For two years, Hera has been coasting through interplanetary space, carried along by orbital mechanics with minimal intervention. That long, quiet cruise phase is now over. The European Space Agency's planetary defence mission is about to do something far more demanding: slow down sharply enough to enter the gravitational neighbourhood of a double asteroid system more than 300 million kilometres from Earth.
A 93-Minute Burn That Cannot Afford to Go Wrong
In the early hours of October 15, the flight dynamics team at ESA's European Space Operations Centre in Darmstadt, Germany, will command Hera's Orbit Control Thrusters to fire continuously for 93 minutes. The burn is oriented toward the target asteroid system and is designed to bleed off enough relative velocity to bring Hera close to the orbital speed of the Didymos binary — a pair consisting of the primary asteroid and its smaller companion, Dimorphos.
ESA describes this as the single most operationally challenging activity since lift-off. Unlike a routine trajectory correction, this is a sustained deceleration burn that demands tight precision in both duration and thruster pointing. The flight dynamics team has spent weeks refining the burn parameters to ensure the approach geometry falls within acceptable margins. Any significant deviation could compromise the entire rendezvous sequence.
Three Spacecraft, One Investigation
Hera will not be working alone once it arrives. Tucked aboard the probe are two CubeSats that will be deployed after orbital insertion: Milani, tasked with spectral and mineralogical surface analysis, and Juventas, which carries a low-frequency radar instrument capable of probing the internal structure of Dimorphos.
The trio forms an integrated reconnaissance system. Their core objective is to characterise the aftermath of the controlled kinetic impact carried out in September 2022 by NASA's DART (Double Asteroid Redirection Test) spacecraft. That mission successfully altered Dimorphos's orbital period — marking the first time humanity had demonstrably changed the motion of a natural body in space. Hera's job is now to quantify what actually happened: the precise mass of the asteroid, the shape and depth of the impact crater, and the mechanical properties of the surface material.
Building the Foundation for Planetary Defence
Didymos and Dimorphos were chosen precisely because they are representative of the broader population of binary near-Earth asteroids that statistically pose a collision risk to our planet. Understanding how such a body responds to a kinetic impulse is not an abstract scientific exercise — it is the groundwork for a credible, evidence-based planetary defence capability.
ESA frames Hera as part of a broader international strategy, complementary to NASA's DART. The datasets that Milani and Juventas gather on surface composition and subsurface structure will feed into computational models that engineers and policymakers could one day rely on when deciding how — and whether — to deflect a genuinely threatening object.
If the October 15 braking manoeuvre proceeds as planned, Hera will begin a gradual approach toward Didymos, eventually settling into close observation orbits. The months ahead will determine how much detail the mission can extract from one of the most consequential asteroid impacts ever deliberately caused.
