"Hera's Final Approach to Dimorphos: Planetary Defence Faces Its First Real Test"

Four years ago, humanity punched an asteroid on purpose. On 26 September 2022, NASA's DART spacecraft slammed into Dimorphos, a 150-metre moonlet orbiting the 780-metre asteroid Didymos, and shortened the little moon's orbit by 32 minutes. It was the first time our species had deliberately changed the motion of a celestial body, and the headlines quite rightly celebrated. But a punch is not an understanding. Nobody has yet seen the crater DART left behind, nobody knows Dimorphos's mass precisely, and nobody knows how much of the orbital change came from the impact itself and how much from the tonnes of debris the collision threw off into space. That is the difference between a stunt and a technique, and it is exactly the gap the European Space Agency's Hera mission is about to close.

Hera launched in October 2024, swung past Mars in March 2025 to steal a little orbital energy, and has spent the last two years chasing a target 140 million kilometres from Earth. This week it completed a major braking burn just 2,300 kilometres from the Didymos system, and a series of smaller transition manoeuvres between 25 and 31 October will bleed off the last of its relative speed. If all goes to plan, the first proximity manoeuvre — arrival, in mission language — happens on 4 November at 01:00 UTC, opening an early characterisation phase that runs to 15 December.

Arrival here does not mean what it usually means. Didymos's gravity is roughly 40,000 times weaker than Earth's, and Dimorphos's about 200,000 times weaker, so Hera cannot simply slip into orbit. Instead it will fly a repeating pattern of hyperbolic arcs, three days out and four days back, passing within 20 to 30 kilometres of Dimorphos at a relative speed of about 12 centimetres per second — slower than a walking pace. Each arc ends with a manoeuvre, and if one fails, the spacecraft simply drifts away rather than crashing. It is flying, in other words, the way you would approach something you are afraid of breaking: slowly, and with an exit always open.

What Hera will do over the following seven months is the part I find most compelling. Its cameras will map the crater DART dug and measure Dimorphos's mass, shape and spin. Two CubeSats will join the work in early 2027: Juventas, which will attempt the first radar sounding of an asteroid's interior, and Milani, which will study dust and surface composition. From February to April, Hera will close to within about a kilometre of Dimorphos's surface. The nominal mission ends on 2 June 2027, and ESA plans a public event in Berlin on 25 November where the first close-up pictures should be shown.

Why does any of this matter to people who will never see an asteroid up close? Because the kinetic impactor is currently the only planetary defence technique ever demonstrated at full scale, and binary systems like Didymos make up roughly 15 percent of known asteroids. If astronomers ever find a real object on a collision course with Earth, the question will not be whether we can hit it — DART answered that — but whether we can predict what the hit will do. The key number is the momentum enhancement factor: how much extra push the ejected debris contributed beyond the spacecraft's own momentum. Get it wrong and a deflection mission could under-deliver, or shove a rubble-pile asteroid apart into several still-dangerous pieces. Hera's measurements will turn that factor from a guess into a calibration, and every future deflection plan will be built on it.

There is a quieter point here too, and it is the one that stays with me. Planetary defence is a field in which live tests come perhaps once a decade, and a failed one cannot be repeated on demand. DART was the easy half of the experiment: go fast, hit hard, watch the orbit change. Hera is the hard half: go slow, look closely, and admit how much we still did not know. The mission's real test is not whether a spacecraft can arrive at a rock — it is whether we are patient enough to learn from a punch we already threw.