Back when Andy Rivkin was in college, he had a few friends in medical school. “I was like, oh man, I don’t want do anything that has too much responsibility,” he says. Instead, he looked to the stars. “Astronomy seemed pretty safe.” And, for a while, it was. Rather than having to make decisions about someone’s root canal or abdominal surgery, he watched worlds flit about in the darkness.

But Rivkin, a planetary astronomer at the Johns Hopkins University Applied Physics Laboratory (APL) in Baltimore, has found himself with more responsibility than he expected. Along with hundreds of others, he is part of the Double Asteroid Redirection Test (DART) mission, an ambitious effort led by NASA and the APL to slam an uncrewed spacecraft into an asteroid to change its orbit. This is a dry run for the real deal: one day, a technological descendant of DART could be used to deflect a planet-threatening space rock, saving millions—perhaps billions—of lives in the process.

On November 23, DART will launch on a SpaceX Falcon 9 rocket from California’s Vandenberg Space Force Base. Sometime next fall, it will smash into its target at 24,000 kilometers per hour. Ground-based astronomers like Rivkin will watch the rendezvous unfold with bated breath, hoping to see the telltale signs of success: a dust cloud, and an asteroid dancing to humanity’s tune for the very first time. Will it work?

“We don’t know what’s going to happen because we’ve never tried it before,” says Michele Bannister, a planetary astronomer at the University of Canterbury in New Zealand.

Success would not mean Earth is automatically protected from rogue asteroids. Despite resting most of our homeworld-protecting hopes on shooting at space rocks, there are no silver bullets in planetary defense. The bizarre and variable geology of asteroids may serve to rebuff our deflection attempts, our network of early-warning telescopes is rife with gaping observational holes, and the politics of deciding who can try to deflect an inbound impactor are fraught with uncertainty.

DART, no doubt, represents a major step forward. But the path to a comprehensive planetary defense plan is a long and winding road, and we have just begun to walk it.

Nixing the Next Tunguska

Despite the prominence of Texas-sized asteroidal antagonists in Hollywood blockbusters, big rocks are not a cause for much concern among levelheaded scientists. Almost all asteroids a kilometer or larger across with orbits approaching Earth have already been found, and none shall seriously threaten us in the next few centuries.

Like much in life, when it comes to planetary defense, it is the small things that matter. The space rock that exploded in midair over the Russian city of Chelyabinsk on February 15, 2013, was estimated to be just 17 meters long—and yet its blast, equivalent to perhaps 470 kilotons of TNT, unleashed a window-shattering shockwave that injured 1,200 people.

This airburst event, the first of its kind in the social media age, caused jaws to drop across the world. “It was sobering,” says Kelly Fast, the Near-Earth Object Observations Program Manager for NASA’s Planetary Defense Coordination Office—an office set up, not coincidentally, just three years after the Chelyabinsk event.

It could have been worse. In 1908, what seems to have been a 60-meter meteor detonated above a remote stretch of Siberia, flattening more than 2,000 square kilometers of forest. Imagine that happening over the city or town you live in: buildings would be reduced to rubble, debris would fly about in hurricane-force winds, and clothing and flesh alike exposed to the initial, scorching flash could burst into flames. It would be comparable to a massive nuclear explosion, minus the radiation.

These small impactors are disconcertingly plentiful. Of those at least 140 meters across, models suggest around 25,000 exist that approach within 190 million kilometers of the sun. Some of these so-called city-killer objects may pass unnervingly close to Earth's orbit. And of these diminutive but destructive near-Earth objects, “we think we’ve found less than half,” Rivkin says.

It is estimated that, every century, there is a 1 percent chance a city-killer will impact Earth. Even if that transpires, most of the planet’s surface is ocean, suggesting that a space rock is most likely to land in the middle of nowhere. But if one of them hits any nation, plunges into a country’s coastline or blows up overhead, it could cause one of the worst natural disasters in human history. Any given year, the odds are on our side, but wait long enough and our luck will run out. Without an effective defense plan, “it’s not a matter of if, but when” a city-killer will make our global civilization have a very, very bad day, says Kacper Wierzchoś, an astronomer at the University of Arizona.

