LISA Pathfinder paves the way for the detection of gravitational waves in space
Yearbook article 2016, Max Planck Institute for Gravitational Physics, Hannover
Author: Jens Reiche
The LISA Pathfinder satellite mission demonstrates core technologies for future gravitational-wave observatories in space like eLISA. These observatories will study low-frequency gravitational waves, which are emitted by, e. g., binary supermassive black holes or galactic binary stars. LISA Pathfinder was launched on December 3, 2015, and commenced its science operations in March 2016. LISA Pathfinder will lead to a comprehensive model of all significant physical noise sources that can be extrapolated to the eLISA mission.
Albert Einstein predicted gravitational waves back in 1916 on the basis of his General Theory of Relativity. He provided a quantitative estimate for the extremely slight effects of passing gravitational waves: they express themselves in periodic changes of distances in the space-time continuum of the General Theory of Relativity. Characteristic relative changes in length are between around 10-18 and 10-24 depending on the type and distance of the source. It was this minute effect that led Einstein to doubt that it would ever be possible to measure gravitational waves directly. On 14 September 2015, the ground-based advanced LIGO detectors successfully detected gravitational waves for the first time ever. They originated from the fusion of two black holes separated by a distance of more than one billion light years.
The spectrum of gravitational waves
Gravitational waves originate from accelerated large masses. These include binary star systems, supernovas, the merger of compact objects such as neutron stars and black holes, and supermassive black holes in the centres of distant galaxies which coalesce with one another.
Essentially dependent on the masses involved, these sources emit gravitational waves whose frequencies span around 22 orders of magnitude, from around 10-18 Hz up to around 104 Hz. In order to investigate as much as possible of this spectrum and the sources, ground-based detectors are required for the short-wavelength portions of the spectrum, as well as space-based observatories for the long-wavelength portions.
eLISA: Measurement of gravitational waves in space
The measurement paths of ground-based interferometric detectors such as GEO600, LIGO and Virgo are restricted to a length of only a few kilometres, thus limiting the sensitivity to the changes in these lengths caused by gravitational waves as well.
This restriction does not apply in space: the planned evolving Laser Interferometer Space Antenna (eLISA) gravitational wave observatory is to comprise three satellites which form the corners of an almost equilateral, triangular laser interferometer with arms several million kilometres in length (Fig. 1). Gravitational waves are to be detected by the changes in the lengths of the arms. The measurement accuracy required still amounts to around 0.001 nm, one hundredth of the diameter of an atom, however. This gives an idea of the metrological challenges – particularly in controlling interfering effects – which hinder the detection of even the strongest gravitational waves.
A further advantage of taking measurements in space is the lack of any seismic activity, which provides an insurmountable limit to the ground-based gravitational wave observations below around one hertz. The frequencies which must be metrologically covered with space experiments such as eLISA range from a few
10-5 Hz through to around 0.1 Hz in the gravitational wave spectrum. Low-frequency gravitational waves and their massive sources can be investigated only with the aid of measurements in space.
Observatories such as eLISA will observe objects such as supermassive black holes with millions of solar masses, which are orbiting each other in close proximity and ultimately fall into each other in the centres of merging galaxies. In addition, the gravitational wave signals in this frequency range are probably dominated by thousands of binary star systems in our Milky Way with orbital periods of only a few minutes.
Technology demonstration with LISA Pathfinder
After more than ten years of development time, LISA Pathfinder (LPF) is to now pave the way for the eLISA gravitational wave observatory. As a range of new technologies is required for eLISA, the critical technologies are to be developed initially as part of the LPF testing mission and tested under space conditions.
To this end, one of the eLISA interferometer arms was scaled down from millions of kilometres to around 40 centimetres so that two test masses which simultaneously function as the end mirrors of the interferometer arm fit into a satellite. Both test masses are additionally located in vacuum containers in order to minimize interferences by residual gases (Fig. 2). The requirements which eLISA places on the measurement system were moreover reduced by a factor of 10: for LISA Pathfinder, the freedom from interferences is defined by the maximum permissible spectral density of the interfering acceleration of the test masses of 3·10-14 ms-2Hz-1/2 in the frequency range between 10-3 and 10-2 Hz.
The main objective of LISA Pathfinder consists in confirming one of the prerequisites of the General Theory of Relativity, namely that the motion of bodies subject only to gravity follows so-called geodetic lines, i.e. the shortest connections in the four-dimensional space-time continuum. For this purpose, LISA Pathfinder will primarily carry out detailed tests of three important technologies, which it is impossible to conduct on Earth or only with restrictions.
