An experimental study of low-velocity impacts into granular material in reduced gravity

In order to improve our understanding of landing on small bodies and of asteroid evolution, we use our novel drop tower facility \citep{sunday2016} to perform low-velocity (2 - 40 cm/s), 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

Wheel-regolith interactions on small-body surfaces

We conduct experiments using a single-wheel testbed and simulations using the Soft-Sphere Discrete Element Method to study wheel-regolith interactions on small-body surfaces. We analyze wheel sinkage and traction on different surface materials and we discuss the influence that lowgravity has on rover maneuverability

Low-velocity impacts into granular material: application to small-body landing

With the flourishing number of small body missions that involve surface interactions, understanding the mechanics of spacecraft - surface interactions is crucial for improving our knowledge about the landing phases of space missions, for preparing spacecraft operations, and for interpreting the results of measurements made during the surface interactions. Given their regolith-covered surfaces, the process of landing on a small body can be considered as an impact at low-velocity onto a granular material in reduced-gravity. In order to study the influence of the surface material, projectile shape, and gravity on the collision dynamics we used two experimental configurations (one for terrestrial gravity experiments and one for reduced-gravity experiments) to perform low-velocity collisions into different types of granular materials: quartz sand, and two different sizes of glass beads (1.5 and 5 mm diameter). Both a spherical and a cubic projectile (with varying impact orientation) were used. The experimental data support a drag model for the impact dynamics composed of both a hydrodynamic drag force and quasi-static resistance force. The hydrodynamic and quasi-static contributions are related to the material frictional properties, the projectile geometry, and the gravity. The transition from a quasi-static to a hydrodynamical regime is shown to occur at lower impact velocities in reduced-gravity trials than in terrestrial gravity trials, indicating that regolith has a more fluid-like behaviour in low-gravity. The reduced quasi-static regime of a granular material under low-gravity conditions leads to a reduction in the strength, resulting in a decreased resistance to penetration and larger penetration depths

The influence of gravity on granular impacts II. A gravity-scaled collision model for slow interactions

Slow interactions on small body surfaces occur both naturally and through human intervention. The resettling of grains and boulders following a cratering event, as well as observations made during small body missions, can provide clues regarding the material properties and the physical evolution of a surface. In order to analyze such events, it is necessary to understand how gravity influences granular behavior. In this work, we study slow impacts into granular materials for different collision velocities and gravity levels. Our objectives are to develop a model that describes penetration depth in terms of the dimensionless Froude number and to use this model to understand the relationship between collision behavior, collision velocity, and gravity. We use the soft-sphere discrete element method to simulate impacts into glass beads under gravitational accelerations ranging from 9.81 m/s^2 to 0.001 m/s^2. We quantify collision behavior using the peak acceleration, the penetration depth, and the collision duration of the projectile, and we compare the collision behavior for impacts within a Froude number range of 0 to 10. The measured penetration depth and collision duration for low-velocity collisions are comparable when the impact parameters are scaled by the Froude number, and the presented model predicts the collision behavior well within the tested Froude number range. If the impact Froude number is low (0 < Fr < 1.5), the collision occurs in a regime that is dominated by a depth-dependent quasi-static friction force. If the impact Froude number is high enough (1.5 < Fr < 10), the collision enters a second regime that is dominated by inertial drag. The presented collision model can be used to constrain the properties of a granular surface material using the penetration depth measurement from a single impact event. If the projectile size, the collision velocity, the gravity level, and the final penetration depth are known and the material density is estimated, then the internal friction angle of the material can be deduced

A novel facility for reduced-gravity testing: A setup for studying low-velocity collisions into granular surfaces

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 to 1.0 m/s2. 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

Mechanical properties of rubble pile asteroids (Dimorphos, Itokawa, Ryugu, and Bennu) through surface boulder morphological analysis.

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

Regolith science with the cameras on the MMX Rover

The JAXA Martian Moons Exploration (MMX) mission [1] has a primary objective to study the formation and origins of Phobos and Deimos. The MMX spacecraft will also deploy a CNES/DLR rover to the surface of Phobos [2,3]. This rover will be the first of its kind to attempt wheeled-locomotion on a low gravity surface. As such, this rover provides a unique opportunity to study not only the surface properties of Phobos, but also regolith dynamics on small-bodies. This information is valuable for understanding the surface processes and geological history of Phobos in addition to being of high importance to the landing (and sampling) operations of the main MMX spacecraft [1]

Atterrissage, enfoncement et roulage sur la surface de petits corps du système solaire

