCC comes in.
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Characterising an asteroid
Asteroids aren’t easy to observe, as they’re so much smaller than planets and can have a wide range of characteristics. They can be solid rock or loose piles of rubble. They’re made of all kinds of materials, which can make them appear brighter or dimmer.
This is a particular problem, as measuring an asteroid’s brightness is one way astronomers estimate how large it is; larger objects reflect more light.
“You could have a very reflective object that appears to be much bigger, relatively, or you could have a much darker object that’s actually very big, but looks like something small,” says Föhring.
Because of the challenges inherent in finding these objects, astronomers work with many data sources, from automatic robotic telescopes to huge all-sky surveys.
“We want to have as many observations as we can,” Föhring says, especially if an object looks like it might hit Earth soon.
“It’s actually more valuable to get positions from multiple locations around Earth, rather than from the same country or continent,” because that gives a more accurate position determination.
“We even get in touch with people from, shall we say, less friendly countries. Because it’s not about politics; it’s about the science. And we all understand that comes first.”
Understanding an asteroid's orbit
When the astronomers have observed as much information as they can, that’s when the mathematicians step in.
“We get everything that the astronomers measure about asteroids and their positions in the sky, and from there we try to reconstruct their 3D orbits in the Solar System,” Fenucci says.
But that isn’t simple either, as there are many uncertainties in the data, and often there is more than one potential orbit that fits the data, creating “a cloud of possible orbits that are compatible with the measurements.”
It’s all a matter of probability to take these possible paths and see how many could cross Earth’s. And it’s done using a program called Aegis.
The software lets the team see if anything is predicted to pass close to Earth in the coming decades and the information is shared on a risk list of current most-threatening objects.
As well as information on these further-off potential impacts, there’s also a system for identifying small objects predicted to strike Earth imminently.
While that sounds scary, there are actually millions of small asteroids in the Solar System, many whipping past Earth on a regular basis.
Most that enter our atmosphere burn up harmlessly before reaching the ground. Still, studying these objects – and the ones that do impact Earth – is valuable for research and to practice dealing with more dangerous events.
The imminent impactor alert system is called Meerkat, Moissl explains, “named after the little mongoose guys, as there’s always one on the lookout to tell the others that something is coming.”
Typically, an imminent impact is spotted a few times per year, with around two to twelve hours of warning.
“It’s like high-speed training,” says Moissl. “Everything happens in a matter of hours.”
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Asteroid alerts, night and day
As working on near-Earth objects requires such varied areas of expertise, the NEOCC office is full of researchers from a wide range of fields.
“It’s a unique environment because you’re putting astronomers and mathematicians together in the same office, and we can interact with each other to tell each other what interesting cases we have,” says Fenucci.
The physical proximity makes for a highly collaborative environment and the team pulls together whenever an urgent event arises.
One example being the time they all scrambled to help one member find information on 1979 XB, a huge asteroid with a poorly understood orbit, which popped up after not being seen for 40 years.
There are some differences between astronomy work and other office jobs, though.
“Observations are at night, so we don’t do that from the office normally. We often schedule things or observe from home,” Föhring says.
Asteroids have no respect for the nine-to-five schedule, so the team has to be flexible about their work times.
“When we have an impact alert, it’s not guaranteed to be at 9am in the morning so we have all day to work on it!”
In practice, these alerts rarely come at convenient times. “The imminent impactors have a bias towards weekend nights,” Moissl jokes.
Though workers at the centre can choose whether or not to receive these notifications, and people often take evenings off for special events, generally speaking, they’re thrilled to get the call as it means the opportunity for more science and learning.
“People who get the wake-up calls are very excited to get them,” Föhring says. “They live for that!”
An asteroid emergency at Christmas
A lot of the time, working in planetary defence is much like any other office job: Zoom meetings, number crunching, dealing with emails. But occasionally, there are dramatic moments that come right out of a Hollywood movie.
Just such a moment occurred one day in December 2024 when the Meerkat quick-alert system flagged up a small impactor set to arrive imminently.
This wasn’t during a regular day at NEOCC, though: it was during the office Christmas party and people had come from all over Europe to celebrate with their colleagues.
Everyone was in the office and they rushed into action with observers scheduling last-minute time on telescopes and mathematicians narrowing down the impact area.
Föhring was able to connect with locals in the region via a webchat to let them know a non-threatening, but noticeable impact was coming and inform them of what to expect.
The team even found a public webcam with a live feed of the area where the impact was expected in Lensk, Russia, showing a town square next to the frozen river, with a Christmas tree on display. It gave the whole event “a very Christmassy feeling,” Moissl says.
