What was the recent scare?
It was from a football field–size asteroid designated 2024 YR4. Detected in December by a telescope in Chile, the space rock was flagged for its size—it’s 130 to 300 feet long, big enough to wipe out a city—and a trajectory that put it on track to possibly hit Earth on Dec. 22, 2032. NASA initially put the chance of an impact at 1.2 percent. The odds soon jumped to 3.1 percent, or 1 in 32, a record high for an asteroid of its size. But after closer study of the asteroid’s orbit, the threat of a hit was downgraded to a negligible 0.004 percent. While humanity’s plans for the 2032 holidays are safe, the scare highlighted a threat that scientists say urgently needs more focus: the millions of giant rocks that are hurtling through space, some of which may be on collision courses with our planet. “Take it as a warning shot across our bow,” said astrophysicist Neil DeGrasse Tyson. “These things are out there.”
How many space rocks pose a risk?
NASA’s Center for Near Earth Object Studies (CNEOS) tracks more than 37,000 asteroids whose trajectories approach Earth’s orbit. Most are the size of a car or smaller and pose no risk, because they will burn up in our atmosphere. On the opposite end of the spectrum are “planet killers”: asteroids a kilometer or more across that could potentially wipe out civilization. About 900 have been identified. In between are a range of potential threats. A 160-foot-wide asteroid could destroy a major metropolitan area; those are thought to strike once every 1,000 years. A 500-foot- wide space rock could inflict mass casualties across a state or a small country; those arrive every 20,000 years. Of course, “these numbers are very approximate,” said planetary geologist Gordon Osinski, “and they don’t really help us figure out when the next one might happen.”
When did the last big one hit?
On Feb. 15, 2013, a roughly 60-foot-wide asteroid entered the atmosphere and exploded 19 miles above the Russian city of Chelyabinsk. It set off a blinding flash and a shock wave that damaged more than 7,000 buildings over 200 square miles, and injured more than 1,600 people, many of them hit by shattered glass. The light from the blast was like “the end of the world,” said Valentina Nikolayeva, a teacher. In 1908, a 130-foot-wide asteroid or comet—the latter is an icy ball of dust and rock—exploded 6 miles above a remote stretch of Siberia, releasing 185 times more energy than the atomic bomb dropped on Hiroshima. Some 800 square miles of forest were leveled in the so-called Tunguska event. Still, that space rock was a pebble compared with the one that wiped out the dinosaurs 66 million years ago. There’s no current fear of anything of that magnitude hitting Earth, but other Tunguska-size threats are out there.
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Can they be stopped?
Possibly. But first we have to see them coming. In 2005, Congress directed NASA to find and track, by 2020, 90 percent of near-Earth objects—asteroids or comets that come within 30 million miles of our planet’s orbit— 460 feet or larger. Right now “we’re at something like 45 percent,” said CNEOS director Paul Chodas. NASA has built a network of telescopes, including the one in Chile that detected 2024 YR4, to identify threats. An infrared space telescope, NEO Surveyor, that will further boost detection is scheduled for launch in 2027. When a new object is found, information is shared with a global web of space agencies and observatories that go to work determining its shape, size, and orbital path. If one is judged to be headed for Earth, the next task is to try to alter its path—something NASA recently proved feasible.
How did NASA do that?
In 2022, it launched the golf cart–size Double Asteroid Redirection Test (DART) spacecraft, which slammed into Dimorphos, a 530- foot asteroid, at 14,000 miles per hour. The collision successfully altered the projectile’s orbit. Researchers are also studying the use of a “gravitational tractor,” a spacecraft that would orbit alongside an asteroid, exerting gravitational pull that would gradually alter the rock’s course. All these efforts require years of advance planning and may not be effective against a giant asteroid. If a space rock is too large for deflection—or due to hit with relatively short notice— humanity would need to use a nuclear bomb to deflect or vaporize it. A 2021 study showed that a 1-megaton nuke launched at least two months before impact could annihilate a 330-foot asteroid. But setting off a nuke in space is no small matter. It “could be very awkward geopolitically,” said Robin George Andrews, author of How to Kill an Asteroid. It’s just one example of how asteroid response requires a globally coordinated effort.
Are countries working together?
We’re making progress. The Chelyabinsk explosion led to the creation of NASA’s Planetary Defense Coordination Office, designed in part to link efforts with other groups such as the U.N.-sponsored International Asteroid Warning Network. And last year for the first time, international representatives attended NASA’s biennial Planetary Defense Interagency Tabletop Exercise, which gamed out the discovery of a massive asteroid with a 72 percent chance of impacting Earth in 2038. Scientists say this cooperation is a step in the right direction, but that many questions remain around a potential global asteroid defense. What’s the best approach? Who would be in charge? “Asteroid impacts are one of the few natural disasters that we actually have the means to both foresee and prevent,” said NASA aerospace engineer Brent Barbee. We must be “as prepared as possible.”
Why the impact zone matters
Some 66 million years ago, an asteroid at least 6 miles wide slammed into Mexico’s Yucatan Peninsula. Digging a crater 125 miles wide, it triggered earthquakes and tsunamis, and firestorms that may have spanned the globe. Gas, soot, and dust blanketed the planet, blotting out the sun and sending global temperatures plummeting. That extinction event wiped out 75 percent of life on Earth—including the dinosaurs. But some experts believe the impact would have been far less calamitous if the rock had landed elsewhere. A 2017 study concluded that the asteroid struck a spot unusually rich in organic sediment, which worsened the blackout effect. The dinosaurs might have survived if not for that happenstance, the researchers believe, and the rise of the mammals—including humans— might never have occurred. “This is maybe a lucky coincidence that everything came into place like it is today,” said geochemist Mario Fischer-Gödde.
