From Earth, the universe seems calm and distant. Most cosmic violence, from supernovae to colliding galaxies, happens far away. Asteroids, on the other hand, regularly pass through Earth’s neighbourhood, often unnoticed. Most are harmless, but a single significant impact can change life on our planet.
Finding, tracking and, if needed, deflecting asteroids is now a vital scientific challenge: the core mission of planetary defence.
Late in 2024, astronomers spotted a new near-Earth asteroid designated 2024 YR4, roughly the size of a 15-story building. While initial calculations revealed a small but real chance this asteroid could hit Earth in 2032, additional observation virtually eliminated any serious impact risk.
For a few tense weeks in early 2025, however, YR4 became a real-life planetary defence drill. Scientists worldwide mobilised to observe and characterise it, and the International Asteroid Warning Network (IAWN) issued its first-ever official asteroid threat notification.
The story did not end there. Updated calculations briefly suggested a 4% chance that YR4 could hit the Moon in 2032. This did not represent a danger, but it highlighted how even unlikely scenarios can carry operational consequences. “A lunar impact remains unlikely, and no one knows what the exact effects would be” says ESA’s Richard Moissl.
The saga proved that our planetary defence systems are improving. We detected the threat, communicated it, mobilised global resources to study it, and ruled out an Earth impact, exactly what we’d want to happen.
Detecting asteroids is a daily task for astronomers. Surveys such as Pan-STARRS, Catalina Sky Survey, and ATLAS scan the sky each night for new moving objects. Once detected, an asteroid’s position is reported, allowing its orbit to be calculated and any close approaches assessed.
Objects that pass especially near Earth receive closer scrutiny, but most are quickly confirmed as harmless as their orbits are refined. If a potential risk emerges, international alert protocols are activated – as happened with 2024 YR4, which briefly triggered coordinated observations worldwide.
Astronomers use the term Near-Earth Object (NEO) for asteroids or comets whose orbits bring them relatively close to our planet (within about 1.3AU – Astronomical Unit – of the Sun). A smaller group deserves special attention: asteroids that are at least about 140m wide, large enough to cause regional devastation, and pass close to Earth in astronomical terms (within 0.05 AU).
So far, scientists have identified over 2,300 potentially hazardous asteroids, and none of the known ones are expected to hit Earth. In fact, more than 99% pose no risk for at least the next century. The real concern lies with objects we have not yet discovered.
Most kilometre-scale asteroids, the planet busters, are already known. But tens of thousands of medium-sized objects, large enough to destroy a city or trigger a tsunami, are still missing from our catalogues. The 2013 Chelyabinsk incident caused by a 20m asteroid that arrived from the Sun’s direction showed how easily such objects can escape detection.
To reduce this blind spot, scientists are turning to infrared space telescopes, such as NASA’s NEO Surveyor (planned for 2026) and ESA’s NEOMIR, expected around 2030, designed to spot asteroids approaching from near the Sun.
Spotting a dangerous asteroid is only the first step. The challenge is knowing what to do next: how to prevent an impact or at least limit the damage. As the joke goes, the dinosaurs went extinct because they didn’t have a space programme.
For us, that changed in September 2022, when NASA conducted the world’s first full-scale asteroid-deflection test. The DART mission deliberately crashed a spacecraft into Dimorphos, a harmless target chosen for experimentation. The result was clear: the asteroid’s orbit changed measurably, proving that deflection is possible with enough warning.
Europe is now taking the next step. ESA’s Hera mission is heading to the same system and is expected to arrive in 2026, where it will study the impact up close and refine our understanding of how effective this technique really is. Together, these missions mark the beginning of active planetary defence.
The case of 2024 YR4 shows how early headlines can mislead when taken out of context. In early 2025, some reports labelled it a potential “city killer”, even though the probability of impact was extremely small, and steadily decreasing as more observations refined its orbit.
Asteroid risk is real, but it must be kept in proportion. Objects can be classified as potentially hazardous based on size and orbit alone, even when the actual chance of collision is close to zero. Besides, in astronomical terms a “close approach” can still mean millions of kilometres away.
This is why scientists take planetary defence seriously while pushing back against unnecessary alarm. Both NASA and European Space Agency emphasise a simple fact: no known asteroid is currently on a collision course with Earth.
Planetary defence is still a young field, but missions like DART and Hera mean humanity now has something the dinosaurs never did – the ability to spot potential threats early and respond with science rather than fear.
