The experiment described involves three spacecraft flying in a specific formation (equilateral triangle with sides measuring one million kilometers) and using lasers shining between them. This configuration and method are designed to detect gravitational waves by precisely measuring the tiny changes in the distances between the spacecraft caused by passing gravitational waves. Voyager-2 and New Horizons are spacecraft designed for planetary exploration, not gravitational wave detection. LISA Pathfinder was a single spacecraft technology demonstrator mission for the LISA concept, proving the feasibility of housing free-falling test masses and measuring their motion with extreme precision, but it did not involve three separate spacecraft measuring distances over millions of kilometers. Evolved LISA (Laser Interferometer Space Antenna) is a planned space mission by ESA and NASA that will consist of three spacecraft forming an equilateral triangle with sides approximately 2.5 million kilometers long. It will use laser interferometry between the spacecraft to detect gravitational waves in the millihertz frequency range, which are expected from sources like merging supermassive black holes and binaries of compact objects in our galaxy. The description in the question, while giving slightly different side length (one million km vs 2.5 million km), accurately captures the core concept of the Evolved LISA mission design for gravitational wave detection in space.