How Planetary Defenders Planned to Stop That City-Killer Asteroid
March 26, 202511 min readA Near-Miss with a City-Killer Asteroid Highlights Gaps in Earths DefensesThe threat from near-Earth asteroid 2024 YR4 may have subsided, but discoveries of other hazardous space rocks are set to soar as new observatories come onlineBy Robin George Andrews edited by Lee Billings Mark Garlick/Science Photo Library/Alamy Stock PhotoIn mid-February 2025 astronomers announced that 2024 YR4a recently discovered asteroid big enough to severely damage or even destroy a citystood a 3.1 percent chance of hitting Earth in 2032. At that moment, it officially became the most dangerous space rock known to science. But its reign of terror was brief: just a week later, additional telescopic observations allowed astronomers to refine their projections of 2024 YR4s orbital path, and the asteroids impact odds crateredto most everyones great relief.But theres more to this story besides the chills and thrills of a near miss. 2024 YR4s threat was no exercise; it tested the mettle of Earths staunchest planetary defenders in ways all too real. And although they passed with flying colors, theres always next time. Astronomers have, to date, found about 16,000 asteroids roughly the size of this one in near-Earth orbits around the suna seemingly impressive figure, save for the fact that some 215,000 more are thought to still be out there undiscovered.Sooner or later, chances are that astronomers will find a menacing asteroid on an actual collision course with our planet. The saga of 2024 YR4 offers a cautionary preview of how the worlds spacefaring nations would need to react when that happens.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.This would have been a delicate case, says Juan Luis Cano, an aerospace engineer with the European Space Agencys Near-Earth Object Coordination Center (NEOCC) in Italy. And thats because, while stopping an asteroid striking Earth is already difficult, doing it in under eight years could have easily pushed our technological capabilities to their outermost limits.This is effectively something we didnt really say to the public because we were pretty sure the risk would be removed, but if it didnt, the situation was not really ideal, says Patrick Michel, principal investigator of Hera, an ESA mission helping to test and characterize asteroid deflection techniques. A successful mission to deflect or destroy 2024 YR4 was not infeasible, Michel says. But it would not have allowed us any error.A Global ResponseAstronomers discovered 2024 YR4 on December 27 of last year, two days after the asteroid made a close approach to Earth. Initial observations quickly put its size between 40 meters and 90 meters. Soon after, three independent orbital dynamics groupsNASAs Center for Near Earth Object Studies in California, ESAs NEOCC and the privately owned Near Earth Objects Dynamics Site in Italydetermined that the odds of an impact on December 22, 2032, reached 1 percent.This is an important threshold, says Kelly Fast, acting planetary defense officer at NASAs Planetary Defense Coordination Office. An asteroid around this size that stands a 1 percent or greater chance of hitting Earth within the next half century is officially a cause for concern under prevailing planetary defense protocols. And, Fast says, everything happened the way it should.The United Nationssupported International Asteroid Warning Network, or IAWNan authority that, among other things, keeps the world informed about impact threatsissued a global alert. Another U.N.-backed group, the Space Mission Planning Advisory Group (SMPAG), composed of countries with space agencies, began to discuss possible mitigation responses. Many U.N. countries who currently do not get along super well were discussing this together, says Michel, who is a member of IAWNs steering committee.Astronomers all over the world began tracking the asteroid and sharing their data, while NASAs Sentry and ESAs Aegis orbital dynamics software programs autonomously and continuously refined 2024 YR4S orbit and its impact odds in full view of the press and public. Interagency cooperation went swimmingly. You had these independent ways of calculating the orbit, Fast says. And to be able to check each other was great.But for the worlds planetary defense groups, the frenzy of activity for the first few weeks of 2025 was all-consuming. 2024 YR4 easily overwhelmed everything else that we were doing, says Kathryn Kumamoto, head of the planetary defense program at the nuclear physicsfocused Lawrence Livermore National Laboratory. Imagine, for a moment, if there had been two or more different threatening asteroids to track rather than just one.Besides such the more the merrier concerns, uncertainties over the size and trajectory of 2024 YR4 posed additional problems.Initial observations of the asteroid relied on its reflected sunlight, and a small, shiny space rock can bounce back the same amount of light as a larger, duller one. This led to 2024 YR4s relatively wide 40-to-90-meter size estimate, which left lots of wiggle room for projections of how much damage it could cause. If a 40-meter asteroid were to score a direct hit on a city, it would cause widespread damage and some fatalities but wouldnt wipe that metropolis off the map. A city-striking 90-meter asteroid would be an order of magnitude more destructive; it would vaporize the impact site and spark a mass casualty event while spreading destruction tens of miles further afield.Astronomers enlisted the giant, infrared-attuned James Webb Space Telescope (JWST) for follow-up observations. They hoped to pin down the actual