Hera launch — to verify DART

ESA's Hera departed on a Falcon 9, bound for Dimorphos to survey the crater DART left when it shifted the moonlet's orbit in 2022.

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The story

On 7 October 2024 ESA's Hera left Cape Canaveral on a Falcon 9. Its destination is the Didymos–Dimorphos binary asteroid — the very place where, in September 2022, NASA's DART struck the small moon Dimorphos at 22,000 km/h and shortened its orbit by 32 minutes. Hera is the survey team for humanity's first deliberate change of a celestial orbit.

The launch itself was a close call. The window opened as Hurricane Milton approached Florida, and the countdown ran in a brief spell of acceptable weather. A day later the launch site would have closed, and the next window was weeks away.

Hera's job is to measure the consequences precisely: the size and shape of the crater DART left, how much material was thrown out, the mass and internal structure of Dimorphos. Only with those numbers can the planetary-defence formula — how hard and how early to hit an asteroid actually heading for Earth — be completed.

After a Mars flyby in March 2025 it arrives at Didymos in December 2026, carrying two CubeSats, Milani and Juventas, that will sample the surface composition and sound the interior by radar. The orbit replay follows the two-year trajectory from launch to the asteroid.

Why it matters

DART proved an asteroid can be pushed; Hera measures how far. They are two halves of one experiment. Ground telescopes could only see the 32-minute change in orbital period — how big the crater is and how heavy Dimorphos is can only be learned on site, and without those the key coefficient of momentum transfer, β, cannot be computed.

How a "rubble-pile" asteroid responds to an impact was itself an open question. The tail of thousands of tonnes of debris seen after DART supported the idea of a loose pile of gravel rather than solid rock; Hera's radar and close-up imaging will settle it.

For ESA it is the first asteroid mission since Rosetta, and a sign that Europe holds one pillar of international planetary defence. The NASA–ESA division of labour, impact then verification, is a template for future threat response.

How to watch

Launch was at 10:52 EDT on 7 October 2024 from Cape Canaveral pad 40. The spacecraft went straight into an Earth-escape trajectory and now follows a heliocentric orbit. It is not something to see with the naked eye; progress is followed through ESA's mission pages and the orbit replay.

The orbit replay draws the trajectory from Earth at launch through the Mars flyby of March 2025 to arrival at Didymos in December 2026. With the probe tracked, the path can be seen bending outward past the orbit of Mars.

Didymos itself, 780 m across, is too faint for small telescopes. Large instruments observed DART's aftermath during the 2022–2023 close approach, and at Hera's arrival the asteroid is about 1.9 AU from Earth.

See this for yourself — replay the moment in the orbit view

Background

Hera descends from ESA's early-2010s AIM (Asteroid Impact Mission). When funding fell through in 2016 DART flew alone, and in 2019 ESA's ministerial council approved a slimmed-down successor named Hera — the goddess of marriage, the partner that keeps the results of DART's "dart".

DART's impact on 26 September 2022 was watched by ground telescopes and the LICIACube CubeSat; the orbital period fell from 11 h 55 min to 11 h 23 min. The 32-minute change, far beyond the 73-second minimum expected, is attributed to the extra push from ejected debris.

Beyond Hera, the planetary-defence roadmap includes ESA's Ramses and NASA's OSIRIS-APEX at the 2029 Apophis approach. The β value Hera measures becomes the reference for those missions and for any real threat calculation.

Hera carries two cubesats, Milani and Juventas. Milani studies the minerals of Dimorphos’s surface and the impact dust with a spectral camera; Juventas probes the asteroid’s interior for the first time with low-frequency radar and is to attempt a landing on Dimorphos at the end. The main spacecraft orbits the Didymos system for six months measuring the size of DART’s crater and the mass of Dimorphos. The three names come from the Greek goddess Hera, the Italian astronomer Andrea Milani and the Roman goddess of youth.

FAQ

When does Hera reach the asteroid?

Scheduled for December 2026, after a Mars flyby in March 2025.

How much did DART change the orbit?

It shortened Dimorphos's orbital period by 32 minutes, from 11 h 55 min to 11 h 23 min.

What is the key quantity Hera measures?

Crater size and the mass and internal structure of Dimorphos, from which the momentum-transfer efficiency β is computed.

Why was the launch a close call?

Hurricane Milton was approaching Florida, and the launch used a brief weather window just before the site closed.

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Times and figures are approximate — verify with official sources when planning travel. Simulation uses J2000 mean elements.