One of the main differences between the configuration of LPF and that of LISA is the fact that the sensitive axis of the two test masses are co-linear, requiring that one of them be controlled (the spacecraft can’t follow both test masses at once!). On LISA, the two sensitive axes in a single spacecraft are close to orthogonal, and as such, the spacecraft can follow both test masses at the same time, resulting in two truly drag-free test masses along their sensitive axis. The additional control needed for the 2nd test mass on LPF introduces a source of disturbance that will not be present on LISA.

In order for us to probe below this noise source to assess the potential performance of LISA, an experiment has been designed in which the actuation of the 2nd test mass is switched off. After a few minutes of free drift, the actuation is restored for a short time to ‘kick’ the test mass back to its nominal position. Then the actuation is switched off again, and the process is repeated. This is a lot like tossing a coin and in fact the test mass will follow parabolic trajectories between the ‘kicks’. The data from this experiment can then be analysed to provide a measurement of the purity of free-fall one can achieve in a more LISA-like configuration. If the actuation noise is a limiting factor, then this experiment will result in periods of time where the relative acceleration of the two test masses is lower than before, and therefore closer to pure geodesic motion.