Hera Asteroid Braking Begins With a Long Engine Burn

On the early morning of October 15, 2026, a spacecraft the size of a small car will fire its engines for more than an hour and a half, pulling off one of the most delicate driving maneuvers ever attempted in deep space. The European Space Agency's Hera mission is about to hit the brakes, slowing to meet the binary asteroid pair that NASA's DART spacecraft struck in 2022 in the first test of whether a spacecraft could deflect an asteroid. According to ESA, the Hera Asteroid Braking burn will bring Hera near to matching the orbital velocity of the two asteroids, setting up its arrival next month.

Hera has been cruising for two years since its launch on October 7, 2024, picking up speed from a gravity flyby of Mars in the spring of 2025. It is now moving relative to Earth at about 12 kilometers per second. To slip into orbit around its target instead of sailing past it, the spacecraft must shed a few hundred meters per second of velocity relative to the asteroids, a change that takes patience rather than power. The first braking maneuver does the heavy lifting.

Why slowing down is the hard part

For the first burn, Hera will rotate so that its ring of Orbit Control Thrusters faces the Didymos pair. Three hydrazine-fed thrusters will then fire continuously for 93 minutes, the longest single burn of the mission so far. Smaller steering jets will pulse through the maneuver to keep the spacecraft pointed correctly while the main engines push, as described in ESA's account of the braking maneuver. Engineers rehearsed the sequence with a short test firing on October 2, 2026, before building the full command sequence.

The Hera Asteroid Braking sequence continues about a week later, on October 22, 2026, with a second, shorter burn scheduled to complete most of the velocity adjustment and correct any shortfall from the first. After that, navigation gets steadily harder: the closer Hera gets, the more precisely controllers must know where it is. The work falls to ESA's Flight Dynamics team, a group of more than 50 mathematicians, physicists and aerospace engineers at the European Space Operations Centre in Darmstadt, Germany, with around 10 people per shift guiding the spacecraft. According to ESA flight dynamics engineer Francesco Castellini, the team ran the test firing, then began generating the full command sequence for the big burn.

The crash scene Hera is driving toward

Hera's destination is the aftermath of a deliberate collision. In September 2022, NASA's DART spacecraft slammed into Dimorphos, a moonlet about 170 meters across, to test whether a kinetic impact could change an asteroid's path. The strike shortened the moonlet's orbit around its larger companion, Didymos, by 32 minutes, proving that humanity can nudge a threatening asteroid. But the impact also raised questions no telescope could answer: how deep a crater did DART carve, how much material was thrown off, and whether the moonlet's shape changed. Mars keeps yielding clues about its past: scientists recently found the longest organic molecules ever detected in ancient Martian rocks.

Answering those questions is Hera's job. The mission is ESA's first spacecraft built for planetary defense, and officials have described it as a crash scene investigation at the site of the DART impact. Hera will survey the pair for at least six months, measuring the crater, the mass, the shape and the orbit of the moonlet. Two shoebox-sized CubeSats, named Milani and Juventas, will ride along for closer looks, including a radar probe of the interior, and both are expected to attempt landings on the moonlet once their surveys are done.

The stakes go beyond one crater. If astronomers ever spot a real asteroid on a collision course with Earth, the DART-Hera pair is the only full-scale rehearsal humanity has. DART proved the punch works; Hera measures exactly what the punch did. Without that second half, future deflection plans would rest on guesswork about how much momentum an impact actually transfers.

What could still go wrong

Nothing about the final approach is routine. DART's impact threw a trail of debris thousands of miles into space, and some of that material could still be drifting near the pair, a hazard for a spacecraft flying ever closer. The gravitational environment around two small, irregularly shaped rocks is also poorly understood, and scientists believe the moonlet may even be tumbling after the strike. ESA flight director Ignacio Tanco has acknowledged the uncertainty, saying the team does not know the operating environment very well, and that finding out is the whole point of the mission.

There is also the sheer precision of the task. Hera must thread its way toward two bodies whose gravity and rotation are not fully mapped, using thrusters that must fire for over an hour without a hiccup, while controllers on Earth watch with a communications delay measured in minutes. A burn that falls short can be corrected a week later; a spacecraft that arrives too fast has no second chance to slow down.

If the burns go as planned, Hera will ease into the neighborhood of the asteroid pair in November and begin the close-up survey that deflection researchers have awaited for four years. The spacecraft that spent two years crossing the solar system will finally get to look at the dent humanity made in an asteroid, and to tell Earth exactly what a successful deflection looks like. The same spirit of measuring rather than guessing runs through other Mars work, from Perseverance's self-driving record to proposals for Mars homes brewed from yeast and gelatin.