Hera arrives at Dimorphos
ESA's Hera is due to reach the asteroid Didymos and its moonlet Dimorphos in late 2026. It will study up close the result of the first planetary-defence test, in which NASA's DART struck Dimorphos and changed its orbit in 2022.
Available views: Orbit ✓ · Sky — (not visible in the sky)
The story
In September 2022, NASA's DART spacecraft crashed head-on into Dimorphos, a small moon circling the asteroid Didymos. It was the first time people deliberately tried to change the orbit of another body in space, and it worked. Measurements with ground telescopes showed that the time Dimorphos takes to go around Didymos had become about 33 minutes shorter than before the impact. ESA's Hera is the spacecraft sent back to the scene to see up close what actually happened.
Hera launched in October 2024 and flew past Mars in March 2025, using the planet's gravity to change its speed and direction. On the way it also photographed Deimos, the smaller moon of Mars. When Hera reaches Didymos, it will not move in close right away. It will first watch how the two bodies move from a distance, then step closer in stages, shrinking its observing distance bit by bit. The investigation is planned to last about six months.
Much could not be seen at the time of the DART impact. Apart from pictures taken in the last few minutes before the crash, there was no way to see the whole shape of Dimorphos or the mark the collision left. From Earth the two bodies look like one point of light, so the orbit time had to be worked out indirectly, by measuring how the brightness changed when one passed in front of the other. Hera is going to fill in those gaps.
Hera does not travel alone. It carries two tiny satellites called CubeSats, each about the size of a shoebox, and releases them at the asteroid. Juventas uses radar to look inside Dimorphos and measures its gravity, while Milani studies the minerals on the surface and the dust floating around the pair. Near the end of the mission, both CubeSats are planned to attempt a landing on the asteroid's surface.
Mission goals
The questions Hera is trying to answer are concrete. How big is the crater the impact made? Exactly how much mass does Dimorphos have? And how much did the impact really change its orbit? Debris flung off by the crash pushed back like a small rocket, so the change in orbit was larger than the push from the spacecraft alone would explain. Measuring that effect precisely is what will let anyone calculate, if a dangerous asteroid ever appears, what to hit it with and how hard.
A small asteroid like Dimorphos is probably not a single solid rock but a loose pile of rubble held together. If so, the impact did not just dig a crater in hard ground; it may have changed the shape of the whole pile. Researchers have in fact suggested that DART may have reshaped Dimorphos, and if that is confirmed, the assumptions behind calculations for pushing asteroids aside will have to change. Juventas's radar, looking directly inside, will help answer this question.
Planetary defence was for a long time a matter of calculations and computer simulations, and DART turned it into a real experiment for the first time. Hera is the final step, measuring the result precisely and turning it into knowledge others can reuse. An experiment is not finished until its outcome has been measured.
Mission chronology and background
DART and Hera were originally one plan. Under a joint NASA–ESA concept called AIDA, the United States would provide the spacecraft that crashes and Europe the spacecraft that observes, with both at the asteroid at the same time. In 2016, however, the original European observer mission did not get funding and was cancelled, and a redesigned mission, Hera, was approved in 2019. As a result, DART made its impact alone, and Hera became an investigator arriving four years later.
The impact was not completely unwatched. LICIACube, a small Italian satellite released from DART beforehand, flew past just after the crash and photographed the material thrown out. The Hubble Space Telescope and telescopes on the ground recorded a long dust tail streaming away from the asteroid. But all of these were views from outside; none of them showed how the body itself had changed.
The next steps in planetary defence follow two paths. One is finding objects approaching Earth earlier, because the sooner a dangerous object is found, the more time there is to respond. The other is making use of chances nature provides, such as the close pass of the asteroid Apophis by Earth in 2029. The numbers Hera measures will serve as reference values along both paths.
Other milestones of this mission
Where to see it — sky and simulator
There is nothing to see in the sky. Didymos and Dimorphos look like a single dot even in large telescopes, and Hera is far fainter. Instead, in the orbit view, choose Hera as the tracked object and set the clock to the arrival period. The replay then shows the path from Earth past Mars out to the asteroid, and where Earth and the asteroid are at the moment Hera arrives.
Two parts of the replay are worth watching. One is the bend in the path as Hera passes Mars. The other is the approach, when the difference in speed between Hera and Didymos shrinks almost to nothing. An asteroid's gravity is so weak that arriving is less like entering orbit and more like flying in formation alongside it, so matching speeds until the closing speed is almost zero is the most important step.
The real data and images will be released by ESA. The exact arrival date and the early approach schedule can change based on navigation, so the date on this page is the planned one. The release of the first close-up pictures, showing the mark DART left behind for the first time, is likely to be the most watched moment of the mission.
See this for yourself — replay the moment in the orbit view
FAQ
Did DART really change the moonlet's orbit?
Yes. Telescope observations after the impact showed that Dimorphos's orbit around Didymos had become about 33 minutes shorter, far more than the success threshold set in advance. Explaining why the change was that large needs measurements made at the asteroid itself.
Is this asteroid a threat to Earth?
No. Didymos and Dimorphos are not on a path to hit Earth, and they were chosen for DART precisely because the test could be done safely. Being a pair that orbit each other was the deciding factor, because the change in orbit is easy to measure from Earth.
Will Hera hit anything?
No. Hera only observes and carries no impactor. Its job is to measure the aftermath with cameras, radar and spectrometers. Near the end of the mission, though, its two CubeSats are planned to attempt a landing on the surface.
How small is Dimorphos?
It is about 150 m across, a little longer than a football pitch, and orbits about 1.1 km from the larger asteroid Didymos. From Earth the two cannot be seen separately, so the orbit time is worked out from the dips in brightness when one passes in front of the other.
Related
- DART impact — the first deflection test
- Hera launch
- Apophis passes close to Earth
- Hera — object index
- Earth — object index
- Deimos — object index
Times and figures are approximate — verify with official sources when planning travel. Simulation uses J2000 mean elements.