What happened with asteroid 2024 YR4?
Asteroid 2024 YR4, about 60 metres across, briefly had a chance of hitting Earth in 2032 of up to about 3 per cent, before further observations ruled it out.
2024 YR4 was discovered on 27 December 2024 by a survey telescope in Chile. Early observations showed it would pass near Earth in December 2032, and calculations gave a small chance of impact.
Rising probability. As more observations came in during January and February 2025, the calculated chance rose, peaking at about 3 per cent. This was the highest impact probability ever calculated for an asteroid of its size, briefly placing it at level 3 on the Torino scale, used to communicate impact risk. Rising probabilities like this are expected early on, because the area of uncertainty shrinks around Earth before moving off it.
Ruled out. By late February 2025, further observations had reduced the chance of Earth impact to essentially zero.
Size. Observations by the James Webb Space Telescope in March 2025 suggested it is about 53 to 67 metres across, similar in size to the object that caused the Tunguska event.
The Moon. Calculations in 2025 showed a small chance, of a few per cent, that 2024 YR4 could hit the Moon in December 2032. Scientists have discussed what such an impact would look like and whether debris could reach Earth's orbit, and further observations are expected to refine the figure.
Lessons. The episode showed how planetary defence systems work: discovery, follow-up observations, international coordination and clear public communication. It was also the first time the International Asteroid Warning Network's threshold for notifying the United Nations was triggered.