Background information - Full Marks for LISA Pathfinder

Hannover, 23. September 2010

LISA Pathfinder – development and demonstration of key LISA technologies
LISA Pathfinder will test in space those key technologies specifically developed for the LISA mission. For this purpose, one laser arm of the three planned LISA satellites is effectively reduced from 5 million kilometers down to around 30 cm and the experiment fitted into one single spacecraft. Each of the three subsequent LISA satellites will contain two laser interferometers and two special test masses. Each laser interferometer will be aligned with the two other satellites and will measure tiniest changes in the distance between the test masses in the different satellites with a precision of about one picometer (1 pm = 10-12 m).

The scientific instruments
for these technologies are provided by two partly complementary payload packages:

  • the LTP (LISA Technology Package, ESA) and
  • the DRS (Disturbance Reduction System, NASA).

The LTP is built by a consortium of European space companies (D, I, UK, ES, NL, CH, F) led by EADS Astrium, Friedrichshafen and contains the following key technologies:

  • inertial sensors to monitor the relative position of the test masses with respect to the satellite
  • laser interferometry to determine the relative positions of the two test masses
  • drag-free control system (DFACS) to adjust alignment of the satellite relative to the test masses by means of Micro-Newton ion thrusters

The core of LISA Pathfinder
The first two of these measurement systems – the inertial sensors and the laser interferometers – form the optical measurement system (OMS).

The OMS is the heart of the payload of LISA pathfinder and includes the following instruments: the laser assembly contains the light source (infrared laser at 1064 nm with a power of 40 mW), an electronic control unit and software. On the optical bench the laser beam runs through a complex system of mirrors and beam splitters which together form a laser interferometer. This allows precise distance measurements between the test mass and the satellite body and also between the two test masses themselves. The LISA mission will employ a similar method to detect gravitational waves from 2020 onwards. These will change the distance of five million kilometers between the satellites. The Data Management Unit (DMU) controls the overall OMS, collects and stores the data and performs an initial data analysis.

The different OMS components have been constructed by different contractors and are now being delivered. They are vetted in a further test regime at system level in the AEI laboratories.

ESA has commissioned the European aerospace company EADS Astrium at Astrium/Great Britain in Stevenage to build the LISA Pathfinder spacecraft, i. e. the payload carrier. One of the payload package the LISA technology package LTP, is produced by Astrium in Germany in collaboration with several contractors, including Tesat Spacecom GmbH.

The second payload package, the Disturbance Reduction System (DRS), is developed in the United States by JPL (Jet Propulsion Laboratory) under the leadership of NASA. It will provide a system of Micro-Newton thrusters complementary to the on-board LTP technology with dedicated control electronics.

LISA, the gravitational wave observatory in space
Gravitational waves are tiny ripples in spacetime. They are generated during cosmic events with very massive objects, e. g. when two black holes merge.

The space mission LISA, a cornerstone mission by ESA and NASA due to launch in 2020, will measure these tiny deviations of space-time at frequencies between 0.1 mHz and 1 Hz. Therefore LISA is complementary to the earthbound detectors that measure at higher frequency bands. LISA will consist of three satellites in an equilateral triangle configuration. Laser interferometry will be used to measure very small changes of their five million km baseline distance caused by a passing gravitational wave.

The exceptional sensitivity of LISA will allow extremely precise measurements and thus a view so far back into the past of the Universe that it cannot be matched by any other technology. Among other cosmic sources, LISA will observe with high precision how black holes merge to form single and larger black holes. LISA will enable gravitational scientists to predict cosmic events such as the merger of two supermassive black holes for their astronomer colleagues so that they can follow the actual event with their telescopes. LISA will also be able to observe those events that lie in the very distant past – even back to the first of their kind ever.

LISA will also help to clarify the "history of the expansion of the Universe“ and make a significant contribution to the explanation of the physical properties of the mysterious dark energy – which supposedly drives the accelerated expansion of the Universe today.

The method to measure the expansion of the Universe with LISA is based on a discovery made in1986 by Prof. Bernard F. Schutz, director at the AEI: he showed that the exact distance of a spinning binary black hole system can be deduced by analyzing its gravitational signals. This is the most reliable method for distance measurements at cosmic scales available to astronomers today.