Listening to the 'sound' of the Universe – highly sensitive core of LISA Pathfinder completed

Tuning into the 'sound' of the universe

If a stone is dropped in water, oscillations spread in waves across the surface of the water from the point of impact. Like stones falling into water, substantial masses that move through space extremely quickly and with rapidly changing acceleration also produce waves. These propagate through space and inevitably announce their presence as very small oscillations in space-time. These gravitational waves – predicted by the German physicist Albert Einstein in his Theory of General Relativity as far back as 1916 – allow us to 'listen to the sound' of the Universe, opening a new and unobstructed window to observe exotic celestial objects.

Among these are supernovae, close binary star systems consisting of white dwarfs, collisions between neutron stars and pulsars or black hole collisions during galaxy mergers. In the core of galaxies, objects such as white dwarfs, neutron stars or black holes, may travel towards the super massive black hole at the centre; if they get close enough, they will start to spiral in as their orbits shrink as a result of the loss of energy and angular momentum carried away by gravitational waves (extreme-mass-ratio inspirals). Gravitational waves from the time immediately after the Big Bang can reveal more about the origin of the Universe. However, Einstein was somewhat doubtful that gravitational waves would ever be detected, as their effects are extremely small; it has not been possible to measure them so far. But today – almost 100 years after Einstein's prediction, mankind is on the verge of making their 'extremely quiet' oscillations 'audible'. It is expected that proof will be furnished in the coming years – initially using ground-based systems, and subsequently with eLISA out in space.

LISA Pathfinder prepares the way for eLISA

The eLISA space observatory will consist of three spacecraft – a mother craft and two daughter craft. They will be positioned in a near-equilateral triangle formation, with each side measuring approximately two million kilometres. The entire triangle will rotate and follow Earth on its orbit around the Sun at distances ranging between 10 and 25 million kilometres. The spacecraft are connected by laser beams – a feat of unprecedented technical precision. The observatory will be able to 'hear' any gravitational waves that pass through their alignment in a frequency range of between 0.1 millihertz and 0.1 hertz. The necessary technology requires initial testing in space, as the mission is extremely complex and involves components that cannot be tested adequately on the ground.

LISA Pathfinder will handle this task. The components used in the scientific payload – the LTP – were developed by several European countries and put together to form a composite payload by Airbus Defence & Space in Friedrichshafen. DLR played a key role in the development of the payload within the framework of the ESA Science Programme. Here, the German contribution received a substantial boost from a grant awarded by the DLR Space Administration to the Max Planck Institute for Gravitational Physics (Albert Einstein Institute) in Hannover.

Highly sensitive measurement system

The LTP control electronics in the LISA Pathfinder orbiter have been installed and tested, and the final and most important unit in the payload, the LTP Core Assembly, will follow shortly. Its construction is now complete, and the unit has been sent from Friedrichshafen to IABG in Munich for integration into the spacecraft and final testing.

During the mission, two identical cube-shaped test masses, each weighing two kilograms, will be suspended in their own LTP vacuum vessel, almost free of internal and external disturbances, hence demonstrating free flight in space. A special gold-platinum alloy has been used for the masses, ensuring that magnetic forces do not have any effect. Using ultraviolet radiation, a contactless discharge system prevents any electrostatic charging of the masses. The caging and venting 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. The laser interferometer will measure the position and orientation of the two test masses relative to the spacecraft and to each other at a precision of approximately one hundred millionth of a millimetre. In addition, there are other, less precise, sensors that also help determine their positions. A Drag-Free Attitude Control System (DFACS) uses the measurement data to control the spacecraft and to ensure it always remains centred on the test masses. Cold gas micronewton thrusters developed for the Gaia astrometry mission, which have the capability of delivering propulsion in extremely fine and uniform amounts, will control the position of the spacecraft.