Mission and System Design for the Manipulation of PHOs with Space-borne Lasers

Owing to their ability to move a target in space without requiring propellant, laser-based deflection methods have gained attention among the research community in the recent years. With laser ablation, the vaporized material is used to push the target itself allowing for a significant reduction in the mass requirement for a space mission. Specifically, this paper addresses two important issues which have remained unanswered by previous studies: the impact of the tumbling motion of the target as well as the impact of the finite thickness of the material ablated in the case of a space debris. We developed an analytical model based on energetic considerations in order to predict the efficiency range theoretically allowed by a CW laser deflection system operating under the plasma formation threshold and in absence of the two aforementioned issues. A numerical model was then developed to solve the transient heat equation in presence of vaporization and melting and assess the efficiency reduction due to the unsteadiness induced by the tumbling motion of the potentially hazardous object (PHO). The model was translated to handle the case where the target is a piece of space debris by considering specific materials such as aluminum and titanium alloys or even carbon fiber and by adapting the finite size of the computational domain along with the propagation of the ablation front. From the results of this later model, pulsed lasers appear better suited to answer the needs of a space debris de-orbiting laser system rather than CW lasers. An empirical ablation threshold is also found that establishes a direct relation between the pulse duration or the heating time (CW case), the delivered flux and the properties of the material. Derived from theoretical consideration, this threshold matches well with the predictions of our numerical model. Moreover, the numerical results are found to agree with published data of thrust coupling coefficient on targets made of aluminium and titanium alloys. In the second part of the paper, we coupled our thrust model within an orbit propagator and considered several redirect scenarios for the case of a small(56m) and a larger(100m) asteroid as well as an 8-ton defunct satellite currently orbiting in a sun-synchronous orbit at a 765km altitude. In each scenario, the laser is assumed mounted on a spacecraft that will first rendez-vous with the target and will then operate from a safe distance (500m). Based on the results, realistic mission architectures are explored. Within the last section, the paper also highlights the advantages offered in term of redundancy and scalability by techniques such as beam combining or formation flying. We show that a medium class mission carrying a CW laser system able to generate 2.4kW of output power could ensure the deflection of a 56m asteroid while a formation of such spacecraft could also achieve the deflection of a larger threat. For the debris case, our preliminary results indicate that a spacecraft carrying an actively Q-switched diodepumped solid state laser (DPSSL) able to generate 3kW of output power would bring the altitude of Envisat down to 400 kilometers in less than 500 days.