Laser Interferometer Space Antenna

Laser Interferometer Space Antenna

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The Laser Interferometer Space Antenna (LISA) is a planned space mission to detect and accurately measure gravitational waves from astronomical sources. LISA was originally conceived as a joint effort between the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 space agency NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

 and the European Space Agency
European Space Agency
The European Space Agency , established in 1975, is an intergovernmental organisation dedicated to the exploration of space, currently with 18 member states...

 (ESA). However, on April 8th 2011, NASA announced that it would likely be unable to continue its LISA partnership with the European Space Agency, due to funding limitations. ESA is planning to begin a full revision of the mission's concept, renamed the Next Gravitational-­Wave Observatory (NGO), with selection of the winning Cosmic Vision
Cosmic Vision
The Cosmic Vision is the paraphrasing name given to the roadmap for scientific space based missions of the European Space Agency in the time frame between 2015 and 2025....

 L-class mission candidate due in February 2012.

If launched, LISA will be the first dedicated space-based gravitational-wave detector; it will measure gravitational wave
Gravitational wave
In physics, gravitational waves are theoretical ripples in the curvature of spacetime which propagates as a wave, traveling outward from the source. Predicted to exist by Albert Einstein in 1916 on the basis of his theory of general relativity, gravitational waves theoretically transport energy as...

s by using laser interferometry
Interferometry
Interferometry refers to a family of techniques in which electromagnetic waves are superimposed in order to extract information about the waves. An instrument used to interfere waves is called an interferometer. Interferometry is an important investigative technique in the fields of astronomy,...

 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
Heliocentric orbit
A heliocentric orbit is an orbit around the Sun. All planets, comets, and asteroids in our Solar System are in such orbits, as are many artificial probes and pieces of debris. The moons of planets in the Solar System, by contrast, are not in heliocentric orbits as they orbit their respective planet...

.
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
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

 band between 0.03 milliHertz to 100 milliHertz, including signals from massive black holes
Supermassive black hole
A supermassive black hole is the largest type of black hole in a galaxy, in the order of hundreds of thousands to billions of solar masses. Most, and possibly all galaxies, including the Milky Way, are believed to contain supermassive black holes at their centers.Supermassive black holes have...

 that merge at the center of galaxies
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...

, or that consume smaller compact objects
Compact star
In astronomy, the term compact star is used to refer collectively to white dwarfs, neutron stars, other exotic dense stars, and black holes. These objects are all small for their mass...

; 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
Big Bang
The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe. According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the young Universe to cool and resulted in...

, and speculative astrophysical objects like cosmic string
Cosmic string
Cosmic strings are hypothetical 1-dimensional topological defects which may have formed during a symmetry breaking phase transition in the early universe when the topology of the vacuum manifold associated to this symmetry breaking is not simply connected. It is expected that at least one string...

s and domain boundaries.

Mission


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 telescope
Telescope
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...

s, two laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...

s and two test masses, arranged in two optical assemblies pointed at the other two spacecraft. This forms three Michelson
Michelson interferometer
The Michelson interferometer is the most common configuration for optical interferometry and was invented by Albert Abraham Michelson. An interference pattern is produced by splitting a beam of light into two paths, bouncing the beams back and recombining them...

-like interferometers, each centered on one of the spacecraft, with the platinum
Platinum
Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...

-gold
Gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...

 test masses defining the ends of the arms. The entire arrangement, which is ten times larger than the orbit of the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

, will be placed in solar orbit at the same distance from the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

 as the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

, but trailing the Earth by 20 degrees, and with its orbital plane tilted relative to the ecliptic
Ecliptic
The ecliptic is the plane of the earth's orbit around the sun. In more accurate terms, it is the intersection of the celestial sphere with the ecliptic plane, which is the geometric plane containing the mean orbit of the Earth around the Sun...

 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
Solar wind
The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...

 on the test masses, each spacecraft is constructed as a zero-drag satellite
Zero-drag satellite
Zero-drag satellites or equivalently "drag-free satellites" are satellites where the payload follows a geodesic path through space only affected by gravity and not by non-gravitational forces such as drag of the residual atmosphere, light pressure and solar wind. A zero-drag satellite has two...

, 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
LISA Pathfinder
LISA Pathfinder is the revised name for SMART-2, a NASA/ESA space probe to be launched in June 2013. SMART stands for Small Missions for Advanced Research in Technology. The aim of the LISA Pathfinder is to test technologies needed for the Laser Interferometer Space Antenna, a joint NASA/ESA...

"), 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
LIGO
LIGO, which stands for the Laser Interferometer Gravitational-Wave Observatory, is a large-scale physics experiment aiming to directly detect gravitational waves. Cofounded in 1992 by Kip Thorne and Ronald Drever of Caltech and Rainer Weiss of MIT, LIGO is a joint project between scientists at MIT,...

) 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.

Science


The main goal of LISA is to use direct measurements of gravitational waves to study astrophysical systems and to test Einstein
Albert Einstein
Albert Einstein was a German-born theoretical physicist who developed the theory of general relativity, effecting a revolution in physics. For this achievement, Einstein is often regarded as the father of modern physics and one of the most prolific intellects in human history...

's theory of gravity. The existence of gravitational waves is inferred from observations of the decreasing orbital periods of several binary pulsar
Binary pulsar
A binary pulsar is a pulsar with a binary companion, often a white dwarf or neutron star. Binary pulsars are one of the few objects which allow physicists to test general relativity in the case of a strong gravitational field...

s, such as the famous PSR 1913+16
PSR 1913+16
PSR B1913+16 is a pulsar which together with another neutron star is in orbit around a common center of mass, thus forming a binary star system. In 1974 it was discovered by Russell Alan Hulse and Joseph Hooton Taylor, Jr., of Princeton University...

. 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. Thanks to its sensitivity in the low-frequency band of the gravitational-wave spectrum, LISA will detect waves generated by binaries within our galaxy (the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...

) and by massive black holes in distant galaxies
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...

.

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
Doppler radar
A Doppler radar is a specialized radar that makes use of the Doppler effect to produce velocity data about objects at a distance. It does this by beaming a microwave signal towards a desired target and listening for its reflection, then analyzing how the frequency of the returned signal has been...

 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
LIGO
LIGO, which stands for the Laser Interferometer Gravitational-Wave Observatory, is a large-scale physics experiment aiming to directly detect gravitational waves. Cofounded in 1992 by Kip Thorne and Ronald Drever of Caltech and Rainer Weiss of MIT, LIGO is a joint project between scientists at MIT,...

, VIRGO
Virgo
-Astronomy:* Virgo Cluster, a cluster of galaxies in the constellation Virgo* Virgo , a constellation* Virgo Stellar Stream, remains of a dwarf galaxy* Virgo Supercluster, a galactic supercluster-Surname:* Virgo...

, and GEO 600
GEO 600
GEO 600 is a gravitational wave detector located near Sarstedt, Germany. This instrument, and its sister interferometric detectors, when operational, are some of the most sensitive gravitational wave detectors ever designed...

, 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
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

 and infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

).

2011 mission changes


On April 8th 2011, it was announced in a press release made through NASA's LISA Project Office, that, based on the United States President's FY12 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 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
Astronomy and Astrophysics Decadal Survey
The Astronomy and Astrophysics Decadal Survey is a review of astronomy and astrophysics literature produced approximately every ten years by the National Research Council of the National Academy of Sciences in the United States. The report surveys the current state of the field, identifies research...

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.

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