In 2013, a small asteroid exploded in the atmosphere over Chelyabinsk, Russia. The sonic boom from the event sent more than a thousand people to the hospital, mostly from flying glass from shattered windows. The Chelyabinsk meteor was a relatively small chunk of space rock—asteroid researchers think it was probably about 20 meters (66 feet) across—but exploding over a city made it a noteworthy event. It's probable many similar asteroids hit Earth on a regular basis, but most don't happen to fly over metropolitan areas; they fall into the ocean or over lightly populated regions.

However, Earth has played target in the cosmic darts tournament before. Meteor Crater in Arizona, the Tunguska impact in Siberia in 1908, and most famously the Chicxulub asteroid in Mexico (which played a part in the extinction of the dinosaurs) are just three of many known examples. That's why many people are looking at viable options for planetary defense: destroying or turning asteroids aside before they can hit Earth. And planetary defense is one reason the United States' National Nuclear Safety Administration (NNSA) has given for not destroying some of its surplus nuclear warheads.

It's easy to be cynical about American nuclear weapons policy, especially now that we're decades since the end of the Cold War. Debates over nuclear winter, mutually assured destruction, and the like feel very distant. So reports that the US wasn't following the stated schedule for decommissioning nukes in the name of planetary defense triggered the skeptical radar, not least since The Atlantic, The Wall Street Journal, and other sources made it sound like the plan was to blow asteroids to smithereens.

There are many good reasons to doubt the wisdom of such a strategy, but it turns out this initial impression—and the impression given by many published articles—was wrong. The real plan is a lot less problematic than trying to obliterate an asteroid.

As a result, there's reason to be less cynical about the prospect of nuking asteroids, though there are still some open questions and fierce debate over planetary defense. To see the fuller picture, it's necessary to look at the risks of asteroid impacts, what we know about asteroids themselves, and what that means for the prospect of pushing them around. So let's examine whether the stated goals of stockpiling nukes are consistent with asteroid mitigation; sadly Bruce Willis will not be involved.

Death from above

Most asteroids and the smaller chunks of rock we call meteoroids are Chelyabinsk-scale threats, but a significant number are bigger—and not all of them are far away. While many big asteroids reside in the Main Belt (sometimes simply called the "asteroid belt") between Mars and Jupiter, researchers have identified a large number of near-Earth asteroids (NEAs), of which about 1,563 are deemed "potentially hazardous." NEAs are asteroids that orbit in a range a little closer to us than Mars. However, while a number of these NEAs cross Earth's orbit frequently—the "Apollo" and "Aten" groups of asteroids—very few large specimens come anywhere close to us. And none of them present an immediate danger to us. After all, the Solar System is a big place and Earth is relatively small.

For that reason, experts are divided on how much we should worry about asteroid impacts right now. The risk is low for the next few decades, but the potential damage is sufficiently high even for small impacts that some think we should focus a lot of effort on mitigation. The dinosaur-killing Chicxulub asteroid may have been 10 kilometers (six miles) across, but we don't need one that big to wreak serious havoc.

So if we want to take the long view, there is a persistent danger. Gravity from Jupiter and (to a lesser degree) Saturn can kick asteroids out of the Main Belt. Some of those are ejected from the Solar System entirely or fall into the Sun, but others end up in shallower orbits, where they might become new NEAs. The rate at which the gas giants are creating NEAs may be slow, but we're well advised to keep watch anyway. If we spotted an asteroid today that could be on a collision course with Earth, say within the next 30 years, it gives us time to prepare now rather than closer to the time of potential disaster.

"The dinosaurs didn't have a space program, much less telescopes, so it didn't end very well for the dinosaurs," says Alessondra Springmann, a planetary scientist who studies asteroids at the University of Arizona. "Hopefully we can find asteroids before they find us."

Asteroid researchers would also like to see them spotted because they want to study the structure of NEAs. "Planets are formed out of the same things the asteroids are formed out of," says Springmann. "But planets have been heated up, they've melted, the planets have surface processes, and Venus, Earth, Mars, the outer planets all have atmospheres." Asteroids, by contrast, are relatively pristine: "They're a whole source of information about Solar System as it formed." NEAs are particularly nice for the practical reason that they're close to us. We don't need to fly space probes (or presumably humans) out past the orbit of Mars to study them.

One fascinating discovery has been made by early NEA studies. "About 15 percent of near-Earth asteroids larger than 200 meters [656 feet] have moons," says Springmann. By measuring the orbits of those moons, researchers can find the mass of the asteroids they orbit using Kepler's Third Law (the same way astronomers measured the mass of the Sun or Jupiter). Combining the mass with radar measurements of the size of NEAs, astronomers have obtained the overall density of the asteroids.

The result: many asteroids are "rubble piles" (more formally known as "gravitational aggregates"), collections of smaller rocks and grains held together by gravity and molecular forces derived from static electricity. With some densities just greater than water, "they're more like conglomerations of styrofoam rather than a big type of boulder," says Springmann.

The aggregate nature has some interesting consequences. According to our theoretical understanding of these bodies, sunlight can heat one side of these asteroids slightly more than the other, increasing their spin until they literally fragment. At least one seems to have broken up entirely, but in less drastic cases, a smaller chunk falls off to make one of the moons astronomers have observed. And we might be able to exploit the effect sunlight uses to steer asteroids away from Earth.

Asteroid variety

Cristina Thomas of Goddard Space Flight Center notes that the sheer variation in asteroid properties makes classification hard. Asteroids range from very dark—blacker than a chalkboard—to light-colored, reflecting as much as 50 percent of sunlight back. A major way to catalog asteroid diversity is by looking at the mineralogy of meteorites, which are smaller chunks that have fallen to Earth, but which are fragments of the same types of rock constituting their larger asteroid cousins.



That's why all of this information is incredibly relevant for planetary defense. Rubble pile asteroids would require different mitigation techniques than solid, rocky, monolithic asteroids, but just because they're loose aggregates doesn't mean they can be easily broken up. "If you smacked an asteroid and it fell apart, it's likely that given enough time, it would re-accrete unless you had a very, very large offense," says Cristina Thomas of NASA's Goddard Space Flight Center. The tool we need to reduce the threat is more a metaphorical shovel than a hammer.

Of course, even if we successfully broke an asteroid into pieces, we'd need to make sure those pieces don't all hit us and cause as much destruction as the original asteroid. From the science fiction films Armageddon and Deep Impact, you might get the idea that we shouldn't close our eyes or fall asleep the best strategy is to use nuclear weapons to blow the threatening space rock to smithereens. This idea isn't off the table completely, but it must be kept as a last resort, something to be considered only if there isn't enough time to put together something less risky to Earth. And, somewhat surprisingly, there's another possible use of nuclear weapons that is less risky.