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Imagine the Universe! LISA Special Exhibit

eLISA: the enhanced Laser Interferometer Space Antenna

diagram of LISA's position in space

This artist rendering shows how a LISA-type set of satellites would orbit the sun, trailing the earth by about 20 degrees. As it orbits, the triangle would appear to "cartwheel", allowing it to scan the full sky. (Credit: AEI/MM/exozet)

The eLISA mission is a next-generation space observatory project proposed by the European Space Agency to observe gravitational waves in the frequency range of 0.0001 to 0.1 Hertz. Scientists from around the world hope to participate in this exciting mission.

Gravitational waves are "ripples in spacetime" caused by the rapid motion of compact massive objects, such as neutron stars and black holes. The eLISA mission would be most sensitive to large-scale events like galaxy mergers, black hole collisions, and neutron star binary systems. The mission consists of three separate spacecraft, which form the three corners of a large triangle in space. Each of the eLISA spacecraft orbits the sun independently, trailing Earth's orbit by about 20 degrees. When a gravitational wave passes through, eLISA would detect slight changes in the distances between the spacecraft using a measurement technique called laser interferometry. The project is expected to launch in the mid-2030s, with a mission life of about five years.

How do You Detect Gravitational Waves?

Laser interferometry enables eLISA to detect tiny changes in distance. Here's how it works: A laser beam from two spacecraft points toward a detector on another spacecraft, 1 million kilometers away. The laser beam precisely measures distances between the two spacecraft, creating two measurement arms. When the distance along the arms changes even slightly, the combined laser beams will show a series of light and dark fringes called an interference pattern. The interference pattern tells scientists that eLISA has detected a gravitational wave. The two arms, formed by the three spacecraft, provide two detectors, which work together to confirm the observation and to get more details about the passing wave.

Why is eLISA so big? It has to be big because gravitational waves stretch and squeeze spacetime iteself. For example, if there were two binary neutron stars spiraling into each other in the Virgo galaxy cluster, they would produce a wave no bigger than a thousandth the radius of a proton. The configuration proposed for eLISA could measure a change in length of 10 to 100 times smaller than the diameter of an atom. To measure the very small changes expected for gravitational waves, the distance between spacecraft must be very large – about 1 million km. This is about two and a half times the distance between the Earth and the moon, which will make the observatory very sensitive!

On Earth, scientists can measure distances that are about 1 million times smaller than what eLISA could do in space, in part because ground-based detectors can use much more powerful lasers. So, to get the same strain sensitivity, the length of an arm can be much smaller, about 4 km. The LIGO project has built two such detectors.

Together, LIGO and eLISA cover different frequency ranges, and thus complement each other.

Spectrum of gravitational wave sources and detectors that can detect them

This chart shows the sources that are associated with each gravitational wave frequency range. It also shows the types of gravitational wave detectors required for different frequencies. Click image for a larger version. (Credit: NASA/J. I.Thorpe)

Ground-based detectors cannot see low frequencies because the ground moves too much (remember, these detectors can see motions that are smaller than atoms). A space-based detector, which is free from ground motion, is the only way to see those low frequencies. Other types of ground-based detectors use the Cosmic Microwave Background (CMB) or arrays of millisecond pulsars to detect gravitational waves. Space-based gravitational wave detectors will enable us to observe the range of frequencies where most of the known sources are located.

Expected Science Results

Once eLISA starts to return data, scientists will compare the data to computer models to interpret what they see. Dr. Robin T. Stebbins and his team of researchers at NASA are creating a "numerical laboratory" of models that represent gravitational wave phenomena. Using these waveform computer codes, scientists will be able to decipher eLISA's message. They will be able to sift through the information, isolating sources of gravitational waves as well as the behaviors that cause them. The scientists behind the eLISA mission hope to detect information about black holes and the Big Bang, how galaxies formed, as well as further, detailed proof of Einstein's famous theory of general relativity.

See what eLISA could tell us about Black Holes, General Relativity, and The Big Bang.

Publication Date: August 2003
Updated: December 2015


A service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA/GSFC

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