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Fitting together Einstein’s cosmic puzzle

LISA Pathfinder will pave the way towards detecting gravitational waves, one of the most elusive forces in space.

Testing on two critical aspects of the LISA Pathfinder mission was completed at the end of 2011: the maintenance of a highly stable environment on the spacecraft and the measurement accuracy of its on-board instruments. This mission – for which Airbus Defence and Space is the spacecraft prime contractor – will pave the way towards detecting gravitational waves, one of the most elusive forces in space.

“The number of things we’ve had to learn that no-one’s ever done before is quite amazing. The gravitational modelling was at an unprecedented level of detail, for example, and we had to develop a magnetically shielded chamber to perform magnetic noise measurements,” says Neil Dunbar, Spacecraft Engineering Manager on LISA Pathfinder at Airbus Defence and Space Stevenage. This programme is itself a precursor to a larger mission for a New Gravitational wave Observatory (NGO), and is expected to be launched in 2014 to test technologies for the later mission on a smaller scale, many of which are firsts.

The electrodes housing (EH) box is a cube measuring about 70mm on a side which will contain a gold-platinum LISA Technology Package proof mass with a gap of only 4mm. This gap will allow the proof mass to float inside the EH once LISA Pathfinder is in space. (© ESA)NGO is a candidate for ESA’s next generation of Cosmic Vision missions, with launch set for the early 2020s. Its goal is to detect gravitational waves – fluctuations in the fabric of space and time – using laser interferometry to measure the distance between free-floating cubes on board spacecraft one million kilometres apart. Under perfect conditions, the cubes would be expected to exactly copy each other’s motions. However, according to Einstein’s general theory of relativity, if a gravitational wave were to pass through space, then a minuscule distortion in the fabric of space itself would be detectable.

The cubes – or test masses – will be arranged two per spacecraft within a triangular three-vehicle constellation. The mission is set to answer questions on the physical laws of the universe, aiming to detect signals from massive black holes, binaries of compact stars in the Milky Way and possibly even the very early phase of the Big Bang. For LISA Pathfinder only one spacecraft with two test masses onboard will be built to prove the technology needed for NGO in space. “It’s the most exciting programme I’ve ever worked on,” adds Ian Honstvet, LISA Pathfinder Project Manager, who is also based in Stevenage.

The first challenge of such a complex mission as LISA Pathfinder is establishing an environment in space stable enough to shield the test masses from all external influences other than gravitational waves and allowing them to fly a perfect gravitational free-fall trajectory. As such, the spacecraft is being constructed around the test masses, 46-millimetre gold-platinum cubes electrostatically suspended inside a housing, which is itself inside a vacuum container within the optical bench subsystem.

The LISA Pathfinder Core Assembly – designed by the LISA Test Package Architect team in Friedrichshafen. The structural model of the LCA is shown from top as integrated inside the spacecraft's cylindrical support structure.

The LISA Pathfinder Core Assembly – designed by the LISA Test Package Architect team in Friedrichshafen. The structural model of the LCA is shown from top as integrated inside the spacecraft's cylindrical support structure.

Challenging developments

“The material used in the cubes is highly reflective, not very susceptible to magnetisation and highly conductive so the cubes don’t build up temperature gradients or pockets of charge,” explains Neil. “The downside is that it’s structurally weak. While this does not pose any significant problems once they are floating freely inside the cage, they could be damaged during launch. As such, they have to be held in place inside the cage using eight fingers and then released only when the spacecraft reaches its final position. “The release is tricky, and development of the caging mechanism to safely hold the masses and gently release them has been a significant challenge for the instrument team in Friedrichshafen,” Neil adds.

Although the achievement of gravitational free fall can only be verified in space, the system level tests carried out at test facilities in Ottobrunn in 2011 have confirmed that key elements of the spacecraft are ready for flight. The tests covered vibration and shock, electromagnetic compatibility and thermal vacuum. Critical requirements, such as the existence of an exceptionally stable thermal environment and extremely clean EMC and magnetic environment, have already been successfully tested and met on the ground.

Vacuum testing in Ottobrunn in October 2011.

Vacuum testing in Ottobrunn in October 2011.

Measuring in the quantum realm

The system level tests carried out at test facilities in Ottobrunn last year verified the mission’s second challenging aspect: the incredible accuracy of the measurements required to monitor gravitational waves. The final mission will measure low-frequency waves with a resolution of 10 picometres, yielding a strain sensitivity of one part in 1021. The precision of the optical metrology subsystem was tested during the thermal simulation testing using mirrors to replace the test masses. During the 2011 tests, a staggering two-picometre resolution was obtained, far exceeding the best performance for an instrument of this type.

Thanks to the dedication of the spacecraft and instrument teams, these tests have also qualified the majority of the spacecraft system for the mechanical and electromagnetic launch environment requirements and against the temperature extremes the spacecraft will experience during the post launch mission phases.

Both Neil and Ian emphasise that there is still a long way to go on LISA Pathfinder. The micropropulsion subsystem, in particular, is still under development, and even after the launch of the nominally six-month mission, the design teams will continue working. Ian concludes: “This will be where the inflight testing measures the actual performance of this demonstration mission and hence the viability of the future NGO mission.”

 

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