size before mid-April, when the asteroids orbital path would take it beyond the sight of JWST and all other telescopes. But by the time JWST turned its gaze to 2024 YR4 in early March, better estimates of the asteroids trajectory had already ruled out a 2032 impact.Thankfully, the improved forecast eliminated one particularly worrisome scenario in which 2024 YR4 would fade from view while still bearing a significant impact risk. Otherwise astronomers could have been forced to wait until the asteroids next close approach, in 2028, to get more definitive observations. That wouldve been four long years in which all that could be said was that the asteroid might hit Earth in a projected impact corridor that stretched from remote patches of ocean and uninhabited desert to densely populated cities such as Lagos, Nigeria, and Mumbai, India.For those hoping to stave off disaster, we couldnt afford to wait for four years for it to come back and then say, okay, we know the answer, Kumamoto says. Given enough time, we can definitely deal with that size of object. And then time was the interesting question here.Deflection and DisruptionRather than wait until 2028 for more information, the planetary defense community wondered if a reconnaissance probe could be launched (or coopted from current active space missions) to catch up to 2024 YR4 ahead of time. Such scouting could determine the asteroids size and its mass and gather crucial hints about its overall structure, which could range from weakly bound rubble pile to mechanically rigid, monolithic rock.All wed need is a camera and some thrusters, Kumamoto says. To save time, groups assessed the potential emergency usage of several spacecraft already enroute to different asteroids, including NASAs OSIRIS-APEX and Lucy spacecraft, ESAs Hera mission and Japans Hayabusa2# probebut none proved to be a good fit for the orbital maneuvers required to reach 2024 YR4.That raised the prospect of something entirely unprecedented: building and launching a threat-mitigation mission against a target about which little was known and that could ultimately prove to be totally harmless.Assuming, however, that 2024 YR4 was bound for Earth in this what-if scenario, we still could have gotten relatively lucky. If its on the smaller end, and its going to hit in the middle of the ocean, maybe thats fine, Kumamoto says. Such an outcome would have produced a powerful shockwave but in an uninhabited area; not even a tsunami would have been guaranteed because most of the asteroid wouldve likely broken apart and burned to ashes while traversing Earths atmosphere. In that case, a good option would have been to do nothing.If the impact site had been narrowed down to a populated area, though, the choice could either have been to evacuate the locality and take the hit or, more likely, to try deflecting or destroying 2024 YR4 before it could strike. Planetary defenders tend to prefer deflection because its gentler and more predictable. Destroying the asteroid (experts use the term disrupting) involves breaking it into pieces, and this only works if the resulting debris misses Earth or is small enough to burn up harmlessly in the atmosphere. Disrupting a larger asteroid into smaller, but still sizable, shards would still be a disasterit would merely transform a cannonball into a spray of buckshot still speeding toward our planet.Another reason many experts prefer deflection is that its the only mitigation method thats been tested on an actual space rock: in 2022 NASAs Double Asteroid Redirection Test (DART) mission spectacularly showed how slamming an uncrewed kinetic impactor spacecraft into a large asteroid can change its orbit. But this method isnt like playing billiards in space, with a single kinetic impactor bashing an asteroid into a radical new direction; rather, such impacts impart small nudges so that one small orbital shift adds up over years to change an Earth-striking asteroid into an Earth-avoiding one.Computer simulations by researchers at Lawrence Livermore suggest that a 90-meter asteroid could be confidently deflected away from Earth by one single DART-esque kinetic impactorbut that this orbital shift would take 10 years to unfold. Asteroid 2024 YR4 may have been 90 meters in length and may have struck Earth in 2032. Eight years is tight, Kumamoto says. And so both deflection and disruption were under consideration for 2024 YR4.Lets say 2024 YR4 was smaller, about 50 meters in size. If it was bound to hit Earths edge as a glancing blow, then a gentler nudge by a single DART-like spacecraft might have succeeded. But if it was destined for a more direct hit, then a more powerful kinetic impactorperhaps several of themmight have been required.If you needed to move it a significant distance, then we run into the possibility that we end up fragmenting the asteroid, says Kumamotoand you dont want to accidentally break it and risk a larger fragment still impacting Earth. But at that 50-meter size range, if you attacked 2024 YR4 with enough force, then you could shatter it into inconsequentially sized pieces that might only startle onlookers and shatter a few windows. If somethings 50 meters and a few 10-meter objects hit [Earth], thats maybe not so terrible, says Andy Rivkin, an astronomer at the Johns Hopkins University Applied Physics Laboratory, who helmed the proposal to use JWST to observe the asteroid.Going NuclearSome experts argue, however, that something as fraught as asteroid mitigation would demand the assurances that seemingly only one extreme measure can provide. When in doubt, going nuclear should suffice.These nuclear options ... are always brought up by our