AEI / MM / exozet; Gravitationswellen-Simulation: NASA / C. Henze
The largest Observatory in the World
The ESA mission LISA (Laser Interferometer Space Antenna) is expected to detect low-frequency gravitational waves from space from 2035 and investigate the physical properties of their sources with great accuracy. Gravitational waves are oscillations in space-time. They are caused by rapid temporal changes in the spatial distribution of very large masses, for example by the merger of two black holes.
They propagate through space at the speed of light and exhibit an oscillation pattern and a frequency curve that are characteristic of their source. The frequencies of gravitational waves that can be observed with LISA lie in a very slowly oscillating range from a few hundred thousandths up to about one Hertz (oscillations per second). This makes it possible to observe sources that are between a thousand and ten million times the mass of our sun, as they emit such slowly oscillating waves.
LISA will complement an existing network of ground-based gravitational wave observatories. These include, for example, LIGO in the USA and Virgo in Italy. The ground-based observatories receive oscillations at significantly higher frequencies of around 30 to several thousand Hertz and therefore observe different sources than LISA.
Gravitational waves make themselves felt as tiny, periodically repeating changes in distances on earth and in space, the amplitudes of gravitational waves. These waves reach us with such an incredibly small strength that they are extremely difficult to measure. Gravitational wave detectors such as LISA or LIGO work with laser beams that travel over defined measuring distances (laser interferometers). When a gravitational wave hits the detector, it stretches and compresses these paths periodically, which can be measured with the help of the laser beam. The change in length for LISA is of the order of just a few femtometres, i.e. one hundredth of the diameter of a hydrogen atom nucleus.
Simulation of the emission of gravitational waves during the merging of two black holes
The simulation shows two black holes colliding and merging. This process releases gravitational waves, which are shown here with different colours depending on their oscillation amplitude. A reddish colouring indicates large amplitudes, with increasingly smaller amplitudes towards blue and black. LISA will be able to register these gravitational waves and those from a large number of similar sources. This will provide a great deal of information about these sources and thus greatly expand our understanding of the development of our universe.
Existing detectors on Earth, such as Virgo or LIGO, work with measuring distances of three and four kilometers respectively. They observe gravitational waves with high frequencies that are emitted by comparatively low-mass cosmic objects. At around 2.5 million kilometers, the measuring distance of LISA will be much larger than that of the earthbound gravitational wave detectors. It can be used to detect gravitational waves at very low frequencies. They originate from cosmic objects that are up to more than ten million times the mass of our sun. LISA will therefore be an important complement of the existing detectors.
In addition to ground-based observatories, LISA will be able to observe very different sources of gravitational waves
In addition to ground-based observatories, LISA will be able to observe very different sources of gravitational waves. These include, for example, extremely short-period binary stars in our galaxy, massive black holes with up to several tens of millions of solar masses in the centers of distant galaxies, the "inspiral" of compact objects such as stars and neutron stars into massive black holes, and black holes with masses comparable to the sun orbiting each other ever more closely. Their amplitudes ("Characteristic Strain") are plotted over the gravitational wave frequencies. The sensitivity of the instrument is shown as a dashed line. In addition to the galactic background (grey) and short-period binary stars in our galaxy (blue dots and stars), the temporal developments of binary systems of massive black holes (MBHBs - Massive Black Hole Binaries) and the inspiral of low-mass objects into massive black holes (EMRIs - Extreme Mass Ratio Inspirals) are shown. Also shown is the evolution over time of the first detected source GW150914 (blue line), which long ago moved out of the LISA frequency band and into that of LIGO. The red and grey lines show a similar development for other sources.
Credit:
LISA consortium (edited figure from Physik Journal 21 (2022) No.2)
The tiny amplitudes of a gravitational wave can only be detected by highly sensitive laser measurements (interferometry). LISA will set up a laser interferometer in space for this purpose, with three identical spacecraft forming an almost equilateral triangle with sides of about 2.5 million kilometers in length. There are two telescopes on board each spacecraft, which will send laser beams to the other two probes and receive laser light from them in turn. Each of the telescopes has a mirrored, cube-shaped test mass made of a special metallic alloy (Gravitational Reference System - GRS) and an optical bench (OB); the test masses float freely in their housings during measurement operation. They also form mirrors for the laser beams emitted by the other probes. External interference on the spacecraft and thus the test masses, caused for example by the radiation pressure of the sun, magnetic fields or changing gravitational forces, is largely eliminated by their structure. Other disturbances are compensated for with the help of a so-called "Drag-Free Attitude Control System" (DFACS) and highly sensitive steering thrusters (cold gas micronewton thrusters). The DFACS measures these disturbances and converts the measurement results into corresponding correction signals for the thrusters.
