The LISA mission is a concept for a future space-based gravitational-wave observatory now being developed. It consists of three spacecraft separated by millions of miles flying in a triangular formation that trails behind Earth as we orbit the sun. Each spacecraft transmits and receives laser beams linking it to the others, and these signals are combined to search for passing gravitational waves -- ripples in space-time -- as they pass through the formation. This model is based on an early LISA concept. The design is now being refined and needed technologies are being developed, so the actual spacecraft may look quite different. NASA is a partner in the mission, which is being led by the European Space Agency. The observatory is expected to launch in the mid-2030s.