American colleagues, Cano says. Thats to be expected. Various national labs that help maintain the U.S. nuclear arsenal have conducted complex supercomputer simulationsand even some lab-based experimentsdesigned to (safely) work out the efficacy of using nukes to deflect or disrupt asteroids. And that work suggests nukes can be remarkably effective.For example, recent Lawrence Livermore research has shown that the close-up detonation of a one-megaton nuke could vaporize a 100-meter asteroid, which would have covered the largest possible version of 2024 YR4. But why stop there? One could opt for an even more ludicrously powerful nuclear blast to ensure the entire asteroid would be reduced to insignificant smithereens. During the high-level discussions about 2024 YR4, disruptioneither using a kinetic impactor or a nuclear devicewere kind of both on the table, Kumamoto says.Nukes can serve for deflection scenarios, too. Irradiating one side of an asteroid with a nuclear blasts x-rays would yield an incandescent jet of rock vapor, which, like a short-lived rocket, would push 2024 YR4 to the side more forcefully than a DART-like impact.Of course, although going nuclear may on paper appear as the best shot at protecting the planet, policy matters complicate things. Aside from the fact that its technically illegal to use nukes in space, the geopolitical precariousness (and environmental risks) associated with strapping them to giant space rockets could be particularly problematic. If we had ourselves in a situation where that was the only option, it would be really challenging, Fast says.If 2024 YR4 was on the larger end of the size range and had remained a danger even after it was observed again in 2028, SMPAG and the community could have presented world leaders with a stark choice: use multiple DART-like kinetic impactorseach requiring a perfect launch and a flawless journey through space to 2024 YR4or deploy a single spacecraft armed with one powerful nuclear bomb. Which choice would be more palatablethe one with more possible points of failure or the simpler one that risks inflaming tensions between nuclear-armed nations?Fortunately, for 2024 YR4, we wont have to find out. But dont rest too easy: more Earth-threatening asteroids are coming.Thats perhaps the most important takeaway lesson from the case of 2024 YR4, says Richard Binzel, an asteroid hazards expert at the Massachusetts Institute of Technology. Soon this same sort of situation will shift from being a novelty to becoming routine.NASAs asteroid-hunting Near-Earth Object Surveyor space telescope will be launched by the end of the decade, and with the U.S.-funded, multipurpose Vera Rubin Observatorywhich will find millions of asteroids in just a few months of operationscoming online this year, many potentially hazardous space rocks will be identified. Because our eyesight is improving with these new surveys, well begin to see whats always been there, Binzel says. Expect more news stories to feature asteroids with rapidly fluctuating impact odds in the coming years.Another lesson? Time is the most important factor for planetary defense, bar none. Most if not all dangerously sized asteroids will miss the planet. The hope is that, with these next-gen telescopes, the few that might hit Earth will be spotted with decades rather than mere years of lead timeallowing scientists and policymakers to muster Earths defenses with considerably less time pressure.Technically, we have never been more aware of the threat of asteroid strikes, nor have we ever been more capable of detecting and preventing them in advance. But perhaps the most concerning lesson to be gleaned from 2024 YR4, and Earths planetary defense readiness, has nothing to do with scientific know-how.Leviticus Lewis was, until recently, Federal Emergency Management Agency (FEMA) detailee at NASAs Planetary Defense Coordination Office. He retired at the end of last year, but he is a veteran of many asteroid strike tabletop exercises conducted by NASA and its national and international partnerssimulated impact scenarios designed to see how relevant experts would handle such a threat. As such, he was kept in the loop throughout the 2024 YR4 saga.The good news, he says, is that the planetary defense fraternity took it pretty seriously. The system kind of worked as it was supposed to. The bad news is that the Trump administration is threatening to gut NASAs funding while taking an axe to other agencies that could help mitigate this type of natural disaster. FEMAs under attack, Lewis says.Although Europe is making moves to shore up its own planetary defense capabilities, the U.S. isfor nowthe leader in planetary defense. It has a dedicated asteroid-hunting observations program, and it has intensively funded and researched the use of DART-like kinetic impactors and nuclear devices to prevent asteroid strikes. But just, for a moment, imagine that 2024 YR4 proved to be a danger with additional observations. The corridor of possible impact locations has never been over U.S. territory but instead stretches over swaths of central Africa, south Asia and parts of South America. Some in the community have wondered if, in todays world, the U.S. government would have offered help freely or instead demanded some quid pro quo for assistanceor simply turned a blind eye, leaving those in danger to fend for themselves.The universe has given us a break now, Lewis says. But what about the next time? What if the U.S. takes its eyes off the ball? The asteroid doesnt care. Its just going to keep coming. We cant depend on the world not being crazy when it happens.
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