Autonomous GNC for asteroid deflection and attitude control via laser ablation

摘要

This paper presents an autonomous guidance, navigation and control system for the deflection and attitude control of a small asteroid via laser ablation. Laser ablation consists of irradiating the surface of the asteroid with a laser beam with sufficient intensity to sublimate the irradiated material. The resulting jet of gas and debris induces a force and a torque thrusting the asteroid off its natural course and changing its tumbling motion. In this paper it is proposed to use the laser to first de-tumble the asteroid. A reduction of the rotational speed of the asteroid increases the yield of the laser ablation process. An autonomous proximity control system is then implemented to keep the spacecraft flying in formation with the asteroid under the effect of the thrust acting on the asteroid, plume impingement, laser recoil and solar radiation pressure. The spacecraft employs and processes the measurements coming from its own on board measurements, given by a laser range finder, high resolution cameras, and an impact sensor. The latter is combined with the attitude information and, thus, used to estimate the plume impingement force, which acts in the same direction of the exerted thrust due the laser ablation. In this way the spacecraft is able to estimate on-board the imparted acceleration and the effectiveness of the laser ablation procedure. An unscented Kalman filter is used to estimate spacecraft position and velocity together with the perturbative accelerations. A second filter is implemented to estimate the asteroid’s rotation by extracting and tracking the motion of asteroid’s features, using either optical flow or spectral methods. These variables are used to implement spacecraft trajectory control in order to permit the laser to work at his optimal focussing distance. Two trajectory control strategies are considered: in the first one, a series of impulse bits maintains the spacecraft within a 0.5 m box from the reference trajectory; the second strategy is based on a continuous low-thrust control. It is shown that both techniques are viable and accurate. The discrete impulsive control does not downgrade the laser performance given the small oscillations with respect to the nominal conditions. Nonetheless low thrust allows the spacecraft to impart a higher momentum onto the asteroid.