Space tethers as testbeds for spacecraft formation-flying

We analyze the dynamics of a tethered demonstrator proposed for testing elements of formation flying for space-based interferometry. A tether connecting the space vehicles offers the dual advantage of being the main structural component that keeps the spacecraft together and acting as the control actuator to reposition the formation. Moreover, when used for a spinning formation the tethered configuration can hold an inertial pointing while keeping the formation together with no expenditure of fuel. In order to minimize the disturbance of the environmental perturbations, the proposed intererometric missions are planned to fly at the Sun-Earth Lagrangian points or in Earth trailing orbit. The demonstrator is meant as a low-cost mission to be flown in low Earth orbit to test actuators, sensors and control algorithms required for formation flying in a representative dynamic environment. The orbital characteristics have been chosen with the goal of minimizing dynamic effects due to Earth's oblateness and to provide over a relatively long period of time almost constant thermal inputs and solar pressure which typical of higher orbits. In this paper, we analyze the effect of external perturbations on the inertial pointing of the formation and preliminary control strategies for deploying the tethered demonstrator.