DART was the impact test; Hera is the forensic survey. NASA’s DART spacecraft deliberately hit Dimorphos, the smaller body in the Didymos binary asteroid system, on 26 September 2022. The collision shortened Dimorphos’s orbit around Didymos by 32 minutes. ESA’s Hera mission is designed to revisit the same system, measure the asteroids and the impact site, and determine how efficiently the collision transferred momentum.
The missions are complementary parts of the international Asteroid Impact and Deflection Assessment (AIDA) collaboration. DART demonstrated that a kinetic impact can change an asteroid moonlet’s orbit; Hera is intended to establish why the change was that size and how reliably the technique could be assessed for other targets.
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The difference in one table
| Comparison | NASA’s DART | ESA’s Hera |
|---|---|---|
| Mission role | Kinetic impactor that deliberately collided with Dimorphos. | Follow-up survey mission examining the Didymos system and DART’s result. |
| Key date | Impact on 26 September 2022. | Launched 7 October 2024; rendezvous with Didymos planned for November 2026. |
| Primary action | Struck Dimorphos at a reported 6.1 km/s. | Map, measure and investigate the asteroids, the impact area and the system’s response. |
| Measured result or planned result | Dimorphos’s orbital period fell from 11 hours 55 minutes to 11 hours 23 minutes, a 32-minute reduction with about one minute of uncertainty. | Determine Dimorphos’s mass, surface properties, porosity and internal structure, and derive the impact’s momentum efficiency. |
| Evidence status | Impact and orbital change are complete and were established through observations from Earth. | Survey work was still ahead as of 13 October 2026. Hera had sighted the system but had not yet reached its planned close-up operations. |
| AIDA position | First, experimental impact phase. | ESA-led follow-up phase. |
That division of labor is the key to understanding the comparison. DART supplied a before-and-after orbital measurement. Hera is intended to supply the physical measurements needed to interpret that change.
What NASA’s DART actually did
It targeted the smaller member of a binary asteroid
Didymos is a binary system: a larger primary body orbited by the smaller moonlet Dimorphos. The target was not the pair’s path around the Sun. DART was aimed at changing Dimorphos’s orbit around Didymos, which gives astronomers a measurable clock for the experiment.
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ESA describes Didymos as approximately 780 metres in diameter and Dimorphos as approximately 150 to 151 metres. Because the pages use slightly different rounded values, “about 150 metres” is the most useful general description for Dimorphos.
The collision was deliberate and fast
DART struck Dimorphos on 26 September 2022 at a reported speed of 6.1 kilometres per second. The spacecraft was the impactor in the experiment: its job was not to orbit the asteroid, land on it or return samples, but to deliver a controlled kinetic hit.
The orbit changed by a measured amount
Before the collision, Dimorphos completed one orbit around Didymos in 11 hours 55 minutes. After the collision, ESA reported an orbital period of 11 hours 23 minutes. The difference is 32 minutes, with approximately one minute of uncertainty in the reported reduction.
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This is a successful experimental result. It shows that an intentional collision can alter the motion of a small asteroid moonlet in a binary system. The result does not, by itself, reveal Dimorphos’s mass, the strength of its interior, how much material was excavated, or exactly how much of the spacecraft’s momentum was transferred through the impact and resulting debris.
Why the period measurement was so useful
A binary system provides a natural reference. Observers can track the timing of Dimorphos’s repeated passages around Didymos and compare the period before and after the impact. A shorter period is direct evidence that the moonlet’s orbit changed, even though the moonlet remained gravitationally bound to the larger body.
The measurement is therefore an orbital outcome, not a complete physical diagnosis. Knowing that the period changed answers whether the experiment worked. Knowing the moonlet’s mass and internal structure is necessary to explain the size of the change and to compare the result with models of other asteroids.
What ESA’s Hera is designed to do
Survey the entire binary system
Hera is an ESA-led planetary-defence mission and the follow-up phase of AIDA. Its primary task is to examine Didymos and Dimorphos at close range after the DART collision. The spacecraft is intended to characterize the system rather than repeat the impact.
ESA’s science plan calls for mapping Dimorphos’s entire surface at a resolution of a few metres. Around the DART crater and nearby impact region, the planned resolution is about 10 centimetres. Those scales serve different purposes: the broader map establishes the moonlet’s shape and surface context, while the finer survey records the local consequences of the collision.
Measure properties DART could not establish from the impact alone
Hera is intended to derive Dimorphos’s mass and investigate its surface character, porosity and internal structure. These properties affect how an asteroid responds when struck. A compact, strong body and a loosely packed, rubble-like body can produce different amounts of ejecta and different momentum transfer even when the impactor and impact speed are similar.
The mission also aims to determine the impact’s momentum efficiency. In practical terms, that means comparing the spacecraft’s incoming momentum with the resulting change in the asteroid system, while accounting for the material thrown away from the impact site. Hera’s close observations are needed to constrain that calculation rather than relying only on the observed orbital period.
