LISA (Laser Interferometer Space Antenna), a space-based detector with million-kilometer long arms will detect low-frequency gravitational waves that cannot be measured by ground based gravitational wave detectors.
The relative distance change between two freely falling bodies caused by a passing gravitational wave is exceedingly small, because spacetime is an extremely stiff elastic medium. For example: gravitational waves of a typical white dwarf binary at a distance of 50 pc create a periodic change of only 10-10 m in distance between two test masses, separated by a sufficiently large distance.
A suitable instrument for measuring such small length changes over a broad band of low frequencies is a laser interferometer with an arm length as large as possible and long integration times, the primary impetus for a space-borne detector. Hence LISA-type detectors can be thought of basically as a Michelson interferometer in Space with an arm length in the order of million kilometers. This arm length allows observation of most of the interesting sources of gravitational waves.
LISA-type missions include
- three spacecraft
- laser interferometry for gravitational wave detection
- millions of kilometer armlength
- drag-free operation
The first mission concept studies for a space-borne gravitational wave observatory can be traced back to activities in the 1980s at the Joint Institute for Laboratory Astrophysics (JILA) leading to a first full description of a mission comprising three drag-free spacecraft in a heliocentric orbit, then named Laser Antenna for Gravitational-radiation Observation in Space (LAGOS). In the early 1990s, LISA was proposed to ESA. At this time LISA consisted of six spacecraft, but showed already the key features of the later LISA and today’s LISA: Interferometric measurement of distances, long baselines (5 × 106 km in those days), drag-free spacecraft based on inertial sensors, and the familiar “cartwheel”-orbits. The number of spacecraft was reduced to the current three in a series of cost-reduction exercises in 1996 and 1997.
In 2001, LISA became part of the Beyond Einstein programme of NASA as one of the great observatories. In 2003, LISA underwent a first of a series of US reviews aimed at technology readiness that culminated with LISA being identified as the mission with the highest readiness in NASA’s Beyond Einstein programme.
When ESA formulated the Cosmic Vision 2015–2025 programme in 2005, and started the assessment phase in 2007, LISA was identified early on as one of the potential candidates for the L1 launch slot. In early 2011, LISA was presented to the advisory structure of ESA as a formal candidate for the L1 launch slot. Shortly after, in a response to the evolving programmatic framework in the US, it was decided that all L missions were to undergo a reformulation under the premise of an “ESA-led” mission.
LISA: the reformulated mission
LISA is a space mission designed to measure gravitational radiation over a broad band at low frequencies, from about 0.1 mHz to 1 Hz, a band where the universe is richly populated by strong sources of gravitational waves. It measures signals from a wide range of different sources that are of strong interest to the astrophysics of black hole and galaxy formation, and also to tests of general relativity and to cosmology: massive black holes merging in galaxies at all distances; massive black holes consuming smaller compact objects; known binary compact stars and stellar remnants; members of known populations of more distant binaries; and probably other sources, possibly including relics of the extremely early Big Bang, which are as yet unknown.