The big thing in physics right now is gravity waves. Ripples in space-time. We've had some success detecting them - but as with so much in astronomy, we can get better detection if we launch our telescope into outer space.
Which is expensive, so ESA decided to do a test run first with a small satellite called Lisa Pathfinder, using test objects (with known mass).
The satellite is tracking the test objects 300 times better than the mission requirements. The current plan is to launch the actual telescope, which will consist of three satellites in linked orbit, in 2034 - but the success of this project might mean it's brought forward.
Let's hope so.