LISA Pathfinder & LISA
LISA Pathfinder was an ESA spacecraft that launched in December 2015. The mission was designed to test technologies for LISA (Laser Interferometer Space Antenna), due to be launched in 2034. LISA aims to detect and study gravitational waves. In April 2016, ESA announced that LISA Pathfinder demonstrated that the LISA mission is feasible.
LISA Pathfinder was a scaled-down model of one arm of the final LISA spacecraft. LISA will consist of three separate spacecrafts, arranged in a triangle formation, and will work as an interferometer: bouncing lasers between the spacecrafts. The distance between the spacecrafts will be 2.5 million km, whilst in the LISA Pathfinder model the distance is only 40 cm.
The scaled-down model did not decrease the accuracy of the measurements. However, the sensitivity to gravitational waves is proportional to the arm length, and so LISA Pathfinder is a billion times less sensitive than LISA itself will be.
Once the LISA Pathfinder test mission was complete, it was deactivated in June 2017. LISA will be built and launched in the 2030s, into an orbit around the Sun. It aims to detect gravitational waves by measuring the distance between each of the three spacecrafts very precisely. LISA will be the first gravitational wave observatory in space (LIGO detects gravitational waves from the surface of Earth).