On September 26, 2022, NASA deliberately crashed a spacecraft into an asteroid millions of kilometres from Earth. The collision was not a mistake or an attempt to destroy the space rock. It was a carefully planned experiment to answer a question that could one day matter to the survival of our planet: can humanity change the path of a dangerous asteroid before it reaches Earth?
The experiment worked. NASA’s Double Asteroid Redirection Test (DART) shortened the time taken by the asteroid moonlet Dimorphos to orbit its larger companion, Didymos, by approximately 33 minutes. Humanity had demonstrated that a spacecraft could deliberately change the motion of a celestial object.
But the collision also created a new scientific challenge. Researchers could measure the orbital change from Earth, yet important details about the impact site, the asteroid’s interior and the amount of material blasted into space remained uncertain.
That is where Europe’s Hera mission comes in. As Hera approaches the Didymos asteroid system in October 2026, scientists are preparing to investigate the aftermath of DART’s historic collision. The objective is to turn a successful demonstration into a much more reliable understanding of how asteroid deflection works.
Why NASA Deliberately Hit an Asteroid
DART was designed to test a technique called the kinetic impactor method. The principle is straightforward: a spacecraft travelling at high speed collides with an asteroid, transferring momentum and changing its motion.
In a planetary-defence scenario, even a relatively small change in an asteroid’s velocity could become significant if it happens early enough. Over months or years, the altered trajectory can produce a substantial difference in where the object eventually travels.
DART targeted Dimorphos, a moonlet approximately 160–170 metres across that orbits the larger asteroid Didymos. The pair was selected because their orbit could be measured accurately from Earth, making it possible to compare their motion before and after impact. Neither asteroid poses an impact threat to Earth.
When DART struck Dimorphos at roughly 22,000 kilometres per hour, the spacecraft did more than push against the asteroid. The collision excavated material from its surface, sending a cloud of rocky debris into space. That escaping material carried momentum with it, producing an additional recoil that increased the impact’s overall effect.
NASA’s subsequent measurements showed that Dimorphos’s orbital period fell from about 11 hours and 55 minutes to approximately 11 hours and 22 minutes. The final measured reduction was around 33 minutes—far greater than the minimum change NASA had set as a success criterion.
The result established that a kinetic impactor can alter an asteroid’s motion. It did not, however, answer every question needed to plan a future mission against a genuinely hazardous object.
The Missing Details Behind DART’s Success
The amount an asteroid moves after a collision depends on more than the spacecraft’s mass and speed. Its surface material, internal structure, density and gravity all influence the outcome.
Consider two asteroids of similar size. One might be a relatively solid body, while the other could be a loosely assembled collection of rocks and dust held together mainly by gravity. The same spacecraft could produce different results against these two targets.
The debris released by the collision also matters. If substantial material escapes into space, its recoil can amplify the momentum transferred to the asteroid. But the amount of debris—and the direction and speed in which it travels—depends on the target’s physical properties.
Researchers have already found evidence that Dimorphos changed shape after the collision. Observations suggest it is a loosely packed object rather than a simple, solid rock. Yet distant observations cannot provide the same level of detail as a spacecraft examining the asteroid up close.
For planetary defence, this is a crucial distinction. Scientists need predictive models that can estimate how a real asteroid would respond to an impact, not just confirmation that one particular experiment changed one particular orbit.
What ESA’s Hera Mission Will Investigate
The European Space Agency’s Hera spacecraft was developed to conduct a detailed follow-up investigation of the Didymos–Dimorphos system. Launched in October 2024, the spacecraft is scheduled to reach the asteroid pair in October 2026.
Hera’s job is not to strike Dimorphos again. Instead, it will examine the asteroid system and the aftermath of the DART impact, collecting measurements that could not be obtained during the original collision.
One major objective is to determine Dimorphos’s mass more accurately. Combined with measurements of the orbital change, this will help scientists calculate how efficiently the impact transferred momentum to the asteroid.
Hera will also map the moonlet’s surface and study its shape, structure and composition. Close-range observations can help researchers determine the dimensions of the impact crater, examine the surrounding terrain and improve estimates of the asteroid’s internal properties.
The spacecraft is designed to deploy two small companion probes, named Milani and Juventas, during its investigation. Their planned work includes studying surface material and using radar measurements to investigate Dimorphos’s internal structure. The mission’s detailed operations will depend on the spacecraft’s arrival, commissioning and deployment sequence.
Together, these observations should help scientists understand why DART produced the orbital change that it did—and how confidently the same physics can be applied to other asteroids.
Why the Follow-Up Could Change Planetary Defence
An asteroid-deflection mission cannot be planned using size alone. Scientists must estimate an object’s mass, composition, rotation, trajectory and physical structure, then work out how much warning time would be available.
Hera will help improve the models used to connect an impactor’s characteristics with the resulting change in an asteroid’s motion. That information could help future mission planners estimate the spacecraft mass, impact speed and timing needed to produce a useful deflection.
The findings may also show where current models need improvement. If Dimorphos’s interior is more complicated than expected, for example, that could affect how researchers calculate the response of similar objects.
These questions are relevant to asteroids that astronomers closely monitor, although monitoring an object does not automatically mean it is on a collision course with Earth. Astronomers must continually refine orbital measurements and evaluate uncertainty before assessing a potential threat.
The case of asteroid 2024 YR4 and the question of an Earth impact illustrates why risk estimates can change as new observations become available. A reliable defence strategy begins with knowing where an object is going and how much uncertainty remains in that prediction.
Detection is equally important. As discussed in our report on the challenge of detecting dangerous asteroids, a deflection technique is most useful when a potential threat is identified early enough to plan and carry out a response.
Could Scientists Deflect Every Dangerous Asteroid?
No. DART demonstrated one promising method, not a universal solution to every possible asteroid threat.
The effectiveness of a kinetic impactor would depend on the size and physical properties of the target, the amount of warning time, the available launch capability and the accuracy of the asteroid’s predicted trajectory. A very large object might require a different approach, multiple spacecraft or a much earlier intervention.
Some asteroids could also be difficult to intercept because of their orbits, while others might be discovered too late for a deflection attempt to be feasible. Even a technically successful impact would require careful follow-up measurements to confirm that the object’s trajectory had changed as intended.
Planetary defence therefore depends on several systems working together: sky surveys to discover objects, telescopes and radar to refine their orbits, computer models to assess risk, and spacecraft capable of responding when necessary.
Research into technologies that could change how humanity operates in space provides a wider perspective on how advanced space systems may shape future capabilities. But asteroid defence must be judged by evidence and engineering constraints, not by the assumption that every threat can be stopped.
The Next Step in Protecting Earth
DART proved that humans can change the motion of an asteroid through a deliberate spacecraft impact. Hera’s investigation is intended to explain the physical details behind that success, from the mass of Dimorphos to the damage caused by the collision and the structure of the asteroid itself.
Those details matter because the next asteroid discovered on a collision course with Earth may be very different from Dimorphos. The more accurately scientists understand how asteroids respond to impacts, the better prepared they will be to assess whether a deflection mission could work—and what it would take to make one succeed.
The experiment has already crossed a historic threshold. The next challenge is to turn that achievement into a dependable planetary-defence capability, backed by measurements, tested models and enough advance warning to act.
Hera will not make Earth invulnerable to asteroids. It can, however, help scientists replace some of the uncertainty surrounding asteroid deflection with the data needed to make better decisions when the stakes are real.
