Key Takeaways
- Hera will arrive at the Didymos asteroid system in November 2026 to study the DART impact aftermath
- NASA’s DART mission struck Dimorphos in September 2022, measurably altering its orbit
- Hera provides the detailed measurements needed to turn DART’s demonstration into a repeatable method
The Aftermath of an Unprecedented Experiment
On September 26, 2022, NASA’s Double Asteroid Redirection Test (DART) spacecraft slammed into Dimorphos, the moonlet of the near-Earth binary asteroid Didymos, at approximately 6.6 kilometres per second. The 570-kilogram spacecraft successfully changed Dimorphos’s orbital period around Didymos by approximately 33 minutes, shortening it from 11 hours and 55 minutes to 11 hours and 22 minutes. That was roughly 25 times more change than the minimum needed to demonstrate that the kinetic impactor technique could work.
The DART mission was a significant success by any measure. Telescopes around the world, including the Hubble Space Telescope and JWST, observed the impact plume that expanded outward from the surface for days. Ground-based radar observations tracked the orbital period change over subsequent weeks. The conclusion was clear: hitting an asteroid with a spacecraft changes its orbit. The question DART could not answer – because DART was destroyed on impact – was exactly how, at what efficiency, and with what lasting physical consequences.
ESA’s Hera mission will answer those questions when it arrives at Didymos in November 2026. Hera launched in October 2024 from Cape Canaveral on a Falcon 9 rocket, giving it approximately two years of cruise before its arrival.
What Hera Will Measure and Why It Matters
The key quantity that DART left undetermined is Dimorphos’s mass. Without knowing the mass, scientists cannot calculate the momentum transfer efficiency of the impactor – the ratio of how much momentum the deflected asteroid gained to how much the impactor brought in. That ratio, called the momentum enhancement factor (beta), depends critically on how much ejecta the impact generated. Ejecta launching off the surface at high velocity carries additional momentum away from the asteroid, amplifying the net deflection beyond what the impactor alone would have provided.
DART observations from Earth showed an enormous ejecta plume, suggesting that ejecta contributed significantly to the total momentum change. But “significantly” is not a number useful for planning future planetary defence missions. Hera needs to measure Dimorphos’s volume precisely – by mapping its shape with its cameras – and combine that with mass estimates from radio tracking of Hera’s trajectory as it orbits close to Dimorphos, where the asteroid’s gravitational field is detectable.
Beyond mass, Hera will characterise the physical state of Dimorphos in detail: surface composition using spectroscopy, internal structure insights from radar, and the morphology of the DART impact crater. Images from DART’s companion cubesat LICIACube captured the ejecta plume immediately after impact but could not resolve the crater itself. Hera will image the crater directly, revealing the size, shape, and character of the excavation.
Hera’s Two CubeSats
Hera carries two CubeSats named Milani and Juventas. Each will separate from Hera at Didymos and conduct independent proximity operations around the asteroid system.
Milani, built by a consortium led by Tyvak International and INAF, carries a hyper-spectral imager to characterize the surface composition of both Didymos and Dimorphos in detail across wavelength ranges not covered by Hera’s main instruments. Mapping compositional variations across Dimorphos will reveal whether the DART impact excavated material from the surface layer only or exposed subsurface material with different properties.
Juventas, built by a consortium led by GomSpace and Emxys, carries a low-frequency radar designed to probe the interior structure of Dimorphos. Asteroids are often loosely bound “rubble piles” rather than solid monoliths, and the interior porosity significantly affects how kinetic impactors transfer momentum. A porous rubble pile absorbs and redistributes impact energy differently than a solid rock, and the beta factor depends on that structure. Juventas’s radar provides the first direct sub-surface look inside a small asteroid.
Both CubeSats represent a growing ambition in ESA’s use of small spacecraft for companion science. Their deployment at Didymos, independently operating and collecting data simultaneously with Hera, multiplies the mission’s data return beyond what a single spacecraft could achieve.
Didymos as a Planetary Defence Reference System
Didymos is a well-chosen target for a planetary defence demonstration. The binary system – a primary body 780 metres in diameter accompanied by a 160-metre moonlet – is a near-Earth asteroid with a well-characterized orbit that has been studied extensively from Earth-based telescopes and radar facilities including Goldstone and Arecibo (before the Arecibo telescope’s 2020 collapse). That pre-existing knowledge base gives context for Hera’s measurements.
The binary nature of the system was essential to DART’s design: measuring Dimorphos’s orbital period around Didymos provided a precise way to quantify the deflection without having to track the asteroid’s heliocentric orbit, which would have required far longer observation periods. Hera benefits from the same binary architecture: orbiting Dimorphos, it can precisely characterize Dimorphos’s mass by measuring its gravitational effect on Hera’s trajectory.
