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Why LISA? ...Gravitational Waves.

What are Gravitational Waves?

This movie shows a simulation of the merger of two black holes and the resulting emission of gravitational radiation. The very fabric of space and time is distorted by massive objects, which is shown here by the colored fields. The outer sheets (red) correspond directly to outgoing gravitational radiation, which was recently detected by the NSF's LIGO observatories. Credit: NASA/C. Henze

Gravitational waves were first theorized by Albert Einstein. They are created during events such as supermassive black hole mergers, or collisions between two black holes that are billion times bigger than our Sun. These collisions are so powerful that they create distortions in spacetime, known as gravitational waves.

Gravitational waves detectable by the LISA mission could also come from other distant systems including smaller stellar mass black holes orbiting supermassive black holes, known as Extreme Mass Ratio Inspirals (EMRIs).

What do Gravitational Waves tell us?

There are many astrophysical phenomena that are either very dim or completely invisible in any form of light that astronomy has relied on for 400 years. Gravitational waves are a powerful new probe of the Universe that uses gravity instead of light to take measure of dynamical astrophysical phenomena. Studying gravitational waves gives enormous potential for discovering the parts of the universe that are invisible by other means, such as black holes, the Big Bang, and other, as yet unknown, objects. LISA will complement our knowledge about the beginning, evolution and structure of our universe.



Why do we need to go to space?

A space-based configuration allows for an extremely large detector to study regions of the gravitational wave spectrum that are inaccessible from Earth.

gravitational wave spectrum
Click Image to Zoom. There are promising detection techniques across the entire gravitational wave spectrum, which is populated by a broad range of astrophysical sources. The spectrum in the region probed by LISA is one of the most interesting, populated by a rich diversity in astrophysical phenomena of interest to astronomers and astrophysicists.

A Different Frequency Range from Different Objects

The gravitational wave spectrum covers a broad span of frequencies. LISA operates in the low frequency range, between 0.1 mHz and 1 Hz (compared to LIGO's frequency of 10 Hz to 1000 Hz). The difference means that the waves LISA is looking for have a much longer wavelength, corresponding to objects in much wider orbits and potentially much heavier than those that LIGO is searching for, opening up the detection realm to a wider range of gravitational wave sources.

LISA has three spacecraft that form an equilateral triangle in space where the sides of the triangle, also called LISA's "arms", extend about a million miles. Therefore, from space, LISA can avoid the noise from Earth and access regions of the spectrum that are inaccessible from Earth due to these extremely long arms. The gravitational wave sources that LISA would discover include ultra-compact binaries in our Galaxy, supermassive black hole mergers, and extreme mass ratio inspirals, among other exotic possibilities.

The Entire Spectrum

We aim to study the entire gravitational wave spectrum, covering a range of gravitational wave sources using experiments on


WHAT is LISA?

LISA is a space-based gravitational wave detector constructed of three spacecraft separated by millions of miles.

LISA's enormous detector size and orbit, trailing behind the Earth as it orbits the Sun, are illustrated here. Credit: AEI/Milde Marketing

LISA's Size and Precision are Out of this World

LISA consists of three spacecraft that are separated by millions of miles and trailing tens of millions of miles, more than one hundred times the distance to the Moon, behind the Earth as we orbit the Sun. These three spacecraft relay laser beams back and forth between the different spacecraft and the signals are combined to search for gravitational wave signatures that come from distortions of spacetime. We need a giant detector bigger than the size of Earth to catch gravitational waves from orbiting black holes hundreds of millions of times more massive than our sun. NASA is a major collaborator in the European Space Agency (ESA)-led mission, which is scheduled to launch in the early 2030s and we are getting ready for it now!


How does LISA Detect Gravitational Waves?

Gravitational wave events will cause the three LISA spacecraft to shift slightly with respect to each other.

LISA laser beam
Click Image To Zoom. LISA will observe a passing gravitational wave directly by measuring the tiny changes in distance between freely falling proof masses inside spacecraft with its high precision measurement system. Credit: AEI/MM/exozet

Catching Waves

A bit like the objects moving on the surface of a pond produce ripples and waves, massive objects moving in space distort the fabric of spacetime and produce gravitational waves. Some of these gravitational wave events will cause the three LISA spacecraft to shift slightly with respect to each other, as they "ride the gravitational waves", to produce a characteristic pattern in the combined laser beam signal that depends on the location and physical properties of the source.

LISA is Extremely High Precision

These signals are extremely small and require a very sensitive instrument to detect. For example, LISA aims to measure relative shifts in position that are less than the diameter of a helium nucleus over a distance of a million miles, or in technical terms: a strain of 1 part in 1020 at frequencies of about a millihertz.

