OrbitalHub

Wikipedia dixit:

“LISA Pathfinder will place two test masses in a nearly perfect gravitational free-fall, and will control and measure their relative motion with unprecedented accuracy. The test masses and their environment will be the quietest place in the Solar System.

LISA Pathfinder is a proof-of-concept mission to prove that the two masses can fly through space, untouched but shielded by the spacecraft, and maintain their relative positions to the precision needed to realize a full-up gravitational wave observatory later. The primary objective is to measure deviations from geodesic motion. Much of the experimentation in gravitational physics requires measuring the relative acceleration between free-falling, geodesic reference test particles.

In LISA Pathfinder, precise inter-test-mass tracking by optical interferometry will allow scientists to assess the relative acceleration of the two test masses, situated about 38 cm apart in a single spacecraft. The science of LISA Pathfinder consists of measuring and creating an experimentally-anchored physical model for all the spurious effects – including stray forces and optical measurement limits – that limit the ability to create, and measure, the perfect constellation of free-falling test particles that would be ideal for the eLISA follow up mission. In particular, it will verify: drag-free attitude control of a spacecraft with two proof masses, the feasibility of laser interferometry in the desired frequency band (which is not possible on the surface of Earth), and the reliability and longevity of the various components—capacitive sensors, microthrusters, lasers, and optics.

For the follow-up mission, eLISA, the test masses will be 2 kg cubes housed in two separate spacecraft one million kilometers apart.”

Video credit: ESA