The size of Dimorphos – 160 metres in diameter – is representative of the asteroid size that poses the most challenging planetary defence problem. Objects larger than a kilometre in diameter are mostly catalogued and their orbits are tracked through programmes like NASA’s Center for Near Earth Object Studies (CNEOS). Objects smaller than about 50 metres either burn up in the atmosphere or cause limited regional damage. The 100-to-300-metre class, of which Dimorphos is a member, can cause regional to continental devastation, is numerous enough to be statistically threatening, and is partially untracked. DART and Hera together are developing the deflection technique applicable to exactly this class.
The Planetary Defence Context in 2026
The broader planetary defence community has made substantial progress since DART’s launch in 2021. The Planetary Defense Coordination Office at NASA coordinates detection, characterisation, and deflection planning. The Vera C. Rubin Observatory in Chile, which began full survey operations in 2025, has been systematically extending the near-Earth asteroid catalogue at an unprecedented rate, identifying previously unknown objects including several in the 100-to-500-metre size range.
Each newly discovered potentially hazardous asteroid requires years of tracking before orbit projections reach sufficient precision to assess impact probability. Hera’s November 2026 arrival at Didymos brings closure to the DART chapter of the planetary defence story – transforming a celebrated single impact event into a scientifically characterized technique with known parameters and uncertainty bounds.
The data Hera returns will feed directly into the ESA Space Safety Programme’s impact modelling tools and into NASA’s planetary defence planning. When future threat scenarios are assessed, the Hera-validated beta factor will be the empirical foundation for estimating how much deflection a given impactor can achieve against a given target.
Summary
Hera’s November 2026 Didymos arrival is the second half of a two-spacecraft, two-agency, 14-year planetary defence experiment. DART proved that humanity can hit an asteroid and change its orbit. Hera will prove that the physics of that process is understood well enough to predict the outcome of future deflection missions before they fly. Given that no known asteroid currently threatens Earth and the question is entirely about whether a deflection would work when needed, that quantitative understanding is the difference between a demonstrated capability and a demonstrated concept. Hera closes that gap.
Appendix: Top 10 Questions Answered in This Article
What is ESA’s Hera mission?
Hera is ESA’s planetary defence mission that will arrive at the binary asteroid system Didymos in November 2026. Its primary purpose is to characterise the aftermath of NASA’s DART impact on the asteroid Dimorphos, providing the physical measurements needed to turn the kinetic impactor demonstration into a repeatable, predictable planetary defence technique.
What did NASA’s DART mission accomplish?
NASA’s DART spacecraft struck the moonlet Dimorphos on September 26, 2022, changing its orbital period around Didymos by approximately 33 minutes – about 25 times more than the minimum required to demonstrate the technique worked. The result confirmed that a kinetic impactor can change an asteroid’s orbit.
Why is Hera needed after DART succeeded?
DART was destroyed on impact and could not measure the aftermath. Critical parameters – especially Dimorphos’s mass and the size and character of the DART crater – could not be determined without a follow-up mission. Without knowing the mass, scientists cannot calculate the momentum transfer efficiency needed to predict outcomes of future deflection missions.
What is the momentum enhancement factor?
The momentum enhancement factor, called beta, is the ratio of the asteroid’s gained momentum to the impactor’s incoming momentum. Beta exceeds one because ejecta launched from the surface at high velocity adds additional momentum beyond the impactor itself. DART’s large ejecta plume suggests a significant beta value, but Hera must measure it precisely.
What CubeSats does Hera carry?
Hera carries two CubeSats: Milani, which carries a hyper-spectral imager to map surface composition, and Juventas, which carries a low-frequency radar to probe Dimorphos’s interior structure. Both will operate independently in the Didymos system.
How big is Dimorphos?
Dimorphos is approximately 160 metres in diameter. Its primary body Didymos measures approximately 780 metres across. The size of Dimorphos is representative of the 100-to-300-metre asteroid class that poses the most challenging known planetary defence problem.
When did Hera launch?
Hera launched in October 2024 from Cape Canaveral aboard a SpaceX Falcon 9 rocket, beginning a two-year cruise to the Didymos asteroid system before its planned November 2026 arrival.
Why is the Didymos binary system useful for planetary defence research?
Dimorphos’s orbital period around Didymos provided a precise measurable quantity to quantify DART’s deflection effect. Hera benefits from the same architecture: orbiting close to Dimorphos allows its gravitational field to be detected in Hera’s trajectory, providing a mass estimate. The pre-existing observational record of Didymos from radar and telescopes adds context.
What radar facility provided key early observations of Didymos?
The Goldstone Solar System Radar facility contributed key pre-mission radar observations of Didymos and Dimorphos. The Arecibo Observatory also provided observations before its collapse in 2020.
How will Hera’s data improve future planetary defence planning?
The Hera-validated momentum enhancement factor for Dimorphos provides an empirical foundation for estimating how much deflection a given impactor can achieve against a given target in future threat scenarios. This converts DART’s qualitative demonstration into a quantitatively predictable technique applicable to actual planetary defence planning.
