Gravitational waves are ripples in spacetime produced by the acceleration of very massive objects, such as black holes coming together and merging. 

Different objects in space produce gravitational waves of different timescales, ranging from milliseconds to billions of years. 

Some of these waves can only be observed from space.

This is the goal of ESA’s future mission LISA, which will be the first space-based gravitational wave observatory.

LISA will study gravitational waves that are produced by merging stellar mass black holes, supermassive black holes and white dwarfs. It will also pick up the waves produced by compact objects, like neutron stars or small black holes, that fall into a supermassive black hole.