Questions about LISA
Open questions about the project, or LISA itself? Some questions have already been asked and answered. If there is a question missing from your point of view, just write to us!
LISA Mission
How did LISA get its name?
It’s an acronym: LISA stands for “Laser Interferometer Space Antenna”.
How long will the LISA mission last?
The LISA mission is designed for 4 years of nominal science operations, with a potential extended mission of up to 6 years. In addition to wear-and-tear of the spacecraft and its instruments, limitations to LISA’s lifetime come from the amount of propellant available to perform the drag-free flight of the spacecraft around the test masses, the long-term stability of the orbits that form the constellation, and communications difficulties associated with increasing distance between the consteallation and Earth.
LIGO has already found gravitational waves, why do we need LISA?
Gravitational wave science is about much more than just verifying the existence of the waves themselves. Long before LIGO made its first detection in 2015, the consensus amongst most physicists was that gravitational waves were real. The real power of gravitational waves is as a new tool for understanding our Universe. The early results from LIGO have already demonstrated this potential by uncovering what appears to be a new population of heavy black holes as well as determining the origin of heavy elements in the Universe through observations of a neutron star merger that was also observed by a large number of electromagnetic telescopes. Since LISA observes in an entirely separate band from LIGO, it can help answer different questions such as: “How did the massive black holes at the centers of galaxies form and grow?, “How have stars in our Milky Way evolved and died?”, and “Is general relativity the correct description of gravity and black holes?”
How does LISA differ from ground-based gravitational wave interferometers like LIGO, Virgo, and KAGRA?
Gravitational wave interferometers all operate on the same physical principle that gravitational waves can be observed by measuring the proper distance between freely-falling objects using beams of light. However LISA will operate in a very different regime to ground-based observatories. LISA’s million-kilometer-scale arm lengths are optimized to observe gravitational waves with milliHertz frequencies. These low-frequency gravitational waves don’t influence detector like LIGO very much since they are optimized to detect frequencies in the tens to hundreds of Hertz. In general, LISA will observe systems with larger masses and increased separations in comparison to those observed by LIGO, Virgo, and KAGRA. LISA sources will also tend to evolve more slowly, allowing longer observations of each source. The two types of observatories complement one another, just like how different types of electromagnetic observatory (e.g. radio, optical, X-ray, etc.) complement one another.
What is NASA’s role in LISA?
LISA is led by the European Space Agency (ESA), which in 2017 selected LISA for study as a large-class mission in the Cosmic Visions Programme. LISA was Adopted as a project by ESA’s Science Program Council in January 2024. Partnering with ESA are NASA and a collection of European National space agencies. NASA will provide three critical hardware elements for LISA: lasers, telescopes, and charge management devices. In addition, NASA is developing a science ground segment to process the LISA telemetry and produce scientific data products for public consumption. NASA scientists, engineers, and managers are working closely with the ESA and European counterparts to ensure that LISA is a success.
What type of rocket is currently intended to carry the three satellites into space?
Ariane 6 and/or Vega-C (co-developed by ArianeGroup and Avio) are possible options.
Technologies
Why are the LISA spacecraft sometimes called Sciencecraft?
The usual structural and thermal analysis of the spacecraft has therefore been extended to include gravitational effects as well to ensure that the requirements on gravity gradient at the position of the test masses is fully met. In addition, the payload controls the position of the spacecraft during science operations, rendering the spacecraft effectively a part of the instrument. The importance of the co-design and the co-operation of spacecraft and payload is captured in the term “sciencecraft”.
What is Time Delay Interferometry (TDI) and how does it work?
