The space probe came barreling in at thousands of miles per hour, its mechanical eyes locked on its target—an asteroid named Dimorphos.
About an hour out, the asteroid looked to the probe’s cameras like nothing more than a faint speck in the darkness of space, slightly larger than a single pixel on your screen. A few minutes out, it began to look distinctly asteroid-like, lumpy and gray. Three seconds out, the asteroid filled the whole view—bright and beautiful, the landscape so rich with texture that you could almost feel the craggy rock against your fingertips.
And then, nothing. The spacecraft crashed into the asteroid, its fancy cameras and all the rest of its delicate machinery smashed to bits.
This was the plan all along. NASA did not send this probe to observe this asteroid or even scoop some samples from its surface to bring back to Earth, as other missions have done. The agency dispatched the spacecraft with the explicit hope of crashing it and changing the asteroid’s trajectory. This is a test run, but a future version of this mission could save Earth from a catastrophic impact by deflecting an asteroid on a collision course. A little bit of practice never hurts.
The asteroid at the heart of the mission—a small one, about 525 feet (160 meters) across—doesn’t pose a hazard to Earth. None of the known asteroids near Earth do—or at least they won’t in the next century. But someday, a mission like this “could save millions of lives,” Angela Stickle, a planetary scientist at the John Hopkins University Applied Physics Laboratory and leader of the team that planned this impact, told me. The mission—known as Double Asteroid Redirection Test, or DART, for short—is the world’s first planetary-defense test. In a grander sense, this is the first time human beings have attempted to alter the orbit of another celestial body in our solar system at all. And so far, it seems to be working; the DART spacecraft, about the size of a vending machine, smacked right into the center of Dimorphos tonight. When the probe struck, the impact slowed down the space rock, shortening its orbit—we’ll find out by how much in the coming days. Other natural disasters may end human civilization, but now, at least, we’re one step closer to preventing the kind of calamity that ended the dinosaurs.
The DART mission launched last year, just before Thanksgiving. The spacecraft spent months cruising toward Dimorphos, which is both an asteroid and a moon; it orbits another, larger asteroid, known as Didymos. Now that the impact is over with, astronomers will spend the coming days and weeks checking data from telescopes to see how the little asteroid’s path changes. Stickle’s team has predicted that the collision will shrink Dimorphos’s 12-hour orbit around Didymos by about 10 minutes. It doesn’t sound like much, but, in a more perilous scenario, a small shift could turn a certain hit into a near miss.
Asteroids are everywhere, circling the sun along with us. The asteroids that are big enough to do global-scale damage are easier to detect and rule out as potential hazards, and astronomers have found most of them, based on analyses of the objects already observed in our solar system. The smaller ones, like Dimorphos and Didymos, are trickier to spot, and scientists have discovered less than half of the total that they estimate exist. This is slightly concerning, because even an asteroid as small as Dimorphos could destroy a major city. But with enough warning, we could, in theory, avoid an asteroid impact.
“In movies and television and literature, asteroids are always the stand-in for acts of God. They’re a metaphor for the things that humans cannot control,” Carrie Nugent, a planetary scientist at Olin College who studies asteroids, told me. “But asteroids are governed by the laws of physics; they’re relatively simple. They are a predictable natural disaster, and therefore a very preventable natural disaster.”
There’s more than one way to deflect an asteroid, Nugent said. One option involves sending an uncrewed spacecraft to detonate an explosive near the looming asteroid. Another involves putting a probe in orbit around an asteroid and allowing the gravitational push-and-pull between the two objects to change the asteroid’s path. The funkiest suggestion Nugent said she’s heard involves spray-painting half of an asteroid white and the other half black; the cosmic street art would cause an imbalance in how much sunlight the object absorbs and gives off, which would in turn produce a change in its orbit. (That one’s less of a sure bet in an Armageddon situation: The process would take millions of years to work, Nugent said.)
Although we have now shown that we can successfully bonk an asteroid off course, there are still plenty of concerns in the realm of planetary defense. Astronomers are worried that the proliferation of satellites in Earth’s orbit could make detecting potentially hazardous asteroids more difficult in the future. And there are legal ramifications to be considered, Nugent said. International law prohibits the detonation of nuclear devices in space, for example. And what happens if one country mounts an asteroid-deflection test and accidentally nudges the object toward another part of the planet, rather than the infinite expanse of space? The affected party might want to, er, sue for damages, and there’s no comprehensive legal framework in place for that now, Nugent said.
Over the years, astronomers have carried out tabletop exercises involving imaginary asteroid threats, gaming out how they’d react to the sudden appearance of a cosmic threat hurtling toward Earth. Those scenarios have played out in conference rooms. To successfully execute a piece of that preparation nearly 7 million miles from Earth—to do the real thing—is thrilling. Planetary science is not usually an applied science, after all. “I love what I do, but a lot of it does not have a direct link to I’m making somebody’s life better with this,” Stickle said. Slamming into an asteroid like this is about as applied as it gets—and, depending on what the universe has in store for us, knowing how to do it might come in handy.