Hera arrives at Dimorphos

ESA's Hera reaches Dimorphos — the asteroid DART deflected in 2022 — to survey the aftermath of humanity's first planetary-defense test.

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The moment of Hera arrives at Dimorphos: https://nowsky.org/e/hera-didymos-arrival — opens at the right place and time, replayable, no login.

Available views: Orbit ✓ · Sky — (not visible in the sky)

The story

In September 2022 NASA's DART spacecraft drove straight into Dimorphos, the small moonlet of the asteroid Didymos. It was the first time humans deliberately changed the orbit of another world, and it worked: ground telescopes measured the moonlet's orbital period afterwards and found it more than half an hour shorter than before. ESA's Hera is the follow-up, sent back to the same place to see up close what actually happened.

Hera launched in October 2024 and picked up a gravity assist at Mars in 2025, using the opportunity to photograph Deimos, the smaller Martian moon, on the way through. On reaching the Didymos system it does not close in immediately. It first watches both bodies from a distance to confirm their motion, then steps inward in stages, lowering its observing range as confidence grows.

A great deal was invisible at the time of the impact. Beyond a few minutes of approach images, there was no way to see the whole shape of Dimorphos or the mark the collision left on it. From Earth the pair is a single point of light, so everything rested on inferring the period from how that point brightened and dimmed. Hera goes to fill in what was missing.

It does not travel alone. Two shoebox-sized CubeSats, Juventas and Milani, ride along and are released during the mission. Juventas carries a low-frequency radar to look inside Dimorphos, while Milani studies mineral composition and the dust floating around the pair. Late in the mission the plan calls for the CubeSats to attempt a landing on the surface.

Why it matters

Hera's questions are concrete. How large is the crater? What is the mass of Dimorphos? And how much of the impact energy actually turned into orbital change? That last figure, the momentum transfer efficiency, is greater than one, because debris thrown off the surface pushes back like a small rocket. Measuring it directly is what lets anyone calculate, next time, what to hit and how hard.

A body like Dimorphos is probably not a single rock but a loose pile of rubble. If so, the impact was less a crater punched into solid ground than a deformation of the whole heap. There is a real possibility that DART changed the moonlet's overall shape, and if that is confirmed, the assumptions behind deflection modelling have to be revised. Juventas and its radar, looking straight into the interior, are how that gets settled.

The historical position is easy to state. Planetary defence was for decades a subject of calculation and simulation, and DART turned it into an experiment. Hera is the part of the experiment where the result is quantified and turned into knowledge others can reuse. An experiment is not finished until somebody measures the outcome.

How to watch

There is nothing to see in the sky. Didymos and Dimorphos register as a single point even in large telescopes, and Hera itself is far fainter. Use the orbit view instead: lock onto Hera, move the clock to the arrival period, and you can follow the route from Earth past Mars out to the asteroid, along with the Earth-asteroid geometry at the moment it gets there.

Two stretches of the replay repay attention. One is the bend in the trajectory at the Mars assist. The other is the approach, where the velocity difference between Hera and Didymos shrinks almost to nothing. Asteroid gravity is so weak that arriving is less an orbit insertion than a formation flight, and watching the closing speed drive toward zero is the interesting part.

The data itself comes out through ESA. The precise arrival date and the schedule of the early approach phases can shift with the navigation solution, so read the date on this page as the planned one. The moment most worth waiting for is the release of the first close-up images — the first time anyone sees the mark DART left behind.

See this for yourself — replay the moment in the orbit view

Background

DART and Hera were designed as one programme. Under the joint NASA-ESA concept known as AIDA, the United States would provide the impactor and Europe the observer, with both spacecraft present at the same time. When European funding fell through at one stage, the observer slipped, so DART flew its impact alone and Hera became the investigator arriving four years later.

The scene was not entirely unwatched in 2022. LICIACube, a small Italian satellite carried by DART and released beforehand, flew past moments after impact and photographed the ejected material, while Hubble and ground telescopes tracked a long dust tail streaming away from the asteroid. All of that, though, was the view from outside; none of it revealed how the body itself had been altered.

The next steps run along two tracks. One is finding near-Earth objects earlier, since an early discovery makes any response far easier. The other is using the opportunities nature supplies, such as the close approach of Apophis in 2029. The numbers Hera pins down will serve as reference values along both of those tracks.

FAQ

Did DART really change the moonlet's orbit?

Yes. Ground-based observations after the impact showed the moonlet's orbital period around Didymos had shortened by more than half an hour, well beyond the threshold set in advance. Explaining why the change was that large, however, requires measurements taken at the asteroid itself.

Is this asteroid a threat to Earth?

No. The Didymos system is not on a collision course with Earth, and it was chosen precisely because the test could be run safely there. Being a binary was the deciding factor: the orbital change is easy to measure from Earth by watching the pair eclipse each other.

Will Hera hit anything?

No. Hera is an observer and carries no impactor. Its job is to measure the aftermath with cameras, radar and spectrometers. Late in the mission, though, its two CubeSats are planned to attempt a touchdown on the surface.

How small is Dimorphos?

It is a body on the scale of roughly a hundred metres, orbiting the larger Didymos. From Earth the two cannot be resolved separately, so the orbital period is inferred from the dips in brightness when one passes in front of the other. Only Hera's close-up imaging will show its full shape.

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Times and figures are approximate — verify with official sources when planning travel. Simulation uses J2000 mean elements.