Europe's Hera spacecraft gets first clear look at the asteroid pair struck by NASA's DART mission

The European Space Agency's Hera spacecraft has photographed the asteroid system it was built to study as two separate objects for the first time, a milestone that comes as the probe brakes toward a rendezvous with the aftermath of humanity's only deliberate asteroid strike. The image, taken on 18 October and published by ESA on 27 October, shows the near-Earth asteroid Didymos and its small moon Dimorphos as distinct points of light rather than the single blurred dot Hera had seen until then.

Dimorphos is the asteroid NASA's DART spacecraft deliberately rammed into in September 2022, in the first full-scale test of whether a spacecraft impact can meaningfully alter an asteroid's path. Hera, launched two years ago, is the follow-up mission sent to examine what that collision actually did to the body it struck, work that mission scientists say is needed before anyone can rely on a similar manoeuvre to protect Earth from a genuine threat.

A system resolved after years of approach

The new picture was captured by Hera's monochrome Asteroid Framing Camera from a distance of 1,280 km, with the Didymos system itself some 197 million km from Earth at the time. In it, Dimorphos covers only about one pixel but sits clearly apart from its larger companion. "With this image the smaller Dimorphos asteroid becomes visible for the first time," said Hera project scientist Michael Kueppers, in the ESA release. "Before this acquisition the spacecraft was still resolving the Didymos system as a single dot. But now, even though Dimorphos is made up of only around one pixel in this initial image, it is clearly well separated from its parent body."

The image followed two braking burns a week apart, on 15 and 22 October, which slowed Hera to a relative velocity of just 20 metres per second with respect to the asteroid pair, according to Italian coverage of the manoeuvres. That gradual deceleration is intended to let the spacecraft close the remaining distance carefully rather than race past its target, a risk ESA has guarded against since revising Hera's arrival schedule to bring the encounter forward by about a month.

Measuring two small worlds

The numbers involved are modest by astronomical standards but matter enormously for the mission's precision. Dimorphos is roughly 151 metres across, about the size of the Great Pyramid of Giza, while Didymos measures some 780 metres in diameter, slightly smaller than Dubai's Burj Khalifa. The two bodies orbit one another at an average separation of 1.15 km, with Dimorphos circling its parent at about 18 centimetres per second. Those figures, laid out in ESA's release, will serve as the baseline against which Hera measures any change once it reaches the asteroids directly.

Finishing an experiment DART started

The reason for all this scrutiny traces back to 26 September 2022, when NASA's DART spacecraft, built and operated by the Johns Hopkins Applied Physics Laboratory, struck Dimorphos head-on at roughly 6.1 km/s. The impact threw a debris plume more than 10,000 km across and shortened Dimorphos's orbit around Didymos by about 33 minutes, a figure later refined to 33.0 ± 1.0 minutes in a peer-reviewed analysis published in Nature. That paper, led by Cristina Thomas, found the deflection was far larger than simple momentum transfer from the spacecraft alone would predict, implying that ejected rock and dust contributed significant extra push.

That finding is itself the puzzle Hera now has to resolve. ESA's release lists the open questions bluntly: did DART leave a crater, or reshape Dimorphos entirely? What is the asteroid's actual composition, internal structure and mass, and which of those properties explains a deflection so much greater than pre-impact simulations predicted? Without answers, DART remains an impressive one-off demonstration rather than a method engineers could confidently repeat against a real incoming asteroid.

Who is watching, and why it matters

The audience for this slow-motion investigation is less the public than the international planetary-defence community, the loose network of space agencies, universities and research institutes trying to establish kinetic impact as a validated tool against near-Earth objects. Scientists attached to Hera's two instrument-carrying CubeSats, Milani and Juventas, are among those most invested in what the spacecraft finds once it gets close enough for detailed measurements. Researchers at Italy's national astrophysics institute, which leads the dust-sensing instrument aboard Milani, have already described the moment of seeing Didymos through Hera's own camera as a milestone worth marking, even before the mission's real science begins.

"For the very first time we are seeing through Hera's own eyes the very double asteroid system that the mission was designed and built for," said Hera mission manager Ian Carnelli. "Our views will much grow sharper as Hera comes closer. We will unveil the first detailed images of Dimorphos – the first object in the Solar System to have had its orbit measurably shifted by human action – on 25 November."

What comes next

ESA has set 25 November as the date for the first detailed, close-up images of Dimorphos, which should finally show whether DART left a visible crater. Over the following months, Hera is expected to carry out what the agency calls a "crash site investigation," using its onboard instruments before releasing the two shoebox-sized CubeSats for an even closer survey. The mission's longer-term ambition is to attempt a landing on one of the two bodies once the close-range survey is complete.

Until then, the spacecraft's own cameras will keep sharpening their view with every kilometre it closes, turning what is currently a system of two barely distinguishable points of light into the detailed subject of planetary defence's first forensic investigation.