February 18, 20254 min readCity Killer Asteroids Earth Impact Risk Rises to Highest Ever RecordedAsteroid 2024 YR4 has a 3.1 percent chance of hitting Earth, astronomers saybut theres no need to panic yetBy Lee Billings edited by Dean VisserThe asteroid 2024 YR4 probably wont come nearly as close to Earth anytime soon as the space rock in this artists illustration. But astronomers cant yet rule out a potentially catastrophic encounter projected for December 2032. Alejandro Miranda/Alamy Stock PhotoA city killersized asteroid called 2024 YR4, which had been discovered swooping uncomfortably near Earth last December, now has an estimated 3.1 percent chance of striking our planet during another close encounter in late 2032, space scientists announced on Tuesday. The escalation makes this sizable space rock the most threatening ever forecasted to impact Earth, although the prospect for catastrophe remains relatively slim: the chance for a direct hit is now one in 32.This is the highest impact probability we have seen for an asteroid of this size or larger, says Davide Farnocchia, an impact hazard expert at the Center for Near Earth Object Studies at NASAs Jet Propulsion Laboratory (JPL). But Farnocchia notes that 2024 YR4s superlative status is unlikely to last. The impact probability might change by the time you write this up, he says.First spotted by a specialized asteroid-alert telescope in Chile on December 27 last year, 2024 YR4 didnt enter the limelight until a month later, when preliminary assessments of its orbit showed the 40- to 100-meter-wide object had a greater than 1 percent chance of colliding with our world on December 22, 2032. The risk corridor stretches from the eastern Pacific Ocean to South Asia, cutting across vast, people-free stretches of sea and desertbut also massive population centers, including Bogot, Colombia, Lagos, Nigeria, and Mumbai, India. Theres even a vanishingly small chance that the incoming space rock could strike the moon.On supporting science journalismIf you're enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.If there is a collision with Earth, whether the asteroid breaks apart in our planets skies or punches a crater into the surface, the immediate effects could resemble a detonating hydrogen bomb, unleashing enough localized devastation to destroy any unlucky metropolis in the way.Its no wonder, then, that the subsequent intermittent upticks to 2024 YR4s impact odds have concerned astronomersas well as the increasingly befuddled public. How could scientists be so uncertain about this space rocks set course around the sun? Why dont they seem very worried about the rising odds that it will hit us? And whats taking them so long to discern the true danger (if there is any)?The simple answer is that pinning down an objects orbit gets easier the longer you look at it, and astronomers havent yet had enough time and opportunity to do that for 2024 YR4. The asteroid is now zooming away from us and has already become too faint in Earths skies for most telescopes to see. But it is still being regularly monitored by multiple large observatories. A team of astronomers will use the keen infrared eyes of NASAs James Webb Space Telescope to further constrain estimates of the asteroids size and trajectory in early Marchand again in early May, just before it dwindles from view (until its orbit brings it close to Earth again in 2028).The relative scarcity of data points lies behind the fluctuating risk assessment, which, until yesterday, had pegged the asteroids probability of impact at 2.6 percent. This boost came from two effects, Farnocchia says: Februarys full moon prevented precise observations for about a week, followed by an influx of fresh data from two facilities (the Magdalena Ridge Observatory in New Mexico and the Nordic Optical Telescope on La Palma, part of Spains Canary Islands, both of which renewed their tracking on February 15). Independent number crunching on all those data occurs at three separate facilities worldwide: NASAs Center for Near-Earth Object Studies at JPL, which is in California, plus the European Space Agencys (ESAs) Near-Earth Objects Coordination Center and the Near Earth Objects Dynamics Site (run by the private company SpaceDyS), which are both in Italy. So far, all three centers have reached the same general conclusions, showing a low but steadily increasing chance of impact.Still, for now I would not be worried, says Detlef Koschny, a planetary scientist at the Technical University of Munich, who, on behalf of ESA, chairs the United Nationsaffiliated Space Mission Planning Advisory Group (SMPAG)an organization tasked with coordinating global responses to asteroid threats. As the uncertainty reduces, the probability for it to cross our planet actually increasesuntil the uncertainty area doesnt cross our planet at all. Imagine 2024 YR4 as a bullet fired down a shooting range and Earth as the bulls-eye on a paper target. An initial projection might forecast the bullet hitting anywhere on the paper, but as the projectile flies downrange, a better estimate predicts that it will hit somewhere in the targets center. The bulls-eye (Earth) will occupy a larger area of this smaller region, and its calculated chance of being struck will rise, even if the bullet (asteroid) is actually off target.This is what happened with Apophis, the previous record-setting potentially hazardous asteroid. After its discovery in 2004, forecasters projected a possible collision with Earth in 2029. Over a few months the probability peaked at 2.7 percent, only to plummet to 0 percent after sufficient further observations. In all likelihood, within months 2024 YR4s rising impact probability will prove to be a similar false alarm (which is perhaps why astronomers so far have stubbornly refused to bestow it with a catchier name). In the meantime, you can call it what you likeand safely ignore ebbs and flows of its odds for catastrophe.The orbit of 2024 YR4 is already certain enough that nobody really reacts to the day-to-day changes, says Timothy Spahr, an astronomer who manages the International Asteroid Warning Network. Yes, the probability [of impact] can change, but in order to really drill down beyond a few percent, well need to increase the observational arc another 30-plus days. The process can seem a little tedious, he admits. But by the time the asteroid fades to black later this year, astronomers should know much more about how worried everyone should be.If 2024 YR4s forecast is still ominous by then, let alone by its next approach in 2028, preventive measures for 2032 may be in order. These could range from evacuating areas in the risk corridor to launching high-stakes space missions to nudge the asteroid off its collision courseor even to blast it to bits. But given that continued observations are likely to rule out the possible impact, Farnocchia concludes, its still premature to talk seriously about deflecting 2024 YR4for now anyway.Additional reporting by Meghan Bartels.