Spacecraft Control Relative to a Free-Flying Mass for Gravitational Experiments in Space

Abstract

A fundamental physics experiment has been proposed whereby a 200 kg cannon-ball is flown in close proximity to a drag-free spacecraft in low earth orbit. The spacecraft contains a cryogenically cooled superconducting differential accelerometer which can detect the gravitational field of the cannonball with a sensitivity of 10-16 m s-2. By using different materials for the accelerometer proof-masses, a possible violation of the Weak Equivalence Principle at short range may be investigated. For a nominal seperation of 1 m, the position of the cannonball must be measured to within 10 μm, or better still, controlled to this level. A simple dynamic model of the coupled two-body system is presented (linearized Hill's equations), and simulations with an LQR controller with integral terms is demonstrated to be effective. Sensing the cannonball position is identified as the critical issue and some preliminary ideas are presented