ESA's Hera probe reached asteroid Dimorphos, which NASA struck

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NASA's DART mission made a cosmic mess four years ago — and now a new spacecraft is ready to scout out exactly what the first-of-its-kind collision accomplished.

The Double Asteroid Redirection Test (DART) spacecraft slammed into a small space rock called Dimorphos on Sept. 26, 2022, in humanity's first test of planetary defense. That's the field dedicated to protecting Earth from the possibility of a calamitous asteroid impact, and DART mimicked how humans could change the orbit of a threatening space rock, taking it off its collision course.

The mission definitely succeeded, shortening the little asteroid's orbit around a larger rock called Didymos by about 33 minutes . But in order to make the most of the experiment, scientists ensured a second spacecraft would follow DART at a more leisurely pace. That decision will pay off this month with the arrival of a European Space Agency (ESA) spacecraft dubbed Hera , which launched in October 2024 , two years after DART's impact.

"Hera wants to be the detective, like Inspector Colombo, going back to the crash scene," said Patrick Michel, a planetary scientist at the French National Centre for Scientific Research (CNRS) and principal investigator of the Hera mission.

An animation of Hera braking as it approaches its destination. (Image credit: ESA) Scientists have been able to do limited monitoring of the Didymos system over the years with telescopes both on the ground and in space. That's how they calculated the basic change to the orbit, and they've built a basic sketch of what happened.

"We know that there was a lot of material kicked up," said Angela Stickle, a planetary scientist at Johns Hopkins University's Applied Physics Laboratory affiliated with both missions. "We know that we probably left a pretty significant mark on Dimorphos."

But the story isn't over and scientists are far from satisfied. No one knows how big a crater the impact left, or whether it completely rearranged all the asteroid's boulders. There are hints Dimorphos is now tumbling, and material may have moved between it and the larger Didymos.

Exactly two years after its launch, ESA's Hera planetary defense mission caught its first glimpse of its destination: the Didymos binary asteroid system, seen from an approximate distance of 147,000 miles (236,500 kilometers) away. (Image credit: ESA) Even the change to the orbit is complicated, said Cristina Thomas, a planetary astronomer at Northern Arizona University affiliated with both missions: The time Dimorphos took to circle Didymos was still changing some six months after impact, for reasons that aren't clear. "This thing that we thought was going to be very cut and dry turned out to be really interesting and complicated," Thomas said.

That's where Hera comes in, with its first arrival maneuver scheduled for Oct. 15. During three careful braking maneuvers, the spacecraft will slow down and approach the space rocks. Assuming everything goes smoothly, by early November, the spacecraft will come within 20 miles (30 kilometers) of the two rocks and begin its early science observations, then move even closer in early December.

The Hera spacecraft seen after separated from the upper stage of its SpaceX Falcon 9 launcher on Oct. 7, 2024. (Image credit: SpaceX) Mission personnel can't wait for their first glimpses of Dimorphos — the first time scientists will visit someplace they've seen before without knowing what they'll find. "For us, it's like a new world," Michel said. "The new image won't have anything to do with the previous one, so that's really exciting."

In addition to mapping the two asteroids and seeing what kind of crater the impact left, scientists will also have their eyes peeled for any pieces of the DART spacecraft, although they don't expect to find anything. "We were coming in really fast, so typically, the whole spacecraft you would expect to just kind of shatter or melt," Stickle said. The only sliver of possibility is that part of the two long solar panels stretching off the spacecraft may have snapped off and survived. (The appendages may also have turned the crater shape into an oval.)

The Hera mission's timeline. (Image credit: ESA) In January, once Hera has settled in, it will release two tiny companions that have piggybacked out to Dimorphos. One, called Milani, will identify the composition of materials on and around Dimorphos. The other, Juventas, will use radar to peer inside Dimorphos — the first time scientists have ever been able to study the interior of a space rock directly. Juventas will end its mission by landing on the small asteroid, and Milani may attempt a landing as well.

A series of images captured by the DART spacecraft as it sped to impact the asteroid Dimorphos on Sept. 26, 2022. (Image credit: NASA/JHUAPL) The trio of spacecraft will operate in tandem throughout early 2027, and the main spacecraft will operate at least through September 2027. All told, they will give scientists an unprecedented look at the pair of asteroids — the first binary asteroid a spacecraft has ever studied in detail. And the Didymos–Dimorphos duo is a good one to visit first. "It's very ordinary, and I mean that in the best possible way," Thomas said.

Ordinary except, of course, for that time humans punched it, opening the door for the Hera mission to explore the previously unremarkable rocks. And finally, that moment is just around the corner.

"Nothing really worries me," Michel said of the spacecraft's arrival. "It's really just excitement now."