The European Space Agency's LISA Pathfinder satellite was shut down this week, after 16 months of service in the hunt for gravitational waves. But this is only the beginning: the Pathfinder satellite was a testbed for technology that will eventually be used in the main LISA mission, which is set to become the largest gravitational wave observatory ever built.

LISA Pathfinder's on-board systems were powered down on July 18, just after 8pm Central European Time. To keep the now-defunct satellite from crashing back to Earth, it was nudged into a safe parking orbit, which will keep it away from the planet for the next 100 years.

"When we were in contact with LISA Pathfinder yesterday evening for the last time and said goodbye to the satellite, it was a unique and emotional moment," says Karsten Danzmann, co-Principal Investigator of the LISA Pathfinder mission. "After years of planning and the launch of the satellite in December 2015 we have been spending many days and nights since early 2016 paving the way for the future of gravitational-wave astronomy with LISA Pathfinder."

The point of Pathfinder was to test the technology that will eventually make its way into the main LISA mission, scheduled for launch in 2034. The scientists report that the project passed with flying colors, with the systems operating optimally and measurements that surpassed expectations. Over the intervening years, the core concepts will be scaled up so LISA can listen in on low-frequency gravitational waves.

These ripples in the fabric of spacetime were first predicted by Einstein over 100 years ago, and were one of the last pieces of the puzzle of general relativity to be directly observed. In 2015, the LIGO facility detected the physical effects of gravitational waves for the first time, before going on to witness them twice more over the years. But in order to better learn about the phenomenon, we need more precise instruments, and by virtue of floating in space, LISA can pick up far smaller distortions than the ground-based LIGO facility.

Although gravitational waves are caused by cataclysmic events like the merging of two black holes, by the time their ripples reach us their effects can only be seen on the subatomic scale. To detect these incredibly tiny wobbles, LIGO and LISA both work off the process of laser interferometry, beaming a laser between two points and watching for any changes in the distance between them.

On LISA Pathfinder, these two points were free-floating cubes made of gold and platinum, spaced 38 cm (15 in) apart. In the LIGO facility, the points were 4 km (2.5 miles) apart, but the future LISA mission blows both of them away. It will be made up of a constellation of three separate satellites beaming lasers to each other in a triangle, where each side is 2.5 million km (1.6 million miles) long. At this scale, the scientists on the project say that LISA would be able to detect gravitational waves coming from virtually anywhere in the universe.

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