In late December 2024, scientists discovered an asteroid that was calculated as having a small chance of hitting Earth in 2032. As the probability of an impact grew, the subsequent media coverage of asteroid 2024 YR4 brought this potential threat from space sharply into focus – an asteroid crashing into Earth was no longer the stuff of science fiction but a real possibility.
Marco Micheli, Astronomer / NEO Observer, is one of six Starion experts in this field who work at the European Space Agency’s (ESA’s) Near-Earth Orbit Coordination Centre (NEOCC), part of ESA’s Planetary Defence Office within the Space Safety Programme. He explains how asteroids are identified as a potential threat by a global community of experts.
How are asteroids identified as a threat?
There are telescopes across the world dedicated to discovering asteroids; they’ve been designed and are operated solely for that purpose. The identification process is mostly automated but with some human validation. Once a new asteroid has been identified, teams of experts at centres such as NEOCC or the equivalent at NASA then estimate the probability of it passing close to Earth.
Usually we can work out within a few days whether these newly discovered asteroids present a real threat. Typically around 95% don’t, with only 4% to 5% of them warranting extra observations and checks because we can’t immediately exclude the possibility of an impact.
At that point an iterative process starts, with scientists observing them for days, weeks or even months, and mathematicians recomputing the probabilities, often daily. The more data we gather, the more precise we can be about their trajectory. In the vast majority of cases, we eventually verify they are not a risk, but some, like asteroid 2024 YR4, pass a certain threshold beyond which things start to get really interesting!
How is ESA involved in these activities?
ESA’s NEOCC is an operational programme that carries out observations of small bodies in our solar system and calculations in order to evaluate whether they present a threat to Earth. The centre was officially inaugurated in 2013, although its web portal was deployed the year before.
NEOCC doesn’t operate in isolation. We’re part of a global community of centres and scientists that collaborate continuously, sharing information and coordinating our observations to be as efficient and effective as possible. Along with all other similar organisations, our data is sent immediately to a single centre in the USA managed by the International Astronomical Union and is published for everybody to use. Essentially, we’re contributing to a worldwide effort to collect as much data as possible in order to characterise the trajectories of potential near-Earth objects and predict their impacts.
Most of us working in this field are scientists and mathematicians, and planetary defence is one of the most open and collaborative scientific fields you will ever find. We share our technical and hardware capabilities and exchange knowledge to ensure that every asteroid gets properly followed up without wasting human and hardware resources.
What roles do the Starion team have within NEOCC?
The work at NEOCC is divided into three main pillars. The first is ‘Observations’ – as the name suggests, these are astronomers who observe asteroids and measure their positions over time. The second pillar is ‘Orbit determination’, where they use the observational data to compute an asteroid’s trajectory and calculate the probability of it impacting Earth. The third pillar is ‘Mitigation’, which covers everything related to preventing or mitigating damage on the ground from a future impact.
Then there is one more section called ‘Information provision’, with a team that ensures all the findings are correctly published in the NEOCC web portal.
When I joined the team here over 11 years ago, there were four of us across two ESA centres. Now there are 15 , of whom six are Starion staff: three of us are observers, two work in orbit determination and the sixth works on the portal. (There are also three trainees/students.)
I’m in the Observation team but we don’t all do exactly the same thing. I’m an ‘astrometrist’: my skills and experience are in asteroid dynamics, which involves precisely measuring their position in the sky using images from telescopes across the world. Meanwhile one of my colleagues, Maxime Devogèle, is an expert on the physical side, focussing on the characterisation of the object: its size, the material it’s made of and so on. For various reasons it’s very hard to measure the size of an asteroid with any good level of confidence – it requires very technically tricky observations.
As an observer, I’m one of a very small community across the world – there are just a couple of dozen of us – and we all know each other personally. Although there are formal ways of avoiding duplicating our efforts and combining our efforts when an asteroid appears to be a significant threat, being friends professionally makes that aspect much easier.
What was special about asteroid 2024 YR4?
Asteroid 2024 YR4 was one of the minority of asteroids that wasn’t quickly excluded after initial observations. Instead, the global community kept observing it every day and the probability kept crawling up every time we got more data.
It was an interesting object because, although it would certainly not wipe out life on Earth, it was still big enough – around 40 to 90 metres across – that an impact would be significant on a regional scale, especially if it hit a city. Also, the possible impact date was in 2032, which is relatively close in time, so there was a real sense of urgency in our observations and calculations.
The big challenge was that it was moving away from Earth and getting fainter every day, so we had to use increasingly larger telescopes to observe it; in the end there were more than 500 observations using 63 observatories, including the James Webb Space Telescope, which lets us see objects when they’ve gone beyond what we can see with telescopes on Earth. We were sharing every detail of our observations and plans to ensure we could extract all the information possible… and the probability of an impact kept rising. Eventually it crossed an internationally agreed threshold of 1% that triggered the involvement of two international organisations set up by the United Nations and, in turn, brought this asteroid to global attention.
The two groups are the International Asteroid Warning Network (IAWN) and the Space Mission Planning Advisory Group. It gives you an idea of how rarely we encounter such a situation if you realise that these were both set up in 2013 but asteroid 2024 YR4 was the first object since then to trigger formal duties for either of them, and the first time IAWN issued a ‘Potential asteroid impact notification’. In fact, this was the second highest level of impact probability since we started monitoring in the late 1990s and the highest since 2004, making the situation very intense.
In the end, the impact probability peaked at around 3% before dropping to zero, with the whole observation and calculation period lasting around 2 months. We’ve now completely excluded the probability of it hitting Earth but not of it hitting the Moon (this currently stands at about 4%), so it’s still of interest to us scientifically. Now, however, it’s moved so far away in its 4-year orbit that we’ll have to wait until it comes close again in 2028 to carry out further studies.
Find out more
You can find out more about the work of these organisations on their websites:
Videos of the 9th IAA Planetary Defense Conference are available online – links for each day can be found at https://iaaspace.org/event/9th-iaa-planetary-defense-conference-2025/. In particular, you can watch an overview of how the risk of impact by asteroid 2024 YR4 was assessed starting at 1:59:40 on day 1 at: https://www.youtube.com/live/OIz_MGOfWxA
