Drag-free Spacecraft Technologies: criticalities in the initialization of geodesic motion

Present and future space missions rely on systems of increasingly demanding performance for being successful. Drag-free technology is one of the technologies that is fundamental for LISA-Pathfinder, a European Space Agency mission whose launch is planned for the end of September 2015. A purely drag-free object is defined by the absence of all external forces other than gravity. This is not a natural condition and therefore a shield has to be used in order to eliminate the effect of undesired interactions. In space, this is achieved by properly designing the spacecraft that surrounds the object, usually called test mass (TM). Once the TM is subjected to gravity alone its motion is used as a reference for the spacecraft orbit. The satellite orbit is controlled by measuring the relative TM-to-spacecraft position and feeding back the command to the propulsion system that counteracts any non gravitational force acting on the spacecraft. Ideally, the TM should be free from all forces and the hosting spacecraft should follow a pure geodesic orbit. However, the purity of the orbit depends on the spacecraft’s capability of protecting the TM from disturbances, which indeed has limitations. According to a NASA study, such a concept is capable of decreasing operation and fuel costs, increasing navigation accuracy. At the same time, a drag-free motion is required in many missions of fundamental physics. eLISA is an ESA concept mission aimed at opening a new window to the universe, black holes, and massive binary systems by means of gravitational waves. This mission will be extremely challenging and needs to be demonstrated in flight. LISA-Pathfinder is in charge of proving this concept by demonstrating the possibility of reducing the non-gravitational disturbance below a certain demanding threshold. The success of this mission relies on recent technologies in the field of propulsion, interferometry, and space mechanisms. In this frame, the system holding the TM during launch and releasing it in free-fall before the science phase represents a single point of failure for the whole mission. This thesis describes the phenomena, operations, issues, tests, activities, and simulations linked to the release following a system engineering approach. Great emphasis is given to the adhesion (or cold welding) that interferes with the release. Experimental studies have been carried out to investigate this phenomenon in conditions representative of the LISA-Pathfinder flight environment. The last part of the thesis is dedicated to the preliminary design of the housing of the TM in the frame for a low-cost mission conceived at Stanford (USA). Analysis and results are through out presented and discussed. The goal of this thesis is a summary of the activities aimed at a successful LISA-Pathfinder mission. The ambition is to increase the maturity of the technology needed in drag-free projects and therefore provide a starting point for future fascinating and challenging missions of this kind.