The eLISA/NGO mission
Electromagnetic waves, created by the acceleration of electrical charges, propagate in the framework of space and time. Gravitational waves in contrast, created by the acceleration of large bulks of mass, are waves of the spacetime fabric itself. They cause a change in the quadrupol moment of the mass distribution or in other words, time-varying changes in distance between free macroscopic bodies.
eLISA/NGO will observe passing gravitational waves directly by measuring the tiny changes of distance between freely falling proof masses inside spacecraft. Key features of eLISA/NGO are interferometric measurement of distances, long baselines of 2 x 106 km, drag free spacecraft based on inertial sensors, and the familiar “cartwheel”-orbits.