Interferometry is a technique that uses the interference of waves to make precise measurements. The wavelength of the interfering waves acts like the tick marks on a ruler for measuring distance. Optical interferometers can make very precise measurements because the wavelength of the light waves they use is small — around one micron for instruments like LIGO and LISA. A fundamental limitation of interferometry is that precision of the measurement is limited by the stability of the waves used in the interferometer. For an optical interferometer, if the wavelength of the light fluctuates, a spurious signal will be generated that mimics physical motion. One way to mitigate the effect of a fluctuating source is to compare pairs of distances using a common light source. This is the underlying concept of the Michelson interferometer that was used by Albert Michelson and Edward Morely to search for the “luminferous aether” in the late 19th century. LIGO uses the same concept in its interferometers over a century later. In order for this technique to work, the lengths of the light paths must be precisely matched. While LISA’s orbits produce approximately-equal arms, they differ by up to a percent and fluctuate by almost the same amount over long time periods due to orbital mechanics. Time Delay Interferometry (TDI) is a technique that was developed in the late 1990s and early 2000s to allow LISA to take advantage of the “common mode rejection” effect despite having unequal arms. TDI takes advantage of the fact that LISA measures the interference in each one-way laser link individually. While each of these signals is dominated by fluctuations in the LISA laser wavelength, those same fluctuations are measured at multiple points in the LISA constellation with varying time delays. By combining these individual measurements and correcting for the time delays, and adding in some rough knowledge of the constellation geometry, a significant amount of suppression of laser wavelength noise can be achieved. The ability to suppress laser wavelength noise through TDI is primarily determined by the precision of the individual interference measurements and the accuracy of the estimates of the LISA arm lengths. TDI has been extensively examined in analytic studies, numerical simulations, and experimental analogues and has been demonstrated to work as expected. The LISA team continues to refine our understanding of this important technique to ensure that it will provide the sensitivity that LISA requires to achieve its science goals.
What is LISA´s payload?
LISA´s payload consists of two identical units on each spacecraft. Each unit contains a Gravitational Reference Sensor (GRS) with an embedded free-falling test mass that acts both as end point of the optical length measurement and as geodesic reference test particle. A telescope transmits the laser light along the arm and also receives the weak light (few hundred pico-Watts!) from the other end. Laser interferometry is performed on an optical bench in between the telescope and the GRS.
What are the test masses made of?
The test masses are 46 mm cubes, made from a dense non-magnetic Au-Pt alloy and shielded by the Gravitational Reference Sensor (GRS). The GRS core is a housing of electrodes, at several mm separation from the test mass, used for precision capacitive sensing and electrostatic force actuation in all non-interferometric degrees of freedom. The GRS also includes fibers for UV light injection for photoeletric discharge of the test mass and a caging mechanism for protecting the test mass during launch and then releasing it in orbit. The GRS technology is a direct heritage from LISA Pathfinder.
What are LISA´s key features?
Key features of LISA are interferometric measurement of distances, million-km long baselines, drag free spacecraft based on inertial sensors, and the familiar “cartwheel”-orbits. Unique are the free-falling test masses inside each spacecraft. The test masses will be undisturbed by forces other than gravitation. A new technology, the so-called “drag-free” operation, allows the spacecraft to follow the test masses, all the while shielding the test masses from spurious forces.
How precisely does the distance between the LISA satellites need to be maintained?
The gravitational waves that LISA is designed to observe have typical timescales of hours. So long as the distance between the satellites is smoothly changing over these time scales, the gravitational waves can be observed as an additional modulation on top of this smooth change. Each satellite is in an independent Keplerian orbit around the Sun with the plane of the triangle inclined at 60 degrees to the plane of the ecliptic. Over the course of the mission, the nominal 2.5 million kilometer distance between each satellite will vary by hundreds of thousands of kilometers. LISA will be able to measure the absolute distance between the satellites to a few centimeters and will measure hour-scale fluctuations at the level of several picometers (1pm = 1 trillionth of a meter), the level required to detect gravitational waves.
How can LISA measure the distance between its spacecraft?
LISA’s distance measuring system is a continuous interferometric laser ranging scheme, similar to systems used for radar-tracking of spacecraft. But for LISA, the direct reflection of laser light, such as in a normal Michelson interferometer, is not feasible due to the large distance of million km between the spacecraft: Diffraction expands the laser beam so much that for each Watt of laserpower sent, only about 250 pW are received. Direct reflection would thus result in an attenuation factor of about 6.25 x 10-20, yielding about one photon in every three days.
How are the three LISA spacecraft able to point at one another?
The orbits of the LISA spacecraft are set up in such a way that the constellation maintains a nearly perfect equilateral triangular shape that is inclined by roughly 60 deg with respect to the ecliptic plane. Once each spacecraft is inserted into its predetermined orbit, tracking from the ground will be used to precisely locate them and determine their relative positions. The spacecraft will then undergo a “constellation acquisition” procedure which begins with one spacecraft turning on its laser while its partner spacecraft scans the sky. At some point during the scan, an acquisition sensor on the partner spacecraft will detect the laser and record its position. The spacecraft will then orient towards that position and turn on its own laser. Once a two-way laser “link” is established, precision interferometric measurements can be used to align the beams. This same procedure is repeated to establish the remaining links in the constellation. This procedure has been verified in simulations and will continue to be refined as the LISA design matures. A variant of this procedure was used to establish the laser link between widely-separated spacecraft on the GRACE-FO mission which launched in 2018.
