What happens when an arrow hits the bullseye? In a game between amateurs, he sends everyone home. But professional players will want to analyze the shot in preparation for re-hitting.
In this case, that arrow is NASA’s Double Asteroid Redirection Test (DART), the spacecraft that last November crashed into the asteroid Dimorphos in hopes of redirecting its course. On March 2, a quintet of magazine articles Nature confirmed what DART controllers had already guessed: The impact of the mission was an overwhelming success.
But DART will not be the last human mission to visit Dimorph or the largest asteroid it orbits, Gemini. The European Space Agency’s Hera will soon follow in DART’s footsteps to assess its aftermath—in far more detail than scientists have so far, with a combination of instruments from Earth and the DART mission’s own sensors .
Now scheduled for an October 2024 departure, Hera is set to lift off from Cape Canaveral on the wings of a SpaceX Falcon 9 rocket. According to the mission’s current itinerary, it will arrive at Dimorphos and Didymos in late 2026 for about six months of sightseeing. Then, if conditions permit, Hera—a car-sized probe equipped with a large radio antenna and a pair of solar panels—will attempt a full landing on Gemini.
Hera will also carry two passengers: a pair of CubeSats named Milani and Juventas. Milani will study the exterior of asteroids. Juventas will explore the interior of asteroids. With three spacecraft, scientists can get three different views of the Dimorpho crash site. The main purpose of the mission is to follow in the shadow of DART and understand what damage humanity’s first asteroid strike actually left on its target.
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Between the now-destroyed cameras of DART, the LICIACube companion, and the telescopes monitoring from Earth’s ground and orbit, we already know quite a bit about the planetary defense test. We can see the trajectory of Dimorphos, both before and after the DART impact. We know that DART changed this, cutting Dimorph closer to Gemini and shortening its orbital period. and we can be where the DART hit the surface of the asteroid, down to a patch the size of a vending machine.
But there’s still a lot we don’t know—most crucially, the mass of Dimorphos before and after its penetration. Scientists cannot calculate the measurement from Earth, but Hera’s instruments will have that ability. Without knowing the mass, we have no way of knowing why, exactly, the DART impact pushed Dimorphos into its new orbit.
“We want to determine, precisely, how much momentum was transferred to Dimorphos,” says Patrick Michel, an astronomer at the Côte d’Azur Observatory in France and principal investigator of the Hera mission.
Hera can also tell us what cosmetic marks the DART left from the crash. It’s possible the impactor simply left a crater or violently shook the asteroid, rearranging a large chunk of its exterior. “Many of us wonder how much of the surface we will be able to identify,” says Andy Cheng, an astronomer at the Johns Hopkins Applied Physics Laboratory who worked on DART.
The problem is, until humans send an observer to the asteroid, we don’t know what the surface holds, Michel says. What the outside of the asteroid looks like now depends on what the inside of Dimorphos looked like when the DART hit it. If the spacecraft dramatically reshaped the asteroid, it’s a sign that the target’s interior was weakly held together. And right now, “we have no idea, really, what’s going on inside,” says Terik Daly, an astronomer at the Johns Hopkins Advanced Physics Laboratory and a member of the DART team. Hera, along with the radar-laden Juventas, will attempt to scan below the rocky surface.
Of course, Hera will not be able to observe everything. Many astronomers have focused on the Dimorphos ejecta—the material ejected from the asteroid during the DART impact—to understand exactly how the impact propelled the asteroid. By the time Hera arrives, at least four years after the crash, most of that ejection will have long since dissipated.
However, knowing more about the asteroid’s interior may help astronomers understand where that ejection came from—and what would happen if we crossed paths with a space rock again. “For example, in the future, if we were to use this technique to deflect an asteroid, then we could make a more accurate prediction [to hit it]says Jian-Yang Li, an astronomer at Pennsylvania State University who worked on DART.
There are also other reasons why Dimorph might not look the same in 2026. Just as the moon pulls and pushes the tides around Earth’s oceans, Gemini’s gravity may be playing with its smaller companion. Scientists believe it is possible that these forces cause Dimorphos to wobble in its orbit. But then again, they won’t be able to notice any of this until Hera comes up close.
As the mission progresses, they might at least be able to set a baseline. Michel says that astronomers on Earth can simulate many of Dimorphos’ possible future trajectories on their computers. “It’s not really a problem that we arrive four years later,” says Michel. “We have the tools to understand if something evolved.”
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Data from the DART impact and Hera’s eyes will certainly help astronomers understand asteroids in their pre- and post-collision state. But they will also help us ward off the specter of death from above. Humans have long feared destruction from space in concert with the dinosaurs, and with DART, planetary defense—the science of combating that fear—took its first step into real-world strategies.
It’s hard to say when we’ll need the ability to deflect a space rock. Astronomers’ predictions show that no object larger than a kilometer will pass by Earth in the next century. But according to Michel, space agencies have not identified 60 percent of the objects that fly there are at least 40 meters long—large enough to destroy an area or a small country.
“We know that, ultimately, such an impact [with Earth] it will happen again,” says Michel, “and we can’t improvise.”