Four years ago, NASA deliberately crashed a spacecraft into an asteroid to see if humanity could nudge a space rock off course. It worked. Now Europe is sending a detective to examine the scene. The European Space Agency’s Hera mission is in the middle of the most demanding phase of its journey since launch: slamming on the brakes so it can settle in beside the asteroid pair Didymos and Dimorphos next month.

Here’s what’s happening, and why this small spacecraft matters for everyone on Earth.

A crash-scene investigation, two years in the making

Hera launched from Cape Canaveral Space Force Station in Florida on October 7, 2024, just ahead of Hurricane Milton. Since then it has been coasting through the solar system, with a pair of deep-space engine burns and a spring 2025 flyby of Mars that bent its path toward its targets.

Its destination is a binary asteroid system. Didymos is the larger body, and Dimorphos is the small moon that orbits it. In 2022, NASA’s DART (Double Asteroid Redirection Test) spacecraft struck Dimorphos and shifted its orbit around Didymos. According to ESA, that made Dimorphos the first object in the solar system to be transformed by human action.

DART proved that a high-speed impact can change an asteroid’s motion. Hera’s job is to find out exactly how much it changed, and why, by studying the asteroid up close.

Why Hera has to slow down

Getting to an asteroid is only half the challenge. Stopping there is the other half. ESA says Hera has been traveling at more than 12 kilometers per second relative to Earth, and to study Didymos and Dimorphos it needs to closely match their speed.

That’s where the braking maneuvers come in. ESA’s plan calls for Hera to rotate so that its ring of Orbit Control Thrusters faces the Didymos system, then fire three of those hydrazine thrusters at once:

  • Braking Maneuver 1 (BRM-1): scheduled for Thursday, October 15, a continuous burn of 93 minutes designed to reduce Hera’s speed relative to the asteroids by a few hundred meters per second. Engineers ran a brief test burn on October 2 to prepare.
  • Braking Maneuver 2 (BRM-2): scheduled for the early morning of Thursday, October 22, a shorter burn of roughly 31 minutes that should complete most of the required change in velocity and correct for any shortfall in the first burn.
  • Transition maneuvers: three smaller burns toward the end of October to bring Hera’s speed relative to the asteroids down to nearly zero.

It’s delicate flying. The spacecraft’s smaller Reaction Control Thrusters also have to fire to keep Hera pointed correctly during the long burns. After each one, ESA’s flight dynamics team uses tracking data from deep-space antennas, including ESA’s 35-meter dishes in New Norcia, Australia, to confirm the maneuver did what it was supposed to.

Arriving early

In an October 2025 update, ESA said Hera’s reliable propulsion system allowed the team to design a more aggressive plan that brakes “later and harder” during the approach. The payoff: Hera is on track to arrive at Didymos in November 2026, about a month earlier than originally planned.

What Hera will do when it gets there

Once it arrives, Hera will act like a forensic investigator. It will measure the asteroids’ shapes, masses and surfaces, and study how Dimorphos responded to DART’s impact. Those details matter because the effectiveness of an impact depends heavily on what an asteroid is made of and how it’s put together.

Hera isn’t alone, either. It carries two suitcase-sized CubeSats, Milani and Juventas, which will be released to perform their own close-up studies. ESA notes that the asteroids’ extremely weak gravity makes operating these tiny spacecraft especially tricky; even the gentle push of sunlight can significantly affect their paths. Hera also has an autonomous navigation system to help it keep the asteroids in view while it’s too far from Earth for real-time commands.

Why it matters for planetary defense

No known asteroid poses a threat to Earth in the near future, but space agencies want a proven toolkit ready long before one ever does. DART showed that a kinetic impactor can move an asteroid. Hera will help turn that one-time experiment into a repeatable, predictable technique by filling in the details scientists need to model future deflections accurately.

In other words, this mission isn’t just about one pair of rocks. It’s about learning how to protect the planet with confidence.

The wrap-up

Hera has spent two years racing through space, and this month it is finally easing off the throttle. If the remaining braking burns go to plan, Europe’s planetary defense probe will arrive at Didymos and Dimorphos in November and begin its close-up investigation of the first asteroid humans ever moved. Keep an eye on ESA’s updates; the first close-up images of the DART impact site could be among the most fascinating space pictures of the year.

Image: DART — Earth's first planetary defense mission by jurvetson via Flickr, Public Domain Mark 1.0