GW-Science
LIGO has already found gravitational waves, why do we need LISA?
Gravitational wave science is about much more than just verifying the existence of the waves themselves. Long before LIGO made its first detection in 2015, the consensus amongst most physicists was that gravitational waves were real. The real power of gravitational waves is as a new tool for understanding our Universe. The early results from LIGO have already demonstrated this potential by uncovering what appears to be a new population of heavy black holes as well as determining the origin of heavy elements in the Universe through observations of a neutron star merger that was also observed by a large number of electromagnetic telescopes. Since LISA observes in an entirely separate band from LIGO, it can help answer different questions such as: “How did the massive black holes at the centers of galaxies form and grow?, “How have stars in our Milky Way evolved and died?”, and “Is general relativity the correct description of gravity and black holes?”
Can LISA science be done from the ground?
No. Both ground motion and time variations in familiar Newtonian gravity from spurious mass motions on the Earth prevent observations below about 1 Hz on the ground. It is necessary to make measurements in space in order to observe many of the important astrophysical sources throughout the Universe.
How can LISA observe so many sources simultaneously? Won’t there be a source confusion problem?
At any one moment, LISA will be sensing gravitational waves from millions of individual sources. The vast majority of these will be binary systems of compact objects in the Milky Way, but signals will also be received from extragalactic sources such as the mergers of massive black holes. Each of these signals has a distinct waveform that depends on the astrophysical properties of the source (masses, spins, orientations, positions, etc.). Thanks to extensive work in theory and modeling, we have very good templates for these sources which we can compare to the LISA data and extract individual signals using a technique known as matched filtering. The entire LISA data set is processed as a hierarchical global fit, where individual sources are added and subtracted to improve the overall fit. The most significant sources are easily identified and characterized. As the signal strength decreases, a point is eventually reached where no additional sources can be confidently extracted. Simulations with mock LISA data suggest that tens of thousands of individual signals will be identified in the full LISA data set with the remaining Milky Way binaries producing an unresolved, but still detected, foreground of gravitational waves in the lower part of the LISA sensitivity band. The LISA community is continuing to conduct mock data challenges of increasing sophistication to hone the data analysis techniques that will be used to solve this problem.
How precisely does the distance between the LISA satellites need to be maintained?
The gravitational waves that LISA is designed to observe have typical timescales of hours. So long as the distance between the satellites is smoothly changing over these time scales, the gravitational waves can be observed as an additional modulation on top of this smooth change. Each satellite is in an independent Keplerian orbit around the Sun with the plane of the triangle inclined at 60 degrees to the plane of the ecliptic. Over the course of the mission, the nominal 2.5 million kilometer distance between each satellite will vary by hundreds of thousands of kilometers. LISA will be able to measure the absolute distance between the satellites to a few centimeters and will measure hour-scale fluctuations at the level of several picometers (1pm = 1 trillionth of a meter), the level required to detect gravitational waves.
LIGO and other ground-based interferometers are enormously complex, isn’t attempting this in space too difficult?
Since gravitational waves are the stretching of spacetime itself, they have the interesting property that the measured displacement between two reference objects scales with the original separation between those objects. In other words, if there is more spacetime to stretch, the total stretch is larger. LISA’s arms are roughly a million times longer than LIGO’s, which means that a gravitational wave of the same amplitude will produce displacements that are roughly a million times larger in LISA. The total displacement is still small, on the order of picometers (one picometer = one trillionth of a meter) but is well within the range of modern metrology techniques. From the metrology perspective, the LISA measurement challenge is “easier” than that of LIGO, which is important given that it has to be robust enough for spaceflight as well as be able to be operated from far away.
What are gravitational waves?
Gravitational waves are ripples in the fabric of space-time generated by some of the most powerful astrophysical events – such as collisions of black holes and exploding stars. Gravitational waves travel at the speed of light through the universe. They allow us to explore the dark side of the universe.
What can we learn from the observed signals?
The gravitational waves that LISA will discover include ultra-compact binaries in our Galaxy, supermassive black hole mergers, and extreme mass ratio inspirals, among other possibilities. LISA will be the first to explore gravitational waves in the frequency range of 0.1 milliHertz to 0.1 Hertz.
