Laser Interferometer Space Antenna
Artist's conception of LISA spacecraft
|Launch date||2034 (planned)|
The Laser Interferometer Space Antenna (LISA) is a proposed space mission concept designed to detect and accurately measure gravitational waves - tiny ripples in the fabric of space-time - from astronomical sources. The LISA Project has now evolved into eLISA (Evolved Laser Interferometer Space Antenna). The project is currently a collaboration of the self-funded and independent eLISA consortium. 
The LISA project was previously a joint effort between the United States space agency NASA and the European Space Agency ESA. However, on April 8, 2011, NASA announced that it would likely be unable to continue its LISA partnership with the European Space Agency, due to funding limitations.  ESA has therefore revised the mission's concept to fit into a European-only cost envelope. The project was chosen as the L3 mission within the ESA Cosmic Vision Program, with a tentative launch date in 2034.
LISA/eLISA will be the first dedicated space-based gravitational wave detector. It will measure gravitational waves directly by using laser interferometry to monitor the fluctuations in the relative distances between three spacecraft, arranged in an equilateral triangle with 5 million kilometer arms, (eLISA: 1 million kilometers) and flying along an Earth-like heliocentric orbit.
LISA/eLISA is designed to directly observe gravitational waves, which are distortions of space-time travelling at the speed of light. Passing gravitational waves alternately squeeze and stretch objects by a tiny amount. Gravitational waves are caused by energetic events in the universe and, unlike any other radiation, can pass unhindered by intervening mass. By 2028 scientists' understanding of the universe will have been dramatically improved by advanced observatories of electromagnetic radiation. Launching LISA/eLISA will add a new sense to scientists' perception of the universe and enable them to listen to a world that is invisible with light.
LISA/eLISA will detect signals e.g. from massive black holes that merge at the center of galaxies, or massive black holes  that consume smaller compact objects; from binaries of compact stars in our Galaxy; and possibly from other sources of cosmological origin, such as the very early phase of the Big Bang, and speculative astrophysical objects like cosmic strings and domain boundaries.
The LISA/eLISA Mission’s primary objective is to detect and measure gravitational waves produced by compact binary systems and mergers of supermassive black holes. LISA/eLISA will observe gravitational waves by measuring differential changes in the length of its arms, as sensed by laser interferometry. Each of the LISA spacecraft contains two telescopes, two lasers and two test masses, arranged in two optical assemblies pointed at the other two spacecraft. This forms Michelson-like interferometers, each centered on one of the spacecraft, with the platinum-gold test masses defining the ends of the arms. The entire arrangement, which is ten times larger than the orbit of the Moon, will be placed in solar orbit at the same distance from the Sun as the Earth, but trailing the Earth by 20 degrees, and with its orbital plane tilted relative to the ecliptic by 60 degrees. The mean linear distance between the constellation and the Earth will be 50 million kilometers.
To eliminate non-gravitational forces such as light pressure and solar wind on the test masses, each spacecraft is constructed as a zero-drag satellite, and effectively floats around the masses, using capacitive sensing to determine their position relative to the spacecraft, and very precise thrusters to keep itself centered around them.
An ESA test mission called LISA Pathfinder (LPF) will prove LISA/eLISA´s key technologies in space. LPF consists of a single spacecraft with one of the LISA/eLISA interferometer arms shortened to about 38 cm, so that it fits inside a single spacecraft. LPF will be launched in 2015. 
The main goal of LISA is to use direct measurements of gravitational waves to study astrophysical systems and to test Einstein's theory of gravity. The existence of gravitational waves is inferred from observations of the decreasing orbital periods of several binary pulsars, such as the famous PSR 1913+16. However, gravitational waves have not yet been directly detected on Earth because of their extremely small effect on matter. Observing them requires two things: a very strong source of gravitational waves – such as the merger of two black holes – and extremely high detection sensitivity. The LISA instrument should be able to measure relative displacements with a resolution of 20 picometers over a distance of 5 million kilometers, yielding a strain sensitivity of better than 1 part in 1020. Due to its sensitivity in the low-frequency band of the gravitational-wave spectrum, LISA will detect waves generated by binary stars within our galaxy (the Milky Way); by binary, supermassive black holes in other galaxies; and by extreme mass ratio inspirals ("EMRIs"), in which a stellar-mass black hole is captured by a supermassive black hole.
Other gravitational-wave experiments
Previous searches for gravitational waves in space were conducted for short periods by planetary missions that had other primary science objectives (such as Cassini–Huygens), using microwave Doppler tracking to monitor fluctuations in the Earth-spacecraft distance. By contrast, LISA is a dedicated mission that will use laser interferometry to achieve a much higher sensitivity. Other gravitational wave antennas, such as LIGO, VIRGO, and GEO 600, are already in operation on Earth, but their sensitivity at low frequencies is limited by the largest practical arm lengths, by seismic noise, and by interference from nearby moving masses. Thus, LISA and ground detectors are complementary rather than competitive, much like astronomical observatories in different electromagnetic bands (e.g., ultraviolet and infrared).
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