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The Japanese MMX sample return mission to Phobos by JAXA will carry a rover developed by CNES and DLR that will be deployed on Phobos to perform in situ analysis of the Martian moon’s surface properties. Past images of the surface of Phobos show that it is covered by a layer of regolith. However, the mechanical and compositional properties of this regolith are poorly constrained. In particular, from current remote images, very little is known regarding the particle sizes, their chemical composition, the packing density of the regolith as well as other parameters such as friction and cohesion that influence surface dynamics. Understanding the properties and dynamics of the regolith in the low-gravity environment of Phobos is important to trace back its history and surface evolution. Moreover, this information is also important to support the interpretation of data obtained by instruments onboard the main MMX spacecraft, and to minimize the risks involved in the spacecraft sampling operations. The instruments onboard the Rover are a Raman spectrometer (RAX), an infrared radiometer (miniRad), two forward-looking cameras for navigation and science purposes (NavCams), and two cameras observing the interactions of regolith and the rover wheels (WheelCams). The Rover will be deployed before the MMX spacecraft samples Phobos’ surface and will be the first rover to drive on the surface of a Martian moon and in a very low gravity environment.Graphic Abstract

Planetary defense efforts rely on estimates of the mechanical properties of asteroids, which are difficult to constrain accurately from Earth. The mechanical properties of asteroid material are also important in the interpretation of the Double Asteroid Redirection Test (DART) impact. Here we perform a detailed morphological analysis of the surface boulders on Dimorphos using images, the primary data set available from the DART mission. We estimate the bulk angle of internal friction of the boulders to be 32.7 ± 2. 5° from our measurements of the roundness of the 34 best-resolved boulders ranging in size from 1.67–6.64 m. The elongated nature of the boulders around the DART impact site implies that they were likely formed through impact processing. Finally, we find striking similarities in the morphology of the boulders on Dimorphos with those on other rubble pile asteroids (Itokawa, Ryugu and Bennu). This leads to very similar internal friction angles across the four bodies and suggests that a common formation mechanism has shaped the boulders. Our results provide key inputs for understanding the DART impact and for improving our knowledge about the physical properties, the formation and the evolution of both near-Earth rubble-pile and binary asteroids. Planetary Defense efforts rely on estimates of asteroids’ mechanical properties, which are difficult to obtain accurately from Earth. Here, authors compare images from space missions to the rubble-pile asteroids Dimorphos, Itokawa, Ryugu, and Bennu and study such properties through boulders on their surface.

We report on the development of a passive sorption pump, capable of maintaining high-vacuum conditions in the InSight seismometer throughout the duration of any extended mission. The adsorber material is a novel zeolite-loaded aerogel (ZLA) composite, which consists of fine zeolite particles homogeneously dispersed throughout a porous silica network. The outgassing species within the SEIS evacuated container were analyzed and the outgassing rate was estimated by different methods. The results were used to optimize the ZLA composition to adsorb the outgassing constituents, dominated by water, while minimizing the SEIS bakeout constraints. The InSight ZLA composite additionally facilitated substantial CO 2 adsorption capabilities for risk mitigation against external leaks in Mars atmosphere. To comply with the stringent particle requirements, the ZLA getters were packaged in sealed containers, open to the SEIS interior through 1 μ m -size pore filters. Results from experimental validation and verification tests of the packaged getters are presented. The pressure forecast based on these data, corroborated by rudimentary in situ pressure measurements, infer SEIS operational pressures not exceeding 10 − 5 mbar throughout the mission.

IDEFIX, the Martian Moons eXploration (MMX) mission Phobos rover, will be the first of its kind to attempt wheeled-locomotion on a low-gravity surface. The IDEFIX WheelCams, two cameras placed on the underside of the rover looking at the rover wheels, provide a unique opportunity to study the surface properties of Phobos, regolith behaviour on small-bodies and rover mobility in low-gravity. The information gained about Phobos’ surface will be of high importance to the landing and sampling operations of the main MMX spacecraft, in addition to being valuable for understanding the surface processes and geological history of Phobos. Here we introduce the WheelCam science objectives, the instrument and the characterisation activities. We also discuss the on-going preparations linked to the analysis and interpretation of the WheelCam images on the surface of Phobos.

