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.