The LISA Pathfinder Mission was a proof-of-concept mission to test and prove the technology needed for LISA's success.


What is LISA Pathfinder?

LISA Pathfinder Mission was a proof-of-concept mission for LISA.

lisa pathfinder artists impression
Click Image to Zoom. LISA Pathfinder operated from a vantage point in space about 1.5 million km from Earth towards the Sun, orbiting the first Sun-Earth Lagrangian point, L1. Credit: ESA - C.Carreau

LISA Pathfinder Exceeded Expectations

LISA Pathfinder was launched on December 3, 2015 as a proof-of-concept that tests that the noise characteristics of free-floating test masses within the spacecraft are small enough compared to an expected gravitational wave signal. Completing its mission in July, 2017, LISA Pathfinder has shown that the low noise levels surpassed the original requirements, demonstrating that key technology for LISA is well underway.


lisa pathfinder characteristics
View Full Screen. This plot shows the result of LISA Pathfinder's two-month experiment in drag-free flight, where the goal is to follow test masses as they fall through space affected only by gravity. LISA Pathfinder reduced non-gravitational forces on the test masses to a level five times better than the mission required and within 25 percent of the requirement for a future space-based gravitational wave detector. The cause of the spike around 0.07 hertz is still under investigation. The line labeled "noise model" represents a simple physical model of the measured performance. It consists of a flat component, dominant at low frequencies, arising from residual gas molecules around the test mass and a rising component, dominant at higher frequencies, representing the limits of the instrument's ability to sense the motion of the test masses. This model explains the vast majority of the observed behavior, providing confidence that such models can be used to extrapolate from LISA Pathfinder to a full-scale future observatory. Credit: NASA's Goddard Space Flight Center. See related article below for details.

What is LIGO?

LIGO is a ground-based observatory that first detected gravitational waves.

LIGO Detections
Click Image to Zoom. LIGO has already significantly increased the number of black holes with known masses, which are measured in terms of the Sun's mass on the y-axis. The observatory has definitively detected three sets of black hole mergers (bright blue). For each event, LIGO determined the individual masses of the black holes before they merged, as well as the mass of the black hole produced by the merger. The black holes shown with a dotted border represent a LIGO candidate event that was too weak to be conclusively claimed as a detection. Details/Credit: LIGO/Caltech

LIGO Made History

On September 14, 2015, the Laser Interferometer Gravitational-wave Observatory (LIGO), a ground-based gravitational wave observatory, made history by detecting the first gravitational waves from the merger of two stellar mass black holes. To date, LIGO has announced detection of a total of three black hole binary systems and one strong candidate system.

aerial view of LIGO
Click Image to Zoom. The LIGO Laboratory operates two detector sites, one near Hanford in eastern Washington, and another near Livingston, Louisiana. This photo shows the Livingston detector site. Credit: LIGO/Caltech

Ground Based vs Space Based

As LIGO and other ground-based detectors increase their sensitivity, it is expected that they will detect additional stellar-mass black hole systems as well as neutron star systems and possibly supernovae. It is important to note that earth-based detectors, due to their limited size, observe in a different portion of the gravitational wave spectrum. We need the enormous size of LISA in space to detect enormous objects creating gravitational waves at lower frequencies in the spectrum. LISA's three spacecraft will create an equilateral triangle in space and the paths between each pair of spacecraft, referred to as LISA's arms, will extend millions of miles. With three arms, LISA will be able to measure the amplitude, direction and polarization of gravitational waves to obtain more information about the sources.


Who's Involved?

Led by ESA, the project is a collaboration of ESA and NASA.

ESA and NASA logos

The European Space Agency (ESA) is leading the LISA mission and NASA will collaborate by leveraging the growing body of knowledge from the U.S. scientific community.

What is NASA's Role?

NASA support for LISA will be at every level, including potential contributions to the design and development of the mission concept, providing portions of the instrument and other flight hardware, and participating in the operations and science data analysis. The details of the specific items and mission elements to be provided by NASA are part of ongoing discussions. Current discussions about NASA's involvement include items that could greatly increase the science output of the mission.


How to Get Involved

neutron star merger simulation of gravitational waves
Click Image to Zoom. Numerical simulation of the gravitational waves emitted during the merger of two neutron stars into a black hole. Credit: Max Planck Institute for Gravitational Physics. More Info

For Scientists

Education and Outreach Resources

Links to gravitational wave education resources for teachers and students.


Citizen Science Projects


Related Overview Articles/Websites