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Successful test drive for space-based gravitational-wave detector

Mission paves the way for planned €1-billion space observatory.

Scientists have long dreamed of launching a constellation of detectors into space to observe gravitational waves — the ripples in space-time predicted by Albert Einstein and observed for the first time earlier this month.

Nature Special: Gravitational Waves

That dream is now a step closer to reality. Researchers working on a €400-million (US$440-million) mission to try out the necessary technology in space for the first time — involving firing lasers between metal cubes in free fall — have told Nature that the initial test drive is performing just as well as they had hoped.

“I think we can now say that the principle has worked,” says Paul McNamara, project scientist for the LISA Pathfinder mission, which launched last December. “We believe that we now are in a good shape to look to the future and look to the next generation.”

“Everything works as we designed it. It’s sort of magical, and you rarely see that in your career as an experimentalist,” says Stefano Vitale, a physicist at the University of Trento in Italy, and a principal investigator for the Pathfinder mission.

Credit: Source: ESA/ATG medialab

The European Space Agency (ESA) financed the test, and hopes ultimately to launch a €1-billion observatory to hunt for gravitational waves. For that mission, lasers would be bounced between three spacecraft set millions of kilometres apart. Each craft would contain a test mass (a metal cube) that would be placed in free fall, protected from any forces except that of gravity. Because gravitational waves stretch and compress space-time, the observatory hopes to be able to see passing waves by using the lasers to detect minute changes in the distance between the free-falling cubes.

Because of its enormous scale, a space-based observatory could detect lower-frequency gravitational waves than can Earth-based experiments — such as the US Advanced Laser Interferometer Gravitational-Wave Observatory, which announced a first successful detection on 11 February. Lower-frequency waves can be triggered by more-powerful events, which scientists hope to study, such as collisions between galaxies and supermassive black holes.

The Pathfinder mission aimed to show — on a much smaller scale — that the basic design works, and to chart its limitations. It uses two test masses (each a 2-kilogram cube of gold and platinum) set 38 centimetres apart, floats them in isolation from everything except the influence of gravity, and tests to see whether changes in their relative movement can be measured with an accuracy of a picometre, 100,000th of the width of a human hair. To keep the cubes in free fall, the spacecraft monitors their motions and uses tiny thrusters to keep itself centred on the masses.

The complexity of such an experiment, carried out millions of kilometres from Earth, meant that sceptics doubted whether it could ever work in space, says Vitale. But data that have been streamed back since 23 February, when Pathfinder began to use the lasers to track its released cubes, show that it not only fulfils its requirements but exceeds them, he says. For now, the team is keeping under wraps details on exactly how well the instruments are performing.

Proving that the basic technology works is only the mission’s first step. Its main science goal, which the Pathfinder team will work on over the coming months, is to understand where 'noise' in the system is coming from. That knowledge will be essential in designing the space-based observatory, which is scheduled for launch in 2034. “The main goal of the mission is not so much to measure how well we’re doing, but to understand how well we’re doing,” says McNamara.

Success of Pathfinder was seen as a prerequisite for building the observatory, which ESA agreed to fund in 2013. But before launching such an ambitious experiment, scientists also considered it desirable that gravitational waves should already have been seen on Earth-based detectors. “It looks like these two conditions have been fulfilled in the same month. So it’s really our month,” adds Vitale.

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ESA/ATG medialab

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ESA/ATG medialab

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Freefall space cubes are test for gravitational wave spotter 2015-Nov-17

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Gibney, E. Successful test drive for space-based gravitational-wave detector. Nature 531, 30 (2016). https://doi.org/10.1038/nature.2016.19452

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