Laser Interferometer Space Antenna
Artist's conception of LISA spacecraft
|Launch date||2034 (planned)|
|Semimajor axis||1 AU|
|Orbital period||1 year|
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's present incarnation is the Evolved Laser Interferometer Space Antenna (eLISA).
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 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 scaled down design was initially known as the New Gravitational-wave Observatory (NGO) for ESA's L1 mission selection. Following this unsuccessful application, the name was changed to eLISA. The project was chosen as the L3 mission within the ESA Cosmic Vision Program, with a tentative launch date in 2034.
eLISA will be the first dedicated space-based gravitational wave detector. It will measure gravitational waves directly by using laser interferometry. The LISA concept has a constellation three spacecraft, arranged in an equilateral triangle with million kilometre arms (5 million km for classic LISA, 1 million km for eLISA) flying along an Earth-like heliocentric orbit. The distance between the satellites is monitored to detect a passing gravitational wave.
A LISA-like mission 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. Launching 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.
Potential sources for signals are merging massive black holes at the centre of galaxies,; massive black holes  orbited by small compact objects, known as extreme mass ratio inspirals; 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 centred 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 Hulse–Taylor binary pulsar. 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 —- less than the diameter of a helium atom —- 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).
The first design studies for gravitational wave detector to be flown in space were performed in the 1980s under the name LAGOS (Laser Antena for Gravitational radiation Observation in Space). LISA was first proposed as amission to ESA in the early 1990s. First as a candidate for the M3-cycle, and later as 'cornerstone mission' for the 'Horizon 2000 plus' program. As the decade progressed, the design was refined to a triangular configuration of three spacecraft with three 5 million kilometer arms. This mission was pitched as a joint mission between ESA and NASA in 1997.
In the 2000s the joint ESA/NASA LISA mission was identified as a candidate for the 'L1' slot in ESA's Cosmic Vision 2015-2025 programme. However due to budget cuts, NASA announced in early 2011 that it would not be contributing to any of ESA's L-class missions. ESA nonetheless decided to push the program forward, and instructed the L1 candidate missions to present reduced cost versions that could be flown within ESA's budget. A reduced version of LISA was designed with only two 1 million kilometer arms under the name NGO (New/Next Gravitational wave Observatory). Despite NGO being ranked highest in terms of scientific potential, ESA decided to fly Jupiter Icy Moon Explorer (JUICE) as its L1 mission. One of the main concerns was, that the LISA Pathfinder mission had been experiencing technical delays, making it uncertain if the technology would be ready for the projected L1 launch date.
Soon afterwards, ESA announced it would be selecting themes for its L2 and L3 mission slots. A theme called "the Gravitational Universe" was formulated with the reduced NGO rechristened eLISA as a straw-man mission. In November 2013, ESA announced that it selected "the Gravitational Universe" for its L3 mission slot (expected launch in 2032).
- "eLISA Consortium portal". Retrieved 2013-11-13.
- "ESA LISA homepage". Retrieved 2010-08-13.
- "LISA Pathfinder mission". Retrieved 2013-05-22.
- "LISA International Science Team website". Retrieved 2010-08-13.
- "NASA LISA homepage". Retrieved 2010-08-13.
- "Selected: The gravitational universe; ESA decides on next large mission concepts". eLISA Consortium. Retrieved 29 November 2013.
- "ESA's new vision to study the invisible universe". ESA. Retrieved 29 November 2013.
- "eLISA, The First Gravitational Wave Observatory in Space". eLISA Consortium. Retrieved 12 November 2013.
- "eLISA, Partners and Contacts". eLISA Consortium. Retrieved 12 November 2013.
- "LISA on the NASA website". NASA. Retrieved 12 November 2013.
- "President's FY12 Budget Request". NASA/US Federal Government. Retrieved 4 Mar 2011.
- Amaro-Seoane, Pau; Aoudia, Sofiane; Babak, Stanislav; Binétruy, Pierre; Berti, Emanuele; Bohé, Alejandro; Caprini, Chiara; Colpi, Monica; Cornish, Neil J; Danzmann, Karsten; Dufaux, Jean-François; Gair, Jonathan; Jennrich, Oliver; Jetzer, Philippe; Klein, Antoine; Lang, Ryan N; Lobo, Alberto; Littenberg, Tyson; McWilliams, Sean T; Nelemans, Gijs; Petiteau, Antoine; Porter, Edward K; Schutz, Bernard F; Sesana, Alberto; Stebbins, Robin; Sumner, Tim; Vallisneri, Michele; Vitale, Stefano; Volonteri, Marta; Ward, Henry (21 June 2012). "Low-frequency gravitational-wave science with eLISA/NGO". Classical and Quantum Gravity 29 (12): 124016. doi:10.1088/0264-9381/29/12/124016.
- "ESA Lisa Pathfinder overview". ESA. 6 June 2013. Retrieved 12 November 2013.
- "eLISA: LPF". eLISA Consortium. Retrieved 12 November 2013.
- "eLISA: Science Context 2028". eLISA Consortium. Retrieved 15 November 2013.
- "Gravitational-Wave Detetectors Get Ready to Hunt for the Big Bang". Scientific American. 17 September 2013.
- "eLISA whitepaper, sec. 5.2 p 40, arXiV:1201.3621v1". eLISA consortium. 17 Jan 2012.
- "eLISA whitepaper, sec. 4.3, p. 25, arXiV:1201.3621v1". eLISA consortium. 17 Jan 2012.
- "eLISA whitepaper, sec 3.3, p 11, arXiV:1201.3621v1". eLISA consortium. 17 Jan 2012.
- "eLISA whitepaper, sec 7.2, p 59, arXiV:1201.3621v1". eLISA consortium. 17 Jan 2012.
- "eLISA whitepaper, sec 1.1, p 5, arXiV:1201.3621v1". eLISA consortium. 17 Jan 2012.
- "eLISA: the mission concept". eLISA Consortium. Retrieved 12 November 2013.
- "eLISA: distance measurement". eLISA Consortium. Retrieved 12 November 2013.
- "eLISA: key features". eLISA Consortium. Retrieved 12 November 2013.
- "eLISA: dragfree operation". eLISA Consortium. Retrieved 12 November 2013.
- "eLISA: Lisa Pathfinder". eLISA Consortium. Retrieved 12 November 2013.
- "ESA: Lisa Pathfinder overview". European Space Agency. Retrieved 12 November 2013.
- Moore, Christopher; Cole, Robert; Berry, Christopher (19 July 2013). "Gravitational Wave Detectors and Sources". Retrieved 14 April 2014.
-  and 
- Danzmann, Karsten; (eLISA Consortium) (2013). "The Gravitational Universe". Retrieved 15 April 2014.
- "Selected: The Gravitational Universe ESA decides on next Large Mission Concepts". Max Planck Institute for Gravitational Physics.