Laser Interferometer Space Antenna - Mission

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 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.