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NASA’s DART was the impact experiment; ESA’s Hera is the follow-up investigation. DART deliberately struck Dimorphos, the smaller body in the Didymos binary asteroid system, on 26 September 2022 and shortened its orbit around Didymos by 32 minutes. Hera launched on 7 October 2024 to examine that system at close range, measure the bodies and impact site, and determine how efficiently the collision transferred momentum. As of 13 October 2026, Hera had sighted the system but had not yet reached its planned November 2026 rendezvous.
Quick comparison: DART and Hera at a glance
| Axis | NASA’s DART | ESA’s Hera |
|---|---|---|
| Mission role | Kinetic impactor that intentionally collided with Dimorphos. | Follow-up survey mission examining the asteroid pair and DART’s effects. |
| Defining event | Impact on 26 September 2022 at a reported 6.1 km/s. | Launch on 7 October 2024; rendezvous with Didymos planned for November 2026. |
| Primary evidence | Remote observations measured a 32-minute reduction in Dimorphos’s orbital period. | Planned close-up measurements of mass, surface, porosity, internal structure and the impact response. |
| Program relationship | Impact phase of the international Asteroid Impact and Deflection Assessment collaboration. | European follow-up phase of the same AIDA experiment. |
| Status at the latest dated update | Completed impact experiment. | Approaching the system; first sighting was reported on 7 October 2026, before rendezvous. |
The simplest distinction is action versus diagnosis. DART supplied the shove. Hera is intended to determine what that shove did to the asteroid and how the result can be interpreted for future planetary-defense missions.
What DART actually tested
The target was a moonlet, not a solitary asteroid
Didymos is a binary system. The larger primary is about 780 metres across, while its smaller companion, Dimorphos, is about 150 metres across; ESA pages round the latter figure to 150 or 151 metres. Dimorphos travels around Didymos, giving observers a repeating clock that can reveal a change in its orbit.
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DART was aimed at Dimorphos. The experiment was not designed to push the entire pair into a different orbit around the Sun. Its measurable objective was to alter Dimorphos’s path around Didymos and then detect that change from Earth-based observations.
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The collision and the measured orbital change
DART struck Dimorphos on 26 September 2022 at a reported speed of 6.1 km/s. Before the collision, one Dimorphos orbit took 11 hours 55 minutes. Afterward, ESA reported a period of 11 hours 23 minutes: a decrease of 32 minutes, with uncertainty of approximately one minute.
That result is a direct demonstration that a deliberately guided spacecraft can change an asteroid moonlet’s orbital period. The measurement came from observing the timing of the binary system, rather than from a spacecraft remaining in orbit around Dimorphos after impact.
What the first result does not tell us
A changed period establishes that the impact worked, but the number alone does not reveal every property that controls the outcome. To assess the collision quantitatively, scientists need better knowledge of Dimorphos’s mass, shape, surface, porosity and internal structure. They also need to examine the crater and surrounding terrain created by the strike.
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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchThose missing physical measurements matter because the spacecraft’s momentum was not the only momentum involved. Material ejected from the surface can add a recoil effect, while a porous or weak interior can respond differently from a dense, solid body. Without measuring the target and the impact aftermath, the orbital-period change cannot by itself provide a complete estimate of the impact’s momentum efficiency or show how reliably the technique would work on another asteroid.
What Hera is designed to investigate
Physical properties of Dimorphos
Hera’s central scientific task is to turn the observed orbital change into a better-understood physical experiment. The mission is intended to derive Dimorphos’s mass and investigate its surface character, porosity and internal structure. Those measurements should help establish how much momentum the collision transferred to the moonlet and how much additional effect came from material thrown away during impact.
The spacecraft is also intended to inspect the DART impact region directly. A close survey can determine the crater’s dimensions and appearance, document nearby geological features and place the remote orbital measurement in a physical context. The result should be a more complete account of both the target and the response.
Surface mapping at two scales
ESA describes a plan to map the entire surface of Dimorphos at a resolution of a few metres. The area around the DART crater is planned for much finer mapping, down to 10 centimetres. The two scales serve different purposes: the broad map supplies a global model for shape and terrain, while the detailed map concentrates on the impact’s local effects.
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These are planned mission capabilities, not completed survey results. They should therefore be read as what Hera is intended to deliver after arrival, not as observations already available in October 2026.
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Two CubeSats add close-proximity measurements
Hera’s operations plan includes two small companion spacecraft, Juventas and Milani. They are designed to separate from the main spacecraft and contribute investigations from close to the asteroid system. Juventas is assigned radar sounding and gravity work, which can help probe beneath the surface and improve estimates of the bodies’ mass distribution. The operations plan describes these activities as part of the mission sequence; they are not results that had been completed at the latest status date.
Mission chronology and the latest status
From impact to follow-up
- 26 September 2022: DART collides with Dimorphos and supplies the kinetic-impact phase of AIDA.
- 7 October 2024: ESA launches Hera for the follow-up survey.
- March 2025: The cruise plan includes a Mars and Deimos flyby.
