ESA’s Hera spacecraft has completed the first of two major braking burns, a 93-minute firing of three thrusters to slow its approach to Didymos and Dimorphos. The European Space Agency confirmed the next day that the maneuver stayed within expected parameters. A second burn is scheduled for October 22; together, the operations should bring Hera nearly to rest alongside two dark rocks only hundreds of meters across.
- First brake completed: The Oct. 15 burn lasted 93 minutes and cut relative velocity by several hundred meters per second.
- Second burn ahead: The Oct. 22 maneuver should last about 31 minutes and supply most of the remaining deceleration.
- Arrival in November: Hera is now set to reach the Didymos system a month earlier than originally planned.
Why Hera Must Slow Down
Hera is traveling at more than 12 kilometers per second relative to Earth. Reaching the asteroids is not simply a matter of pointing at them; Hera must match their velocity to enter controlled proximity. ESA calls the braking campaign “the most difficult activity the mission has undertaken since liftoff.” Until now, the journey mostly involved accelerating; this phase demands precise deceleration.
The plan calls for two large braking burns and three smaller adjustments. The Oct. 15 burn trimmed relative velocity by several hundred meters per second. The Oct. 22 burn, lasting around 31 minutes, will provide most of the remaining braking and correct any deviation from the first. Three minor maneuvers later in the month should bring relative velocity close to zero.
The team carried out a test burn on Oct. 2 to prepare for the first maneuver, according to Francesco Castellini, an ESA flight dynamics engineer. Verification is performed from the New Norcia station in Australia, one of ESA’s three 35-meter deep-space antennas, by measuring distance and the Doppler effect.
From Launch to Asteroid Rendezvous
Hera lifted off on Oct. 7, 2024, from Cape Canaveral as Hurricane Milton approached Florida and the weather forecast offered only a 15% chance of favorable conditions. Since then, the spacecraft has performed two major deep-space maneuvers and a Mars flyby in March 2025. The flyby delivered a push of nearly one kilometer per second and allowed Hera to commission its instruments; it also photographed Deimos, one of the Martian moons.
The February-March operation consumed the most fuel: 123 kilograms of hydrazine across three burns over roughly four weeks, producing a velocity change of 367 meters per second. Arrival is now expected in November, a month earlier than the original plan, because Hera is braking later and harder.
Several weeks of approach will follow. Hera will have to locate two small, dark objects by itself. Sylvain Lodiot, who leads outer solar system mission operations and planetary defense at ESA, said:
“The spacecraft is proving to be flexible and dynamic.”
A Crime-Scene Investigation at Dimorphos
Hera’s destination is the aftermath of a planetary-defense experiment. In September 2022, NASA’s DART spacecraft deliberately slammed into Dimorphos and changed its orbit around Didymos. The orbital period fell from 11 hours and 55 minutes to 11 hours and 23 minutes—32 minutes shorter, as NASA confirmed.
The impact worked, but researchers do not know exactly why it worked so well. Key unknowns include Dimorphos’s actual mass, its internal structure, and how much material was ejected, which amplified the effect. Without those data, DART remains a successful anecdote; with them, asteroid deflection becomes a calculable and repeatable technique. Dimorphos is the first object in the solar system transformed by human action.
Hera will close in by stages: 25 to 30 kilometers in November and December, up to 10 kilometers by the end of the year, and then to about 4 kilometers. Michael Müller, a flight dynamics engineer, explained:
“Hera operates more autonomously, and its autonomy increases the closer it gets to the asteroids.”
The mission also carries two suitcase-sized CubeSats. Juventas will probe Dimorphos’s interior with radar and measure its gravity, while Milani will study the dust. Both will attempt to land on the asteroid and are ESA’s first deep-space CubeSats.
Spain has a substantial role. According to TEDAE, GMV leads a seven-country consortium responsible for guidance, navigation and autonomous control, proximity-operations and mission analysis, and Juventas’s guidance. Thales Alenia Space España developed the communications subsystem, capable of working at 500 million kilometers; Sener supplied the low-gain antenna; and Alter handled engineering, testing and electronic components. Madrid’s European Space Astronomy Centre participates in image planning during gravity assists, and tracking includes the 35-meter Cebreros antenna in Ávila.
Hera rarely produces headlines yet sustains everything else. DART supplied the spectacle; Hera supplies the decisive follow-up by measuring what happened so deflection can be repeated. China and NASA pursue related work. A clean burn is not dramatic, but failure would leave Hera moving at 12 kilometers per second away from its target.