Full marks for LISA Pathfinder in final exam
The test mission for the gravitational-wave detector in space LISA has reached an important milestone after a rigorous review of the whole mission
LISA Pathfinder (LPF) passed its final examination at the end of the development phase with flying colors: the whole concept - from the scientific instruments and the operating system to the space hardware - stood up to the intense scrutiny of ESA-reviewers in a final 'Critical Design Review’. A launch in late 2012 or early 2013 should now be possible. The Critical Design Review was conducted at the European Space Research and Technology Centre (ESTEC) of the European Space Agency (ESA) in Noordwijk, the Netherlands.
Gravitational waves tell of star explosions, the collision of black holes and even the Big Bang itself. The space-based gravitational wave detector LISA (Laser Interferometer Space Antenna) will observe the gravitational waves from coalescing binary black holes among other sources, beginning its search for the elusive signals in 2020. Working in conjunction with other astronomical methods and gravitational wave observatories on Earth, we will then be able to observe the unknown and uncharted ‒ the so-called "Dark Side of the Universe".
LISA Pathfinder will test the technology for observing gravitational waves in space from late 2012 onward, and was now stringently examined itself, including:
- the highly sensitive laser interferometric system
- the concept of free floating test masses
- the data transmission software
- the control systems, data analysis procedures as well as
- the rocket, ground segment, satellite guidance and mission logistics
"LISA Pathfinder is unique ‒ the spacecraft is a scientific and technological tour de force. Once launched, we can't recall it for repairs or to improve on something. So we have to make certain that all components will survive the conditions of launch and space flight intact and that they will all work and interact as planned", explains Prof. Dr. Karsten Danzmann, director at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute) and head of the Institute for Gravitational Physics of the Leibniz Universität Hannover. "So we have been reviewing the whole mission in all stages of development right from the start, together with ESA and independent referees."
"We know what each individual component is supposed to be doing and we compare the projected numbers with the actual test performance. This final review did not spring any surprises on us and we are very pleased with the result", continues Karsten Danzmann who is also one of the two Principal Investigators for LISA Pathfinder and the speaker for Europe for the large LISA mission to follow. These reviews determine the continuation or cancellation of certain lines of development or even of the complete mission.
The review process
In a veritable review marathon, every single aspect of the mission is examined again and again, subjected to stern scrutiny during each stage of development before it is cleared for launch at the very end. A milestone in this review process is the so-called "Critical Design Review": a final examination and evaluation of mission concept and implementation which was now completed by ESA experts.
The LISA Pathfinder mission payload is the LISA Technology Package (LTP). This was developed and built in close collaboration of scientists and industrial partners. The Albert Einstein Institute in Hannover has led the scientific development effort for LTP under Prof. Dr. Karsten Danzmann, also leading the science mission design for LISA Pathfinder together with the University of Trento. The Albert Einstein Institute has received funds for industrial contracts from the Deutsches Zentrum für Luft- und Raumfahrt DLR (German Aerospace Center).
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.