Before then, humanity was limited to scanning the night sky for hazardously sized asteroids—and if one was spotted heading to Earth, little could be done except to try and get those in the eventual target zone out of harm’s way. But on that September day, at precisely 7:14 p.m. eastern time, NASA’s Double Asteroid Redirection Test, or DART mission, reached its crescendo: its uncrewed spacecraft smashed into a (harmless) asteroid named Dimorphos, and changed its orbit—a dry run for a real-life emergency that would require space agencies to deflect an asteroid away from the planet.

DART was launched on a SpaceX Falcon 9 and was the first-ever planetary defense test, one that illustrated that scientists and engineers could rearrange the cosmos to make it more habitable for humans. And next week, the European Space Agency is going to check NASA’s homework by launching its own mission to Dimorphos.

Its spacecraft, Hera, is not only one of the most technologically sophisticated sleuths to visit the sort of asteroid that could one day threaten us. It’s also a dress rehearsal for a reconnaissance mission—one that could scrutinize an Earthbound space rock in advance of any deflection attempt, thereby bolstering humanity’s chances of success. Hera is slated to launch on a Falcon 9 from Cape Canaveral Space Force Station in Florida on Monday, October 7th, at 10:52 a.m. ET.

ESA, which excels in planetary science missions, had fumbled its chance to fly its own planetary defense mission alongside DART—and with NASA leading the charge, ESA became a valuable, but secondary, player in the game. But should Hera work wonders, they may once again be on equal footing with their transatlantic partners. 

No pressure then. “Everyone’s excited,” says Patrick Michel, the Hera principal investigator at the University of Côte d’Azur. “And a little bit scared.”

DART’s objective was simple: point a spacecraft at an asteroid, hit it, and change its orbit. That asteroid was the 525-foot-long asteroid, Dimorphos, which zipped around the larger space rock Didymos. Neither threatened Earth, but Dimorphos was the perfect target: it’s roughly the size of asteroid colloquially referred to as a city killer, in that should one impact a city, that metropolis would be annihilated. 

About 25,000 of these exist in near-Earth orbits, and about 14,000 are yet to be found. If we wait long enough, and do nothing, the odds of a city getting razed by a space rock rise to 100 percent. So: why not do something about it? Let’s find those asteroids before they find us, and then blast any heading our way.

NASA and the Johns Hopkins University’s Applied Physics Laboratory in Maryland, who designed, built, and flew the mission, wanted to crash the car sized DART into Dimorphos to shift its orbit around Didymos. While this transpired, telescopes on Earth (and in space) would watch on to check if they succeeded. A little CubeSat named LICIACube – designed by the Italian Space Agency – would also hitch a ride on DART until just before its terminal plunge, leaping off to survive and photograph DART’s impact.

All of this went exactly according to plan. Not only did the semi-autonomous DART spacecraft hit its bullseye, but it changed the orbit of Dimorphos far more significantly than the minimum amount NASA had set as a threshold for success. It appeared that so much debris was ejected during the impact that it acted like a rocket booster, vigorously thrusting Dimorphos out of the way. 

But images taken by LICIACube and Earth’s observatories revealed a more complex story: boulders the size of houses were caught flying in formation with the recently bruised Dimorphos, and a comet-like tail of coruscating matter stretching into space persisted for months. The DART team were prepared for plot twists, but what they chronicled still came as a shock. “I didn’t expect such a long tail. I didn’t expect it to be so persistent,” says Cristina Thomas, a planetary astronomer at Northern Arizona University and the lead of the observations working group for the DART mission. 

Stranger still, the entire asteroid seemed to respond to the impact less like a monolithic solid and more like a fluid. Changes in how it reflected sunlight indicated that the impact caused the asteroid to almost splash about, creating tsunamis of rock that washed across it. “Maybe we didn’t leave a crater. Maybe we reshaped it. And that’s kind of mind-blowing,” Thomas says.

DART may have been a triumph, but it was a brute force experiment. You can’t just punch any old asteroid as emphatically as possible to protect the planet. You want to deflect it, not hit it so aggressively that you inadvertently fragment it into smaller but still dangerously sized pieces flying off in all directions. 

No matter what asteroid a spacecraft has visited, “we’ve been surprised every time we’ve showed up,” says Andy Rivkin, a planetary astronomer at the Applied Physics Laboratory, the co-investigation team lead on the DART mission, and a Hera participating scientist. That’s fine when you’re conducting science; less so when you’re trying to save the world.

