Hera asteroid mission – close flyby of Mars and its moon Deimos
March 13, 2025
Hera asteroid mission – close flyby of Mars and its moon Deimos
Animation - the Martian moon Deimos crosses the surface of Mars
Martian moon Deimos seen crossing the face of Mars in this sequence of Asteroid Framing Camera images acquired during the Hera mission's 12 March 2025 gravity-assist flyby of Mars. The car-sized Hera spacecraft was about 1000 kilometres away from Hera as these images were acquired. Deimos orbits approximately 23,500 kilometres from the surface of Mars and is tidally locked, so that this side of the moon is rarely seen.
Monochromatic image of Deimos passing in front of Mars, acquired by the Asteroid Framing Camera (AFC)
Martian moon Deimos appears dark, framed by the brighter planet Mars behind it, in this visible light monochromatic Asteroid Framing Camera image acquired by ESA’s Hera spacecraft during its gravity-assist flyby on 12 March 2025. The car-sized planetary defence spacecraft was approximately 1000 kilometres from the 12.4-kilometre-diameter Deimos moon when this hyperspectral image was acquired. Deimos orbits at approximately 23,500 kilometres from the surface of Mars and is tidally locked, so that this side of the dark moon is rarely seen. At the top of the image is the bright Terra Sabaea region, close to the Martian equator, outlined by darker regions around it, with part of the 450 kilometre-diameter Huygen crater seen to the right side of the image. To the bottom right corner is part of Hellas Basin, among the largest known impact craters in the Solar System with a diameter of 2300 kilometres and a depth of more than seven kilometres.
It is assumed that around 15 percent of asteroids are binary (or triple) asteroid systems. Many of the smaller bodies are so-called ‘rubble pile asteroids’. They originated from asteroids that were completely destroyed by a collision and whose fragments then came together again in a loose gravitational state. Due to the relatively low mass and gravity of the bodies involved, the smaller asteroids orbit their ‘parents’ at a comparatively low speed of less than one metre per second. This opens up the possibility of measurably shifting the orbit of one of these asteroid moons – something that would not be possible with such precision for a single asteroid in a much faster-moving solar orbit. Hera will use this opportunity to find out whether asteroids posing a threat to Earth can be deflected.
On 12 March 2025, the Hera spacecraft used Mars's gravity to accelerate its journey to the Didymos/Dimorphos binary asteroid system.
The probe's camera system, developed and built in Germany, captured new images of the Martian moon Deimos.
The flyby serves as excellent preparation for the Martian Moons eXploration (MMX) mission, scheduled to launch in 2026.
Focus: Space, Mars and its moons, asteroid defence, exploration
On 12 March 2025, the European Space Agency (ESA) Hera space probe conducted a flyby of Mars and its moons, Phobos and Deimos. This event marks a significant milestone on Hera's journey to the Didymos asteroid system, where researchers aim to gain critical insights for developing reliable asteroid defence techniques. During the flyby, Hera used Mars's gravity to optimise its trajectory. Its closest approach to Mars occurred at a distance of approximately 5000 kilometres, but the probe passed even closer to Deimos, coming within approximately 1000 kilometres and allowing it to capture detailed images of both the Red Planet and its mysterious smaller moon from a new perspective. The Asteroid Framing Camera (AFC) system, built by Jena-Optronik and scientifically supervised by the German Aerospace Center (Deutschen Zentrum für Luft- und Raumfahrt; DLR), was used for imaging. The newly gathered data will contribute to the scientific study of Deimos, while also allowing calibration of Hera's instruments ahead of its primary mission in the Didymos system.
The German Space Agency at DLR is coordinating Germany's contributions to the Hera mission with funding from the Federal Ministry for Economic Affairs and Climate Action (BMWK). The DLR Institute of Planetary Research's Jean-Baptiste Vincent is the Principal Investigator (PI) for the AFC system, and the institute is also co-leading the Hera data analysis working group. The AFC system is designed with built-in redundancy, consisting of two identical cameras aboard the spacecraft. Developed by Jena-Optronik, the cameras are operated by ESA in close collaboration with DLR. The AFC captures monochromatic (black and white) images in the visible spectrum, serving both scientific research and spacecraft navigation purposes.
