This is the interior of an electrode housing box. It may not look much, but the gold–platinum test cube will fly aboard the European Space Agency’s LISA Pathfinder mission in an attempt to achieve the best free-fall ever.
What the hell is that good for, you may ask? Launching later this year, LISA Pathfinder wil help gravitational waves – ripples in the fabric of space-time that are predicted by Albert Einstein’s general theory of relativity. According to ESA, the mission’s goal is to achieve the best free-fall ever, reducing all the non-gravitational forces acting on two test masses and controlling any residual effect with unprecedented accuracy. This is how it will work:
LISA Pathfinder’s test masses are two identical cubes of solid gold–platinum alloy, measuring 4.6 cm on a side and almost 2 kg each. Once in space, they will have no mechanical contact with their immediate environment. Each cube is surrounded by an ‘electrode housing’, its walls lying several millimetres from the cube on all six sides. The boxes will track the positions of the test masses and apply tiny adjustments if needed. The housings are part of highly sophisticated equipment that includes a laser metrology system and several microthrusters to apply small shifts to the spacecraft’s position to keep it centred on the masses. In fact, achieving a near-perfect gravitational free fall is very complex even in space, as forces other than gravity will disturb the motion of the cubes, including pressure from sunlight and particles from the solar wind.
The data collected by LISA Pathfinder will reveal all the spurious effects that can affect the purely gravitational motion of two test masses in space. In a full-scale gravitational wave observatory, the test masses would be contained in individual spacecraft separated by a million kilometres. Knowledge of all the non-gravitational forces acting on them will be needed to calibrate and optimise such a future experiment, enabling the detection of possible variations in the position of the test masses caused by a passing gravitational wave.
[CGS SpA via ESA]