The team watched together as the asteroid harmlessly came down 10 hours after the first observation. “It was an amazing day.”
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A close call in summer 2024
Sometimes, though, the surprises aren’t so jolly. In the summer of 2024, there was an unnerving close call when automated systems reported a large object potentially more than 50m (almost 165ft) in size and with a one-percent impact probability.
Worst of all, there was a ticking clock, as the potential impact was predicted to occur in just 40 days’ time.
An object that large, arriving that soon, is the nightmare scenario because there’s no way to launch any kind of space-based intervention in the available time. But, at least to begin with, the team wasn’t that worried as an automated system flagging up a potential impact isn’t that rare.
“These things happen sometimes – it’s usually false alerts,” Moissl says. So the typical procedure is to put more telescopes on it and assume that, as more observations come in, the impact probability will drop.
But the impact probability didn’t drop. In fact, it kept rising throughout the day, reaching as high as 10 per cent. People dropped everything and rushed to find out more while working with NASA’s Planetary Defense Coordination Office.
As the probability continued to climb, they considered that they may soon have to issue an alert that an impact was on its way.
“The timer was counting down to telling the world about this potential hazard,” Moissl says.
Fortunately, before anything was announced or a panic could begin, a key person called in. Their job was to check automated warnings against known man-made objects in orbit and they had been away from their messages while driving home from vacation.
They took one look at the data and set everyone’s mind at ease – the object wasn’t an asteroid at all, but ESA’s JUICE spacecraft.
Having performed some complex manoeuvres, JUICE was now heading back towards Earth to perform a flyby, as planned, and would certainly not be crashing into the planet.
“That was really exciting,” Moissl says.
The asteroid that was predicted to hit in 2032
You might assume that protecting Earth from fireballs falling from space is a terrifying job, but the scientists at NEOCC don’t see it that way.
“Luckily, so far, we haven’t had a majorly disastrous object,” Föhring says.
They take their responsibility seriously, though, because even potential impacts that are years away can have real-world effects on people right now.
The asteroid 2024 YR4, which will make a close pass of the Earth in 2032, made headlines when it was discovered to have an impact probability that went as high as 1 in 32.
If the NEOCC team couldn’t pin down its trajectory before 2024 YR4 went out of observation range, the world would have to wait until its next close pass in 2028 to find out whether the asteroid would hit Earth. And if it did, where?
Moissl struggles to imagine what it must be like to live in one of the cities that could be in the asteroid’s path, not knowing whether it’ll be wiped out in the next decade. The worry is real, as is the economic impact and social upheaval for the people considering leaving their homes.
Being able to mitigate that gives Moissl a sense of purpose. “For me, that’s the best part about our work.”
If anything, it’s the Earthly matters that feel more demanding than the cosmic ones. When 2024 YR4 was in the news, NEOCC team members were frequently being called by the media for comment – while they were trying to work out the asteroid’s likely course.
“It was stressful sometimes,” Fenucci admits, while Moissl says it required mental resilience to handle the attention of an anxious public while also calculating asteroid trajectories.
But that same pressure is part of what makes people in this field excel. When a new urgent target is identified, “there’s a level of excitement,” Moissl says. “It’s very intense – there’s a lot of work in a very compressed amount of time.”
When the fate of the planet is at stake, doing a good day’s work is an incomparable reward. Having spent weeks working on the 2024 YR4 data, the team was finally able to confirm that the asteroid wouldn’t be impacting Earth after all, just before it slipped out of observation range.
“We got the confirmation from the system that the impact probability was sinking below one per cent,” Moissl says.
Even though the team was making jokes about the fun part of their job now being over; the relief was palpable. “That was the most joyous moment.”
Though the threat of 2024 YR4 has passed, it's only a matter of time before another asteroid comes along to threaten the planet.
Fortunately for us though, the team at NEOCC, along with similar organisations around the world, are keeping a watchful eye on the sky to provide the early warning we need to take the necessary action.
Meet the asteroid hunters
Richard Moissl is the Head of the European Space Agency’s (ESA) Planetary Defence Office and oversees the efforts of Near-Earth Object Coordination Centre (NEOCC). Previously, he was part of ESA’s Hera mission, which launched a spacecraft in October 2024 to investigate the effectiveness of asteroid deflection tests.
Dora Föhring is an astronomer who works on follow-up observations of newly-discovered asteroids. She is also working on the commissioning of ESA’s new survey telescope, Flyeye, as part of her role at NEOCC.
Marco Fenucci is a mathematician who works on calculating the orbits of observed asteroids. He’s a near-Earth object dynamicist at the NEOCC, so he takes data from the astronomers and uses it to model orbits and predict where the asteroids will be in future.
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