DLR planetary researchers are responsible for planning scientific observations, calibrating instruments in flight, generating data products such as topographical models and asteroid maps, and conducting scientific analyses.
During the flyby, Hera deployed two additional camera systems alongside the asteroid-framing cameras:
The Hyperscout-H near-infrared spectrometer, which captures data across 25 visible and near-infrared spectral bands to help characterise mineral composition.
The Thermal Infrared Imager (TIRI), provided by the Japanese space agency JAXA, which images at mid-infrared wavelengths to measure surface temperature and identify physical properties such as roughness, particle size distribution and porosity.
Background information: Swing-by – course correction using planetary momentum
A swing-by, or gravity assist, is a spaceflight manoeuvre in which a spacecraft uses the gravity of a planet or moon to alter its speed and trajectory, accelerating and changing direction due to the gravitational pull of the celestial body. This technique saves fuel, enabling efficient travel to distant locations within the Solar System. Hera is using the Mars swing-by to optimise its journey to the Didymos asteroid system.
How did the Martian moons form? Searching for answers with the MMX mission
Orbiting approximately 23,400 kilometres above the Martian surface, Deimos is the smaller and more distant of Mars's two moons. This irregular, 'lumpy’ body measures roughly 12 kilometres across and has a particularly dark surface.
The origin of the Martian moons remains scientifically uncertain. Spectral data from their surfaces resemble those of asteroids, supporting the theory that they are captured asteroid bodies. However, other characteristics – such as their nearly circular orbits, which closely align with Mars's equator – suggest formation from debris following a massive asteroid impact. In terms of orbital dynamics, asteroid capture scenarios are difficult to explain convincingly, making the impact theory the currently favoured hypothesis.
The Japan Aerospace Exploration Agency's (JAXA) Martian Moons eXploration (MMX) mission aims to clarify how Mars's moons, Phobos and Deimos, formed and provide insights into planetary formation processes within the Solar System. DLR, in collaboration with the French space agency CNES, is making a significant contribution by providing the 25-kilogram IDEFIX rover. MMX is scheduled to depart for the Martian system in 2026, with the goal of landing on Phobos for the first time in 2029, collecting surface samples and returning them to Earth in 2031.
On the DLR side, the Institute of Robotics and Mechatronics leads the project, working alongside the institutes of Space Systems, Lightweight Systems, System Dynamics and Control, Optical Sensor Systems, Planetary Research, and the Microgravity User Support Center (MUSC).
The rover will examine the geological, physical and mineralogical properties of Phobos's surface, testing its mobility in low-gravity conditions. Its data will help calibrate orbiter measurements and assist in preparing for the landing of the exploration module. Communication with Earth will be managed via the orbiter, with rover operations controlled jointly from the CNES control centre in Toulouse and DLR MUSC in Cologne. The rover is equipped with the NavCAM stereo camera, miniRAD radiometer, RAX Raman spectrometer, and two WheelCAM cameras, which will monitor interactions between the rover’s wheels and the surface.
Hera mission – a joint European project featuring German technology
Hera is Europe's contribution to the Asteroid Impact and Deflection Assessment (AIDA) project, a joint endeavour between the European Space Agency (ESA) and NASA. The project aims to demonstrate that an asteroid's orbit can be altered using a kinetic impactor. In July 2021, NASA launched the DART mission, which successfully impacted Dimorphos − the smaller body of the binary asteroid system − on 26 September 2022. To complete the experiment, ESA launched Hera in October 2024. The probe is scheduled to arrive at the Didymos-Dimorphos system in 2026, where it will spend at least six months comprehensively characterising the binary asteroid.
Germany is the largest contributor to the Hera mission, providing 37.5 percent of the funding. The spacecraft was developed and built by OHB in Bremen. A newly developed antenna, made of carbon-fibre-reinforced polymer, was provided by HPS, while the two asteroid-framing cameras were supplied by Jena-Optronik GmbH. The Technische Universität Dresden is heavily involved in the development of the radar experiment on the Juventas CubeSat. German researchers are also part of the Hera Science Team, working on the scientific evaluation of mission data. The German Space Agency at DLR coordinates these contributions with funding from the Federal Ministry for Economic Affairs and Climate Action (BMWK).
