Laser Interferometer Space Antenna
Artist's conception of LISA spacecraft
The Laser Interferometer Space Antenna (LISA) is a proposed space mission concept designed to detect and accurately measure gravitational waves from astronomical sources. It is currently a collaboration of the self-funded and independent eLISA consortium; a proof-of-concept mission, LISA Pathfinder, designed to demonstrate the technology necessary for a successful full mission is due for launch in 2014.
The LISA 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 began a full revision of the mission's concept and renamed it as the New (or Next) Gravitational-wave Observatory (NGO). It was one of the three candidates for the first (L1) Cosmic Vision L-class mission but it lost to the Jupiter Icy Moon Explorer (JUICE). It may be a candidate for the next L-class mission, a white paper making the case for such a mission shall be submitted for ESA's L2/L3 mission selection. This current mission concept is named the evolved Laser Interferometer Space Antenna (eLISA).
If launched, LISA will be the first dedicated space-based gravitational-wave detector; it will measure gravitational waves 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, and flying along an Earth-like heliocentric orbit. Passing gravitational waves create oscillations in the inter-spacecraft distances, as measured by light, in directions transverse to the direction of wave propagation. LISA will be sensitive to waves in the frequency band between 0.03 millihertz to 100 millihertz, including signals from massive black holes that merge at the center of galaxies, or 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 Mission’s primary objective is to detect and measure as yet unobserved gravitational waves produced by compact binary systems and mergers of supermassive black holes. LISA 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 three 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. This technology was pioneered by the TRIAD satellite in 1972. A single satellite ("LISA Pathfinder"), initially scheduled to be launched in 2013, would test drag-free operation as it will be implemented in LISA. However, due to NASA's uncertain further involvement in the mission, it is unclear whether the satellite will be launched at a different date or at all.
Discussion of a gravitational-wave mission based on laser measurements between separate spacecraft began in 1974. However, it was not until 1981 that a preliminary mission concept somewhat similar to the present LISA mission design was developed. After some study in the US, ESA commissioned a mission study in 1993, and recommended LISA as a Cornerstone mission in its "Horizon 2000 Plus" program in 1994. Since 1997 LISA has been studied and planned jointly by ESA and NASA.
Ground-based detectors like the Laser Interferometer Gravitational-Wave Observatory (LIGO) seek to detect high-frequency gravitational waves from stellar-sized systems, such as spinning neutron stars, supernovae, and the final minutes of the gravitationally-driven inspiral of neutron stars and black holes. By contrast, LISA will observe lower frequency waves from larger or more massive systems, such as compact-object binaries with large orbital separations, and supermassive black-hole binaries in the final months of coalescence.
LISA also can test gravitational waves detection at different frequencies, such as to test the Vanishing-dimension theory.
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).
2011 mission changes 
|This section is outdated. (October 2012)|
On April 8, 2011, it was announced in a press release made through NASA's LISA Project Office, that, based on the United States President's Fiscal Year 2012 Budget Request, NASA would likely be unable to continue participating in the LISA project. As a result, the European Space Agency has ended its study of LISA as a partnership at the scale initially proposed in the New Worlds New Horizons decadal survey. Revised mission concepts will be considered in a selection process commencing in February 2012. A decision is expected in April 2012.
A future minor role for NASA in the ESA-led mission has not been ruled out. NASA's Astrophysics Division plans to continue base funding for the LISA study team through FY11, assuming not-larger-than-anticipated cuts from the U.S. Congress.
LISA was originally recommended in the 2010 U.S. National Research Council decadal report on astronomy and astrophysics as one of two large space missions to be implemented by NASA in the upcoming decade. The report recommended that LISA start in 2016, after a successful LISA Pathfinder flight and selection in the ESA Cosmic Vision program, which would have enabled launch in 2025.
New Gravitational Wave Observatory (NGO) 
A paper published in January 2012 discussed the European New Gravitational wave Observatory (NGO) mission (derived from the previous LISA proposal) and referred to this derivative proposal by an informal name “eLISA”, and proposed that it will survey in the low-frequency gravitational wave band (about 0.1 mHz to 1 Hz) with sufficient sensitivity to detect interesting individual astrophysical sources out to cosmological distances represented by a redshift z = 15.
The NGO constellation would comprise three spacecraft operating in a V formation, with the central craft having two free-falling “test masses” that define the endpoint of the two interferometer arms while the other two craft have the end-points of the interferometer arms. The three craft would orbit the Sun and initially have a near-equilateral triangular formation with an armlength of 1 million kilometers. The goal is detection in a frequency range of 0.03 mHz to 1 Hz.
See also 
- "eLISA NGO News". eLISA Consortium. 25 May 2012. Retrieved 28 December 2012.
- "LISA Pathfinder factsheet". ESA. 11 June 2012. Retrieved 26 June 2012.
- "Time to choose a billion-euro space mission". BBC News. 2012-04-02. Retrieved 2012-04-03.
- "LISA and Changes in the Cosmic Vision Programme". Retrieved 2011-05-04.
- Stebbins, Robin (13 July 2011). "Status of ESA’s Next Gravitational-Wave Observatory (NGO, a.k.a. LISA Light, EuLISA)". NASA. Retrieved 31 August 2011.
- Jonathan Amos (2012-05-02). "Esa selects 1bn-euro Juice probe to Jupiter". BBC News Online (BBC). Retrieved 2012-05-13.
- "A New Astronomy: eLISA Mission Concept". Retrieved 14 May 2013.
- "LISA - Laser Interferometer Space Antenna - Mission". Retrieved 2010-08-13.
- "LISA: Unveiling a hidden Universe - Assessment Study Report". ESA. February 2011. Retrieved 2012-11-05.
- Ziemer, John K.; Merkowitz, Stephen M. "Microthrust Propulsion of the LISA Mission". JPL TRS 1992+. Pasadena, CA : Jet Propulsion Laboratory, National Aeronautics and Space Administration, 2004. Retrieved 11 October 2011.
- "LISA Pathfinder project". Retrieved 2010-08-13.
- For citations to early studies of space-based gravitational-wave interferometers, see page 440 of Thorne's article in Hawking and Israel, Three hundred years of gravitation, Cambridge University Press, Cambridge (1987), and page 116 of Bender's chapter in Ciufolini, Gorini, Moschella, and Fré, Gravitational Waves, IOP Publishing, Bristol (2001).
- "President's FY12 Budget Request". Retrieved 2011-05-04.
- "Cosmic vision:Timeline for selection of L-class missions". ESA. Retrieved 2012-04-23.
- "Astro2010: The Astronomy and Astrophysics Decadal Survey". Retrieved 2010-08-13.
- "ESA Science & Technology: Cosmic Vision". Retrieved 2008-02-05.
- Doing science with eLISA:Astrophysics and cosmology in the millihertz regime, PDF, Pau Amaro-Seoane et al, arXiv:1201.3621v1 [astro-ph.CO] 17 Jan 2012, accessed 20 January 2012
- "NASA LISA homepage". Retrieved 2010-08-13.
- "ESA LISA homepage". Retrieved 2010-08-13.
- "LISA Pathfinder mission". Retrieved 2013-05-22.
- "LISA International Science Community portal". Retrieved 2010-08-13.
- "LISA International Science Team website". Retrieved 2010-08-13.
- "eLISA website of the international scientific community led by the Max Planck Institute for Gravitational Physics (Albert Einstein Institute) 2012".