The highly sensitive measurement system of LISA Pathfinder
Two identical cube-shaped test masses weighing about two kilograms each will be free-floating in their own vacuum canisters for the duration of the mission. They will be almost free of all internal and external disturbances and will thus allow the demonstration of the precise measurement of free-falling masses in space. A special gold-platinum alloy has been used for the masses to eliminate any influence from magnetic forces. Using ultraviolet radiation, a contact-free discharge system prevents electrostatic charge build-up on the test masses. The caging and grabbing mechanism – responsible for protecting the test masses from intense vibrations during launch, releasing them in a highly controlled setting, and capturing them as necessary – is a particular challenge in this context. A laser interferometer will measure the position and orientation of the two test masses relative to the spacecraft and to each other with a precision of approximately 10 picometers (one hundred millionth of a millimetre). In addition, there are less precise capacitive inertial sensors that also help determine their positions. The positional data is used by a Drag-Free Attitude Control System (DFACS) to control the spacecraft and ensure it always remains centred on one test mass. The actual position of the satellite is controlled through cold gas micronewton thrusters, which have the capability of delivering propulsion in extremely fine and uniform amounts. The thrust generated is in the micronewton range – this equates to the weight of a grain of sand on Earth.
Paving the way for new astronomy
LISA Pathfinder paves the way for eLISA, a large space observatory for the direct observation of one of the most elusive astronomical phenomena – gravitational waves. These tiny distortions of space-time were predicted by Albert Einstein and their observation requires an extremely sensitive and highly precise measurement technology. Space observatories like eLISA will measure gravitational waves in the millihertz range. They will complement ground-based detectors such as GEO600, aLIGO, and Virgo, which observe at higher frequencies in the audio range. Gravitational wave observatories will probe unknown domains – the “dark side of the universe” – in concert with other astronomical methods. One example is the formation, growth and merger of massive black holes. It will also be possible to further test Einstein’s Theory of General Relativity and search for unknown physics.