How does LISA differ from LIGO and other ground-based gravitational wave interferometers?

LISA and LIGO operate on the same basic physical principle that gravitational waves can be observed by measuring the proper distance between freely-falling objects using beams of light. However, the two detectors operate in very different regimes. LISA's million-kilometer-scale arm lengths are optimized to observe gravitational waves with milliHertz frequencies. These low-frequency gravitational waves don't influence the smaller LIGO detectors very efficiently, since they are optimized to detect frequencies in the tens to hundreds of Hertz. LISA will generally observe systems with larger masses and increased separations than LIGO. LISA sources will also tend to evolve more slowly, allowing longer observations of each source. The two types of observatories complement one another, much as different types of electromagnetic observatories (e.g. radio, optical, x-ray, etc.) complement one another.

How mature is LISA's technology?

The basic LISA concept has been under development for several decades and much of the required technology has been developed during that time. Most notably, LISA Pathfinder, a dedicated technology demonstrator mission led by the European Space Agency (ESA), provided an in-flight validation of several key LISA technologies. Pathfinder's primary objective was to place a LISA-like test mass in near-perfect free fall and to measure residual accelerations using a second freely-falling test mass. The satellite operated using the drag-free control technique in which the spacecraft is commanded to fly around the test mass without disturbing it. During operations in 2016 and 2017, Pathfinder demonstrated free-fall performance at the femto-g level (one quadrillionth of the acceleration due to gravity on Earth). The free-fall requirements for LISA are derived from the Pathfinder results with appropriate margin. Pathfinder additionally provided validation of a number of component technologies relevant to LISA including sub-picometer laser metrology over short distances, precision micropropulsion, non-contact charge control, and dynamical control of a multi-body kinematic system. Elements of LISA's long-baseline metrology system were demonstrated by the Laser Ranging Instrument on GRACE-FO, a NASA/German partnership for an Earth Geodesy Satellite. During initial tests of the LRI payload in 2018, nanometer-level ranging was demonstrated over a roughly 300 kilometer baseline. ESA, NASA, and a number of European National agencies are currently funding technology development for items that were not demonstrated on LISA Pathfinder or GRACE-FO. An example is the LISA telescope, which is used to maximize the light transfer between the spacecraft and must be sufficiently stable so as to not disturb the interferometric measurement.

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 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.

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 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.

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.

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.

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 (approximatley 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.

How long will the LISA mission last?

The LISA mission is designed for 4.5 years of nominal science operations, with a potential extended mission of up to 5 additional 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.

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.

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). If you are a student, you may consider applying for an internship at a NASA center involved with LISA (via intern.nasa.gov).