- February 2026: The operations timeline includes a second deep-space manoeuvre.
- 7 October 2026: ESA reports Hera’s first sighting of the Didymos system from approximately 236,500 kilometres away.
- November 2026: ESA’s planned rendezvous window with the Didymos system.
What the October 2026 sighting means
At about 236,500 km, Hera detected the system as a bright point. ESA said the two bodies were not yet separately resolved in the camera image and that the spacecraft would need to approach more closely before resolving them. This is a navigation and approach milestone, not evidence that Hera had already begun its close-up survey.
The latest dated status therefore has two parts: DART’s impact and orbital measurement are completed, while Hera’s detailed characterization remains ahead of the planned rendezvous. Claims that Hera has already measured the crater, mass or interior structure go beyond the status available on 13 October 2026.
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DART provides the controlled disturbance
Planetary-defense experiments need a known intervention followed by an observable response. DART supplied that intervention by striking a selected target at a known time and speed. The 32-minute period reduction is the initial response measurement, showing that the orbit around Didymos changed by an amount that could be detected.
Hera supplies the physical explanation
Hera’s measurements are intended to explain why the change had that size. A mass estimate affects the momentum calculation; a shape model affects the geometry of the impact; surface and porosity measurements indicate how the target may have absorbed or expelled energy; and internal-structure data help distinguish a relatively coherent body from a loosely assembled one.
This division of labor is why the missions are treated as one international experiment under AIDA. DART’s result is not being replaced by Hera. Instead, Hera is designed to make the result more useful by attaching physical measurements to it.
Reading the 32-minute result correctly
It is an orbital-period change
The reported 32 minutes is the difference between two periods: 11 hours 55 minutes before impact and 11 hours 23 minutes afterward. It describes how long Dimorphos takes to circle Didymos. It does not describe a 32-minute change in the pair’s journey around the Sun, a change in Earth’s orbit or a distance travelled by the spacecraft.
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It includes a stated uncertainty
ESA gives an uncertainty of about one minute for the reported reduction. That qualification belongs with the headline result: the measured change is 32 minutes, with approximately one minute of uncertainty, rather than an infinitely precise value.
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It is not yet a universal performance guarantee
The outcome demonstrates that kinetic impact can change an asteroid moonlet’s orbit under the conditions of this experiment. It does not establish that every asteroid would respond by the same amount. Target mass, structure, surface material, impact angle and the amount of ejecta can all affect momentum transfer. Hera’s planned measurements are intended to improve the efficiency estimate and help determine how well the method can be assessed for other targets.
Understanding the asteroid system
Didymos is the primary body
Didymos is the larger member of the pair, approximately 780 metres in diameter according to ESA’s mission descriptions. Dimorphos is the smaller companion, approximately 150 metres across, and its orbit around Didymos supplies the timing signal used to evaluate DART.
Rounded dimensions are deliberate
Different ESA pages use slightly different rounded values for Dimorphos, including 150 m and 151 m. For general discussion, “about 150 m” accurately reflects the scale without implying a precision the published rounded figures do not support.
The binary arrangement makes the experiment measurable
Because Dimorphos repeatedly circles Didymos, observers can compare the timing of successive orbital events before and after the collision. A solitary asteroid would not provide the same straightforward orbital-period clock. The binary configuration therefore allowed the impact response to be measured without requiring a second spacecraft to remain beside the target throughout the experiment.
What Hera’s future measurements could establish
A better momentum-efficiency estimate
Hera is intended to combine the observed orbital change with direct measurements of Dimorphos’s mass and the impact site. That combination can constrain the efficiency with which DART’s momentum was transferred to the moonlet. The crater and ejecta evidence can indicate how much extra recoil came from material leaving the surface.
A physical model of the target
Global mapping at metre-scale resolution, detailed imaging near the crater and subsurface and gravity investigations from the companion spacecraft are intended to build a physical model of Dimorphos. Such a model can help scientists relate the response to the asteroid’s shape, density distribution, surface texture and internal arrangement.
Evidence for applying the method elsewhere
ESA states that the main goal of Hera is to help fully validate the kinetic-impactor method for possible use against other targets. “Validate” here means improving the confidence of the calculations and the understanding of target-dependent effects. It does not mean that one successful impact makes the technique automatically suitable for every future asteroid.
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A useful comparison separates four questions: What did each spacecraft do? What was actually measured? What remains planned? And what conclusion is justified by those measurements? DART answers the first two for the intervention itself: it reached Dimorphos, struck it at 6.1 km/s and produced a measured change in the moonlet’s period. Hera addresses the remaining physical questions by surveying the target and the aftermath at close range.
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This distinction also prevents a common timeline error. DART’s result is already an observation, whereas Hera’s crater images, mass estimate and interior measurements are mission objectives stated in the future tense as of October 2026. A mission can have a clear design and an important planned role without having delivered its final science.
What a planned Hera survey involves
- Approach imaging: Hera first acquires and tracks the Didymos system, progressing from an unresolved bright point toward separately resolved views.