Multiple metal alloys, that is, Ti‐6Al‐4 V, 316 L stainless steel, MS1 maraging steel, A2 tool steel, Inconel 625 with TiC and TiB2 reinforcement, and AMZ4 bulk metallic glass, were additively manufactured through laser powder bed fusion and tested as potential excavating tools for future robotic spacecraft landing on icy planetary bodies. Mechanical specific energy as a function of blade hardness was measured for each excavating tool as it trenched through soft and hard salt, where the salt is a regolith simulant for extraterrestrial ice. A2 tool steel, MS1 maraging steel, and bulk metallic glass cutting tools were shown to perform well in the experiments. A method for using the cutting tool as a sensor was also demonstrated. Multiple metal alloys, additively manufactured through laser powder bed fusion, were tested as potential excavating tools for future robotic spacecraft landing on icy planetary bodies. Mechanical specific energy as a function of blade hardness was measured for each excavating tool as it trenched through soft and hard salt, where the salt is a regolith simulant for extraterrestrial ice.Tool steel, maraging steel, and bulk metallic glass cutting tools were shown to perform well in the experiments and a method for using the cutting tool as a sensor was also demonstrated.

The Discrete Element Method (DEM) is frequently used to model complex granular systems and to augment the knowledge that we obtain through theory, experimentation, and real-world observations. Numerical simulations are a particularly powerful tool for studying the regolith-covered surfaces of asteroids, comets, and small moons, where reduced-gravity environments produce ill-defined flow behaviors. In this work, we present a method for validating soft-sphere DEM codes for both terrestrial and small-body granular environments. The open-source code Chrono is modified and evaluated first with a series of simple two-body-collision tests, and then, with a set of piling and tumbler tests. In the piling tests, we vary the coefficient of rolling friction to calibrate the simulations against experiments with 1 mm glass beads. Then, we use the friction coefficient to model the flow of 1 mm glass beads in a rotating drum, using a drum configuration from a previous experimental study. We measure the dynamic angle of repose, the flowing layer thickness, and the flowing layer velocity for tests with different particle sizes, contact force models, coefficients of rolling friction, cohesion levels, drum rotation speeds and gravity levels. The tests show that the same flow patterns can be observed at Earth and reduced-gravity levels if the drum rotation speed and the gravity-level are set according to the dimensionless parameter known as the Froude number. Chrono is successfully validated against known flow behaviors at different gravity and cohesion levels, and will be used to study small-body regolith dynamics in future works.

In order to improve our understanding of landing on small bodies and of asteroid evolution, we use our novel drop tower facility to perform low-velocity (2-40 cm s^-1), shallow impact experiments of a 10 cm diameter aluminum sphere into quartz sand in low effective gravities (~0.2-1 m s^-2). Using in situ accelerometers, we measure the acceleration profile during the impacts and determine the peak accelerations, collision durations and maximum penetration depth. We find that the penetration depth scales linearly with the collision velocity but is independent of the effective gravity for the experimental range tested, and that the collision duration is independent of both the effective gravity and the collision velocity. No rebounds are observed in any of the experiments. Our low-gravity experimental results indicate that the transition from the quasi-static regime to the inertial regime occurs for impact energies two orders of magnitude smaller than in similar impact experiments under terrestrial gravity. The lower energy regime change may be due to the increased hydrodynamic drag of the surface material in our experiments, but may also support the notion that the quasi-static regime reduces as the effective gravity becomes lower.

This work presents an experimental design for studying low-velocity collisions into granular surfaces in low-gravity. In the experiment apparatus, reduced-gravity is simulated by releasing a free-falling projectile into a surface container with a downward acceleration less than that of Earth's gravity. The acceleration of the surface is controlled through the use of an Atwood machine, or a system of pulleys and counterweights. The starting height of the surface container and the initial separation distance between the projectile and surface are variable and chosen to accommodate collision velocities up to 20 cm/s and effective accelerations of ~0.1 - 1.0 m/s^2. Accelerometers, placed on the surface container and inside the projectile, provide acceleration data, while high-speed cameras capture the collision and act as secondary data sources. The experiment is built into an existing 5.5 m drop-tower frame and requires the custom design of all components, including the projectile, surface sample container, release mechanism and deceleration system. Data from calibration tests verify the efficiency of the experiment's deceleration system and provide a quantitative understanding of the performance of the Atwood system.