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European Space Agency OK's gravity-wave mission

21 Jun 2017

Ambitious plan to detect gravitational waves in space using laser interferometry penciled in for 2034 launch.

The European Space Agency (ESA) has moved a step closer to launching a laser-based system for detecting gravitational waves, after selecting the planned “LISA” mission for its science program.

LISA, short for laser interferometer space antenna, will form a key part of ESA’s “Cosmic Vision” plan running through 2035. It will feature a trio of satellites arranged in a triangle and separated by 2.5 million kilometres in space, and has a provisional launch date of 2034 – assuming that design and costing stages proceed as expected.

G-wave space observatory
The giant optical system would aim to build on the recent detection of gravitational waves by the Earth-based laser interferometers that make up the “LIGO” consortium, but would be able to sense ripples in space-time of different frequencies, generated by different types of cosmic events.

ESA says that the LISA Pathfinder mission launched in late 2015 had successfully demonstrated that some of the key technologies needed to detect those waves were feasible.

“This includes free-falling test masses linked by laser and isolated from all external and internal forces except gravity, a requirement to measure any possible distortion caused by a passing gravitational wave,” said the agency.

The optical bench on board that initial mission was developed by UK scientists at the universities of Glasgow and Birmingham. LISA Pathfinder carried two test masses, in the form of small gold-platinum cubes in free fall. Similar components should form part of the eventual space interferometer.

Its optical bench is made from a 20 x 20 cm block of Zerodur ceramic glass, and features a set of 22 mirrors and beam-splitters bonded to its surface to direct the laser.

Paul McNamara, LISA Pathfinder Project Scientist at ESA, said: "The whole point of LISA Pathfinder was to validate the technology for LISA, which requires test masses to be kept motionless to unprecedented levels of accuracy

"This seemed so difficult and could not be tested on the ground, so we had to send the laboratory to space."

ESA now says that LISA Pathfinder will complete its mission on June 30, having “successfully demonstrated the technology to build ESA's future space observatory of gravitational waves”.

G-wave spectrum
While LIGO was able to detect the echoes of ancient black-hole collisions corresponding to a frequency of around 100 Hz, the Earth-bound systems are only able to access a very narrow window within the gravity-wave “spectrum”.

“To fully exploit this new window on the cosmos, it is crucial to be able to detect gravitational waves at low frequencies, between 0.1 mHz and 1 Hz,” says ESA. Those frequencies would be generated by mergers of the supermassive black holes that sit at the center of galaxies – and require interferometer “arms” so long that they could only be constructed in space.

The LISA mission concept would be based around six test masses, two on each satellite. “Each pair of test masses will be located at the end of the arm of the constellation, and will be linked to the others over millions of kilometers via lasers,” says the space agency.

Stefano Vitale, principal investigator for the LISA Technology Package and a researcher at the University of Trento and INFN in Italy, said: “With the astonishing success of LISA Pathfinder, we now know how to build a mission like LISA.”

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