A pathfinder for gravitational waves

After the launch on 2 December, LISA Pathfinder (LPF) will take up a parking orbit close to Earth and separate from the upper stage of the rocket around two hours later. As of 6 December, a series of six orbit-correction manoeuvres will commence, which will lift the apogee of the elliptical orbit higher and higher over the next five days.

Finally, LPF will leave Earth’s orbit completely and drift on a transfer orbit towards the so-called Lagrangian point L1 – around 1.5 million kilometres away from Earth heading towards the Sun. After journeying for around 40 days, the satellite will arrive there and turn into an orbit around the Lagrangian point after having separated from its propulsion module on 22 January 2016.

The Lagrangian point was selected due to a special feature: it is where the gravitational forces of Sun and Earth cancel each other out. This region of space thus provides the ideal environment for the main task of the LISA Pathfinder: to position two test masses in perfect zero gravity, and to measure and control their positions with unprecedented precision.

This scientific feat is only achieved through state-of-the-art technology comprising inertial sensors, a laser metrology system, a drag-free control system and an ultra-precise micro-propulsion system. All these methods are essential for the planned gravitational wave observatory eLISA.

Two identical cube-shaped test masses

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. Cosmic radiation and electric stray fields from the test-mass housings lead to electrostatic charging, however, which can be removed again, contact-free, with the aid of UV radiation.

The caging and grabbing mechanism – responsible for protecting the test masses from intense vibrations during launch of LISA Pathfinder, 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 centered on one test mass. “The Max Planck Institute for Gravitational Physics in Hannover played a leading role in the development and construction of this optical scientific instrumentation was developed”, says Karsten Danzmann, Director at the Institute ad Co-Principal Investigator for the LISA Pathfinder Technology Package, the scientific heart of the satellite.

In addition, so-called inertial sensors detect the positions with lower precision. The measurement data are used to control the probe by means of a “Drag-Free Attitude Control System” so that it always remains centred on one of the test masses.

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. This extremely sensitive control is necessary in order to compensate external, non-gravitational interfering effects such as the radiation pressure of the sunlight or the changing solar wind, and to maintain the position of the satellite around the test masses under zero gravity.

Data evaluation in Hanover

The principal scientific mission of LISA Pathfinder starts on 1 March 2016 and will last at least six months. During this time, the scientists want to carry out a large number of individual experiments, each based on the results of the previous one. The aim is to measure the almost perfect zero gravity by determining the interfering accelerations which do not originate from the gravitation, identify significant sources of interference and minimize them further if required.

The Max Planck Institute for Gravitational Physics is taking a leading role in developing the evaluation software, which plays a key part in extracting important information from the measurement data. The Institute has a control room in Hanover for this purpose. Since it is crucial that the data be evaluated immediately in order to configure the subsequent investigations, researchers from the Institute are also working a round-the-clock shift system at the Darmstadt control centre of the European Space Agency ESA.

LISA Pathfinder will pave the way for eLISA, a large-scale gravitational wave observatory designed to detect one of the most elusive phenomena in astronomy – gravitational waves. Proving the existence of these tiny distortion in spacetime, predicted by Albert Einstein in 1916, requires an extremely sensitive and highly precise measurement technology.

Space observatories such as eLISA will hunt for gravitational waves in the millihertz range, as emitted by pairs of extremely massive black holes or binary star systems consisting of white dwarfs. They will thus complement ground-based detectors such as GEO600, aLIGO, and Virgo, which are looking to detect gravitational waves in less massive objects at higher frequencies.

In concert with other astronomical methods, these gravitational wave observatories will probe unknown cosmic domains – the “dark side of the universe”. With eLISA, astronomers want to observe the formation, growth and merger of massive black holes in 20 years. It should also be possible to track the evolution of galaxies throughout the entire past of the universe can also. eLISA will also further test Einstein’s Theory of General Relativity and search for unknown physics.

Contact

Dr. Benjamin Knispel
Pressekontakt

Max Planck Institute for Gravitational Physics (Hannover), Hannover
Phone: +49 511 7621-9104

Email: benjamin.knispel@aei.mpg.de

Dr. Jens Reiche
Max Planck Institute for Gravitational Physics (Hannover), Hannover
Phone: +49 511 762-5844

Fax: +49 511 762-2784

Email: jens.reiche@aei.mpg.de