A new physical model on the capillary phenomenon of granular particles

Similar to the capillary phenomenon of liquid, granular particles can move up to a certain height along a vertically vibrating tube. The certain height, which is called the equilibrium height, is related to some parameters, e.g., the inner diameter of the tube, the amplitude, and the vibration frequency. In this paper, a theoretical model is proposed to explain the physical origin of the capillary phenomenon and the effects of the inner diameter of the tube, the amplitude, and the vibration frequency on the equilibrium height. In this model, the volumes of the inflowing and outflowing particles in a vibration period are calculated, which can significantly broaden our understanding in the flow of particles in the bottom of the tube. In order to prove the assumption of this physical model that the particles in the bottom of the tube move in the form of sine, several experiments are conducted. The granular climbing heights at different granular positions and different time stages are measured. The results show that granules move in the form of sine, which almost coincides with the motion of the tube. Moreover, motivated by the sampling on the asteroid regolith based on this mechanism, the sampling efficiencies for various vibration amplitudes and frequencies are discussed based on the new proposed model. It is found that there is an optimum frequency at which sampling is the most effective.

Feasible region and stability analysis for hovering around elongated asteroids with low thrust

This paper investigates properties of low-thrust hovering, including the feasible region and stability, in terms of the dynamical parameters for elongated asteroids. An approximate rotating mass dipole model, by which the description of the rotational gravitational field is reduced to two independent parameters, is employed to construct normalized dynamical equations. The boundaries of the feasible region are determined by contours representing the magnitude of the active control. The effects of a rotating gravitational field and maximal magnitude of the low thrust on the feasible hovering regions are analyzed with numerical results. The stability conditions are derived according to the forms of the eigenvalues of the linearized equation near the hovering position. The stable regions are then determined by a grid search and the effects of the relevant parameters are analyzed in a parametric way. The results show that a close hovering can be easier to realize near the middle of the asteroid than near the two ends in the sense of both required control magnitude and stability.

Measuring the mechanical properties of small body regolith layers using a granular penetrometer

Small bodies in the solar system are known to be covered by a layer of loose unconsolidated soil composed of grains ranging from dusty sands to rugged boulders.Various geophysical processes have modified these regolith layers since their origin.Therefore,the landforms on regolith-blanketed surfaces hold vital clues for reconstructing the geological processes occurring on small bodies.However,the mechanical strength of small body regolith remains unclear,which is an important parameter for understanding its dynamic evolution.Furthermore,regolith mechanical properties are key factors for the design and operation of space missions that interact with small body surfaces.The granular penetrometer,which is an instrument that facilitates in situ mechanical characterization of surface/subsurface materials,has attracted significant attention.However,we still do not fully understand the penetration dynamics related to granular regolith,partially because of the experimental difficulties in measuring grain-scale responses under microgravity,particularly on the longer timescales of small body dynamics.In this study,we analyzed the slow intrusion ofa locomotor into granular matter through large-scale numerical simulations based on a soft sphere discrete element model.We demonstrated that the resistance force of cohesionlessregolith increases abruptly with penetration depth after contact and then transitions to a linear regime.The scale factor of the steady-state component is roughly proportionalto the internal friction of the granular materials,which allows us to deduce the shearstrength of planetary soils by measuring their force depth relationships.When cohesion is included,due to the brittle behavior of cohesive materials,the resistance profile is characterized by a stationary state at a large penetration depth.The saturation resistance,which represents the failure threshold of granular materials,increases with the cohesion strength of the regolith.This positive correlation provides a reliable tool for measuring the tensile strength of granular regolith in small body touchdown missions.

Robust extended Kalman filter with input estimation for maneuver tracking

This study investigates the problem of tracking a satellite performing unknown continuous maneuvers. A new method is proposed for estimating both the state and maneuver acceleration of the satellite. The estimation of the maneuver acceleration is obtained by the combination of an unbiased minimum-variance input and state estimation method and a low-pass filter. Then a threshold-based maneuver detection approach is developed to determinate the start and end time of the unknown maneuvers. During the maneuvering period, the estimation error of the maneuver acceleration is modeled as the sum of a fluctuation error and a sudden change error. A robust extended Kalman filter is developed for dealing with the acceleration estimate error and providing state estimation. Simulation results show that, compared with the Unbiased Minimum-variance Input and State Estimation(UMISE) method, the proposed method has the same position estimation accuracy, and the velocity estimation error is reduced by about 5 times during the maneuver period. Besides, the acceleration detection and estimation accuracy of the proposed method is much higher than that of the UMISE method.

