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Space-based gravitational wave detector one big step closer

The LISA Pathfinder exceeded expectations – and after only a couple of months. Belinda Smith reports.

An artist's impression of LISA Pathfinder, which just successfully tested a key technology needed to build a gravitational wave detector in space.
ESA / C. Carreau

The European Space Agency's plan to build an ultra-sensitive gravitational wave detector in space just took a major step forward, with the Laser Interferometer Space Antenna (LISA) Pathfinder mission successfully housing a pair of metallic cubes in free-fall for two whole months.

Last September, the ground-based Laser Interferometer Gravitational-Wave Observatory (LIGO) picked up the ever-so-faint ripples in space-time produced from the cataclysmic smash of black holes.

But a detector in space, far from the rumblings of the Earth's crust or trucks on nearby roads, will be able to pick up much fainter signals – and this is what the LISA mission aims to achieve.

First, though, the LISA Pathfinder must tick all the technology boxes.

Launched in December and lodging in its orbit between the Earth and sun in late January, the LISA Pathfinder sucked all the gas molecules out of its chamber and dropped two gold and platinum cubes into its centre.

A high-resolution laser measured the position of the cubes and fed the information to the craft's Drag-Free and Attitude Control System, which fired microthrusters to manoeuvre the surrounding spacecraft to fly in formation with and protect the cubes.

This means the cubes floated in what is essentially free-fall – with the rest of the spacecraft "falling" with it – also called drag-free flight. It did so for two months from the start of March.

This plot shows the result of LISA Pathfinder's two-month experiment in drag-free flight, where the goal was to follow test masses as they fall through space affected only by gravity. LISA Pathfinder reduced non-gravitational forces on the test masses to a level five times better than the mission required and within 25% of the requirement for a future space-based gravitational wave detector. The cause of the spike around 0.07 hertz is still under investigation.
NASA's Goddard Space Flight Center

The LISA mission, should it go ahead, will comprise masses connected by lasers, but over millions of kilometres.

As gravitational waves stretch the space-time between the masses, lasers pinging between them will be ever-so-slightly out of sync – much in the way that the four-kilometre arms of the LIGO stretched last year.

But ground-based observatories, while exceptionally sensitive, are limited to higher frequency gravitational waves of 100 cycles per second or so.

More exotic events, such as the merger of supermassive black holes, each millions or billions of times the mass of the sun, should produce low-frequency waves – perhaps around one cycle per second.

And this is where a space-based LISA detector will excel. "A full-scale observatory with LISA Pathfinder's performance would achieve essentially all of the science goals," says LISA Pathfinder team member Ira Thorpe.

The findings were published in Physical Review Letters.

Further reading:
Probing creation with waves
Catching gravitational waves
The physics event of a lifetime

Belinda smith 2016 2.jpg?ixlib=rails 2.1
Belinda Smith is a science and technology journalist in Melbourne, Australia.
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