Until October, Hera's target was a single point of light. It is now two. On the evening of Oct. 18, the European Space Agency's planetary-defence spacecraft used its monochromatic Asteroid Framing Camera to image the binary asteroid system Didymos from about 1,280 km away. At that range the small moonlet Dimorphos stands apart from its larger parent for the first time in Hera's own data. ESA published the result on Oct. 27 under the title "First light: Hera resolves two asteroids instead of one."
The image itself isn't much to look at. Dimorphos covers roughly one pixel. ESA's Michael Kueppers notes, though, that the pixel is clearly separated from Didymos, and that separation is the milestone. A spacecraft that can tell the two bodies apart has crossed from approach into actual reconnaissance of the system it was built to study.
A single pixel, but the right one
The sizes explain why a resolved pair is harder to get than it sounds. Didymos is about 780 m across. Dimorphos is about 151 m, which ESA compares to the Great Pyramid of Giza. The two orbit each other about 1.15 km apart on average, with Dimorphos moving at only around 18 cm/s. From more than a thousand kilometres away, that gap is a very small angle.
The whole system is also about 197 million km from Earth (around 122 million miles, in Universe Today's conversion). Everything Hera does at this point runs on long-distance commanding and careful navigation.
"For the very first time we are seeing through Hera's own eyes the very double asteroid system," mission manager Ian Carnelli said in ESA's release. He also set the next public milestone: the first detailed images of Dimorphos are to be unveiled on Nov. 25.
Slowing down to look
The sighting came alongside a series of braking manoeuvres. ESA says two thruster firings a week apart cut Hera's speed relative to the asteroid pair to 20 m/s. That is slow enough for the spacecraft to close in gradually instead of flying past. The burns follow the earlier approach phase, when Didymos first appeared to Hera as an unresolved speck. Now the target has structure, and the spacecraft is moving slowly enough to make use of it.
Hera launched in October 2024. Universe Today notes that the mission is led by ESA, with cooperation from NASA and Japan's JAXA.
What DART left behind
Dimorphos matters because of what happened to it on Sept. 26, 2022. NASA's DART spacecraft hit the moonlet at 6.1 km/s in humanity's first purposeful change to the motion to change the motion of a celestial object. It worked, and by a wide margin.
NASA reported soon after that the impact shortened Dimorphos's orbit around Didymos by 32 minutes, from 11 hours 55 minutes to 11 hours 23 minutes, with an uncertainty of about plus or minus 2 minutes. The agency's minimum success threshold had been a change of 73 seconds, so NASA said the result beat it by more than 25 times. ESA's release now gives the slowdown as around 33 minutes, and Universe Today quotes 32 to 33 minutes. Both fall within the original error bars. ESA says the collision sent a plume of debris across 10,000 km of space.
That result answered the headline question: yes, a kinetic impactor can push an asteroid. It left the harder engineering questions open. What is the precise makeup, structure and mass of Dimorphos, and which properties explain a deflection much greater than impact simulations predicted? Did the impact leave a crater, or reshape the moonlet entirely? Telescopes on Earth can measure an orbital period very precisely. Seeing a crater on a 151 m object 197 million km away is another matter.
Why It Matters
Planetary defence by kinetic impact only becomes a usable tool if the result can be predicted ahead of time, not just measured afterwards. DART was the experiment. Hera is the follow-up that checks the work, and its job is to turn one dramatic number into a model that could be applied to a different asteroid on a real collision course.
For that, Hera needs to measure Dimorphos directly: its size, shape, mass and surface, plus whatever DART carved out of it. Close-range data would tie the observed orbital change to the target's actual properties. A future mission could then size an impactor for a threat instead of hoping the margin is as generous as it was in 2022. A resolved one-pixel moonlet is the first step on that path. The Nov. 25 images will show how much of the impact's aftermath is visible up close.
What comes next
Detailed imaging is only the start of Hera's work. The spacecraft carries two shoebox-sized CubeSats that it will release into the system. According to Universe Today, Milani is meant for mineralogical analysis of the impact site, and Juventas will carry ground-penetrating radar to look beneath the surface. That interior view is something the main spacecraft's cameras can't provide. ESA says the mission will eventually progress to landing.
The near-term date to watch is Nov. 25. If the first close-up images match what the one-pixel first light suggests, Dimorphos will change from a data point in an orbital-period plot to a place with a surface that can be studied. For a moonlet whose main claim to fame is being hit by a spacecraft, that's overdue.