LIGO and other ground-based interferometers are enormously complex, isn’t attempting this in space too difficult?
Since gravitational waves are the stretching of spacetime itself, they have the interesting property that the measured displacement between two reference objects scales with the original separation between those objects. In other words, if there is more spacetime to stretch, the total stretch is larger. LISA’s arms are roughly a million times longer than LIGO’s, which means that a gravitational wave of the same amplitude will produce displacements that are roughly a million times larger in LISA. The total displacement is still small, on the order of picometers (one picometer = one trillionth of a meter) but is well within the range of modern metrology techniques. From the metrology perspective, the LISA measurement challenge is “easier” than that of LIGO, which is important given that it has to be robust enough for spaceflight as well as be able to be operated from far away.
What makes the Gravitational Universe so exciting?
The Gravitational Universe is a new window in astronomy. Powerful sources of gravitational waves are being used to probe a universe that cannot be explored by other means. Significant advances in astronomy have been made by looking at the Universe using electromagnetic radiation as a probe. But with gravitational waves, we can also study the dark universe, analogous to listening for objects that do not produce light. LISA will enable us to explore the dark universe through gravitational waves.
What will we learn from the Gravitational Universe?
Gravitational waves are ripples in the fabric of space-time generated by some of the most powerful astrophysical events – such as exploding stars and collisions of two black holes at the centres of galaxies. Gravitational waves travel at the speed of light through the universe, unhindered by intervening mass – to gravitational waves the universe is transparent. That is why gravitational waves are the cosmic messengers that allow us to explore the so far dark side of the universe.
Data Analysis
How are the scientific products being delivered?
In strong interaction with the Data Analysis Groups of the LISA Consortium, the DPC will implement, execute and control the data analysis pipelines and deliver the scientific products (such as catalogues of identified gravitational waves) to the consortium. To do so, it’s main focus will be on developing tools to support: software development, test and validation; pipeline integration and deployment on computing infrastructures; data management, tracing and archiving; simulation activities.
How can we extract science from the data?
Key components of LISA´s data analysis are the ability of creating high-fidelity waveforms for gravitational wave sources, having a well-understood signal simulator for the mission, and being able to extract the source parameters from the simulated signals. In so-called Mock LISA Data Challenges (MLDC) scientists already demonstrated the feasibility of LISA data analysis.
How does LISA localize sources and how well will it do so?
LISA is an all-sky instrument, with the sensitivity to gravitational waves only weakly depending on the location of the source in the sky. Localization of individual sources comes from two main effects. The first is the motion of the LISA constellation around the Sun, which introduces shifts in both frequency (Doppler effect) and amplitude (sweeping the LISA sensitivity pattern across the sky). These shifts encode information about the sky position of the source in the waveform that LISA observes. Since most LISA sources are observed for months or years, there is sufficient modulation to provide localization. The second effect is that, for the higher frequency sources that LISA observes, the wavelength of the gravitational waves is similar to or smaller than the size of the LISA constellation. This means that different parts of the constellation experience the gravitational wave at slightly different times, which again encodes information about the location of the source. The precision of LISA’s localization of a particular source depends on many factors including the type of source, the particular parameters of the source, and the duration of the observation. For the best-localized sources, the final localizations may be on the order of a few arcminutes. Degree-scale localization will be more typical and the more numerous faint sources will be localized less well. Interestingly, LISA’s localization of a particular source will improve over time, which will open up some novel observing strategies for potential EM counterparts of events such as mergers of massive black holes.
How much data will LISA generate and how will it get to the ground?
LISA’s data and telemetry requirements are relatively modest when compared to many other astrophysics missions. While the precise details are being developed as part of the mission formulaiton process, the rough numbers are known. During normal operations, only one of the three LISA spacecraft will be in contact with the ground. In addition to transmitting its own data, the spacecraft will serve as a relay for data from the other two spacecraft, which will share data over a dedicated inter-constellation link. This is efficient because the separation between spacecraft (2.5Mkm) is roughly 20x smaller than the distance to Earth (approximately 50Mkm). The required data rate to Earth is appoximately 150kbps, or about the speed of a good household modem in the late 1990s. Daily contact will be made with the constellation for a period of roughly 8 hours, resulting in a aggregate data rate of roughly 4GB/day. This will include the primary outputs from the science instrument, auxilliary channels used to monitor the science instrument, and general spacecraft housekeeping data for the full constellation. This data will be processed on ground to produce LISA’s basic measurement product, time-delay interferometry (TDI) variables, which contain the full set of gravitational wave signals in the LISA band as well as residual instrumental noise. The four fundamental TDI variables will be sampled with a rate of a few Hertz, resulting in a data rate of roughly 60MB/day. The TDI data will be used to generate further downstream products such as source catalogs, alerts, etc.
