System modeling and simulation of TianWen-2 sampling on an asteroid regolith surface

The China National Space Administration plans to launch the asteroid sample-return mission TianWen-2 with the target being the near-Earth asteroid 2016 HO3. The TianWen-2 spacecraft mainly focuses on surface sampling on the near-Earth asteroid 2016 HO3. the whole TianWen-2 spacecraft will be controlled to achieve touchdown and sampling rather than releasing a small lander or ejecting a projectile. Small celestial bodies usually have an extremely low surface gravity and are covered with granular material a centimeter in size or smaller in the form of regolith. The dispersion, friction, and nonlinear dissipation of inelastic collision between particles make a regolith surface behave like a solid or a fluid at the macroscopic level, exhibiting phenomena such as deformation, flow, and ejection. Thus, sampling on a low-gravity regolith-covered surface with unknown physical properties is particularly challenging and risky. In order to successfully implement an in situ exploration, it is of great significance to analyze the influence of regolith soil on the sampling process of the spacecraft on the surface of the asteroid and comprehensively understand and evaluate spacecraft dynamics and control behavior during sampling. In a research article recently published in Space: Science & Technology, researchers from China Academy of Space Technology studied the interaction between spacecraft and a regolith surface based on granular dynamics, multibody dynamics, and the control coupling method, in the context of the touchdown sampling of the TianWen-2 spacecraft on 2016 HO3.

First, granular dynamics modeling and experimental validation for an asteroid regolith surface are presented. The experimental validation is a microgravity low-speed intrusion experiment using a drop tower. The height of the drop tower is 116 m, and a microgravity time of 3.6 s can be obtained from the free-fall experiment. The microgravity level is less than 10⁻² g, and the experiment load can reach 70 kg. The experiment system mainly consists of a vacuum chamber, a granular bed, a loading device, a control unit, an image measurement system, and a force measurement system (Fig. 1). By measuring the variations of the loading force with respect to the penetration depth, the bearing strength of the granular bed can be obtained. The granular dynamics modeling is performed with the discrete element method (DEM). The geometric parameters in the modeling and simulation are basically consistent with the experimental settings. The setup procedures of the target granular bed contain three steps. (1) The static friction coefficient between particles is set to zero and the gravity to 9.8 m/s². A rectangular box is filled with spherical particles which are allowed to fall freely under gravity and deposit. (2) When the particles settle down and stabilize under gravitational acceleration (the maximum velocity of the particles is no more than 10⁻⁴ m/s), the static friction coefficient is reset to the target value. (3) The gravitational acceleration is lowered gradually and simulated step by step to obtain a stable granular bed under the same microgravity level as the experiment, i.e., 0.02 g. In this way, we build a 400 mm × 300 mm × 200 mm granular bed with 909909 normal size-distributed particles (mean radius of 1.5 mm, standard deviation of 0.07) obtained by statistical results. Bearing strength from DEM simulation and the experimental results at 3 different impact velocities are compared in Fig. 2. It can be seen that the simulations agree with experimental results well, although there are some differences in particle properties. Therefore, a static friction coefficient of 1 and a rolling friction coefficient of 0.15 are adopted as the parameter setting basis for the surface mechanics model of 2016 HO3 in the system-level simulation.

Then, multibody dynamics modeling is presented. The multibody structure system of the spacecraft is shown is Fig. 3. The flexible solar arrays are modeled by the finite element method and connected to the mainbody by fixed joints. The flexible solar array is usually considered a linear structure with a small deformation. According to the Craig–Bampton substructure analysis method, its high-dimensional finite element model can be transformed into a low-dimensional hybrid coordinate form with less computation time. The sampler is divided into a spring–mass–damper representation and a sampler tip. The spring–mass–damper is connected to the mainbody and sampler tip respectively by a bushing element, which describes the joint stiffness. The model coefficients of the sampler can be measured by a hardware test. The kinematic equation of the system can be established according to the topological structure and connection relationship, and the quasi-Lagrangian equation is used to describe the motion of the base.

Finally, simulation is presented and results are discussed. Fig. 4 shows an overview of the sampling process. The spacecraft always starts its descent from a 30- to 40-m height; and the position and attitude are controlled to land at a final target point using the reaction control system. In the sampling phase, the sampler maintains contact with the surface of the asteroid for sample collection. After sampling, the spacecraft is controlled to ascend and finish the sampling mission. The rotation period of the asteroid is estimated to be 0.5 h, and the local gravitational level is about μ = 0.115 m³/s². The landing site is chosen near the equator. The asteroid surface mechanical model is built using DEM. The properties of the granular medium in DEM simulation refer to the drop-tower experiment correction results. A gravity of 4.6 × 10⁻⁵ m/s² is assumed for 2016 HO3 and considered in the DEM simulation. The system-level cosimulation model for sampling on asteroid regolith surface is shown in Fig. 5. According to the characteristics of different time scales of the models, the integration time step of each model is set separately, i.e., variable step size of the granular dynamics model (determined by the Rayleigh time step, generally less than 10⁻⁴ s), 0.001-s fixed step of the multibody dynamics model, and 0.01-s fixed step of the GNC model. Results are shown in Fig. 6. When the sampler tip is in contact with the particles, the contact force experiences a sharp impulse of 76.8 N and gradually decays to zero after 6 s. The penetration depth of the sampler tip is 10.4 cm. The spring is compressed to a maximum 12.5 cm and also has a residual compression of 7.6 cm after the sampler tip separates from the particles. During the contact, the kinetic energy that the spacecraft had before touchdown is mainly transferred to elastic energy of the spring and kinetic energy of the particle flow and is partially dissipated by friction and damping. It can also be found that reasonable distribution and storage of impact energy is of great significance, which has an important impact on the sinking depth, sample collection time, and other critical indicators.