- Global mapping: The mission plans coverage of Dimorphos’s surface at a few metres per pixel or comparable resolution, providing context for its overall shape and terrain.
- Impact-site inspection: The region around DART’s crater is planned for imaging down to 10 cm, allowing the local result to be compared with the orbital change.
- Close-proximity investigations: Juventas and Milani are planned to separate from the main spacecraft and add measurements near the bodies. Juventas’s assigned radar and gravity work is intended to probe beneath the surface and improve mass-distribution estimates.
- Combined interpretation: Scientists can then connect mass, shape, surface, porosity, internal structure and crater observations with the period reduction measured after DART.
The sequence explains why Hera is more than a second set of photographs. Its measurements are designed to reduce the physical uncertainties that remain after a remote timing result.
What each mission does not do
DART does not provide a complete close-up inventory of Dimorphos’s interior, because it was the spacecraft that ended its mission through the collision. Hera is not the spacecraft that delivered the impact and did not cause the 32-minute change. Neither mission, in the comparison covered here, demonstrates that every asteroid can be deflected by the same spacecraft size, speed or impact geometry.
The target also matters. The reported change concerns Dimorphos’s orbit around Didymos, not the binary pair’s orbit around the Sun. Keeping those reference frames separate is essential when explaining what “deflection” means in this experiment.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Common points of confusion
- “Hera caused the orbital change.” No. DART made the collision. Hera is the follow-up spacecraft intended to measure and interpret the result.
- “DART destroyed Dimorphos.” The reported result is an altered orbit around Didymos. The mission evidence summarized here does not describe Dimorphos as destroyed.
- “The 32 minutes is a change in the asteroid’s solar orbit.” It is a change in Dimorphos’s period around Didymos.
- “Hera has already inspected the crater.” As of 13 October 2026, Hera had only detected the system from roughly 236,500 km and had not yet separately resolved the two bodies.
- “The method is completely proven for all asteroids.” DART demonstrated a measurable orbital response in this system. Hera’s planned mass, structure and impact-site measurements are needed to assess how broadly the result can be applied.
- “DART and Hera are rival missions.” They are sequential parts of AIDA: one performs the intervention and the other investigates it.
What to watch after rendezvous
The most informative milestones will be the transition from approach images to separately resolved views, followed by Hera’s close survey of both bodies. Readers should look for results addressing four questions:
- What is Dimorphos’s measured mass and how does it refine the momentum calculation?
- What does the DART crater look like at the planned 10 cm scale?
- Do surface, porosity and internal-structure measurements indicate a compact body or a more loosely assembled one?
- How do the combined observations change estimates of kinetic-impact efficiency for other potential targets?
Until those observations are reported, descriptions of Hera’s crater measurements or final validation should remain in the future tense.
The practical takeaway
DART and Hera answer different parts of the same planetary-defense question. DART showed that a spacecraft travelling at 6.1 km/s could alter Dimorphos’s orbit around Didymos, producing a measured 32-minute shortening of the period. Hera is the mission intended to determine the target’s physical properties, inspect the impact site and establish how efficiently the collision transferred momentum. DART delivered the experimental result; Hera is designed to supply the context needed to judge what that result means beyond this one asteroid system.
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What is the main difference between NASA’s DART and ESA’s Hera?
DART was the kinetic impactor: it deliberately collided with Dimorphos on 26 September 2022. Hera is the follow-up survey mission, intended to examine the asteroid pair and the impact site and to measure the physical properties needed to interpret DART’s result.
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Did DART successfully change Dimorphos’s orbit?
Yes. ESA reported that Dimorphos’s orbital period around Didymos shortened from 11 hours 55 minutes to 11 hours 23 minutes, a reduction of 32 minutes with about one minute of uncertainty.
What will Hera find out that DART could not?
Hera is intended to measure Dimorphos’s mass, surface character, porosity and internal structure, while inspecting the DART crater. Those data can improve the estimate of momentum efficiency and show how the target’s physical nature affected the response.
Has Hera already reached the asteroid system?
Not at the latest dated update. ESA reported on 7 October 2026 that Hera had detected the system from about 236,500 km, but the two bodies were still a single unresolved bright point. Rendezvous was planned for November 2026.
How large are Didymos and Dimorphos?
Didymos is approximately 780 metres across. Dimorphos is about 150 metres across; ESA pages use rounded values of 150 or 151 metres.
What does the 32-minute figure refer to?
It is the change in Dimorphos’s orbital period around Didymos. It is not a change in the binary pair’s orbit around the Sun or a distance travelled by DART.
Why are two missions needed?
The impact measurement shows that the orbit changed, but it does not by itself reveal the target’s mass, structure or the crater’s physical characteristics. Hera’s close survey is designed to supply that missing context and improve assessment of kinetic impact for other asteroids.
The Bottom Line
DART performed the impact and measured a 32-minute shortening of Dimorphos’s orbit. Hera is the follow-up mission intended to measure the target, inspect the crater and determine what that result says about kinetic-impact planetary defense.
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