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Inspect the DART impact site
The crater and its surroundings are central to Hera’s survey. A close-up view can show the crater’s dimensions, the distribution of disturbed material and the local surface texture. Together with measurements of Dimorphos’s mass and interior, those observations can distinguish whether the impact response was dominated by a simple spacecraft strike or amplified by material ejected from the surface.
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ESA’s operations plan includes two small spacecraft, Juventas and Milani, separating from Hera for close-proximity investigations. Juventas is assigned radar sounding and gravity measurements, which are particularly relevant to probing the moonlet’s interior and refining its mass. These are planned mission activities, not completed observations as of 13 October 2026.
How the missions fit the AIDA experiment
AIDA links two nationally led missions into one planetary-defence investigation. DART supplied the active disturbance: it hit Dimorphos and produced the initial orbital-period change. Hera supplies the follow-up characterization: it is intended to examine the target, the crater and the physical response that produced the measured change.
Calling Hera a second impact mission would be misleading. Its purpose is observation and measurement. Calling DART a complete validation of kinetic impactor technology would also go too far. DART demonstrated an orbital change in this particular binary system; Hera’s measurements are intended to show how efficient the impact was and how confidently the result can inform assessments of other asteroids.
Mission chronology and status
26 September 2022: DART’s impact
DART collided with Dimorphos at 6.1 km/s. Ground-based observations subsequently established that the moonlet’s orbital period around Didymos decreased from 11 hours 55 minutes to 11 hours 23 minutes.
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7 October 2024: Hera launch
ESA launched Hera to conduct the follow-up survey. The spacecraft then began its cruise toward the Didymos system.
March 2025: Mars and Deimos flyby
The cruise plan included a flyby of Mars and its moon Deimos in March 2025. This was a navigation and mission-operations milestone during the journey, not the asteroid survey itself.
February 2026: Second deep-space manoeuvre
Hera’s operations timeline included a second deep-space manoeuvre in February 2026 as the spacecraft prepared for arrival.
7 October 2026: First sighting of the system
ESA reported that Hera detected the Didymos system from approximately 236,500 kilometres away. At that distance, Didymos and Dimorphos still appeared as one bright point in Hera’s camera image; the spacecraft needed to approach further before the bodies could be resolved separately.
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ESA planned Hera’s rendezvous with the Didymos system for November 2026. As of the latest dated status available on 13 October 2026, the rendezvous and close-up survey had not yet been completed. Any statement about Hera’s measurements should therefore use future-tense wording.
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What DART established—and what it left open
Established: a controllable orbital change is possible
The 32-minute reduction is the clearest completed result from the pair of missions so far. It is an observed change in Dimorphos’s orbit around Didymos, not a calculated prediction. The approximately one-minute uncertainty belongs to ESA’s reported reduction.
Not yet established: the impact’s full efficiency
The period change does not uniquely identify the amount of momentum transferred. To interpret it, scientists need the mass of Dimorphos, the geometry and scale of the crater, the amount and direction of ejected material, and information about the moonlet’s structure. Hera is designed to provide those constraints.
Not yet established: how representative the response is
Asteroids differ in size, shape, composition and internal arrangement. A result from a roughly 150-metre moonlet in a binary system cannot automatically be applied to every possible target. Hera’s measurements are intended to improve the physical basis for judging how the kinetic-impact method might perform elsewhere, but they do not erase the differences between individual asteroids.
Why mass, porosity and internal structure matter
The same incoming spacecraft momentum can produce different orbital effects depending on the target. Mass determines how much velocity change is associated with a given impulse. Porosity indicates how much empty space exists between pieces of material. Internal structure helps reveal whether the body behaves more like a coherent rock or a weakly bound aggregate.
Those properties also influence ejecta. Material blasted away from the impact site can carry momentum in the opposite direction from the incoming spacecraft, increasing the net push on the asteroid. Measuring the crater and the disturbed surface helps estimate that contribution. Without such measurements, the orbital-period change is real but the physical pathway that produced it remains only partly constrained.
How to read the two missions without confusing their results
DART’s result is an orbital measurement
When reports say DART changed Dimorphos’s orbit, they mean the time required for Dimorphos to circle Didymos became shorter. They do not mean DART moved the entire Didymos system onto a new orbit around the Sun.
Hera’s result will be a physical survey
When reports describe Hera’s objectives, they refer to measurements the spacecraft is intended to make after arrival: surface maps, impact-site observations, mass and gravity information, radar data and assessments of the moonlet’s structure. Those objectives should not be presented as completed findings before the survey takes place.
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“Successful” has two different meanings here
DART was successful in the narrow experimental sense that the collision produced a measured orbital-period reduction. The broader question—how efficiently the impact changed the orbit, why the response had that magnitude and how transferable the result is—depends on Hera’s follow-up measurements.
What each spacecraft can and cannot answer
DART can answer whether a deliberately aimed spacecraft reached Dimorphos and whether the moonlet’s orbital timing changed afterward. It cannot remain nearby to weigh the moonlet, sound its interior or map the crater at close range because the collision was its final operation.
Hera can make those close observations, but it cannot recreate the original pre-impact state. Its scientists will reconstruct that state from DART’s approach data, observations made from Earth and the condition of the system they encounter. The two missions therefore work best as a sequence: one creates a controlled event, and the other documents its physical consequences.