Hollywood’s preferred defense solution—nuclear bombs—probably could work, as high-fidelity simulations have shown that a sufficiently powerful blast could either knock an asteroid out of Earth’s way or tear it into harmlessly tiny pieces. Using nukes to deflect or disrupt an asteroid, however, is widely considered to be a red-tape-wrapped last resort, a desperate Hail Mary lobbed toward an imminent threat that astronomers detected far too late for other more subtle interventions to suffice. “A kinetic impactor is what we think of right now as our top solution,” says Cristina Thomas, a planetary astronomer at Northern Arizona University—in other words, using a speedy-but-inert projectile to deflect an asteroid many years in advance.

Scientists have simulated playing billiards with asteroids countless times. But there is only one way to know for certain whether we can fling one out of Earth’s way: venture into the darkness, find an asteroid, and give it a good thwack.

Humanity versus Dimorphos

DART, a car-sized box with two winglike solar panels, will soon be heading toward a binary asteroid system. Didymos, nearly 800 meters across, is orbited by a moonlet, Dimorphos, which is 160 meters long. That little moonlet is DART’s target.

About a month out, Didymos will just barely register in DART’s camera. Four hours prior to impact, the spacecraft’s guidance system—a technological cousin of those used to steer missiles on Earth—“takes the wheel and guides us in,” Rivkin says. Shortly thereafter, Dimorphos will swim into view as a blurry but distinct speck of light. About two minutes out, Rivkin explains, the autonomous pilot “takes its hand off the wheel and its foot off the brakes.”

An illustration of DART’s core components, showing the primary spacecraft approaching and colliding with Dimorphos.
Illustration of DART’s core components, showing the primary spacecraft approaching and colliding with Dimorphos. A trailing CubeSat, as well as Earth-based telescopes, will watch for the resulting crater and debris plume. Credit: NASA/Johns Hopkins APL

DART will take and transmit snapshots of its rapidly approaching final destination until the very last instant, before disintegrating into a cloud of shrapnel and superheated plasma in an epic—but entirely noiseless—collision. In space, no one can hear you go “boom.”

Ideally, DART’s momentum gets transferred to Dimorphos, leaving behind an impact crater, and shifting the moonlet’s almost-12-hour orbit around Didymos by at least 73 seconds. A pint-sized CubeSat, released by DART 10 days prior, will observe the violence up close, while ground-based astronomers keep an eye on the binary asteroid system from afar until it fades from view in spring 2023.

Astronomers cycled through several target candidates for DART but settled on Dimorphos for several reasons. The first is one of safety: changing Dimorphos’s orbit cannot change the orbit of Didymos to put it on an intersecting path with Earth. The second is that Dimorphos is a bit like the hand on a massive clock, with Didymos in the clock’s center; despite being hundreds of millions of kilometers away, astronomers on Earth will be able to easily see if the “hand” is ticking around the clock differently postimpact. Just two months of observations will reveal how effective the deflection has been. Dimorphos is also in the size range of asteroids that can squash entire cities, putting it in “the sweet spot from a planetary-defense perspective,” Thomas says.

DART is an odd endeavor by any standards, a brief candle purpose-built for snuffing. Unlike the typical interplanetary mission, which lasts many years, it will operate in space for just 10 months. No extensions await it, because DART “has a very definite end,” says Elena Adams, the mission systems engineer on DART at APL. “In this case, if you keep it going”—in other words, if the spacecraft misses its target—“you really messed up.”

The most distilled definition of success here, then, is simply hitting the target and measuring the shift in Dimorphos’s orbit. But what if Dimorphos refuses to play ball?

The Many Devils of Deflection

On Independence Day, 2005, NASA’s Deep Impact spacecraft fired a projectile into comet Tempel 1, generating a fireball and giant debris plume that allowed scientists to glimpse the interior of a cometary nucleus for the first time. Humanity’s attack run on Tempel 1 found that cometary nuclei can be remarkably fluffy, a notion bolstered by the European Space Agency (ESA) vehicle Philae’s 2014 landing on another rather puffy comet, 67P/Churyumov–Gerasimenko. Such low-density targets pose a problem for planetary defense. “How do you push something like that? How do you fight with foam on a beach?” Bannister says.