What are data analysis pipelines?
A data pipeline is a set of actions that ingest raw data from disparate sources and move the data to a destination for storage and analysis. A pipeline also may include filtering and features that provide resiliency against failure. There are essentially three major types of pipelines along the transportation route: gathering systems, transmission systems, and distribution systems.
What are LISA Data Challenges (LDC)?
LDCs are based on blind challenges of increasing complexity – from a few sources in the first challenge to the full combination of all likely sources in the data stream in the most recent fourth challenge. Scientific research groups from all over the world developed, tested and implemented a wide variety of techniques. As a result a proof-of-concept for LISA data analysis is strongly tested and ready to go.
What is Time Delay Interferometry (TDI) and how does it work?
Interferometry is a technique that uses the interference of waves to make precise measurements. The wavelength of the interfering waves acts like the tick marks on a ruler for measuring distance. Optical interferometers can make very precise measurements because the wavelength of the light waves they use is small — around one micron for instruments like LIGO and LISA. A fundamental limitation of interferometry is that precision of the measurement is limited by the stability of the waves used in the interferometer. For an optical interferometer, if the wavelength of the light fluctuates, a spurious signal will be generated that mimics physical motion. One way to mitigate the effect of a fluctuating source is to compare pairs of distances using a common light source. This is the underlying concept of the Michelson interferometer that was used by Albert Michelson and Edward Morely to search for the “luminferous aether” in the late 19th century. LIGO uses the same concept in its interferometers over a century later. In order for this technique to work, the lengths of the light paths must be precisely matched. While LISA’s orbits produce approximately-equal arms, they differ by up to a percent and fluctuate by almost the same amount over long time periods due to orbital mechanics. Time Delay Interferometry (TDI) is a technique that was developed in the late 1990s and early 2000s to allow LISA to take advantage of the “common mode rejection” effect despite having unequal arms. TDI takes advantage of the fact that LISA measures the interference in each one-way laser link individually. While each of these signals is dominated by fluctuations in the LISA laser wavelength, those same fluctuations are measured at multiple points in the LISA constellation with varying time delays. By combining these individual measurements and correcting for the time delays, and adding in some rough knowledge of the constellation geometry, a significant amount of suppression of laser wavelength noise can be achieved. The ability to suppress laser wavelength noise through TDI is primarily determined by the precision of the individual interference measurements and the accuracy of the estimates of the LISA arm lengths. TDI has been extensively examined in analytic studies, numerical simulations, and experimental analogues and has been demonstrated to work as expected. The LISA team continues to refine our understanding of this important technique to ensure that it will provide the sensitivity that LISA requires to achieve its science goals.
LISA Community
How can I get involved with LISA?
If you are a professional researcher, you may want to join the LISA Consortium. In the US, you should check out NASA’s Gravitational Wave Science Interest Group (GWSIG) https://pcos.gsfc.nasa.gov/sigs/gwsig.php. If you are a student, you may consider applying for an internship at a NASA center involved with LISA (intern.nasa.gov)
What is NASA’s role in LISA?
LISA is led by the European Space Agency (ESA), which in 2017 selected LISA for study as a large-class mission in the Cosmic Visions Programme. LISA was Adopted as a project by ESA’s Science Program Council in January 2024. Partnering with ESA are NASA and a collection of European National space agencies. NASA will provide three critical hardware elements for LISA: lasers, telescopes, and charge management devices. In addition, NASA is developing a science ground segment to process the LISA telemetry and produce scientific data products for public consumption. NASA scientists, engineers, and managers are working closely with the ESA and European counterparts to ensure that LISA is a success.
What is the LISA Consortium?
The LISA Consortium is a large international collaboration that combines the resources and expertise from scientists in many countries all over the world. Together with ESA and NASA, the LISA Consortium is working to bring the LISA Mission to fruition.
Who is working on LISA?
LISA is led by the European Space Agency, with NASA as a partner. In Europe, the Max Planck Albert Einstein Institute is the lead for the LISA Consortium. In the US, NASA Goddard is the lead center, with Marshall and JPL also contributing. On both continents, scientists at many (tens of) institutions are contributing technology and observational science.