DART Smashed Into an Asteroid at 6 Kilometres Per Second, Here Is What Hera Found When It Arrived

On 26 September 2022, a fridge-sized spacecraft travelling at roughly 6.1 kilometres per second hit a 160-metre-wide asteroid called Dimorphos. Nobody was trying to save Earth from that particular rock. Dimorphos posed no threat. NASA's Double Asteroid Redirection Test, DART, was a proof-of-concept: could a kinetic impactor actually change an asteroid's orbit? The answer came back within weeks, and it was not what the mission team had planned for.
Before impact, scientists calculated that a successful deflection would need to shift Dimorphos's orbital period around its larger companion, Didymos, by at least 73 seconds. That was the minimum threshold to call the mission a success. DART moved it by 33 minutes. The pre-impact orbital period was 11 hours and 55 minutes. After impact, it dropped to 11 hours and 22 minutes. That is a change roughly 25 times larger than the minimum target. The spacecraft itself delivered only part of that push. The rest came from something the models had underestimated: ejecta.
When DART hit Dimorphos, the collision blasted hundreds of thousands of tonnes of rock, dust, and debris off the surface and into space. That material did not just scatter, it streamed outward in a specific direction, acting like an exhaust plume. The recoil from that plume amplified the original impact force significantly. Telescopes on Earth, including ground-based observatories and the recently launched James Webb Space Telescope, tracked the debris tail for weeks. It stretched tens of thousands of kilometres. The lesson embedded in that tail: the asteroid's own material did most of the deflecting work, not the spacecraft's mass alone.
Knowing that DART worked is one data point. Understanding why it worked as well as it did, and whether the same result would hold for a different asteroid with a different composition, requires a second visit. That is what ESA's Hera mission is doing. Hera launched in October 2024 and is scheduled to arrive at the Didymos-Dimorphos system in late 2026. It carries two CubeSats, Milani and Juventas, which will descend close to Dimorphos's surface to gather data no Earth-based telescope could produce.

The measurements Hera is collecting include the precise mass of Dimorphos, the internal structure of the asteroid using a low-frequency radar instrument, the exact dimensions and depth of the crater DART left behind, and the distribution of the ejecta that accumulated back on the surface. Mass matters enormously. The deflection models that predicted 73 seconds were working with estimated mass figures. Without knowing the true mass, scientists cannot fully back-calculate the momentum transfer. Hera will close that gap. The crater itself is also scientifically critical: its shape and size will reveal whether Dimorphos is a solid rock, a rubble pile loosely held together by gravity, or something in between. Each internal structure responds differently to an impactor, and the response determines how much deflection a future mission could actually achieve against a real threat.
The amplification effect from ejecta is the most consequential finding from DART, and it is also the most variable. Ejecta output depends on porosity, surface cohesion, and the angle of impact. A rocky, dense asteroid sheds less material than a loosely packed rubble pile. A rubble pile might absorb more of the impact energy internally, reducing the plume, or it might shed far more, amplifying the push further. Nobody knows yet which scenario produces more deflection. Hera's radar instrument, called GRASS on the Juventas CubeSat, will produce the first subsurface picture of Dimorphos. That image will tell planetary defence planners which category of asteroid they are actually dealing with when they look at a future threat.

ISRO has not announced a dedicated planetary defence mission, but India's deep-space tracking infrastructure, including the Indian Deep Space Network at Byalalu near Bengaluru, has contributed to international asteroid observation programmes. The global nature of planetary defence means data from any mission feeds every space agency's threat-assessment models. DART's results are already informing how those models are built.
Planetary defence is now a confirmed discipline, not a theoretical one. The DART result demonstrated that a kinetic impactor can move an asteroid. Hera is converting that demonstration into engineering data precise enough to design a real deflection campaign against a real threat. The outstanding questions are not whether deflection works, DART settled that, but how much lead time a mission needs, how many spacecraft would be required against a larger body, and whether the ejecta amplification effect is reliable enough to factor into mission design as a feature rather than a bonus.

The answers Hera sends back will determine the architecture of every planetary defence mission that follows. The crater DART left on Dimorphos is, in that sense, less an end point than a calibration mark: the first confirmed measurement in a discipline that did not have any before September 2022.
What DART actually proved is narrower and more useful than the headline suggested. It did not prove Earth is safe. It proved that one specific technique, applied to one specific type of target, produced a measurable and calculable result. Hera is now turning that result into a number precise enough to bet a planet on.