Asteroids, too, hold disquieting structural surprises. When NASA’s OSIRIS-REx spacecraft briefly touched down on the asteroid Bennu in 2020 to grab some rock samples, it almost sank into the target spot as if the surface was comprised of “melted butter,” says Patrick Michel, the principal investigator of Hera, a follow-up ESA-led mission, slated to arrive at Didymos in 2026 to examine DART’s consequences up close. Asteroids lacking sufficient gravity to squeeze their innards—perhaps including those city-killers one kilometer or less in size—could be like “rocks flying in formation,” Bannister says. This perversely means that in many respects small space rocks are harder to deal with than large ones, in which gravity’s heavy hand overwhelms most material properties. So, when trying to deflect a city-killer, Bannister says, perhaps we should be thinking: “how do you move a school of fish, not: how do you throw a mountain?”

All of this is pertinent to DART. Megan Bruck Syal, a planetary defense researcher at the Lawrence Livermore National Laboratory, has repeatedly simulated its fated impact. “On the surface, the DART experiment seems really simple,” she says. But only one thing is certain: no outcome is assured, because so many of Dimorphos’s fundamental properties remain unknown.

Mission planners are reasonably confident that DART’s hushed demise will successfully convey a billiardlike kick to Dimorphos, which seems hefty enough to be sufficiently squeezed by gravity’s clutches. But in the case of a slightly less substantial object, a kinetic impactor could just shoot right through, like a bullet through a cake, blowing it into small but still dangerous chunks. A successful deflection for such threats could require multiple, more gentle impacts rather than a one-and-done wallop.

Another huge unknown is Dimorphos’s appearance. It could be shaped like a potato, a dog bone, a rubber duck, two bowling balls stuck together, or something else entirely. A colleague recently gifted Adams a donut-shaped fridge magnet, a wink to how often asteroids surprise scientists once unveiled up close by some deep-space robotic emissary. A near-spherical or even potatolike shape would be optimal for a clean hit, whereas the uneven distribution of mass from more complex morphologies would raise the chance of a glancing blow, one that could just “spin up the moonlet and not actually change its orbit,” says Olivier de Weck, a systems engineering researcher at the Massachusetts Institute of Technology.

In the specific and benign case of Dimorphos, all these uncertainties are mostly academic. But in the event of a deflection attempt for a true city-killer, they could prove critical. We could, for instance, successfully deflect a potentially hazardous asteroid only to inadvertently put it on a new orbit that makes it more likely to hit Earth in the long run. There are points in space around our planet known as gravitational keyholes, wherein Earth’s pull on the asteroid sets the errant space rock on an assuredly destructive journey. “Once you go through a keyhole, the probability of hitting the Earth is virtually 100 percent,” says de Weck. This, to put it mildly, constitutes a major hurdle for any preemptive strikes against nascent impact threats.

Forewarned Is Forearmed

The emerging calculus is formidable indeed: Protecting ourselves from the most numerous and tricky (and thus most dangerous) space rocks requires more than making shots in the dark, especially when each “shot” is a multimillion-dollar deflection attempt. Ensuring success requires first scouting out the threat to learn any given space rock’s exact mass and ability to absorb a weighty impact.

Some of that work can be done from Earth, but as Dimorphos is deviously demonstrating, tiny objects are hard targets for remote studies. It is far better—albeit more difficult—to get up close and personal with any adversarial asteroid before trying to hit it at all. This was, in fact, ESA’s original plan, before schedule slips ensured that its reconnaissance spacecraft would arrive only after DART’s dramatic impact. In the future, miniaturized kinetic impactors could even be sent alongside scientific scouting missions, meant to merely nudge target asteroids to estimate how they would respond to more powerful deflective blows. “We have to go and characterize them better before we rest humanity’s fate in that one golden shot,” de Weck says.

