An asteroid that could level a city now has a 3.1% chance of striking Earth in 2032, according to NASA data, making it the most threatening space rock ever recorded by modern forecasting.
Despite the rising odds, experts say there's no need for alarm. The global astronomical community is closely monitoring the situation and the James Webb Space Telescope is set to fix its gaze on the object, known as 2024 YR4, next month.
"I'm not panicking," Bruce Betts, chief scientist for the non-profit Planetary Society told AFP.
"Naturally when you see the percentages go up, it doesn't make you feel warm and fuzzy and good," he added, but explained that as astronomers gather more data, the probability will likely edge up before rapidly dropping to zero.
2024 YR4 was first detected on 27 December last year by the El Sauce Observatory in Chile.
Astronomers estimate its size to be 40-90 meters wide, based on its brightness. Analysis of its light signatures suggests it has a fairly typical composition, rather than being a rare metal-rich asteroid.
The International Asteroid Warning Network (IAWN), a worldwide planetary defence collaboration, issued a warning memo on 29 January after the impact probability had crossed one percent. Since then, the figure has fluctuated but continues to trend upward.
NASA's latest calculations estimate the impact probability at 3.1%, with a potential Earth impact date of 22 December 2032.
That translates to odds of one in 32 - roughly the same as correctly guessing the outcome of five consecutive coin tosses.
The last time an asteroid of greater than 30 meters in size posed such a significant risk was Apophis in 2004, when it briefly had a 2.7% chance of striking Earth in 2029 - a possibility later ruled out by additional observations.
Surpassing that threshold is "historic," said Richard Moissl, head of the European Space Agency's planetary defence office, which puts the risk slightly lower at 2.8%.
Webb observations in March
"It's a very, very rare event," he told AFP, but added: "This is not a crisis at this point in time. This is not the dinosaur killer. This is not the planet killer. This is at most dangerous for a city."
Data from the Webb telescope - the most powerful space observatory - will be key in better understanding its trajectory, said the Planetary Society's Betts.
"Webb is able to see things that are very, very dim," he said - which is key because the asteroid's orbit is currently taking it out towards Jupiter, and its next close approach won't be until 2028.
Unlike the 10-kilometer-wide asteroid that wiped out the dinosaurs 66 million years ago, 2024 YR4 is classified as a "city killer" - not a global catastrophe, but still capable of causing significant destruction.
Its potential devastation comes less from its size and more from its velocity, which could be nearly 40,000 miles per hour if it hits.
If it enters Earth's atmosphere, the most likely scenario is an airburst, meaning it would explode mid-air with a force of approximately eight megatons of TNT - more than 500 times the power of the Hiroshima bomb.
But an impact crater can't be ruled out if the size is closer to the higher end of estimates, said Mr Betts.
If it does hit, possible impact sites include over the eastern Pacific Ocean, northern South America, the Atlantic Ocean, Africa, the Arabian Sea, and South Asia, the IAWN memo states.
The good news, experts emphasise, is that there is ample time to prepare. If necessary, spacecraft could be sent to deflect the asteroid - technology that was successfully demonstrated in NASA's 2022 DART mission, which altered the course of a non-threatening asteroid.
