What happens when a dart hits the bullseye? In a game among amateurs, it sends everybody home. But professional players will want to analyze the shot in preparation to fire again.
In this case, that dart is NASA’s Double Asteroid Redirection Test (DART), the spacecraft that crashed last November into the asteroid Dimorphos in hopes of redirecting its course. On March 2, a quintet of papers in the journal Nature confirmed what DART’s controllers had already guessed: The mission’s impact was a smashing success.
But DART won’t be the last human mission to visit Dimorphos or the larger asteroid which it orbits, Didymos. The European Space Agency’s Hera will soon follow in DART’s trail to appraise its aftermath—in far more detail than scientists, with their combination of instruments from Earth and the DART mission’s own sensors, have managed so far.
Now scheduled for an October 2024 departure, Hera is slated 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 around six months of sightseeing. Then, if conditions allow, Hera—a car–sized probe outfitted with a large radio antenna and a pair of solar panels—will try to make a full landing on Didymos.
Hera will also carry two passengers: a pair of CubeSats named Milani and Juventas. Milani will study the asteroids’ exteriors; Juventas will probe the asteroids’ interiors. With three spacecraft, scientists can get three different views of the crash site on Dimorphos. The mission’s chief purpose is to follow in DART’s shadow and understand what damage humanity’s first asteroid strike actually left on its target.
Between DART’s now-destroyed cameras, its companion LICIACube, and telescopes watching from Earth’s ground and orbit, we already know quite a bit about the planetary defense test. We can see Dimorphos’ orbit, both before and after DART’s impact; we know that DART altered it, cutting Dimorphos closer to Didymos and shortening its orbital period; and we can home in on where on the asteroid’s surface that DART struck, down to a patch the size of a vending machine.
But there’s still a lot we don’t know—most critically, Dimorphos’s mass before and after it was infiltrated. Scientists can’t calculate the measurement from Earth, but Hera’s instruments will have that ability. Without knowing the mass, we have no way of knowing why, precisely, DART’s impact pushed Dimorphos into its new orbit.
“We want to determine, accurately, how much momentum was transferred to Dimorphos,” says Patrick Michel, astronomer at the Côte d’Azur Observatory in France and Hera’s mission principal investigator.
Hera might also tell us what cosmetic scars DART left from the crash. It’s possible that the impactor simply left a crater, or that it violently shook up the asteroid, rearranging a large chunk of its exterior. “A lot of us are wondering how much of the surface we’ll even be able to recognize,” says Andy Cheng, an astronomer at the Johns Hopkins Applied Physics Laboratory who worked on DART.
The problem is that, until humans send an observer to the asteroid, we don’t know what the surface holds in wait for us, Michel says. What the asteroid’s exterior looks like now depends on what Dimorphos’s interior looked like when DART struck it. If the spacecraft dramatically reshaped the asteroid, it’s a sign that the target’s insides were weakly held together. And right now, “we have no clue, really, what’s happening inside,” says Terik Daly, an astronomer at the Johns Hopkins Advanced Physics Laboratory and DART team member. Hera, along with the radar-packing Juventas, will try to scan below the rocky surface.
Of course, Hera won’t be able to observe everything. Many astronomers have focused on Dimorphos’s ejecta—the material kicked up from the asteroid upon DART’s impact—to understand how exactly the strike nudged the asteroid. By the time of Hera’s arrival, at least four years after the crash, most of that ejecta will have long dissipated.
Still, knowing more about the asteroid’s innards can help astronomers understand where that ejecta came from—and what would happen if we crossed paths with a space rock again. “For example, in the future, if we had to use this technique to divert some asteroid, then we could do a more precise prediction [to hit it],” says Jian-Yang Li, an astronomer at Pennsylvania State University who worked on DART.
There are also other reasons why Dimorphos might not look the same way in 2026. Just as the moon pulls and pushes the tides around Earth’s oceans, Didymos’ gravity might play with its smaller companion. Scientists think it’s possible that those forces might cause Dimorphos to wobble in its orbit. But again, they won’t be able to observe any of this until Hera actually gets 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’s possible future orbits 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.”
The data from DART’s impact and Hera’s eyes certainly will help astronomers understand asteroids in their pre- and post-collision states. But they’ll also help us prevent the specter of death from above. Humans have long feared destruction from space in line with the dinosaurs, and with DART, planetary defense—the science of stemming that fear—made its first step into real-world strategies.
It’s hard to say when we’ll need the ability to deflect a space rock; astronomers’ projections show that no object larger than a kilometer is set to pass Earth in the next century. But, according to Michel, space agencies haven’t identified 60 percent of the objects flying by that are at least 40 meters long—large enough to devastate a region or a small country.
“We know that, eventually, such an impact [with Earth] will happen again,” Michel says, “and we cannot improvise.”