ESA’s Hera spacecraft will make a close-up, post-impact survey of the Didymos binary asteroid system, with Dimorphos as its principal target. It is designed to map both bodies and the DART impact site, determine Dimorphos’s mass and physical properties, measure the system’s gravity and dynamics, and investigate mineral composition and internal structure with two CubeSats. Those measurements will show how the 2022 impact changed Dimorphos’s motion and how reliably a kinetic impact could work as a planetary-defence technique.
What Hera is investigating
Hera is the follow-up mission to NASA’s Double Asteroid Redirection Test (DART). DART intentionally struck Dimorphos on September 26, 2022. Hera did not perform that impact; its job is to examine the result at close range and turn a remotely observed experiment into a physically measured one.
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Didymos is the larger primary asteroid and Dimorphos is its smaller companion. Because the two bodies orbit one another, Hera can study more than a crater. It can examine the shapes, surfaces, masses, gravity fields and orbital dynamics of the complete binary system. Dimorphos receives most of the attention because it was the impact target, but Didymos provides the dynamical context needed to interpret what happened.
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1Scan for outdated or missing drivers - takes under a minute2Repair Windows errors before they cause bigger problems3Fix the driver behind crashes, sound loss and screen glitchesGround-based telescopes already detected a change in Dimorphos’s orbital period around Didymos after the collision. That observation established that DART altered the moonlet’s motion. Hera is intended to determine the physical reasons for the size of that change, rather than simply measure the change again.
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The measurements Hera is planned to make
| Measurement area | How Hera’s mission will investigate it | What it can reveal |
|---|---|---|
| Shape and surface | High-resolution imaging and laser mapping from the Hera spacecraft | Three-dimensional form, surface features and the physical setting of the impact |
| DART impact site | Close visual and topographic survey | The crater’s dimensions, morphology and relationship to surrounding terrain |
| Mass and gravity | Radio-science Doppler measurements, spacecraft tracking and inter-satellite links with the CubeSats | Dimorphos’s mass and the gravity fields of both bodies |
| Surface gravity | Juventas’s GRASS instrument after landing on Dimorphos | A direct measurement of local surface gravity, if the planned landing and measurement succeed |
| Mineral composition | Milani’s prospecting instruments | The mineral make-up of Dimorphos’s surface and nearby material |
| Subsurface and interior | Juventas radar sounding and radio-science analysis | Internal layering, structure and clues to porosity |
These techniques are complementary. A camera can show what the surface looks like, but not by itself how much material lies beneath it. Gravity data constrain mass and density, radar can probe below the surface, and mineral sensing helps identify what the body is made of.
How Hera will map the bodies and the impact site
High-resolution visual imaging
Hera’s visual survey will document Didymos, Dimorphos and the region altered by DART in much greater detail than Earth-based observations can provide. Images will establish the location and appearance of boulders, ridges, depressions and other surface features. Repeated views from different positions can also help build a three-dimensional model rather than a flat photograph.
Laser mapping
Laser mapping will add topographic information to the images. By measuring the distance to the surface, the spacecraft can construct a precise shape model and determine elevations across the bodies. That model is important for calculating volumes, interpreting slopes and comparing the impact site with undisturbed terrain.
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The DART crater is one of the key unresolved features in the experiment. Hera’s survey is intended to establish its shape and dimensions and to show how ejecta and fractured material are distributed around it. Researchers can then relate the crater to Dimorphos’s surface texture and structure. The crater is not merely a visible souvenir of the collision: its geometry records how much material was displaced and how efficiently the impact transferred momentum.
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Why Dimorphos’s mass is central
To quantify DART’s momentum enhancement, scientists need an accurate mass for Dimorphos. The spacecraft’s impact delivered momentum directly, but material blasted away from the surface could have supplied additional recoil. The ratio between the resulting change in Dimorphos’s motion and the momentum delivered by DART depends on the target’s mass and on how the impact interacted with the asteroid.
Hera will use radio science to estimate the mass. Small changes in the spacecraft’s velocity alter the Doppler shift of its radio signal. Tracking those shifts while Hera moves through the binary system provides information about the gravitational pull of Didymos and Dimorphos. Communication links between Hera and its CubeSats will provide additional measurements of the local gravity field.
Mass alone is not enough. Combining mass with a mapped volume allows scientists to estimate bulk density. That result helps distinguish a relatively solid body from a highly porous aggregate of rocks and voids. The distinction affects how an impact creates a crater, ejects debris and changes an orbit.
How the mission will study gravity and orbital dynamics
Radio-science tracking
Radio science treats the spacecraft’s communications system as a precision motion sensor. As Hera responds to the gravity of the two asteroids, its signal changes in measurable ways. Analysis of those changes can constrain the masses and gravitational parameters of the bodies and improve knowledge of the binary orbit.
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Inter-satellite measurements
The CubeSats will not simply take independent observations. Links between Hera and the smaller spacecraft can measure their relative motion while they experience the system’s gravity. This gives a second geometrical perspective and can help separate the gravitational effects of Didymos and Dimorphos.
