Originally a joint ESA/NASA mission to detect gravitational waves from objects such as black holes to consist of three spacecraft flying in triangular formation 5 million km apart, 20 degrees (50 million km) behind the Earth in its orbit. Together, they act as a Michelson interferometer to measure the distortion of space caused by passing gravitational waves. Lasers in each spacecraft will measure minute changes in the separation distances of free-floating solid blocks, known as ‘proof masses’, within each spacecraft. A gravitational wave passing through the spacecraft will change the separation between the proof-masses to vibrate. These tiny movements are observed by means of laser interferometry. Technical challenges involved are now being addressed by the LISA Pathfinder mission. NASA pulled out of the project in 2011 and the project was renamed Evolved Laser Interferometer Space Antenna (eLISA) with a tentative launch date in 2034.