CNES projects library

February 14, 2024


The Hera mission is set to deliver new insights into the formation and evolution of asteroids by studying the geophysical characteristics of the Didymos-Dimorphos binary system. Hera will investigate the moonlet Dimorphos up close to observe the effects of the impact of the U.S. DART mission. The mission also aims to study possible planetary defence techniques.

Would we able to protect our planet against the threat of a near-Earth asteroid impact? In 2013, the international AIDA (Asteroid Impact and Deflection Assessment) programme set out to test a planetary defence technique that would deflect a celestial object from its course. On 26 September 2022, the U.S. DART impactor mission slammed into Dimorphos, the moonlet in the Didymos binary asteroid system, to change its orbital period and assess the feasibility of deflecting such a body. ESA’s Hera mission now aims to observe the effects of this impact in situ and study the binary asteroid’s geophysical characteristics in order to learn more about the formation and evolution of asteroids, and to investigate planetary defence techniques. The main Hera satellite will be controlled from the European Satellite Operations Centre (ESOC), and the Milani and Juventas cubesats from CNES.


  • 1,150 kg mass of Hera
  • cubesats
  • 6 instruments on cubesats
  • 7 French research labs contributing



In July 1994, fragments from comet Shoemaker-Levy 9 smashed into Jupiter, generating a spectacular fireball show and raising the spectre of a threat that Earth had forgotten. While the risk of an Earth-crossing asteroid hitting our planet remains statistically low—less than one in every 500,000 years for a near-Earth asteroid (NEA) more than 1,000 metres across—the potentially catastrophic effects should encourage us to envision a strategy to counter the threat.

Through the international AIDA (Asteroid Impact and Deflection Assessment) initiative, for which one of the two coordinators is from France, space agencies are seeking to develop a planetary defence technique for a scenario in which an NEA might impact Earth. The method chosen as the least costly and most mature is the kinetic impact technique that consists in sending a spacecraft on a collision course with an asteroid to deflect it. To test out this concept, NASA crashed its DART (for Double Asteroid Redirection Test) impactor into Dimorphos, the moonlet in the Didymos binary asteroid system 11 million kilometres from Earth, on 26 September 2022. Hit by the impactor travelling at a speed of six kilometres per second, Dimorphos’s orbital period around its primary asteroid was shortened by 33 minutes (or 2.67 mm/s). The test thus provided experimental proof of the ability to deflect an asteroid’s course. Exactly how effectively it was deflected, which depends on the asteroid’s as yet undetermined mass, remains to be seen. Hera’s mission is to complement this unique experiment with an in-depth investigation of the binary asteroid’s geophysical properties and precise measurements of the effects of DART’s impact to gain valuable knowledge that could be applied to planetary defence.

The Hera satellite is tasked with observing changes brought about by DART’s impact, in particular the amount of momentum imparted to Dimorphos, as this reflects how effective the deflection was. Only part of DART’s energy was imparted to Dimorphos to change its path; the rest sent up a dust plume rising several tens of thousands of kilometres above the asteroid’s surface. Morphological deformations such as the excavation of a crater, cone edges, ejecta or even the asteroid’s shape also have a bearing on how much energy was transferred by the impact. Precisely identifying how DART’s energy was transferred to the asteroid will enable us to ascertain factors likely to improve planetary defence technology.

Hera also has scientific value as the first ever mission dedicated to observing a planned collision and the second rendezvous with a binary asteroid, which will occur just four years after the impact. Any structural changes and morphological deformations will thus still be very “fresh”, and some may indeed be ongoing. Measuring them precisely will deliver new insights into the collision processes behind the evolution of the solar system and its planets. Hera will also conduct the first ever radar survey of an asteroid’s interior with the JuRa instrument, for which the IPAG planetology and astrophysics institute in Grenoble is the French Principal Investigator. Data from this survey will be matched against the theoretical interpretations and models on which our current knowledge is founded.

Navigating autonomously around a low-gravity asteroid is one of the technological challenges facing the Hera satellite. This will be crucial when the spacecraft is far from Earth, as commands might take too long to arrive from the ground control centre and prevent it from achieving the mission’s goals. Further, an inter-satellite link supporting communications between the spacecraft and its two cubesats is a first in interplanetary space. Likewise, a cubesat has never landed on such a small body before. Operations on a body approximately 150 metres across will be performed in very low gravity, thought to be a million times lower than on Earth. In this respect, Hera is also a technology demonstration mission.  


CNES is contributing funding for development of the JuRa low-frequency radar at IPAG in Grenoble, has a team working on cubesat operations planning in Toulouse and is providing support to French scientists involved in the mission.