Juventas and GRASS
Juventas carries GRASS, an instrument ESA describes as intended to make a direct measurement of gravity on an asteroid surface after the CubeSat lands on Dimorphos. A surface measurement would provide a local check on the gravity field inferred from spacecraft tracking. Its value will depend on the landing outcome, instrument operation and the exact location and orientation of the spacecraft.
What Hera will learn about Dimorphos’s interior
Porosity and bulk structure
Porosity describes how much of a body’s volume is empty space rather than solid material. A porous “rubble-pile” structure can respond very differently to an impact from a more coherent rock. Hera’s mass and shape measurements will constrain bulk density, while radio-science results will contribute to estimates of internal structure and porosity.
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Juventas is planned to perform radar sounding within an asteroid. Radar signals can travel into the near subsurface and return information about boundaries or changes in material. The observations may reveal layering, large voids, buried blocks or other internal contrasts, although the final interpretation will depend on signal quality and the complexity of Dimorphos’s geology.
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Why interior information matters
A crater’s appearance can be misleading without knowledge of the target’s internal arrangement. The same impact energy could produce different outcomes in a compact monolith, a fractured body or a loosely packed aggregate. Interior measurements therefore help connect the visible crater and ejecta pattern to the actual momentum transferred to Dimorphos.
How Milani will investigate composition
Milani is the CubeSat assigned to prospect the mineral make-up of Dimorphos. Its observations will complement the main spacecraft’s images by examining what the surface materials are made of. Composition can help identify whether exposed material differs from the surrounding terrain and can provide context for interpreting ejecta from the DART impact.
The available mission descriptions establish composition as an investigation objective, not a completed result. Hera’s data should not be used in advance to claim a specific mineral inventory or a definitive geological history for Dimorphos.
How the measurements fit together
- Build the shape model. Visual and laser mapping will establish the dimensions, topography and surface context of Didymos, Dimorphos and the impact area.
- Determine the gravitational environment. Radio tracking, spacecraft dynamics and CubeSat links will constrain the masses and gravity fields.
- Estimate density and porosity. Mass combined with volume will indicate how much internal void space the body may contain.
- Probe below the surface. Juventas radar sounding and radio-science analysis will test possible internal arrangements.
- Characterize the material. Milani will examine mineral composition, while imaging will relate material units to visible terrain.
- Reconstruct the impact outcome. Researchers will compare the crater, ejecta, target properties and measured orbital change to determine how momentum was transferred.
No single instrument can answer all of these questions. The scientific result will come from combining independent measurements that constrain the same physical event.
What is already known, and what Hera is still meant to establish
The historical fact is that DART collided with Dimorphos on September 26, 2022, and telescopic observations measured a change in Dimorphos’s orbital period around Didymos. That is an observed dynamical effect.
The close-range measurements described for Hera remain mission objectives in the cited ESA and NASA material. The mass, detailed crater properties, internal structure, mineral composition and direct surface-gravity result should therefore be written as planned investigations unless a later official mission update confirms completion. A planned measurement is not the same as a published finding.
Why these measurements matter for planetary defence
Kinetic impact is attractive because it requires no explosive device: a spacecraft strikes an asteroid and changes its velocity. But the outcome depends on properties that are difficult to infer remotely, including mass, density, porosity, surface texture and internal structure.
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Hera is intended to provide the physical calibration that DART could not obtain from the impact spacecraft alone. If researchers know the target’s mass and understand the crater and ejecta, they can calculate the momentum enhancement more reliably. That lets them assess how much of the observed orbital change came from the spacecraft’s direct impact and how much came from recoiling debris.
The result will not be a universal performance number for every asteroid. Dimorphos is one specific binary target with its own composition and structure. Its value is that it can turn a successful demonstration into a better-characterized experiment, showing which target properties control the effectiveness of a kinetic impact.
Important limits when interpreting Hera’s results
- Dimorphos is a particular target. Results from this system cannot automatically be generalized to every near-Earth asteroid.
- Measurements have model dependence. Mass, density and interior structure are inferred by combining observations and physical models; different models may fit the data to different degrees.
- CubeSat operations are conditional. Juventas’s landing, GRASS measurement and radar sounding depend on successful deployment, navigation, contact with the surface and instrument operation.
- Composition is not the same as strength. Knowing which minerals are present does not by itself specify how the body is fractured or how it will respond to an impact.
- Mission plans are not completed results. Official status updates are needed before assigning a completed measurement or final value to Hera.
Bottom line for readers
Hera will measure the Didymos system at several physical levels: its shape and crater, its mass and gravity, its surface minerals, and its subsurface and internal structure. The purpose is to explain the orbital change produced by DART and determine how efficiently a kinetic impact transferred momentum to Dimorphos. Imaging shows the aftermath; radio science weighs the bodies; Milani examines composition; and Juventas extends the investigation into surface gravity and the interior.
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