You are probably thinking of the planned Laser Interferometer Space Antenna (LISA) [1]. Launch is scheduled for 2034 but the technical demonstration mission LISA Pathfinder [2] was already done between 2015 and 2017 and exceeded its targets with regard to sensitivity. Other proposed space-based gravitational wave detectors are TianQin [3] and DECIGO [4].

[1] https://en.wikipedia.org/wiki/Laser_Interferometer_Space_Ant...

[2] https://en.wikipedia.org/wiki/LISA_Pathfinder

[3] https://en.wikipedia.org/wiki/TianQin

[4] https://en.wikipedia.org/wiki/Deci-hertz_Interferometer_Grav...