LISA Pathfinder, formerly Small Missions for Advanced Research in Technology-2 (SMART-2), is an ESA spacecraft that was launched on 3 December 2015. The mission will test technologies needed for the Evolved Laser Interferometer Space Antenna (eLISA), an ESA gravitational wave observatory planned to be launched in 2034. The scientific phase started on 8 March 2016 and will last 6 months. In April 2016 ESA announced that LISA Pathfinder demonstrated that eLISA mission is feasible.

The estimated mission cost is €400 million.

LISA Pathfinder will place two test masses in a nearly perfect gravitational free-fall, and will control and measure their relative motion with unprecedented accuracy. The laser interferometer measures the relative position and orientation of the masses 40 centimetres apart to an accuracy of less than 0.01 nanometres, a technology estimated to be sensitive enough to detect gravitational waves by the follow-on mission, eLISA.

LISA Pathfinder is an ESA-led mission. It involves European space companies and research institutes from France, Germany, Italy, The Netherlands, Spain, Switzerland, UK, and the US space agency NASA.

LISA Pathfinder is a proof-of-concept mission to prove that the two masses can fly through space, untouched but shielded by the spacecraft, and maintain their relative positions to the precision needed to realise a full gravitational wave observatory planned for launch in 2034. The primary objective is to measure deviations from geodesic motion. Much of the experimentation in gravitational physics requires measuring the relative acceleration between free-falling, geodesic reference test particles.

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