Ultimately, when it comes to battling an asteroid, you want to be prepared, not flabbergasted. That means scientists need to get to know these asteroids inside and out, “should this one day be necessary to protect Earth,” says Michael Küppers, the Hera project scientist at ESA. And that’s where Europe’s new asteroid detective comes into play.

Originally, alongside DART, ESA was going to fly a spacecraft of its own, named AIM—the Asteroid Impact Mission. It was to arrive before DART and document the impact, then survey the wreckage. Unfortunately, in 2016, member states failed to agree on AIM’s funding, and the mission was cancelled. Michel, keen to keep the fire lit, pitched a streamlined version of AIM to ESA—and he prevailed. 

In 2019, the other member states gave the new spacecraft concept their benediction and euros. AIM rose from the ashes, reborn as Hera—and this October, it will fly into the firmament on a SpaceX Falcon 9 rocket from the Kennedy Space Center in Florida. After a Mars flyby in March 2025, during which it will calibrate its instruments and conduct some opportunistic science, Hera will catch up to Dimorphos in October 2026, with its full investigation commencing just shy of Christmas that year.

Due to the financial kerfuffle in 2016, the spacecraft would now get to Dimorphos post-impact, with the toaster sized LICIACube filling in for AIM’s original impact documentarian role. It’s a shame Hera can’t witness the history-making fireworks, but thanks to its instrument suite – courtesy of 18 ESA member states, and Japan’s space agency – it’s overqualified for the job of interrogating Dimorphos: its multiple cameras, which can perceive dozens of wavelengths, will craft a portrait of the asteroid that reveals its geological composition; its laser-based radar system will characterize the space rock’s topography and mass, while aiding Hera’s flight; its radio science experiment will watch how Dimorphos wobbles to get a better idea of its density, internal structure, and motion around Didymos.

If Hera is Sherlock, then Juventus and Milani are its Watsons. These two shoebox sized CubeSats, each just a little heavier than a sausage dog, are hitching a ride to Dimorphos with the heftier Hera—and while the primary spacecraft gets on with its work, these two will kickstart their own inquiries. 

Each have radio experiments, collaborating with Hera’s own, to probe the asteroid’s motion and density. Juventus, focused on investigating the subsurface of Dimorphos, will do so with its ground-penetrating radar and an instrument that detects subtle shifts in gravity. Milani is more of an astrochemist, sniffing out the minerals, water and organic matter on Dimorphos, Didymos and in the debris field now enveloping them.

“For me, this is the future of planetary exploration: to send the mothercraft that we want to keep safe, then deploy some cheaper CubeSats, with which you can take more risks,” Michel says. Eventually, both CubeSats will try to land on Dimorphos, or die trying.

Certainly, the surface-level studies of Dimorphos – an asteroid vandalized by humanity – will be intriguing. “We already know that the first images won’t look like the Dimorphos we know,” says Michel. “What we’re going to see has never been seen before.” 

But it’s the measurement of the asteroid’s mass, and the mapping of its internal structure, that’s paramount to planetary defense researchers. 

The original mass of Dimorphos is an estimate based on mostly remote observations. (It’s something like 200 Statues of Liberty.) Values like mass and density play into calculations of how much of a boost that the impact ejecta spray donated to Dimorphos. For now, the DART team think that the asteroid was deflected so much that it was as if 3.5 DART spacecraft hit it. That’s great news for a bona fide asteroid deflection mission: you get a lot more bang for your buck, so to speak.

As to why so much material rocketed off the impact site, scientists think its internal structure is to blame. You may think of asteroids as solid chunks of rock, but no—many of them seem to be boulders flying in close formation.

In 2019, as part of a mission to retrieve a sample of pristine matter from the 3,000-foot-long asteroid Ryugu, Japan’s Hayabusa2 spacecraft blasted a crater in it with a copper bomb. And in 2020, while sampling the 1,610-foot-long asteroid Bennu, NASA’s OSIRIS-REx spacecraft briefly flew onto and prodded it. On both occasions, scientists were shocked to find that both asteroids didn’t behave like mountains of stone, but ball pits in low gravity. The comparatively Lilliputian Dimorphos – which has a different overall composition to the carbon-rich Ryugu and Bennu – is similarly strange.

