Object injection in geodesic conditions: In-flight and on-ground testing issues Original Research Article

In absence of forces, any object moves along the straightest possible path in space–time, called geodesic. The presence of a gravity field generates a space–time distortion that reflects on a curvature of the geodesics. Outer space yields a privileged environment to achieve high levels of geodesic purity, thanks to the absence of many non-gravitational force disturbances typical of the Earth, mainly due to the atmosphere, micro-seismic activity, stray electro-magnetic fields, etc. Many experiments in the field of Fundamental Physics, General Relativity and Earth Observation are performed through space missions, in which objects are either set in geodesic conditions or their deviation from a geodesic is measured. In both cases, stray non-gravitational forces acting on them must be reduced to a negligible level. Such missions share the need to bring proof masses to a free-floating condition inside a reference sensor, starting from an Earth environment in which they are subjected to the constraining action against the gravity field and a launch phase characterized by high inertial loads. This results in the need to apply to the proof mass forces that are progressively reduced of several orders of magnitude, down to the lowest possible level associated to the geodesic status. The novel issues that concern the last step of this sequence, defined as “injection in geodesic conditions”, make this phase object of study in the field of engineering, in which multiple disciplines are involved. Different approaches are followed in these missions to perform the injection in geodesic motion phase, depending on the proof mass mechanical boundary conditions. Being recognized as a mission critical phase, different on-ground testing approaches have been developed to verify and reduce its criticality. This paper deals with the design and on-ground testing approaches of the injection in geodesic conditions phase developed in the frame of some scientific space missions, chosen as meaningful cases. Limited to the injection of free floating proof-masses, the paper does not consider other existing instruments as for example the pendulum accelerometers, in which the mass motion is constrained by mechanical links.