One day, in the near or far future, an asteroid about the length of a football stadium will find itself on a collision course with Earth. If we are lucky, it will land in the middle of the vast ocean, creating a good-size but innocuous tsunami, or in an uninhabited patch of desert. But if it has a city in its crosshairs, one of the worst natural disasters in modern times will unfold. As the asteroid steams through the atmosphere, it will begin to fragment—but the bulk of it will likely make it to the ground in just a few seconds, instantly turning anything solid into a fluid and excavating a huge impact crater in a heartbeat. A colossal blast wave, akin to one unleashed by a large nuclear weapon, will explode from the impact site in every direction. Homes dozens of miles away will fold like cardboard. Millions of people could die.
Fortunately for all 8 billion of us, planetary defense—the science of preventing asteroid impacts—is a highly active field of research. Astronomers are watching the skies, constantly on the hunt for new near-Earth objects that might pose a threat. And others are actively working on developing ways to prevent a collision should we find an asteroid that seems likely to hit us.
We already know that at least one method works: ramming the rock with an uncrewed spacecraft to push it away from Earth. In September 2022, NASA’s Double Asteroid Redirection Test, or DART, showed it could be done when a semiautonomous spacecraft the size of a small car, with solar panel wings, was smashed into an (innocuous) asteroid named Dimorphos at 14,000 miles per hour, successfully changing its orbit around a larger asteroid named Didymos.
But there are circumstances in which giving an asteroid a physical shove might not be enough to protect the planet. If that’s the case, we could need another method, one that is notoriously difficult to test in real life: a nuclear explosion.
Scientists have used computer simulations to explore this potential method of planetary defense. But in an ideal world, researchers would ground their models with cold, hard, practical data. Therein lies a challenge. Sending a nuclear weapon into space would violate international laws and risk inflaming political tensions. What’s more, it could do damage to Earth: A rocket malfunction could send radioactive debris into the atmosphere.
Over the last few years, however, scientists have started to devise some creative ways around this experimental limitation. The effort began in 2023, with a team of scientists led by Nathan Moore, a physicist and chemical engineer at the Sandia National Laboratories in Albuquerque, New Mexico. Sandia is a semi-secretive site that serves as the engineering arm of America’s nuclear weapons program. And within that complex lies the Z Pulsed Power Facility, or Z machine, a cylindrical metallic labyrinth of warning signs and wiring. It’s capable of summoning enough energy to melt diamond.
About 25,000 asteroids more than 460 feet long—a size range that starts with midsize “city killers” and goes up in impact from there—are thought to exist close to Earth. Just under half of them have been found.
The researchers reckoned they could use the Z machine to re-create the x-ray blast of a nuclear weapon—the radiation that would be used to knock back an asteroid—on a very small and safe scale.
It took a while to sort out the details. But by July 2023, Moore and his team were ready. They waited anxiously inside a control room, monitoring the thrumming contraption from afar. Inside the machine’s heart were two small pieces of rock, stand-ins for asteroids, and at the press of a button, a maelstrom of x-rays would thunder toward them. If they were knocked back by those x-rays, it would prove something that, until now, was purely theoretical: You can deflect an asteroid from Earth using a nuke.
This experiment “had never been done before,” says Moore. But if it succeeded, its data would contribute to the safety of everyone on the planet. Would it work?
Monoliths and rubble piles
Asteroid impacts are a natural disaster like any other. You shouldn’t lose sleep over the prospect, but if we get unlucky, an errant space rock may rudely ring Earth’s doorbell. “The probability of an asteroid striking Earth during my lifetime is very small. But what if one did? What would we do about it?” says Moore. “I think that’s worth being curious about.”
Forget about the gigantic asteroids you know from Hollywood blockbusters. Space rocks over two-thirds of a mile (about one kilometer) in diameter—those capable of imperiling civilization—are certainly out there, and some hew close to Earth’s own orbit. But because these asteroids are so elephantine, astronomers have found almost all of them already, and none pose an impact threat.
Rather, it’s asteroids a size range down—those upwards of 460 feet (140 meters) long—that are of paramount concern. About 25,000 of those are thought to exist close to our planet, and just under half have been found. The day-to-day odds of an impact are extremely low, but even one of the smaller ones in that size range could do significant damage if it found Earth and hit a populated area—a capacity that has led astronomers to dub such midsize asteroids “city killers.”
If we find a city killer that looks likely to hit Earth, we’ll need a way to stop it. That could be technology to break or “disrupt” the asteroid into fragments that will either miss the planet entirely or harmlessly ignite in the atmosphere. Or it could be something that can deflect the asteroid, pushing it onto a path that will no longer intersect with our blue marble.
Because disruption could accidentally turn a big asteroid into multiple smaller, but still deadly, shards bound for Earth, it’s often considered to be a strategy of last resort. Deflection is seen as safer and more elegant. One way to achieve it is to deploy a spacecraft known as a kinetic impactor—a battering ram that collides with an asteroid and transfers its momentum to the rocky interloper, nudging it away from Earth. NASA’s