Workers within a clean room at the Johns Hopkins University Applied Physics Laboratory prepare the DART spacecraft for shipment to its launch site at Vandenberg Space Force Base in California. Credit: NASA/Johns Hopkins APL/Ed Whitman

Such precursor missions are only possible if a malevolent asteroid is spotted many years prior to its Earth impact date. Which adds spine-chilling urgency to astronomers’ overlooked and underfunded efforts to find the missing half—or more—of our solar system’s population of city-killers. And while current facilities and the next-generation Vera C. Rubin Observatory are up to this task, they might not be for much longer given the seemingly unstoppable proliferation of satellite mega constellations, whose sunlight-reflecting members create blind spots in the night sky. Light pollution from mega constellations is “a huge problem that needs to be solved,” says Federica Spoto, who researches asteroid dynamics at the Harvard-Smithsonian Center for Astrophysics in Massachusetts. “And I don’t think we’re solving it.”

Fortunately, an upcoming space telescope, NASA’s Near-Earth Object Surveyor, will operate beyond the contaminating reach of the mega constellations. Launching in the next few years—some might say “just in time”—this infrared observatory will peer ahead of and behind Earth’s orbit, spying asteroids normally concealed by the sun’s glare. If all goes well, it should find 90 percent of near-Earth objects 140 meters across and larger. “Then we can really determine whether we have an imminent threat,” Michel says.

And although deflection may be the method of choice for the world’s cadre of anti-asteroid experts, more nuanced defensive measures are being investigated. “We want more tools in the toolbox,” says Rivkin. “We want not just the hammer, but the screwdriver.”

Some promising ideas are shockingly simple. The photons within sunlight impart a small amount of momentum on asteroids, ever-so-slightly altering their orbits. Painting an asteroid white to boost its reflectivity would have the net effect of generating twice the photonic push an all-black asteroid would experience. With enough advance notice, a fresh coat of ivory paint could safely banish an Earth-bound asteroid to the shadowy abyss. Another idea is to park a spacecraft around an asteroid and use its gravity to slowly pull the rock out of Earth’s way. But the piloting of a so-called gravity tractor spacecraft would have to be remarkably precise, and it would only work for small asteroids.

Canceling the Apocalypse

Using a kinetic impactor, for the time being, is the least complicated option available to avert disaster. It is also relatively inexpensive. DART’s total budget is about $320 million, “which is not even the cost of a football stadium,” Michel says. If DART succeeds in deflecting Dimorphos, then a possible near-term future in which many DART-like missions remain on standby, each ready to launch on one of several readily available commercial spacecraft, is easy to envisage.

But “it’s not enough to demonstrate technology,” Michel says. The world still needs to set up a system in which the entire planet responds to the threat of an incoming asteroid in as much unison as possible. Which country, or countries, should be involved in the deflection or disruption attempt? At present, although many nations are involved in the search for near-Earth objects and are participating in DART and Hera, America is leading the way on asteroid deflection technology.

Which countries should aid in any possible impact zone evacuations? When and how should the world decide that trying to deflect or disrupt an asteroid is riskier than simply letting it hit and then assisting the affected nations in their efforts to rebuild? Working groups at the United Nations Office for Outer Space Affairs, as well as biennial tabletop exercises that role-play a potential asteroid impact, are making earnest, but so far paltry, efforts at answering these sorts of enormous questions.

Humanity is some way off from having a full-blown asteroid protection network. But DART’s launch is another key milestone in the evolution of planetary defense, once seen as esoteric and perhaps a little silly. “When I was in graduate school in the 1990s, there was a small number of people who were interested, and everyone else treated it as kind of a crank field,” Rivkin says.

But so was astrobiology—and now space science is consumed by the interplanetary and even interstellar search for alien life. Thanks to the Chelyabinsk event and other dramatic close encounters with impactors, “planetary defense itself has also undergone a real sea change,” Rivkin says. And, for what may well be the first time ever in Earth’s multibillion-year history, some of its inhabitants could soon no longer be powerless against an insidious cosmic threat.

“This is one natural hazard that we can actually quantify and potentially retire,” Bannister says. “That’s an amazing goal we can work for. We can’t do that with earthquakes. We’ll never do that with volcanoes.”

Death-by-asteroid is, by any metric, highly unlikely during any person’s lifetime. And yet, scientists and engineers want to kill off that threat once and for all simply because they can. “If it’s one less thing that anxious people have to worry about when they’re trying to sleep, I think that’s worth it,” says Rivkin. “It’s one less piece of existential dread.”