Hunting killer asteroids

Earlier this year, an asteroid appeared to be on a collision course with Earth. Jenny Winder looks at how astronomers came to declare it a near- miss and how Earth can be defended from dangerous rocks from space

- Jenny Winder

On 27 December 2024, the Asteroid Terrestrial-impact Last Alert System (ATLAS) telescope in Rio Hurtado in Chile discovered asteroid 2024 YR4 speeding past Earth. The detection could have been too late; only two days before, on Christmas Day, the asteroid had been just 828,800km (515,000 miles) from Earth. As astronomers plotted out the path of the asteroid, they realised Earth might not always be so lucky - there was a chance the asteroid could impact our planet in December 2032.

• Telescopes across the globe have been gathering data on YR4 ever since. Chief among these was the Nordic Optical Telescope in La Palma which analysed the asteroid's size, shape and rotation, as well as its motion and exact position. This helped to refine its current orbit and predict its future trajectory. By February 2025, calculations seemed to confirm that its highly elliptical orbit would have a 3 per cent chance of impact with Earth. In April, the James Webb Space Telescope observed YR4 and calculated that it is between 53 and 67 metres across (174-220ft). It has an estimated mass of 220 million kg (485 million lb) and rotates once every 19.5 minutes - relatively fast for an asteroid. Its magnitude changes by 0.42 as it rotates, indicating it probably has an elongated shape. It orbits Earth once every 3.99 years, meaning it will revisit Earth once more in 2028 before the 2032 pass that caused alarm.

While an asteroid this size isn't an extinction-level threat, it could cause severe damage at a local level. An asteroid of this size is expected to impact Earth every few thousand years and can cause devastation either by impacting Earth's surface or by exploding in our atmosphere.

Warning from Chelyabinsk