Dynamical behavior of flexible net spacecraft for landing on asteroid

A new era of up-close asteroid exploration has been entered in the 21st century.However,the widely rugged terrain and microgravity field of asteroids still pose significant challenges to the stable landing of spacecraft and may even directly lead to the escape of the explorer.Owing to the substantial energy dissipation arising from the interaction among multiple bodies,the flexible net,which is a typical multibody system,may be capable of overcoming the above problems.In this study,a dynamical model was established to analyze the movement of the flexible net spacecraft(FNS)near and on the asteroid comprehensively.First,we investigated the dynamical environment of the target asteroid by combining the polyhedron method and spherical harmonics parametric surface modeling approach.Thereafter,we constructed the multibody dynamics model of the explorer using the linear Kelvin–Voigt method.Subsequently,we studied the collision process between the FNS and asteroid based on the spring–damper contact dynamics model.The trajectory and speed of the FNS could be derived by solving the system dynamic equations in parallel.Finally,we analyzed the deformation,descent,jumping motion,and surface movement process of the FNS during the movement.Consequently,a promising scheme is provided for asteroid exploration missions in the future.

Entropy method of measuring and evaluating periodicity of quasi-periodic trajectories

This paper presents a method for measuring the periodicity of quasi-periodic trajectories by applying discrete Fourier transform(DFT) to the trajectories and analyzing the frequency domain within the concept of entropy. Having introduced the concept of entropy, analytical derivation and numerical results indicate that entropies increase as a logarithmic function of time. Periodic trajectories typically have higher entropies, and trajectories with higher entropies mean the periodicities of the motions are stronger. Theoretical differences between two trajectories expressed as summations of trigonometric functions are also derived analytically. Trajectories in the Henon-Heiles system and the circular restricted three-body problem(CRTBP) are analyzed with the indicator entropy and compared with orthogonal fast Lyapunov indicator(OFLI). The results show that entropy is a better tool for discriminating periodicity in quasiperiodic trajectories than OFLI and can detect periodicity while excluding the spirals that are judged as periodic cases by OFLI. Finally, trajectories in the vicinity of 243 Ida and 6489 Golevka are considered as examples, and the numerical results verify these conclusions. Some trajectories near asteroids look irregular, but their higher entropy values as analyzed by this method serve as evidence of frequency regularity in three directions. Moreover, these results indicate that applying DFT to the trajectories in the vicinity of irregular small bodies and calculating their entropy in the frequency domain provides a useful quantitative analysis method for evaluating orderliness in the periodicity of quasi-periodic trajectories within a given time interval.

Timeline Club:An optimization algorithm for solving multiple debris removal missions of the time-dependent traveling salesman problem model

With the increase of space debris,space debris removal has gradually become a major issue to address by worldwide space agencies.Multiple debris removal missions,in which multiple debris objects are removed in a single mission,are an economical approach to purify the space environment.Such missions can be considered typical time-dependent traveling salesman problems(TDTSPs).In this study,an intelligent global optimization algorithm called Timeline Club Optimization(TCO)is proposed to solve multiple debris removal missions of the TDTSP model.TCO adopts the traditional ant colony optimization(ACO)framework and replaces the pheromone matrix of the ACO with a new structure called the Timeline Club.The Timeline Club records which debris object to be removed next at a certain moment from elitist solutions and decides the probability criterion to generate debris sequences in new solutions.Two hypothetical scenarios,the Iridium-33 mission and the GTOC9 mission,are considered in this study.Simulation results show that TCO offers better performance than those of beam search,ant colony optimization,and the genetic algorithm in multiple debris removal missions of the TDTSP model.

A rapid method for validation and visualization of agile Earth-observation satellites scheduling

This paper describes a rapid method for validation and visualization of agile Earth-observation satellites scheduling.Benefited from the previous work,various algorithms are proposed for scheduling the observations of agile satellites.However,the satellite maneuvers are three-dimensional,this characteristic makes it difficult for the operation engineers to validate and interpret the scheduled solutions.They have to plot these attitude data to analyze different situations such as an observing phase or a slew maneuver.Finally,one tries to imagine the three-dimensional situations from many one-dimensional plots,which is time-consuming and susceptible to errors.Moreover,now it is low-efficiency to deal with the data about ephemeris,targets,etc.,because different software platforms are required.In this research,a validation and visualization method is suggested to overcome this barrier.It is successful to integrate the Satellite Tool Kit ActiveX and the C#programming language.Based on the embedded scheme,all the interaction and assessment can be visualized.Practical techniques for modelling satellite objects,sensor objects,target objects and satellite attitudes are presented.Such a method has been applied for Chinese agile satellites project,and a software interface has been developed.The simulation results indicate that the proposed method is intuitive and efficient.Note that this method is general,and thus it can be applied to other Earth observation missions.Enough details are provided for interested readers to develop the software interface.

Solar sail attitude control using shape variation of booms

Active attitude control of solar sails is required to control the direction of the force generated by Solar Radiation Pressure(SRP). It is desirable to control the attitude through propellantfree means. This paper proposes a new method for attitude control of solar sails: A boom consisting of "smart" structural material can be deformed by the piezoelectric actuator, and Solar Radiation Pressure torque will be generated due to shape variation of sail membrane caused by boom deformation. The method has the advantages of simple structure, small disturbance and small additional load, and is not limited by the size of the solar sail. The case of rendezvous with the Asteroid 2000SG344 is used to verify the attitude control around the pitch and yaw axes.