2022, Volume 9, Issue 4
Small celestial bodies in the Solar system, such as asteroids and comets, have become the key targets in the field of deep space exploration. The exploration of small celestial bodies is of great significance for studying the formation and evolution of the solar system, the origin of life, planetary defense, and the exploitation of space resources. With the continuous development of aerospace technology, the way of small-body exploration has gradually shifted from flyby and orbiting to close-proximity detection methods, such as landing, sampling, and touring. The construction of a high-accuracy gravitational field model in the vicinity of the given small body is crucial to the design and implementation of such close-proximity exploration missions and to the study of dynamics near the small bodies. Thus, this paper first reviews the two-hundred-year history of development of the modeling of gravitational field, and elucidates the basic principles and drawbacks in the different methods. The relationship between the gravitational field near the binary asteroid system and the dynamics of such system is second reviewed. This paper also reviews the techniques for modelling the gravitational interactions in the study of the dynamics of the binary asteroid system. These techniques provide fundamental tools for the binary system mission designs and the study of the formation and evolution of the binaries. Finally, the future research trends are discussed.
Closely flying by asteroids can help to capture asteroid surface images, measure asteroid spectra, and obtain physical and chemical properties of asteroids. In particular, flying by multiple asteroids with potential hazards to the earth in one mission will significantly improve the understanding of the characteristics of potentially hazardous asteroids, and it is also of great significance to asteroid defense missions. In this paper, the trajectory of the multiple asteroid flyby mission of potentially hazardous asteroids was optimized. Firstly, the time and position distribution of asteroids passing through the ecliptic plane were analyzed, and the basic strategy of asteroids flyby in the ecliptic was determined. The time of asteroids crossing the ecliptic was taken as the time of asteroids’ flyby. Secondly, the sequential flyby sequence was optimized via beam selection tree search algorithm, and an optimization model for fast and effective solution of asteroid sequential flyby mission trajectory was established. Simulation results show that missions launched from 2024 to 2028 can fly by at least 18 potentially hazardous asteroids, especially the launch window in September 2027, which can fly by 21 potentially hazardous asteroids within a ten-year mission duration.
This paper applied a polyhedron-ellipsoid model to study the dynamics of resonant orbits near the binary system, with the binary system 66391 Moshup as an example. A series of resonant orbital families were calculated the shooting method and continuation method, and the stability and bifurcation of the orbital families were analyzed. Finally, homoclinic connections between resonant orbits were computed by using invariant manifolds. The research shows that no strict planar resonant orbit exists because of the asymmetry of the gravitational field, and the orbital stability and bifurcation have also been affected. In addition, the availability of designing transfer trajectories through resonant orbits was also demonstrated.
During asteroid exploration missions, the spacecraft needs to make an investigation into the characteristics of the asteroid after approaching it. In this paper, the performance of the fly-around mapping orbits and flyby mapping orbits were evaluated. The stable fly-around orbits and hyperbolic flyby orbits at different distances were analyzed using numerical methods. A method to calculate the coverage rate of the orbits was proposed considering the sunlight condition and topographical shading. Then mapping orbits were analyzed in the perspective of coverage rate, mapping time, fuel cost and robustness. Taking nearly spherical asteroid Bennu 101955 and elongated asteroid Eros 433 as examples, the performance of the fly-around orbits and flyby orbits were evaluated, which will provide reference for the mapping phase in future asteroid explorations.
Deep learning algorithm has a higher recognition rate for navigation landmarks such as small meteor craters than traditional algorithms, but it is difficult to achieve matching under various image changes. To solve this problem, a description method of recognition prediction box based on feature descriptor was proposed, and the matching of recognition results was completed. Firstly, the circular support region of the recognition prediction frame was determined and a 10-dimensional feature descriptor with rotation and translation scale and luminance invariance was constructed and the prediction frame was matched by the relative distance between descriptor vectors. The results show that the proposed algorithm is robust to images under different transformations, and the correct matching rate of the prediction frame is over 90%. It may provide the reference for the asteroid exploration navigation system.
A robust and efficient landmark matching algorithm was proposed in this paper to deal with the extreme environment near the target asteroid. First, the matching error of the landmark generated by the SPC (StereoPhotoClinometry) technology was analyzed, and the influence of landmark position error and cameras’ pose error on the matching results was discussed. Then, based on the error analysis, the optimal landmark points were selected, and a weighted normalized cross-correlation (WNCC) algorithm was proposed to obtain accurate matching results robustly and efficiently. Finally, high-fidelity synthetic image sequences were generated to compare the performance of WNCC and the widely used NCC (Normalized Cross-Correlation) algorithm in the previous asteroid missions under a wide range of image scales, viewing geometries, and lighting conditions. The numerical results demonstrate the advance of the proposed method in terms of efficiency, robustness, and accuracy.
Aiming at the requirements of high frame rate, high resolution and high ranging accuracy in small celestial body detection, the characteristics of laser detection technology were deeply analyzed, and a hybrid solid-state laser 3D terrain mapping and navigation integrated design method was proposed. High imaging frame rate was realized by single photon array device and 532nm fiber laser, large field of view and sub-pixel resolution were realized by multi-mode scanning of two-dimensional voice coil motor fast mirror, and high-precision beam expansion and diffraction of laser beam were realized by Damman grating beam splitter. The results show that the laser ranging accuracy is better than 3 cm (3 sigma), the frame rate is 4 Hz, and the imaging resolution is as high as 1 100×1 100. The proposed method can give consideration to both topographic mapping and navigation, realize muti-function, light and miniaturized design, and greatly reduce resource consumption. It has good guiding significance for the implementation of small celestial body exploration missions.
To solve the problem that it is difficult to carry out experimental verification for optical image navigation of small celestial body detector landing section, a small celestial body terrain dynamic simulation and landing scene simulation system was designed based on virtual reality, and the detector attitude design and landing sequence image acquisition were realized. The small celestial body model was established by using three-dimensional modeling technology, and the weighted least square method was used to realize the smooth grid of the model; linear interpolation was used to process the map, and texture mapping was realized by combining spherical mapping with cube mapping; the virtual scene simulation was developed, and the detector attitude design was realized according to the rotation matrix method. Experiments show that the simulation system can meet the demand for image navigation verification of the landing section of the probe, and can realize dynamic observation of small object topography and landing image acquisition with high graphic quality and good real-time performance, and the effectiveness of the system is verified by specific example simulation.
According to the requirements of near-Earth Asteroid kinetic impact mission, an autonomous GNC scheme is proposed, which contains the high-precision extraction method for the line of sight （LOS） of the asteroid center, high-precision autonomous relative navigation based on LOS measurement of the asteroid center, and iterative prediction guidance method. In this scheme, the LOS measurement of the asteroid center is used to estimate the relative position and velocity between the impactor and the asteroid in the direction perpendicular to the LOS. Although the elative position and velocity errors along the LOS are not estimated, the high-precision impact can be realized by the iterative prediction guidance method. The mathematical simulation shows that the proposed scheme can ensure the impactor to hit a near-Earth asteroid with the diameter 50 meter, the impact accuracy is better than 4 meter, which meets the mission requirements.
To address the problem that wheeled mobile rovers are difficult to adapt to the weak gravitational environment of small bodies. The jumping mode of the rover with cubic configuration was analyzed. A single-step bouncing strategy was proposed to exploit its barrier-crossing capability. At the same time, a variable-step A* algorithm was proposed to plan the surface movement path of the rover. The simulation results show that the rover can effectively cross the obstacles. The new algorithm reduces the path nodes and is more efficient for high-density obstacle terrain.