Hera reaches the asteroid humanity deflected on 4 November

On 4 November, if the trajectory holds, Hera will reach the Didymos system after travelling 450 million kilometres. This is no ordinary arrival: the small object it will study, Dimorphos, is the only body in the Solar System whose orbit has been measurably shifted by human action. The European mission is not repeating the strike. It is trying to understand exactly what happened in 2022 so that the next attempt does not have to improvise.

What happened

On 26 September 2022, NASA's DART spacecraft, weighing about 580 kg, slammed into Dimorphos at roughly 22,000 km/h. The impact was no accident: it was the first test of kinetic-impact planetary defence. Dimorphos posed no threat to Earth. The aim was to find out whether a collision could change its path. The answer was yes: the impact shortened by 33 minutes the roughly 12 hours the asteroid takes to complete one orbit of Didymos, according to figures reported by La Vanguardia and ESA.

Hera lifted off on 7 October 2024 from Cape Canaveral, Florida, on a Falcon 9. On 12 March 2025 it flew past Mars to use the planet's gravity assist and save fuel. Since then it has been travelling towards the asteroid pair. On 18 October 2026, at a distance of 1,280 km, its cameras resolved Didymos and Dimorphos as two separate objects for the first time, according to ESA.

Before that image, the spacecraft had braked. Two thruster firings a week apart slowed it to 20 metres per second relative to the asteroids, allowing it to edge closer in a controlled way. It is now about 197 million km from Earth; on the final approach it must fine-tune its position alongside Dimorphos.

Questions remain. Did the impact leave a crater or reshape the whole asteroid? What are its exact mass, internal structure and density? Why was the deflection larger than simulations predicted? Hera carries two identical cameras, a visible-and-infrared instrument, a Japanese-built thermal sensor and a Portuguese laser altimeter. It will also derive the asteroid's mass from the tiny variations its gravity induces in the spacecraft's radio signal.

The map: the footprint of impacts

The map brings together the 28 largest confirmed impact structures on Earth, with their diameter and age, based on the Earth Impact Database. It is not a risk map or a forecast: it is the geological record of what past collisions have left behind.

The largest is Vredefort in South Africa, 160 km across and about 2,023 million years old. Next comes Chicxulub in Mexico, 150 km wide and 66.05 million years old, linked to the extinction of the dinosaurs. Sudbury in Canada measures 130 km; Popigai in Russia and Manicouagan in Canada are around 100 km. These are scars from objects far larger than Dimorphos, which is 151 metres across.

That difference in scale is the point. The map says nothing about the probability of another asteroid strike, nor about the risk from Dimorphos, which poses no threat. What it shows is why planetary defence exists: Earth still bears the marks of ancient impacts, and the way to avoid a future one is not to destroy the object but to detect it years in advance and nudge it just enough. DART proved the method works; Hera must explain why it worked so well.

What comes next

Arrival is scheduled for 4 November. ESA expects to publish the first detailed images of Dimorphos on 25 November, when the spacecraft is closer. The science phase will then begin: mapping the surface, measuring the gravitational field and analysing the composition.

Between mid-January and early February 2027, Hera will release two CubeSats. Juventas carries a radar to probe Dimorphos's interior and establish whether it is a compact body or a loosely bound pile of debris. Milani will study the mineral composition and the dust particles around the asteroid. At the end of their operations, both will attempt to land on it.

ESA's nominal plan ends on 2 June 2027. The agency has not yet decided how it will close the mission: move the spacecraft away from the system or attempt a landing on Didymos. In no scenario will Hera return to Earth.