Si l’observation depuis la Terre a pu révéler le grand nombre et la diversité des petits corps du système solaire, seule leur exploration in-situ permettra de répondre aux questions qu’ils soulèvent aujourd’hui sur leur composition physique et chimique. Cela explique la recrudescence de missions dédiées aux petits corps (Osiris-Rex, Hayabusa-2, Mars Moon Explorer, etc.) ainsi que les efforts déployés par les agences internationales (DLR, NASA, JAXA) pour permettre l’exploration de leur surface. De par leur faible gravité (entre 10-6 et 10-2 g) les astéroïdes (e.g. Eros), les comètes (e.g. Churyumov-Gerasimenko) et les petits satellites (e.g. Phobos) se révèlent des candidats difficiles pour une mobilité de surface. En milligravité (i.e. autour de 10-3g), le poids très faible d’un véhicule ne permet des tractions que 1,000 fois plus faibles que celles dont bénéficient les rovers martiens et lunaires. Surtout, le comportement du régolithe des petits corps est peu compris et donc difficile à prévoir, notamment à cause du rôle majeur joué par les forces de cohésion à ces échelles de gravité. Le comportement du régolithe tient alors davantage à celui d’une poudre cohésive comme la farine qu’à celui d’un gravier fin. Cette thèse explorera donc la faisabilité et la performance attendue d’un véhicule à roue à la surface d’un petit corps. L’étude reposera sur la modélisation du sol par méthodes à éléments discrets (DEM). Cette approche consiste à simuler l’interaction détaillée des grains de régolithe un-à-un (friction, résistance au roulement, cohésion, etc.). A partir d’un code DEM préexistant (e.g. ESyS-Particle) qui sera identifié au commencement de l’étude, la thèse examinera et implémentera les modifications nécessaires à apporter pour modéliser les interactions spécifiques d’une roue avec le régolithe d’un petit corps. L’étape de modélisation franchie, la thèse appliquera l’outil développé à la résolution du problème du roulage : traction et manœuvrabilité (simplifiée) dans différents types de régolithe, à différents niveaux de gravité. Les travaux seront conduits avec l’ISAE-Supaéro : l’encadrante est une experte du régolithe des petits corps, des méthodes DEM ainsi que du développement d’expériences (vols paraboliques, tours de chute) permettant la validation des simulations numériques. Au terme de la thèse, nous aurons répondu aux questions suivantes : quelle est la force de traction que l’on peut attendre d’un régolithe en milligravité ? comment cette traction évolue-t-elle quand la gravité change ? comment la cohésion du régolithe affecte-elle l’écoulement autour de la roue ? Ces réponses nous permettront de déterminer le seuil de gravité minimum, selon le régolithe attendu, en dessous duquel le roulage devient impraticable ou trop inefficace pour être préféré à d’autres solutions (e.g. les « hoppers » Mascot (DLR), Hedgehog (JPL) ou Minerva (JAXA)). Les missions vers les petits corps constituent des opportunités de collaborations inter-agences très intéressantes et abordables en coût et en complexité. La compétence sur la locomotion en milligravité peut se révéler cruciale pour participer en tant que partenaire à ce genre de mission. C’est également un domaine de coopération actif entre le CNES (à DSO/DV/IF) et le DLR (à Oberpfaffenhofen) qui a déjà une certaine expérience dans le domaine avec leurs outils de simulation multi-corps et leurs outils de modélisation DEM. Cette thèse consoliderait l’axe d’étude et cette coopération.Even though observations from the Earth have revealed the great number and diversity of small bodies (asteroids and comets) in the Solar System, only an in-situ exploration of these bodies, accompanied if possible by a sample return, can answer many of the open questions today about their physical and chemical composition. This explains the resurgence of missions dedicated to small bodies (OSIRIS-REx, Hayabusa-2, Mars Moon Explorer, etc.) and the efforts of international space agencies (ESA, NASA, JAXA, DLR, CNES) to explore their surfaces. Asteroids (e.g. Eros), comets (e.g. Churyumov-Gerasimenko), and small satellites (e.g. the Martian moon Phobos) prove to be difficult candidates for surface mobility due to their low gravity (100 to a million times lower than the gravity of the Earth, g). In milli-gravity (i.e. around 10-3 g), the very low weight of a vehicle means that the traction is 1,000 times lower than that of the Martian and lunar rovers. Additionally, the behaviour of the regolith (the layer of more or less fine grains found on the surface of these bodies) of the small bodies is poorly understood and is, therefore, difficult to predict. This is especially true because the properties of the regolith are often unknown before we arrive at the body, and also due to the potentially major role played by cohesive forces at these levels of gravity: the behaviour of the regolith may actually be more like that of a cohesive powder than that of a fine gravel. This thesis will, therefore, explore the feasibility and the expected performance of a wheeled vehicle on the surface of a small body. The study will be based on soil modeling using discrete element methods (DEM). This approach consists in simulating the detailed interaction between individual regolith grains (friction, rolling resistance, cohesion, etc.). From a pre-existing DEM code that will be identified at the beginning of the study, the thesis will examine and implement the necessary modifications to model the specific interactions of a wheel with the regolith of a small body. Once the modeling step has been completed, the thesis will apply the tool developed to solve the problem of rolling: traction and (simplified) maneuverability in different types of regolith, at different levels of gravity. During this thesis, we will tackle the following questions: what is the traction force that one can expect from a regolith in milligravity? How does this traction evolve when gravity changes? How do the characteristics of the regolith flow and the resulting tracks at the surface vary with the regolith properties? How do the cohesion of the regolith and the wheel geometry affect the flow around the wheel? These studies will allow us to determine the minimum gravity threshold, depending on the expected regolith, below which rolling becomes impracticable or too inefficient to be preferred to other solutions such as Mascot hoppers (DLR), Hedgehog (JPL) or Minerva (JAXA)

Using rover wheels to study regolith dynamics

In this study, we present several ways in which a rover wheel can be used as a tool to study regolith dynamics. We demonstrate specific analysis methods by conducting numerical simulations of a simple rover wheel traversing a bed of regolith. The simulation data is used to analyze the general flow behavior of granular material around a wheel in a reduced-gravity environment

Froude scaling for rovers on small body surfaces&#160;

International audienceWe study rover-regolith interactions in low-gravity environments by conducting soft-sphere discrete element method (SSDEM) simulations with a simplified rover wheel and a bed of spherical particles. The simulations reveal that rover performance scales according to the Froude number, or a dimensionless parameter which accounts for the size of the wheel, the rotational velocity of the wheel, and gravity. This relationship provides valuable insight into how to operate rovers and analyze wheel-regolith interactions during future rover missions