Capacitive inertial sensors with extremely low-noise electronics will detect the positions and orientation of the free-floating test masses, which represent the mirrors at the ends of the laser interferometers, relative to the satellite and to each other. The test masses consist of a special gold-platinum alloy (73% Au, 27% Pt) with an edge length of 46 mm, each weighing around 2 kg. In order for the test masses to be able to follow the geodetic lines without disturbance, they require complex screening from all possible interfering forces. These include the radiation pressure of the Sun, electric and magnetic fields, and thermal effects. Moreover, the mass distribution in the satellite must be balanced very accurately in order to minimize gravitational gradients.
The Drag-Free Attitude Control System (DFACS) has a feedback loop with the inertial sensors to largely compensate the interfering forces acting on the satellite and the test masses. These include, for example, minimal electric stray fields on metal surfaces, static charging in the satellite (caused by cosmic radiation), pressure differences (caused by tiny temperature gradients in the residual gas) around the test masses, and the radiation pressure of the laser onto the test masses. Compensation is by way of micronewton thrusters, whose tiny thrust is extremely constant and can also be regulated in very fine steps. Cold gas and colloid thrusters are used here. The technology is also interesting for other missions of fundamental physics, such as the “Microscope” space mission which is currently being built.
Finally, LISA Pathfinder is also to demonstrate the successful use of highly sensitive laser interferometry in space which, in parallel with the inertial sensors, measures the position and orientation of the test masses with the highest precision (around 0.01 nm).
Development of LISA Pathfinder
Selected by ESA in October 2000 as SMART-2 mission for the technology demonstration for LISA, initial developments of critical technologies started in 2001. This was followed from 2004 onwards by the development and implementation of the LISA Pathfinder overall mission with the space probe and the LISA Technology Package (LTP) and the Disturbance Reduction System (DRS) as the American contribution, both payloads of the mission.
The launch was initially planned for the middle of 2009. It soon turned out, however, that the demands placed on the mechanical, thermal and electric freedom from interference (low system noise) were much closer than expected to the limit of what is technically feasible. Various unforeseen technical challenges had therefore to be overcome, thus delaying the launch date with the new European launch vehicle Vega to 3 December 2015.
The international LISA Pathfinder team
LISA Pathfinder and its payload, the LTP, was developed as a technology mission headed by the European Space Agency ESA with crucial contributions of different institutions from ESA member states.
While the ESA – as is usual with scientific space missions – is responsible for the satellite, the launch and the mission operation, institutions and companies of the interested ESA member states build and finance the payloads. As payload and space probe are very closely interrelated on LISA Pathfinder and the two of them represent a single measurement instrument in a way, the ESA has also played a significant role in the development of the LTP in this case.
The scientific management of LISA Pathfinder is shared between the University of Trento in Italy (Stefano Vitale) and the Max Planck Institute for Gravitational Physics (Albert Einstein Institute) in Hanover (Karsten Danzmann). In addition to these two institutes and the aerospace companies commissioned by them, further scientific contributions and additional supplies of hardware to the LTP are supplied by research institutes and universities from Great Britain, Spain, Switzerland, France and the Netherlands. The German contribution particularly comprises the execution of system tasks for the LTP development and the provision of the optical measurement systems and the laser system.
The German contribution was funded by the Space Agency of the Deutsches Zentrum für Luft- und Raumfahrt e.V. with funds from the German Federal Ministry for Economics and Technology on the basis of a resolution of the German Bundestag under the funding code ID 50 OQ 0501.
Mission schedule for LISA Pathfinder
At 1:04 am local time (5:04 am CET) on 3 December 2015, LPF was launched into space on a Vega rocket from the European space station near Kourou in French Guyana – almost 100 years to the day after the publication of the General Theory of Relativity (Fig. 4).
After the successful launch and the long journey to the measuring point, a halo orbit about the Lagrange point 1
around 1.5·106 km away from Earth towards the Sun, the individual payload systems were successfully switched on one after the other and checked to see whether they were operational in the second week of January 2016. Following this – on 22 January 2016 – LISA Pathfinder was detached from the propulsion module and then entered into the halo orbit.
A particularly important step at the beginning of February was the release of the two test masses which were secured by a complex fixing mechanism during the launch. The procedure of a carefully directed release, capture and control by electrostatic forces was successful. Afterwards, the cubes floated freely at a distance of a few millimetres from the walls of the housing without any mechanical contact to the satellite.
Subsequently, the manufacturing industry proved that all systems were working as planned and that the critical technologies for eLISA had been successfully produced, tested under space conditions and fulfilled the requirements in full. At the beginning of March, the satellite started its scientific measuring operations as planned. The mission scientists will carry out hundreds of experiments in rapid succession until the end of September 2016.
According to current plans, the LPF mission is to be concluded by the end of 2016. The results will flow directly into the development of eLISA, which is to definitively start from this year onwards.