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Common misconceptions
Misconception: Hera will hit the asteroid again
Hera is a survey spacecraft. Its planned operations involve observing and measuring the system, with Juventas and Milani contributing close-proximity investigations. The deliberate collision was DART’s role.
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Random freezes, missing sound and display glitches usually trace back to one bad driver. Find and replace yours safely.Free scan · under a minuteMisconception: DART destroyed Dimorphos
The reported result is a changed orbit around Didymos. The available mission description does not say that Dimorphos was destroyed. Hera’s purpose includes examining the body that remained after the impact and characterizing the crater and surrounding surface.
Misconception: the 32-minute figure is a universal asteroid-deflection result
Thirty-two minutes is the measured reduction for Dimorphos in this specific experiment, with about one minute of uncertainty. It is not a standard amount by which every asteroid would move after a similar collision.
Misconception: Hera has already delivered close-up images
On 7 October 2026, ESA reported only a first sighting from about 236,500 kilometres, where the two bodies were not yet separately resolved. The planned November 2026 rendezvous was still ahead at the latest status date.
What readers should watch for in Hera’s findings
The most consequential future results will be the measured mass of Dimorphos, evidence about whether its interior is compact or highly porous, the detailed dimensions and morphology of the DART crater, and the gravity and radar data supplied by the spacecraft and CubeSats. Together, these observations should allow scientists to connect the 32-minute period reduction to a quantified impact response.
Those results will not replace DART’s measurement; they will explain it. DART answered whether a kinetic impact could alter the moonlet’s orbit. Hera is intended to answer how the asteroid responded, how much momentum the collision transferred, and how well the experiment can inform analysis of other potential targets.
Practical questions about the comparison
Which mission came first?
DART came first operationally. It struck Dimorphos on 26 September 2022. Hera launched on 7 October 2024 and was traveling to the same system for its planned November 2026 rendezvous.
Did DART and Hera visit different asteroids?
No. Both missions concern the Didymos binary system, consisting of the approximately 780-metre primary Didymos and its approximately 150-metre moonlet Dimorphos. DART hit Dimorphos; Hera is designed to study both bodies and the impact site.
Why did DART need a moonlet instead of a solitary asteroid?
Dimorphos’s repeated orbit around Didymos provides a timing signal. Measuring that period before and after the collision makes a small change easier to identify than trying to infer a subtle change in the trajectory of an isolated asteroid using only distant observations.
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1Scan for outdated or missing drivers - takes under a minute2Repair Windows errors before they cause bigger problems3Fix the driver behind crashes, sound loss and screen glitchesCan the 32-minute change be called a deflection?
It is an orbital deflection of Dimorphos around Didymos. It should not be described as a 32-minute shift in the binary pair’s orbit around the Sun, nor as a universal prediction for a hazardous asteroid.
What does a 10-centimetre survey add?
The planned 10-centimetre mapping near the crater is much finer than the few-metre mapping planned for Dimorphos’s full surface. It is intended to document local crater shape, disturbed terrain and surface texture, while the broader map supplies the moonlet’s overall context.
When can Hera’s findings be treated as results?
Only after Hera has reached the system, conducted its observations and ESA or the mission team has released measurements. On 13 October 2026, the latest dated status available here, Hera had seen the system from roughly 236,500 kilometres but had not yet separately resolved the two bodies or completed the planned rendezvous.
Frequently Asked Questions
What is the main difference between NASA’s DART and ESA’s Hera?
DART was the kinetic impactor: it deliberately struck Dimorphos on 26 September 2022. Hera is the follow-up survey mission, intended to measure the binary system and determine how the impact produced its orbital change.
Did DART successfully change Dimorphos’s orbit?
Yes. ESA reported that Dimorphos’s orbital period around Didymos decreased from 11 hours 55 minutes to 11 hours 23 minutes, a 32-minute reduction with about one minute of uncertainty.
What will Hera find out that DART could not?
Hera is intended to measure Dimorphos’s mass, surface, porosity and internal structure, map the body and crater, and derive the impact’s momentum efficiency. DART supplied the orbital change but was not a close-up survey spacecraft.
Has Hera reached Didymos yet?
As of 13 October 2026, no close-up survey had been completed. ESA reported a first sighting from about 236,500 kilometres on 7 October, with Didymos and Dimorphos still unresolved as separate points. The planned rendezvous was November 2026.
Will Hera impact Dimorphos?
No. Hera is designed to observe and measure the system. DART performed the deliberate collision; Hera’s planned close-proximity investigations include the Juventas and Milani CubeSats.
Does the 32-minute result apply to every asteroid?
No. It is the measured response of this roughly 150-metre moonlet in the Didymos binary system. Asteroid size, shape, mass, porosity and internal structure can all change the result.
The Bottom Line
Bottom line: DART changed Dimorphos’s orbit; Hera is designed to explain that change. The 32-minute reduction is a completed orbital measurement, while Hera’s planned maps, gravity, radar and structural observations are intended to determine the impact’s efficiency and how useful the result is for assessing kinetic impact on other asteroids.
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