MMX (Martian Moons eXploration) is a robotic sample return mission of JAXA (Japan Aerospace Exploration Agency), CNES (Centre National d' Études Spatiales), and DLR (German Aerospace Center) with the launch planned for 2024. The mission aims to answer the question of the origin of Phobos and Deimos, which will also help to understand the material transport in the earliest period of our solar system, and of how was water brought to Earth. Besides JAXA's MMX mothership, which is responsible for sampling and sample return to Earth, a small rover which is built by CNES and DLR to land on Phobos for in-situ measurements, similar to MASCOT (Mobile Asteroid Surface Scout) on Ryugu. The MMX rover is a fourwheel driven autonomous system with a size of 41 cm x 37 cm x 30 cm and a weight of approximately 25 kg. Multiple science instruments and cameras are integrated in the rover body. The rover body has the form of a rectangular box. Attached at the sides are four legs with one wheel per leg. When the rover is detached from the mothership, the legs are folded together at the side of the rover body. When the rover has landed passively (no parachute or braking rockets) on Phobos, the legs are autonomously maneuvered to bring the rover in an upright orientation. One Phobos day lasts 7.65 earth hours, which yields about 300 extreme temperature cycles for the total mission time of three earth months. These cycles and the wide span of surface temperature between day and night are the main design drivers for the rover. This paper gives a detailed view on the development of the MMX rover locomotion subsystem

The MMX Rover, developed by CNES and DLR, will fly to and explore the surface of the Martian Moon Phobos within the JAXA Martian Moon Exploration Mission. It will be the first wheeled locomotion system in a milli-g environment. In the development of the rover, simulations have been used to test and develop its robotic activities. This paper presents the multi-physics simulations that are being used. The overall simulator setup and its main components are discussed. To provide appropriate simulations for the various topics while maintaining a unified simulator, a modular approach was required. The different modules and their role will be outlined. For this, Dymola's implementation of the Modelica modeling language provides the basis, especially regarding multi-body dynamics, and the possibility to include external libraries, e. g. for environment interaction, control logic and visualization. Finally, examples for the simulator used in driving, uprighting, alignment and separation will be presented. These examples illustrate the approach on experiment design, setup and result evaluation. To date the MMX Rover simulator is regarded as an indispensable development and analysis tools, especially since representative lab experiments are much limited when designing a robotic system for milli-g operations. It is also planned to be used during operations phase for planning and analysis.

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