Study On Energy Optimal Trajectory And Maneuver Strategy For Soft Landing On Irregular Asteroids

With the development of space technology,the targets of human's deep space exploration activities have been extended from the moon and other seven planets in the solar system to many small celestial bodies,including asteroids.Asteroids may provide a large number of rare earth resources for human beings.At the same time,the exploration of asteroids has important scientific significance.Because the shape of asteroids is often very irregular,the distribution of gravitational field is very complex,which brings many difficulties for spacecraft soft landing on asteroids.Current researches have some shortcomings,such as over-simplification of gravitational field model,low computational efficiency and poor robustness of open-loop control strategy,which make the corresponding spacecraft orbital maneuver strategy unsuitable for practical application.In order to solve the above problems,a piecewise linear orbit maneuver strategy is proposed in this paper.The problem of soft landing of spacecraft on asteroids is studied.First,the space around the asteroid is divided into several grid units,and in each unit,the gravitational field of the asteroid is linearly fitted to transform the non-linear gravitational field of the asteroid into a piecewise linear one.Then,the problem of the non-linear energy optimal orbital maneuver strategy is converted into the piecewise linear energy optimal orbital maneuver strategy problem.Finally,this paper proposes an approach based on Pontryagin minimum principle to solve the corresponding piecewise linear problem.Using the proposed piecewise linear orbit maneuver strategy,simulation examples are designed for the asteroid Eros 433 to verify the effectiveness and robustness of the proposed strategy.The simulation results show that when the third-party perturbation is ignored and the motion state of spacecraft is observed accurately,the designed piecewise linear orbit maneuver strategy can accomplish the soft landing task well.The energy consumption during the landing process is almost the same as that of the global optimal orbit maneuver strategy,while the solving time is greatly shortened.When the third-party perturbation is neglected,but there are errors in spacecraft state observation,the piecewise linear orbit maneuver strategy proposed in this paper can still achieve the soft landing target,while the maneuver strategy obtained by solving the non-linear control problem directly can not achieve the predesigned target.Considering the influence of solar perturbation,the soft landing performance of using the non-perturbation nonlinear control strategy directly in the perturbed system is slightly worse than using the strategy in a perturbation-free system;after introducing the filtering state,the influence of perturbation can be eliminated under the condition of accurate observation of spacecraft state,but the calculation time is greatly increased;while the piecewise linear control strategy proposed in this paper is less affected by solar perturbation,and the accomplishment of soft landing mission is similar to that without considering solar perturbation.