ESA's Hera Mission Approaches Asteroid Didymos for First Post-Impact Survey of NASA's DART Deflection Experiment

Humanity's First Planetary Defense Follow-Up Mission

In November 2026, the European Space Agency's Hera spacecraft will arrive at the binary asteroid system Didymos-Dimorphos, conducting the first detailed post-impact survey of NASA's DART mission—the first time humanity deliberately altered the orbit of a celestial body. As of August 2026, Hera is in transit and on track for its rendezvous, marking a critical milestone in the development of planetary defense capabilities.

The DART (Double Asteroid Redirection Test) mission, launched by NASA in November 2021, intentionally crashed a spacecraft into Dimorphos—the smaller moonlet of the Didymos binary system—on 26 September 2022. The impact successfully shortened Dimorphos's orbital period around Didymos by approximately 33 minutes, demonstrating that kinetic impact technology can change an asteroid's trajectory. However, DART's observations were limited to the brief period before and immediately after impact. Many fundamental questions remain unanswered: What is the exact size and shape of the crater left by DART? How did the impact alter Dimorphos's internal structure? What is the asteroid's precise mass and composition?

Hera was designed to answer these questions.

The Hera Spacecraft and Its Instruments

Hera was launched on 7 October 2024 aboard a SpaceX Falcon 9 rocket from Cape Canaveral, Florida. The spacecraft carries a suite of scientific instruments designed to characterise the Didymos system in unprecedented detail:

Two CubeSats—Juventas and Milani—will be deployed from Hera upon arrival at Didymos. Juventas carries a low-frequency radar designed to probe Dimorphos's internal structure, potentially revealing whether the asteroid is a solid body or a rubble pile—a critical distinction for understanding how kinetic impactors transfer momentum. Milani will perform spectral analysis to determine the asteroid's mineralogical composition.

The DART Legacy

NASA's DART mission was a triumph of engineering and a proof of concept for planetary defense. By deliberately crashing a 610-kilogram spacecraft into Dimorphos at approximately 6.1 kilometres per second, DART demonstrated that a kinetic impactor can change an asteroid's orbit—a capability that could one day be used to deflect a hazardous asteroid away from Earth.

However, the efficiency of the deflection—the 'momentum enhancement factor'—was significantly higher than expected. DART shortened Dimorphos's orbit by 33 minutes, far more than the predicted 73 seconds. This was because the impact ejected a large amount of debris from the asteroid's surface, and this ejecta contributed additional momentum beyond the spacecraft's own impact. Understanding this momentum enhancement is essential for planning future deflection missions, and it is one of the key questions Hera will address.

The amount of ejecta depends on the asteroid's surface properties, internal structure, and composition—factors that cannot be fully determined from Earth-based or flyby observations. Hera's sustained, close-range observations will provide the data needed to refine models of kinetic impact deflection.

Planetary Defense Context

Near-Earth objects (NEOs)—asteroids and comets whose orbits bring them close to Earth—represent a low-probability but high-consequence threat. NASA's Planetary Defense Coordination Office and ESA's Planetary Defence Office maintain catalogues of NEOs and assess their impact risk. As of 2026, approximately 38,000 NEOs have been discovered, with new ones added at a rate of roughly 3,000 per year.

The vast majority of discovered NEOs are small and pose no threat. However, the 2013 Chelyabinsk meteor—a roughly 20-metre asteroid that exploded over Russia, injuring approximately 1,500 people—demonstrated that even relatively small objects can cause significant damage. Larger objects, in the 100-metre to 1-kilometre range, could cause regional or global devastation.

DART and Hera represent the first end-to-end demonstration of a planetary defense technique: DART showed that we can change an asteroid's orbit, and Hera will show how effectively we did it and what we can learn from it. Together, they establish a foundation for future planetary defense missions that could protect Earth from a genuine impact threat.

What Comes After Hera

Hera's observations at Didymos will continue for approximately six months following arrival in November 2026. The data collected will be made available to the global scientific community and will inform the design of future planetary defense missions.

ESA and NASA are already planning the next generation of planetary defense capabilities. NASA's NEO Surveyor space telescope, designed to discover potentially hazardous asteroids and comets, is in development. ESA is contributing to the global effort through its Space Situational Awareness programme and is exploring concepts for a rapid-response deflection capability.

The Hera mission also represents a model for international cooperation in planetary defense. The DART-Hera campaign was a joint effort between NASA and ESA, with each agency contributing complementary capabilities—NASA provided the impactor, ESA provides the follow-up survey. This division of labour could serve as a template for a future real-world deflection mission, where speed and coordination would be critical.

Sources

• ESA, Hera mission overview, esa.int/Space_Safety/Hera/Hera_mission_overview
• ESA, Hera, esa.int/Space_Safety/Hera, August 2026
• EarthSky, Hera mission will arrive at asteroid impact site in November, earthsky.org, July 2026
• Wikipedia, Hera (space mission), en.wikipedia.org/wiki/Hera_(space_mission)
• The Planetary Society, Hera, ESA's asteroid investigator, planetary.org/space-missions/hera
• eoPortal, Hera, eoportal.org/satellite-missions/hera