Hera races toward Didymos: Europe enters the final phase of the first detailed investigation of an asteroid whose trajectory was changed by humans
The European Space Agency is entering one of the most sensitive phases of its planetary defense mission. The Hera spacecraft, launched on 7 October 2024, continues its journey toward the binary asteroid system Didymos, where by the end of 2026 it is expected to carry out the first thorough imaging and measurement of the consequences of the impact of NASA’s DART probe. It is a unique destination in the Solar System: Dimorphos, the smaller member of the Didymos system, became the first celestial body whose orbit was measurably changed by human action. That is precisely why Hera is not just another scientific mission to asteroids, but a key step toward turning the deflection of dangerous objects by kinetic impact from theory and a single successful experiment into a verified and repeatable defense method.
According to ESA data, Hera performed its second major deep-space maneuver in February 2026, required to align its trajectory with the orbit of the Didymos system around the Sun. This operation was crucial for the continuation of the flight toward the target, and confirmation of its success was provided by the Estrack deep-space antenna network. Operational teams in Darmstadt thus closed one of the most important stages of the interplanetary journey and are now focusing on the final preparations for arrival near the asteroid. In the coming months, the center of the work is shifting to autonomous navigation, the upgrade of key systems, and precise planning of the approach to bodies that are small, dark, and extremely demanding to observe.
Why Didymos matters for Earth
Didymos and its smaller companion Dimorphos have in recent years become the focal point of international efforts in the field of planetary defense. The large asteroid Didymos is approximately 780 meters in diameter, while Dimorphos is about 151 meters wide. It was precisely into Dimorphos that NASA’s DART spacecraft crashed on 26 September 2022, with the goal of testing whether the direction or speed of motion of a potentially dangerous object could be changed by a controlled impact of a space probe. The experiment succeeded, but for scientists that was not the end of the story, but the beginning. The impact showed that it is possible to alter the orbit of a small celestial body, but a whole series of questions remained open: what is the actual mass of Dimorphos, what is its internal structure like, how large was the effect of the ejected material, and did the impact leave a classic crater or practically reshape the entire body.
That is exactly why Hera has the role of a field investigator arriving at the site of an already performed experiment. While DART showed that the method can work, Hera must determine why it worked exactly as it did and how the result can be applied to future threats. This is crucial for any serious planetary defense strategy. It is not enough to know that an asteroid can be hit; it is necessary to understand how much its trajectory will change after the impact, how the composition and structure of the body affect that, and how great the difference is between one compact asteroid and a body composed of loose rocks and dust. Without those data there are no reliable models, and without models there is no serious operational plan in the event that an object posing a real risk to Earth is discovered in the future.
DART changed more than was initially thought
The results of observations to date show that the effect of DART was greater than the minimum expected. NASA first confirmed that the orbital period of Dimorphos around Didymos had been shortened significantly more than the set success threshold. A later, more detailed analysis showed that the period ultimately stabilized at about 11 hours, 22 minutes, and 3 seconds, which is 33 minutes and 15 seconds less than before the impact. This confirmed that a kinetic impact can produce a very clear and measurable effect, but also that the ejected material played a major role in the overall transfer of momentum. In other words, DART did not act only as a projectile that struck the target, but the enormous quantity of rocky material ejected into space after the collision also produced additional “thrust”.
Even more importantly, new research published in early March 2026 showed that the consequences of the impact were not limited only to the mutual relationship between Didymos and Dimorphos. According to NASA’s JPL laboratory, the orbit of the entire binary system around the Sun was also changed, although it was a very small change, measured in only a fraction of a second. Scientifically speaking, that result has enormous significance: for the first time, it was confirmed that a human spacecraft measurably changed the trajectory of a celestial body around the Sun. That does not mean it was a dramatic shift that could be seen “with the naked eye”, but it confirms the fundamental logic of planetary defense: even a very small change in speed, if applied early enough, can over time produce a large deviation from the original trajectory and make the difference between an impact and a safe miss.
What Hera must discover on site
The greatest value of the European mission lies in the fact that it will bring data that cannot be obtained solely through telescopic observations from Earth. Hera will carry out detailed mapping of the surface of Didymos and Dimorphos, measure their mass, shape, density, and gravitational properties, and try to clarify how much Dimorphos was actually changed after the impact. Earlier NASA and international studies had already suggested that this is not a solid monolith, but a body resembling a “rubble pile”, that is, a loose collection of rocks and smaller blocks held together by weak gravity. Precisely such a structure may be the reason why the impact caused the ejection of a large quantity of material and significantly amplified the effect of the collision.
If Hera confirms that after the impact Dimorphos changed its shape, its distance from Didymos, and perhaps part of its internal structure, scientists will obtain an exceptionally important set of data for calibrating future simulations. This is important not only for understanding the specific system, but also for all future defense scenarios against near-Earth objects. Namely, one of the key problems of planetary defense is that potentially dangerous asteroids differ significantly from one another. Some are rocky and more compact, some are more porous, and some may be binary systems like Didymos. A defense technique that works well on one object does not necessarily have the same effect on another. Hera therefore measures not only “what happened”, but also “why it happened exactly that way”.
