ESA's Hera probe begins close survey of the asteroid NASA deliberately crashed into

The European Space Agency's Hera spacecraft has begun close-range study of the asteroid moonlet that NASA deliberately smashed a probe into four years ago, ESA confirmed this week, marking the start of the first detailed forensic survey of a planetary-defense experiment anywhere in the solar system.

Hera reached the binary asteroid system Didymos and its small moon Dimorphos in recent weeks, arriving roughly a month earlier than originally planned after mission controllers opted for a more aggressive braking approach during the spacecraft's final approach to the Didymos system. It is now circling the pair from a distance of roughly 25 to 30 kilometers and is expected to close to about 20 kilometers next month, beginning what ESA calls a "crash site investigation."

What the numbers show

Didymos, the larger body in the pair, is about 780 meters across; Dimorphos, its moonlet, is roughly 151 meters wide, smaller than many football stadiums. On 26 September 2022, NASA's DART spacecraft struck Dimorphos head-on at more than 6 kilometers per second as a test of whether a kinetic impact could redirect a hazardous asteroid. The collision shortened Dimorphos's orbit around Didymos from 11 hours and 55 minutes to 11 hours and 23 minutes, a change of roughly 32 minutes that was far larger than engineers had predicted, evidence that debris blasted off the surface added extra momentum beyond the impact itself, according to the DART mission's own overview, maintained by Johns Hopkins University's Applied Physics Laboratory for NASA.

Hera got its first look at the pair as two separate objects on 18 October, when its monochrome Asteroid Framing Camera resolved Dimorphos as a single pixel beside its larger companion from about 1,280 kilometers out, according to ESA's account of the milestone, which described the moment the two bodies first came apart in the spacecraft's imagery. By 5 November, the spacecraft had closed to within about 60 kilometers and produced its first clear view of terrain on Didymos that DART's cameras never saw, a vantage point space.com described in its report on Hera's first close-up look at the system.

Why a four-year-old collision still matters

DART, which stood for Double Asteroid Redirection Test, was the first time any space agency had intentionally altered the trajectory of a celestial body. Neither Didymos nor Dimorphos poses any threat to Earth; the pair was chosen because its mutual orbit let scientists measure the deflection precisely by timing how the moonlet's path around its parent asteroid changed. NASA declared the test a success within weeks, but DART's own cameras were destroyed on impact, and the Italian-built LICIACube that watched from a distance could only capture the immediate aftermath. What has been missing ever since is a close, patient look at the crater DART left behind, the shape of any debris field, and the internal structure of the rubble pile that absorbed the blow — the gap Hera was built to close, as ESA laid out when it announced the mission ahead of its 2024 launch.

Hera launched on 7 October 2024 from Cape Canaveral and used a Mars flyby in March 2025 to bend its trajectory toward Didymos, along the way photographing the Martian moon Deimos. The main spacecraft carries a dozen instruments of its own and is ferrying two shoebox-sized CubeSats, Milani and Juventas, which will separate for still closer passes: Milani to study the mineralogy of Dimorphos and its surrounding dust, Juventas to probe beneath the surface with radar. ESA has described the eventual goal as making the Didymos pair the best-characterized asteroids ever studied.

Who is watching closely

The audience extends well beyond the Hera team. Planetary-defense researchers worldwide, including those at NASA's Planetary Defense Coordination Office, are treating the mission's findings as the data set that will determine whether a kinetic impactor can be relied upon as a repeatable technique, rather than a one-off demonstration, should a genuinely hazardous asteroid ever be found on a collision course with Earth. The mission's findings will also feed structural models used by engineers designing any future deflection attempt, since how an asteroid's interior responds to an impact — solid core or loose rubble pile — determines how much force a real mission would need.

"The first thing we said was wow!" said Patrick Michel, Hera's principal investigator, describing the team's reaction to the spacecraft's initial close-up image of Didymos, as reported by Gizmodo.

ESA mission manager Ian Carnelli struck a similar note when the spacecraft's cameras first separated the two asteroids in October, saying the team was "seeing through Hera's own eyes" for the first time after two years of transit. ESA mission scientist Michael Kueppers has said the views will "grow sharper as Hera comes closer" in the weeks ahead, a process already visible in the jump from a single-pixel moonlet in mid-October to a resolved crescent of terrain by early November, detailed in Gizmodo's report on the spacecraft's closest view yet of the DART site.

What comes next

Mission planners have scheduled the first detailed, dedicated images of Dimorphos itself for 25 November, before Hera tightens its orbit to roughly 20 kilometers in December. From there, the spacecraft is expected to spend at least six months mapping both bodies in increasing detail, hunting for the crater DART left behind, measuring Dimorphos's precise mass and shape, and eventually releasing Milani and Juventas for passes close enough that one or both craft may attempt an actual touchdown on the moonlet's surface. Hera's approach has already involved imaging fainter background asteroids to calibrate its instruments along the way, part of the preparation described in an earlier space.com dispatch on the spacecraft's approach to the crash site. If the survey confirms that DART's extra-strong deflection can be explained and reproduced, it would give space agencies their first validated playbook for nudging a dangerous asteroid off an Earth-bound course, rather than a single encouraging data point from one encounter in 2022.