Schematic diagram of the Inertial Sensor Head
The LISA Pathfinder Inertial Sensor Head is designed to keep a cubic (46 mm × 46 mm × 46 mm) gold:platinum test mass, located at the centre, free from all external forces except gravity.
Packed in a titanium vacuum enclosure, roughly 43 cm tall and 18 cm wide, are the support structure, capacitive sensing and actuation electrodes, and mechanisms to hold, position and release the test mass once the spacecraft is on orbit. The Caging and Venting Mechanism has eight 'fingers' that hold the test mass during the launch of the spacecraft. Once the spacecraft is on-orbit, the 'fingers' relinquish control of the test mass and the Grabbing, Positioning and Release Mechanism takes over.
Feedback loops and micro-propulsion thrusters are used to keep the spacecraft centred around the test mass. A charge management system comprising fibre-coupled UV lamps will counteract the build up of electrostatic forces from cosmic rays and solar particles by discharging the test mass at regular intervals.