LISAPathfinder test masses released : a major step on the road
The gravitational skies seem to be auspicious these days. While the LIGO collaboration was announcing the discovery of gravitational waves, the LISAPathfinder team was going through a very delicate process : the release of the two test masses which was completed successfully this morning.
Behind the polished press release of ESA, let me explain what was at stake, and why everyone in the eLISA project is relieved and very joyful today. Indeed, you can see in the tweet below the reaction of Stefano Vitale, the scientist in charge of the mission, and César García, the project manager (we had met him in Kourou last November a few hours before launch, see the video of the hangout).
— ESA_LPF (@ESA_LPF) February 16, 2016
LISAPathfinder is testing the basic principle behind the eLISA mission which is to be able to measure variation of distances between two test masses which are only submitted to gravitation, in other words which are freely floating in the cosmos. These test masses are small cubes of gold-platinum of 46 mmm side.
During the experimental phase, these masses are floating in a cage, called electrode housing. Thanks to these electrodes, the satellite is constantly monitoring the position of the test mass, and operates its external microthrusters in order to change its own position and reposition itself in such a way that the test mass stays at the centre of its cage. In this way, the satellite protects the test mass from external perturbations.
But there is one difficulty: whereas the test mass is floating once on site, it has to be tightly locked during launch: otherwise, the strong vibrations would shake it within its housing, which would provoke irreparable damage.
And here is the tricky engineering problem, which gave nightmares to ESA (and, in an earlier stage, NASA) teams: this was known to everyone in the mission as the “infamous caging mechanism”. How do you release the test mass once you have tightly locked it? The difficulty is that, once tightly pressed, the test mass sticks to the metal finger that presses on it. But one has to release the mass very softly, because only very small forces can be acted upon.
The solution that was finally adopted relies on a two-stage process.
Throughout LISAPathfinder’s launch, , and the six-week cruise to its work site, each cube was held firmly in place by eight ‘fingers’ pressing on its corners. On 3 February, the locking fingers were retracted and a valve was opened to allow any residual gas molecules around the cubes to vent to space. Each cube remained in the centre of its housing held by a pair of rods softly pushing on two opposite sides.
The rods were finally released from one test mass yesterday and from the other this morning, leaving the cubes floating freely, with no mechanical contact with the spacecraft.
Congratulations to the project manager, César García, and all the technical teams involved in this success !
It will be another week before the cubes are left completely at the mercy of gravity, with no other forces acting on them. Before then, minute electrostatic forces are being applied to move them around and make them follow the spacecraft as its flight through space is slightly perturbed by outside forces such as pressure from sunlight.
On 23 February, the team will switch LISA Pathfinder to science mode for the first time, and the opposite will become true: the cubes will be in free fall and the spacecraft will start sensing any motion towards them owing to external forces. Microthrusters will make minuscule shifts in order to keep the craft centred on one mass.
The final word to Stefano Vitale: “Releasing LISA Pathfinder’s test masses is another step forward in gravitational wave astronomy within this memorable month: the test masses are, for the first time, suspended in orbit and subject to measurements”.