Europe’s Hera spacecraft is flying to Dimorphos, the small asteroid moon NASA’s DART spacecraft deliberately struck on 26 September 2022. Hera’s close-up measurements will show what the impact did to the asteroid and how reliably a similar kinetic-impact technique could deflect a hazardous asteroid in the future. The Didymos system is not on a collision course with Earth.
What is Hera investigating?
Hera is examining the aftermath of NASA’s Double Asteroid Redirection Test (DART). Dimorphos orbits the larger asteroid Didymos, and DART was aimed at Dimorphos as a controlled planetary-defense experiment. ESA describes Hera’s mission as a “crash scene investigation” of the impact crater and surrounding surface.
DART demonstrated that a spacecraft can change an asteroid moonlet’s motion. Hera will determine the physical details behind that result: Dimorphos’s mass, shape, surface, interior structure, impact crater and the way debris was ejected. Those measurements are needed to turn one successful experiment into a better-understood technique rather than assuming every asteroid would respond identically.
ESA’s mission overview explains that Hera will measure Dimorphos’s mass and shifted orbit at close range and investigate the impact site in detail: ESA Hera mission overview.
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Did NASA really change an asteroid’s orbit?
Yes. NASA reports that Dimorphos’s orbital period around Didymos shortened from 11 hours 55 minutes to 11 hours 23 minutes—a measured change of 32 minutes after DART’s collision. This is the moonlet’s local orbit around Didymos, not the time the pair takes to orbit the Sun.
| Measurement | What changed | Reported value |
|---|---|---|
| Dimorphos around Didymos | Orbital period after DART | 11 hours 55 minutes before impact to 11 hours 23 minutes after; 32 minutes shorter |
| Didymos–Dimorphos around the Sun | Solar orbital period after impact | Changed by 0.15 seconds |
NASA’s DART mission summary provides the 32-minute result: NASA Science: Planetary Defense – DART. A separate NASA analysis reports the 0.15-second change in the binary system’s solar orbit: NASA’s DART mission changed the Didymos system’s solar orbit.
How did crashing a spacecraft move Dimorphos?
DART was a kinetic impactor: it transferred momentum directly when it hit Dimorphos. The impact also blasted rock and dust into space. That ejecta carried additional momentum in the opposite direction, giving the collision more effect than the spacecraft’s mass alone would produce.
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In NASA’s 2026 account of the study, the momentum-enhancement factor was about two, meaning the escaping debris roughly doubled the impact’s push in that analysis. The reported change in the binary system’s orbital speed was 11.7 microns per second, or 1.7 inches per hour, according to study lead Rahil Makadia as quoted by NASA.
The original DART expectation was that Dimorphos’s orbit would shorten by about 10 minutes; the observed result was the larger, 32-minute change reported after the impact. The two figures should not be treated as competing measurements: one was a pre-impact estimate and the other an observed outcome.
What will Hera measure up close?
Dimorphos’s shape and impact site
Hera will map the surface and inspect the crater—or determine whether the collision left a crater or substantially reshaped the moonlet. Before close-range observations, scientists could not establish those details from DART’s impact data alone.
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Mass, composition and internal structure
Knowing Dimorphos’s mass and density, and whether it is loosely assembled or more solid, is essential for predicting how much momentum a future impactor could transfer. Hera’s instruments will help connect the observed orbit change to the asteroid’s physical properties.
Two companion CubeSats
Hera carries two CubeSats that can operate closer to the asteroids than the main spacecraft:
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- Milani will survey mineral composition.
- Juventas will use radar to probe inside an asteroid.
ESA’s frequently asked questions describe why a close inspection is necessary: ESA Hera Frequently Asked Questions.
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Is Dimorphos going to hit Earth?
No. NASA says neither Didymos nor Dimorphos poses a threat to Earth. DART was a planned test, not an emergency response, and the impact did not place the system on a collision course with our planet. NASA also states that DART could not have redirected the asteroid pair toward Earth.
Planetary defense depends on warning time. A deflection mission could help only if astronomers discover a hazardous object early enough to design, launch and operate a response. The result also depends on the target’s structure: NASA notes that a more solid asteroid might eject less material and therefore receive a smaller momentum boost.
NASA’s current planetary-defense explanation covers the safety context and remaining uncertainty: NASA Science: Planetary Defense – DART.
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When will Hera reach the asteroid system?
ESA lists Hera’s launch date as 7 October 2024 and its rendezvous with the Didymos system for November 2026. The ESA Hera mission page includes a 17 March 2026 update saying the spacecraft was “on course for asteroid rendezvous.” That wording reflects ESA’s dated status update, not a live telemetry guarantee; mission timing and operational status can change.
Check ESA’s mission page for the latest schedule and status: ESA Hera mission page and status updates.
What DART proved—and what Hera must still establish
| DART directly demonstrated | Hera still needs to determine |
|---|---|
| A spacecraft collision measurably shortened Dimorphos’s orbit around Didymos by 32 minutes. | Dimorphos’s mass, density, composition and internal structure. |
| The impact produced ejecta that added momentum to the collision. | The exact crater, debris distribution and whole-body response. |
| The binary system’s orbit around the Sun changed by 0.15 seconds in period. | How transferable the result is to asteroids with different strength and structure. |
ESA summarizes the goal as making kinetic impact a better understood and repeatable planetary-defense technique. The agency’s wording is deliberately forward-looking: Hera is gathering the data needed to assess repeatability, not claiming that every future asteroid can be deflected in the same way.
Why this “crash scene” matters
The phrase describes a planned investigation, not an asteroid accident. DART supplied the controlled collision; Hera is the follow-up forensic mission that can measure the target in three dimensions and relate the orbit change to the impact physics. Together, the missions provide a practical test of whether a kinetic impactor could one day alter the path of a genuinely threatening asteroid.
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