The present status and prospects in the research of orbital dynamics and control near small celestial bodies

Small celestial body exploration is of great significance to deep space activities. The dynamics and control of orbits around small celestial bodies is of top priority in the exploration research. It includes the modeling of dynamics environment and the orbital dynamics mechanism. This paper introduced state-ofthe-art researches, major challenges, and future trends in this field. Three topics are mainly discussed： the gravitational field modeling of irregular-shaped small celestial bodies, natural orbital dynamics and control, and controlled orbital dynamics. Finally, constructive suggestions are made for China’s future space exploration missions.

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.

Motion reconstruction of the small carry-on impactor aboard Hayabusa2

Subsurface exploration is one of the most ambitious scientific objectives of the Hayabusa2 mission.A small device called small carry-on impactor(SCI)was developed to create an artificial crater on the surface of asteroid Ryugu.This enables us to sample subsurface materials,which will provide a window to the past.The physical properties of the resulting crater are also useful for understanding the internal structure of Ryugu.Accurate understanding of the crater and ejecta properties,including the depth of excavation of subsurface materials,requires accurate information on impact conditions.In particular,the impact angle is a critical factor because it greatly influences the size and shape of the crater.On April 5,2019,the Hayabusa2 spacecraft deployed the SCI at 500 m of altitude above the asteroid surface.The SCI gradually reduced its altitude,and it shot a 2 kg copper projectile into the asteroid 40 min after separation.Estimating the position of the released SCI is essential for determining the impact angle.This study describes the motion reconstruction of the SCI based on the actual operation data.The results indicate that the SCI was released with high accuracy.

Guidance, navigation, and control of Hayabusa2 touchdown operations

Hayabusa2 is a Japanese sample return mission from the asteroid Ryugu.The Hayabusa2 spacecraft was launched on 3 December 2014 and arrived at Ryugu on 27 June 2018.It stayed there until December 2019 for in situ observation and soil sample collection,and will return to the Earth in November or December 2020.During the stay,the spacecraft performed the first touchdown operation on 22 February 2019 and the second touchdown on 11 July 2019,which were both completed sucssfully.Because the surface of Ryugu is rough and covered with boulders,it was not easy to find target areas for touchdown.There were several technical challenges to overcome,including demanding guidance,navigation,and control accuracy,to realize the touchdown operation.In this paper,strategies and technical details of the guidance,navigation,and control systems are presented.The flight results prove that the performance of the systems was satisfactory and largely contributed to the success of the operation.

Design and flight results of GNC systems in Hayabusa2 descent operations

Hayabusa2 is a Japanese sample return mission from the near-Earth asteroid Ryugu.The Hayabusa2 spacecraft was launched on December 3,2014,and reached the asteroid on June 27,2018.It remained there until November 13,2019 for in situ observation and soil sample collection and will return to the Earth in November or December 2020.During its stay at the asteroid,Hayabusa2 performed descent operations 16 times.This paper presents an overview of a guidance,navigation,and control method used in such descent operations.The method consists of on-board and on-ground guidance systems to control the spacecraft and an image-based navigation technique that uses a shape model and ground control points of the asteroid.Flight results in the first touchdown operation are shown as an example,which demonstrate that the method showed a good performance overall and contributed to the success of the mission.