“The experiment will employ a trio of spacecraft flying in formation in the shape of an equilateral triangle that has sides one million kilometres long, with lasers shining between the craft”. The experiment in question refers to
Explanation
The description mentions three spacecraft in an equilateral triangle formation with million-kilometre sides using lasers for measurement. This matches the design of the Evolved Laser Interferometer Space Antenna (eLISA) mission. eLISA is a proposed space observatory aiming to detect gravitational waves using precisely positioned spacecraft.
Voyager-2 and New Horizons are single spacecraft on exploration missions, not designed for gravitational wave detection.
LISA Pathfinder was a precursor mission to test technologies for eLISA, not the full mission itself.
Therefore, based on the details provided, the experiment refers to the Evolved LISA mission.