A difficult approach to dark, tiny, and faintly visible bodies
Unlike large planets or their large satellites, from the perspective of space navigation Didymos and Dimorphos represent a very awkward target. These are small, dark bodies that do not reflect much light and are difficult to detect at a great distance. ESA therefore points out that during the final approach Hera will have to actively search for the asteroids and keep them in its field of view while approaching the system. The approach itself will last about three weeks, and during it the guidance, navigation, and control systems will be tested to their very limits.
An additional challenge is created by the extremely weak gravity in the system. According to ESA estimates, the gravity of Didymos is about 40,000 times weaker than Earth’s, and on Dimorphos approximately 200,000 times weaker. In such an environment, a classic orbit does not look like an orbit around a planet. Hera will in fact move around the system’s common center of gravity very slowly, at speeds on the order of centimeters per second, with constant trajectory corrections. This means that even the smallest errors, software failures, or incorrect position estimates will have greater consequences than in an environment of stronger gravity. That is precisely why the final part of the mission is not only scientific work, but also a demanding engineering test of autonomy, sensors, and control.
New upgrades and the role of European CubeSats
Before reaching its destination, Hera is also passing through an important phase of onboard software upgrades. ESA states that extensive modifications have been prepared that should enable the spacecraft to operate more safely in close encounters with asteroids. This includes improvements for the laser altimeter, which will continuously measure the distance from the surface, as well as functions needed for monitoring and confirming the deployment of the two CubeSats that Hera carries with it. These are the small companion spacecraft Milani and Juventas, which should expand the amount and type of data collected in the Didymos system.
Milani will focus on the spectral and compositional properties of the surface, while Juventas carries a radar intended to peer into the interior of Dimorphos and for the first time attempt to image the subsurface structure of an asteroid from close range. ESA emphasizes that these will be the first European CubeSats in deep space. Their deployment will not be merely a technical addition to the main mission, but an important part of the scientific strategy. If the main orbiter records the system from the outside, the auxiliary spacecraft can provide data from different angles, lower altitudes, and specific instruments that Hera itself does not have in the same form. This increases the precision of the overall model of the system and better explains the effect of DART’s impact.
Planetary defense beyond movie scenarios
The topic of asteroid impacts is often associated in public perception with cinematic spectacle, but in reality it is long-term work of early detection, precise measurement, and cool engineering assessment. ESA and NASA have repeatedly stressed that the probability of a major impact is small, but that the consequences of such an event would be serious. History reminds us that even bodies only a few tens of meters in diameter can cause great damage, such as the explosion over Chelyabinsk in 2013 or the event in Tunguska in 1908. For that reason, planetary defense is not an exotic fringe topic, but is gradually growing into a separate field of space policy and security.
In that context, Hera and DART form a logical two-part experiment. The first part showed that human technology is capable of hitting and redirecting a small target in deep space. The second part must determine what the actual effectiveness of such a method is, how to scale it, and under what conditions it would work best. That is precisely why experts emphasize the importance of the so-called momentum enhancement factor, that is, the additional effect that occurs when material ejected after the impact “pushes” the asteroid more than the projectile alone would. If that factor can be understood well enough, future defense missions will be able to be planned with much less uncertainty.
Why autumn 2026 is crucial
According to the current mission plan, in October 2026 Hera should begin a series of precisely timed engine burns that will take it from interplanetary cruising into the asteroid encounter phase. The actual arrival near the system is planned for late autumn 2026, and some of ESA’s official materials mention November as the month of arrival. That will be the moment when the years of design, flight, communication checks, and maneuvers completed so far transition into operational work directly next to the target. For the European space industry and scientific community, this is both a technologically and politically important event, because it will show how ready Europe is to take a leading role in a field that combines science, security, and international cooperation.
If everything goes according to plan, Hera will not only send back attractive images of the binary asteroid, but will deliver the data needed to create the first seriously confirmed “user manual” for the kinetic deflection of an asteroid. That is the crucial difference between symbolic success and real operational capability. DART proved that the target can be hit. Hera must now confirm that humanity understands the consequences of such an impact well enough to one day apply the same method in a situation where the safety of Earth might depend on it.
Sources:- European Space Agency (ESA) – official overview of Hera mission operations, including the maneuver in February 2026 and the planned encounter with Didymos (link)- European Space Agency (ESA) – official overview of the Hera mission, with data on the launch, the mission objective, and the basic characteristics of the Didymos-Dimorphos system (link)- European Space Agency (ESA) – answers to frequently asked questions about the mission, with data on arrival, distance from Earth, and navigation under conditions of very weak gravity (link)- NASA – analysis of changes in the shape and orbit of Dimorphos after the DART impact, including the final shortening of the orbital period by 33 minutes and 15 seconds (link)- NASA JPL – new research published on 6 March 2026 showing that DART also changed the orbit of the Didymos binary system around the Sun (link)- NASA Science – official page about Didymos and Dimorphos with basic data about the system and the objectives of the DART experiment (link)- Johns Hopkins Applied Physics Laboratory – overview of DART’s scientific results and summary of papers on geology and the consequences of the impact in the Didymos-Dimorphos system (link)
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