“They are what you call rubble piles,” says Küppers. “Their cohesion must be extremely low, like, I don’t know, a cappuccino foam.” 

That’s both a blessing and a curse. If you want to deflect asteroids like these away from Earth, then understanding just how likely they are to fountain debris and propel themselves further afield post-impact is vital. But, being so loosely bound by their own gravity, you also need to get an idea of how much it would take to accidentally fragment them into a shotgun blast of smaller asteroids. “For the same impact energy, we can just make a crater, or totally disrupt it,” Michel says.

The Hera mission will really nail down the internal structure of Dimorphos. Is it only rubbly on the surface, or is it rocky froth all the way down? By finding out, planetary defenders can get a better idea of what similar asteroids may be like, should we need to slam a spacecraft into them to shield the world. 

But even after Hera has unearthed Dimorphos’ buried secrets, scientists won’t confidently claim that all city killer sized asteroids are alike. “If we have to save the Earth, I’m not sure I’d base my statistics on three bodies,” says Michel, referring to Bennu and Ryugu. 

Ideally, if an Earthbound city killer is spotted many years in advance, you would want to send a different probe to the asteroid first to get a better idea of its mass and internal structure. That way, when the deflection attempt is performed by another spacecraft, it’s more finely tuned to work on that specific asteroid. Hera may be arriving after DART’s dramatic death. But its design isn’t dissimilar to the sort of robotic scout that would be flown ahead of an impactor spacecraft in an actual crisis.

Although Hera’s examination of Dimorphos is important, it’s arguably the groundwork it is laying for future planetary defense missions that will be its greatest legacy.

For several decades, planetary defense was considered to be a subset of planetary science, the curiosity-driven study of objects in the solar system. Planetary defense missions automatically involve scientific discovery, but their goal – to protect billions from a cosmic catastrophe – is markedly different. Despite that distinction, and despite a longstanding and deep well of support from Congress and scientists within NASA, it’s only been in the last few years that planetary defense missions – like DART, and the under-construction, space-based asteroid hunting observatory, Near-Earth Object Surveyor – have been liberated from competing against pure science missions and given their own dedicated funding stream. 

The same cannot be said for ESA. According to Michel, as AIM’s premature demise underscored, many ESA delegates don’t yet consider planetary defense a priority. It remains underfunded, with staff often working on it part time while doing their regular academic work. “This is one of the only risks we can predict and prevent,” he says. But because the everyday odds of a catastrophic asteroid impact are low, some of Michel’s colleagues think it’s more of an expensive hobby than a globe-saving pursuit. “I know some scientists who… think it’s totally bullshit to study,” he says.

Michel’s hope is that this apathy quickly becomes a fringe position. Should Hera perform as expected, it will demonstrate that Europe, too, wants to help protect all eight billion of us from a future cataclysm. It’s a showcase of ESA’s ability to quickly develop a recon-style asteroid mission—a feat that can be accomplished for a mere $390 million. DART cost $325 million, so in total, even with NASA and ESA’s asteroid-hunting observation program thrown in, “the whole program of planetary defense is less than an aircraft carrier price tag,” Michel says.

America may be the vanguard. But “planetary defense is an international issue,” says Thomas. ESA has long been a key partner, but seeing it rise to the occasion with Hera is welcome news—and Michel wants it to be the mission that, finally, convinces even the most skeptical ESA delegates convert to the cause of planetary salvation.

“A few years ago, we had nothing,” he says. Today, “there is momentum. Now it’s our job to make sure we use it.”

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Robin George Andrews is a photographer, public speaker, and experimental volcanologist-turned-science journalist. He regularly writes about space and geosciences for outlets including the New York Times, Atlantic, National Geographic, Scientific American, and Atlas Obscura. He is also the author of Super Volcanoes: What They Reveal about Earth and the Worlds Beyond (Norton, 2021). He lives in London, England. Robin now has a new book out!

In How to Kill an Asteroid, award-winning science journalist Robin George Andrews—who was at DART mission control when it happened—reveals the development of the technology that made it possible, from spotting elusive asteroids and comets to figuring out their geologic defenses and orchestrating a deflection campaign. In a propulsive narrative that reads like a sci-fi thriller, Andrews tells the story of the planetary defense movement, and introduces the international team of scientists and engineers now working to protect Earth.