Electrostatic Detumble of Space Objects

The growing interest in active debris removal and on-orbit servicing is driven by the increased utilization of Earth orbits, the increased satellite population already in orbit, and the increased possibility of on-orbit collisions. Electrostatic actuation is one of many promising mechanical, flexible, and touchless on-orbit actuation technologies required to interface with target objects. Motivated by large on-orbit rotation rates that render mechanical connections more risky and target structures that may be damaged or destroyed by flexible capture systems, touchless methods provide exciting opportunities to drive the target dynamics towards desired states prior to servicing or removal operations. Prior work has demonstrated that electrostatic actuation holds promise should the concept be extended to 3-dimensional on-orbit target tumbles. Advancing electrostatic actuation onto orbit introduces key questions about the applicability to various target geometries, the choice of mission-relevant relative motion trajectories, and the relative navigation and sensing requirements required to obtain necessary state information. This work addresses these questions and provides novel insight into detumble formulations, relative motion descriptions, and state estimation capabilities. The accumulation of the presented work significantly advances the understanding and demonstration of electrostatics as a viable on-orbit actuation technology. Derived and studied are electrostatic detumble control laws for extending the previously 1- dimensional axisymmetric control law to a 3-dimensional axisymmetric target tumbles using both projection angle and Lyapunov stable control approaches. It is therefore possible to predict the final deep space detumble attitude and enables study of tugging and pushing steady-state behavior and performance predictions for deep-space and on-orbit detumble applications. Presented are the first demonstrations of electrostatic detumble in on-orbit simulations and the proposed deep-space control formulations presented in this work are shown to be sufficient for on-orbit electrostatic detumble applications. Examples include electrostatically detumbling a 1000 kg representative cylindrical target and 3000 kg representative box-and-panel spacecraft in as few as four days from 2 degrees/second rotation rates. In order to simulate electrostatic detumble on-orbit and to study the influences of electrostatic actuation perturbations on servicer-target relative orbits, the new Linearized Relative Orbit Element (LROE) control formulation is developed. The novel state vector captures the geometrical insight of the Clohessy-Wiltshire (CW) equation analytical solution constants and employs Lagrangian Brackets to evolve the otherwise constant state as osculating elements. Presented studies demonstrate that an elegant feedback control formulation enables reconfiguration between relative orbits using continuous accelerations within the capability of currently used low-thrust systems. Further demonstrated are trajectory fuel consumption variations as a function of relative orbit phasing. Similar to classical orbit elements, the LROE state provides geometrical insight into the most efficient relative orbit reconfiguration maneuver placements. The electrostatic detumble control developments assume full and accurate knowledge of the relative dynamics and charging behavior. Studied is the attainability of the relative position between servicer and target and the target craft potential during detumble. A two-time scale filter is presented and implemented for estimating the target electrostatic potential using bearings and range measurements. A two-time scale filter allows for an inner, fast state estimator to work on a different measurement and update time scale than the outer, slow state estimator. The outer estimator also benefits from refinement in the fast states which occur between outer estimator updates. The faster LROE state is estimated in the inner estimator and provided to a full LROE and electrostatic potential estimator. The success of the two-time scale filter demonstrates that the target spacecraft potential may be obtained from relative motion perturbations without the need for additional space weather modeling. However, the filter performance should improve by adding improved charging modeling and auxiliary measurement types. The ability to obtain relative state information and electrostatic potential refinement while electrostatic actuation is active injects confidence into the viability of the LROE relative orbit and the electrostatic detumble controllers.