The spacecraft are actively controlled to remain centered on the test masses along the interferometric axes, without applying forces on the test masses on these axes.
To establish this “drag-free operation”, a housing around the test mass senses the relative position of test mass and spacecraft, and a control system commands the spacecraft’s thrusters to follow the free-falling mass.
Drag-free operation reduces time-varying disturbances to the test masses caused by force gradients arising in a spacecraft that is moving with respect to the test masses. The amplitude spectral density of the residual acceleration of the test mass characterises the disturbance reduction, the first basic function of the science instrument.
The test masses are 46 mm cubes, made from a dense non-magnetic Au-Pt alloy and shielded by the Gravitational Reference Sensor (GRS). The GRS core is a housing of electrodes, at several mm separation from the test mass, used for nm/√ Hz precision capacitive sensing and nN-level electrostatic force actuation in all non-interferometric degrees of freedom. The GRS also includes fibers for UV light injection for photoeletric discharge of the test mass and a caging mechanism for protecting the test mass during launch and then releasing it in orbit. The GRS technology is a direct heritage from LISA Pathfinder.