19 July 2017
Computer simulation of the gravitational waves radiated by the merging of two black holes: The LISA Pathfinder technology demonstration paved the way to the direct measurement in space of gravitational waves, which will happen from 2034 using the LISA gravitational-wave observatory. The gravitational waves that LISA will pick up cannot be detected on Earth, as they would be swamped by seismic interferences and other factors.
The gravitational-wave observatory LISA will ‘observe’ infinitesimal space-time ripples – known as gravitational waves – and hence track down the most energy-rich and powerful astrophysical events in our universe. From 2034, gravitational waves will be investigated using laser interferometry between three satellites, located at distances of around 2.5 million kilometres from each other. Until recently the observatory was only a mission concept, but it has now been selected by ESA's Science Programme Committee (SPC) as the third large mission (L3) as part of ESA's Cosmic Vision Programme.
LISA Pathfinder consists of a science module (top left in the exploded view) and the propulsion module (bottom left), which was ejected upon reaching final orbit. The optical measurement system in LISA Pathfinder – the LISA Technology Package (LTP) – is shown on the right. It comprises two gold/platinum test masses, each enclosed within an electrode housing in compact vacuum chambers of the mission’s science module. The optical bench of the laser interferometer is arranged between the test masses (interval 38 centimetres).
A set of 22 mirrors and beam splitters is installed on a block made of Zerodur glass ceramics with a square base of 20 centimetres edge length – the optical bench of LISA Pathfinder’s interferometer. The two gold-coated test masses on either side of the optical bench are used as the mirrors for the interferometer and are located approximately 38 centimetres apart. The laser interferometer produces highly precise measurements of the position and attitude of the masses relative to each other and the orbiter.
Airbus Defence & Space.
Ready for LISA: These are the unequivocal, initial results of the LISA Pathfinder technology demonstration. The diagram shows the residual force disturbances (test mass acceleration noise), depending on the frequency of the temporal change. Here, measurements are shown in colour, and the graph also marks the requirements for both LISA Pathfinder and LISA. At many frequencies, LISA Pathfinder already satisfied – or even exceeded – the requirements for LISA. At the end of the mission, even the measurements at low frequencies satisfied or were close to the requirements for LISA.
ESA/LISA Pathfinder Collaboration.
The last command to LISA Pathfinder was sent at around 8:00 pm Central European Time on 18 July 2017, after 16 months of scientific operation, marking the end of a sophisticated technology demonstration in space. The Space Administration at the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) and the Max Planck Society funded the German contribution to this European Space Agency (ESA) mission. Paving the way for laser interferometry in space and the detection of gravitational-wave, LISA Pathfinder clearly pushed the boundaries of technical feasibility and took an important step toward the planned gravitational-wave observatory LISA (Laser Interferometer Space Antenna). LISA will be designed to 'observe' infinitesimal spacetime ripples – known as gravitational-wave – and hence track down the most energy-rich and powerful astrophysical events in our universe. From 2034, LISA will measure these occurrences using laser interferometry between three satellites, each located at around 2.5 million kilometres from the other two, forming a triangle in space. So far, LISA has only been a mission concept, but now it has been selected by ESA's Science Programme Committee (SPC) as the third large mission (L3) as part of its Cosmic Vision Programme.
LISA Pathfinder’s core assembly worked flawlessly
As the mission name suggests, the purpose of the LISA Pathfinder mission when it was launched in December 2015 was to pave the way for the LISA gravitational-wave observatory. The mission involved the testing of key technologies for LISA that would not have been possible to operate adequately on Earth due to gravity and other interferences. Some of them are installed in the so-called LISA Technology Package (LTP). This complex payload – LISA Pathfinder's core – was developed by Airbus Defence & Space GmbH in Friedrichshafen with the provision of important components and other contributions from several European countries. The centrepiece – the LTP Core Assembly – was also built and tested in Friedrichshafen and then integrated in the orbiter by the firm IABG in Ottobrunn. "This technology worked extremely well. Already the first measurements at the end of February 2016 during the start of the spacecraft's nominal operation demonstrated that we would significantly exceed some of the mission objectives," says Dr Hans-Georg Grothues, LISA Pathfinder project manager at DLR, looking back. This prompted the decision in June 2016 to extend the mission into mid-2017. "We were therefore able to conduct other long-term measurements, some of them lasting several weeks, which significantly improved on the measurement results once more. Towards the end of the mission, we largely achieved the requirements for the upcoming LISA mission and even exceeded a few of them," Grothues adds. Initial findings have already been published in a scientific journal.
Technological heart is ready for use on LISA
The tests exceeded expectations to such an extent that some of the LTP technology can now be deployed with only minor changes in LISA. "The performance of the inertial sensors for LISA’s laser interferometer was particularly pleasing, as was the drag-free-attitude-control-system (DFACS)," explains Grothues. The DFACS receives signals from the inertial sensors and then, in a feedback loop, uses the European cold gas thruster and the US-American colloid thruster by NASA's Jet Propulsion Laboratory (JPL) to keep the orbiter in balance: this means that the spacecraft and payload create an inseparable unit. The inertial sensors contain free-floating, cube-shaped test masses made of a special gold/platinum alloy with a mass of around two kilograms. They form the mirrors at the ends of the laser interferometer arms, whose light is generated by a particularly low-noise laser by the German firm Tesat Spacecom GmbH. The critical release of the test masses, which had to be fixed in place by a locking mechanism during launch, also proceeded almost without a hitch. Repeated capture, positioning and release operations of the test masses were also performed successfully over the course of the mission.
Using a huge laser triangle to track down gravitational waves
Three satellites positioned at the corners of an almost equilateral triangle, with an edge length of about 2.5 million kilometres, will form the "arms" of LISA’s laser interferometer. The distances between the test masses in the satellites change marginally when a gravitational wave passes through this constellation. "These incredibly small changes in distance are barely the size of the nucleus in a hydrogen atom. But now we know that we can detect and analyse them – and therefore gravitational waves as well – using highly sensitive laser interferometry technology in space. Thanks to LISA Pathfinder, our precise understanding of these deviations will now be incorporated in the design of the LISA gravitational-wave observatory," emphasises Grothues. Although the length of LISA Pathfinder's arms was reduced drastically to 38 centimetres in order to fit the interferometer in the mission's science module, the "LTP still permitted the representative measurement of many effects and interferences acting on the free-floating masses that will be characteristic for LISA as well," Grothues states confidently.
The European Space Agency (ESA) was overall responsible for the LISA Pathfinder mission. Airbus Defence & Space Ltd. in Great Britain built the orbiter and implemented the mission planning on ESA's behalf. Besides ESA, research institutes and industrial firms from Germany, Italy, Great Britain, Spain, Switzerland, France and the Netherlands made key contributions to the development of the LISA Technology Package (LTP) under the auspices of Airbus Defence & Space GmbH in Friedrichshafen. Airbus in Friedrichshafen also built and tested the payload centrepiece – the LTP Core Assembly – which was then integrated in the satellite by the firm IABG in Ottobrunn from the Munich region. Acting alongside Airbus Defence & Space GmbH, the Max Planck Institute for Gravitational Physics / Albert Einstein Institute (AEI) in Hannover contributed significantly to the German contribution, which was funded by the Max Planck Society and the DLR Space Administration on behalf of the Federal Ministry for Economic Affairs and Energy (BMWi).
Last modified:19/07/2017 16:00:44