Another important aspect is the contactless control of unwanted electrostatic charging of the test masses using UV light (discharge). This charging is primarily caused by cosmic particle radiation, which is omnipresent in space. Together with the necessary, low-interference control and detection electronics (including a so-called phasemeter), the elements described form the "Moving Optical SubAssembly" (MOSA). The telescope, OB and GRS are tracked by this moving mount in such a way that the laser connection to the opposite spacecraft of the interferometer is always maintained. Direct back-reflection of the laser beam over 2.5 million kilometers is not practical, even with the very low beam expansion, due to the extremely low beam energy at the receiver; instead, a phase-coupled "fresh" beam is sent back from the receiving probe.
GRS and OB, as well as other technologies now being used in LISA, were already successfully tested in space between 2015 and 2017 during the LISA Pathfinder technology demonstration mission.
The Origin of Gravitational Waves
Sufficiently strong gravitational waves that can be detected on Earth are caused by very rapid temporal changes in very large masses that are at least comparable to the mass of the sun. These are, for example, binary stars in our Milky Way that orbit each other in an extremely short period of just a few minutes, black holes that tear stars apart or even merge with other black holes. The masses of black holes range from a few to millions of solar masses, especially in the case of black holes in the centres of distant galaxies. Neutron stars, compact remnants of massive stars at the end of their lifetimes, can also merge and generate gravitational waves. Finally, gravitational waves are also expected to have been emitted during processes very shortly after the Big Bang. Similar to waves on a water surface, all these gravitational waves propagate through space at the speed of light and can be detected by LISA. These observations of gravitational waves will make a significant contribution to our knowledge of the development of the universe, the formation of galaxies and also of gravity itself.
Also of great importance is the planned search for signals from gravitational wave events and permanent sources of gravitational waves in visible light and other areas of the spectrum of electromagnetic waves, such as gamma and X-rays or radio waves through telescopes on the ground and in space. Finally, neutrinos from gravitational wave events will also be searched for (neutrinos are elementary particles with extremely low mass that can be produced during processes in the vicinity of gravitational wave events). The aim is to obtain as comprehensive a picture as possible for the astrophysical interpretation of the sources of gravitational waves.
The LISA laser interferometer consists of three spacecraft arranged in an (almost) equilateral triangle with sides measuring 2.5 million kilometers
The LISA laser interferometer consists of three spacecraft arranged in an (almost) equilateral triangle with a side length of 2.5 million kilometers; as an entire configuration, they follow the Earth on its orbit around the sun at a distance of around 50 million kilometers (anti-clockwise).
International Cooperation and German Participation in LISA
As an L3 mission in ESA's science programme, LISA is being developed and built with the participation of NASA and with contributions to the payload from more than ten European countries. A scientific LISA consortium is significantly involved in this development and is also setting up the mission's data processing and archiving. The German contribution to LISA consists of the provision of the payload's central phasemeter by the Max Planck Institute for Gravitational Physics / Albert Einstein Institute (AEI) in Hanover. In addition, the AEI will supply a critical opto-mechanical mechanism for the payload in co-operation with Dutch partners. Finally, the AEI will also support the mission with many system design issues. The AEI is also the Principal Investigator of the mission. AEI's participation in LISA is significantly supported by grants from the German Federal Ministry of Economic Affairs and Climate Action (BMWK), represented by the German Space Agency at DLR.
Mission Data
Mission
LISA (Laser Interferometer Space Antenna)
Mission topics
Detection of gravitational waves and characterisation of their sources
Launch date
August 2035
Launch site
Kourou (French Guiana)
Launcher
Ariane 6.4
Mission duration
min. 6.25 years (incl. 4.5 years nom. operations) / 6 years mission extension
Orbit
Heliocentric drift orbit (Earth distance > 50 million km)
Launch mass
approx. 8,300 kg (three spacecraft)
Payload mass
approx. 830 kg (payload per spacecraft)
Dimensions
4.8 m x 3.0 m x 1.1 m (one spacecraft) / triangle with a side length of 2.5 million km
Electrical power consumption
approx. 2,300 W (spacecraft with payload, full payload operation)
Telemetry rate
approx. 270 kbit/s (X-band, downlink per spacecraft)
