Martian Moons eXploration (MMX) is a mission of the Japanese space agency JAXA, with contributions from NASA, ESA, the French space agency CNES, and the German Aerospace Center (DLR). As the third Japanese sample-return mission, it aims to follow the successful tradition of the asteroid missions Hayabusa and Hayabusa2. The launch of MMX is planned for September 2024 using an H-3 rocket from the Tanegashima Space Center in Japan. The probe is expected to reach Mars orbit in August 2025. Phobos and Deimos will be observed, and the MMX rover will be deployed on Phobos to collect surface samples. These samples will be returned to Earth in 2029. The overarching goal of the MMX mission is to clarify the origin of the Martian moons. It is believed that a certain percentage of the surface material on Phobos originates from Mars (ejecta from impacts that later settled on the moon). Therefore, MMX is also a Mars sample-return mission.
Mars, Phobos, Deimos. Source: NASA
The MMX probe consists of several modules:
The MMX spacecraft and its three main modules. Image: JAXA.
A Rover from Europe
The approximately 25-kilogram rover of the MMX mission is a joint project of CNES and DLR. Its task is to investigate the physical and mineralogical properties of surface materials on-site and demonstrate mobility technology in low gravity. The measurements on the surface of Phobos also serve as ground reference for the orbiter's instruments and help prepare for the landing of the exploration module.
During the first test descent to the surface of Phobos (rehearsal), the rover will detach from the exploration module at a height of about 50 meters and then descend slowly to the surface. After multiple bounces – the exact scenario depending on the currently unknown ground properties – the rover will come to rest in a random orientation and autonomously raise itself using the "legs" of the propulsion system. Once it is standing on its wheels, the solar panels can be deployed, and the rover will be operational.
Operation on the surface of Phobos mainly involves phases of battery charging, driving and taking measurements with the scientific instruments, and transmitting the data. Telemetry (data and commands) needs to be sent and received through the mother spacecraft since direct contact with Earth is not possible. The operation of the rover is conducted through the control centers of CNES in Toulouse and at our MUSC (Microgravity User Support Center) of DLR in Cologne.
A particular challenge for the rover is driving in low gravity. The gravitational pull of Phobos is about two thousand times weaker than that of Earth. Therefore, the movement must be slow and cautious to avoid "taking off." The currently planned driving speed is less than one centimeter per second. This speed will be adjusted according to the surface conditions once more is known about their firmness and roughness.
Illustration of the MMX rover. Image: CNES, DLR.
Exploring Phobos in Depth
The rover carries four scientific instruments: two navigation cameras, wheel cameras (WheelCAMs), a radiometer (miniRAD), and a Raman spectrometer (RAX).
The MMX rover has a total of four camera heads. The two navigation cameras look forward and provide a 3D representation of the terrain ahead of the rover. This is important for obstacle detection and route planning, and the high-resolution images also have significant scientific value. The other two cameras are located underneath the rover and look at the contact areas of the two left wheels. These cameras allow for observations of track depth, profile impressions, and the movement of regolith during driving, providing insights into the surface characteristics. The wheel cameras can capture short video sequences to analyze the rover's driving behavior in detail. By illuminating with different-colored LEDs, color images can be obtained at night.
The miniRAD radiometer from the DLR Institute of Planetary Research is designed to detect surface radiation in the thermal infrared wavelength range across six wavelength bands. Its main objective is to determine the surface temperature, which is strongly influenced by both solar radiation conditions and the thermal properties of the materials. Particularly, the thermal conductivity of the materials determines how quickly and significantly the surface temperature changes throughout a Phobos day. By measuring the radiation emitted, conclusions can be drawn about the material properties. The rover can target various locations, allowing for separate determination of the thermal properties of loose material (regolith) and rocks. Since the thermal properties are closely related to the mechanical properties, the prevailing grain size of the regolith can be determined, providing insights into its formation and deposition. The thermal properties also provide indications of porosity, allowing for information on mechanical strength. This way, a direct comparison with the analysis of asteroid and meteorite samples becomes possible.
In addition to measuring surface temperature, miniRAD can characterize the optical properties of the surface. Specifically, it aims to determine the emissivity of the materials, their radiation efficiency, in three spectral bands. These data complement the information collected by the Raman spectrometer and enable an initial mineralogical characterization.
The RAX (RAman spectroscopy for MMX) Raman spectrometer is a collaborative development of the DLR Institute of Optical Sensor Systems, JAXA under the leadership of the University of Tokyo, and the Spanish space agency INTA. RAX will characterize the mineral composition of the Phobos surface. Mineralogy is a key discipline for investigating the processes that celestial bodies may have undergone. The occurrence and abundance of different minerals are examined as they provide insights into the geochemical, thermal, or radiation processes that led to their formation. The Raman measurements on the Phobos surface can be compared with measurements on Mars to test various formation hypotheses. The samples brought back to Earth will be analyzed in laboratories to complement the RAX measurements. Furthermore, comparing both sets of data will determine the representativeness of the returned samples. With this data, the MMX mission will significantly enhance our understanding of Mars and its moons. Ultimately, we hope to uncover how Mars likely acquired its moons
The LCC (Lander Control Center) at DLR-MUSC in Cologne during the Hayabusa2/MASCOT mission.