Gravity space mission passes big test

By Jonathan Amos
BBC Science Correspondent

Image source, ESA
Image caption, Artist's impression: Lisa Pathfinder is stationed more than a million km in the direction of the Sun

The mission to demonstrate technologies needed to detect gravitational waves in space has been a stunning success.

The Lisa Pathfinder satellite was sent into orbit to test elements of the laser measurement system that would be used on a future observatory.

Performance objectives were exceeded on the very first day the equipment was switched on.

"During commissioning, the requirements were being met already," co-principal investigator Karsten Danzmann said.

"We hadn't tweaked anything; we'd just turned everything on to see if the laser was running and, bang, there it was. And the performance has just got better and better ever since," he told BBC News.

There is currently enormous excitement around gravitational waves - the ripples in space-time generated in cataclysmic cosmic events, such as the merger of black holes and the explosion of giant stars.

The existence of these phenomena was first confirmed last year at the Advanced Ligo facilities in the US.

They picked up a very subtle disturbance in their laser interferometers as waves from far-distant, coalescing black holes passed through the Earth.

The success has been lauded as one of the great scientific breakthroughs in decades.

Ripples in the fabric of space-time

Image source, NSF
Image caption, Artwork: Advanced Ligo detected coalescing black holes more than a billion light-years from Earth
  • Gravitational waves are a prediction of the Theory of General Relativity
  • Their existence had been inferred by science but only recently directly detected
  • They are ripples in the fabric of space-time produced by violent events
  • Accelerating masses will produce waves that propagate at the speed of light
  • Detectable sources ought to include merging black holes and neutron stars
  • Ligo fires lasers into long, L-shaped tunnels; the waves disturb the light
  • Detecting the waves opens up the Universe to completely new investigations

Researchers would like to take this same capability into space itself, to be able to observe the waves that are generated by types of event which are beyond the sensitivities of ground laboratories.

The coming together of gargantuan black holes as whole galaxies crash into each other is the kind of source this future mission would target.

But before so expensive a venture is approved, it has to be shown that the key technologies can work on the scale proposed. Hence, the European Space Agency's (Esa) Lisa Pathfinder mission.

The satellite was launched in December equipped with a single instrument designed to measure and maintain a 38cm separation between two small gold-platinum blocks.

These "test masses" were unclamped once in orbit and allowed to go into free-fall inside the spacecraft. A laser interferometer - in essence a very precise ruler - was then set the task of tracking the cubes' behaviour.

The results of this metrology experiment have now been detailed in a paper in the journal Physical Review Letters.

"Our requirement was that we had to measure the position of the test masses to nine picometres per root hertz, and our on-orbit performance is actually about 30 femtometres - so we're about a factor of 300 better than requirements," explained Esa project scientist Paul McNamara.

To put those numbers in more user-friendly terms: a picometre is a millionth of a millionth of a metre; 30 femtometres is tinier still, equivalent to about the diameter of a couple of gold atom nuclei.

Image source, AIRBUS DS
Image caption, Pathfinder's instrument. Other technologies are needed but these can all be tested on the ground

The laser instrument has witnessed the smallest of accelerations in the test masses, such as those resulting from the impact of residual gas molecules still bouncing around inside Pathfinder in the vacuum of space.

"We see relative accelerations lower than a 10th of a millionth of billionth of Earth's gravity," said Dr McNamara.

"It's a very quiet environment; there's no seismic noise, there's no machinery switching on and off, no-one walking past. It's a perfect lab that we're in."

All this gives confidence that a mission proper to measure gravitational waves in space will work.

This would be done by sensing the ripples' very delicate disturbance of gold blocks separated not by 38cm, as in Pathfinder, but by a few million km; and across three spacecraft flying in formation.

It is this configuration that would make the future mission, dubbed simply Lisa, able to detect the long-wavelength, low-frequency signals that are beyond the range of Earth facilities like Advanced Ligo.

"With Lisa Pathfinder we have learnt not only to walk but to jog pretty well. So, we are now ready for the big marathon, to do the big race," said senior Esa official Fabio Favata.

"We haven't been sitting still. The technology on Lisa Pathfinder is key, but the agency has also been working on other technologies that will enable us to carry out this future mission."

Laser science: Measuring the distance between gold blocks

Image source, ESA
Image caption, A cutaway impression of the laser interferometer system inside Lisa Pathfinder
  • Lisa Pathfinder's payload is a laser interferometer, which measures the behaviour of two free-falling blocks made from a platinum-gold alloy
  • Placed 38cm apart, these "test masses" are inside cages that are very precisely engineered to insulate them against all disturbing forces
  • When this super-quiet environment is maintained, the falling blocks will follow a "straight line" that is defined only by gravity
  • It is under these conditions that a passing gravitational wave would be noticed by ever so slightly changing the separation of the blocks
  • Lisa Pathfinder was designed to demonstrate picometre sensitivity, but the satellite cannot itself make a detection of the ripples
  • To do this, a space-borne observatory would need to reproduce the same performance with blocks positioned a million-plus km apart

Lisa, or whatever name it is eventually given, is tentatively scheduled for launch in 2034 (although this could come forward).

Esa is currently in discussion with its US counterpart, Nasa, about a collaboration.

"Nasa has established a study team to see just how the US could participate," said study chair David Shoemaker from the Massachusetts Institute of Technology.

"Clearly, there's a bit of a dance to figure out what best comes from the US versus Europe. It's a question of working out where skills lie, and how the US fits into the plan developing in Europe.

"We're aware that the Europeans really want to firm up the way the mission is divvied up by the end of the calendar year, and so that sets a timescale for the discussions, between the scientists on both sides of the Atlantic and between the heads of the agencies which are starting to get more lively."

The success of Lisa Pathfinder is a fillip to British academia and industry.

The universities of Glasgow, Imperial College London, and Birmingham provided core parts of the instrument, while the satellite itself was assembled at Airbus Defence and Space in Stevenage.

Building Pathfinder was a painstaking process. Its own gravity could have disturbed the experimental blocks and so the layout had to be very carefully designed so that the tugging force exerted by the onboard equipment was evenly balanced in all directions.

"Our mass log in Stevenage where we wrote down what everything weighs has 10,000 entries," said Airbus programme manager Christian Trenkel. "We are very proud of what we have achieved."

Image source, Airbus DS
Image caption, The full Lisa mission is currently set for 2034, but efforts will be made to bring this date forward and follow me on Twitter: @BBCAmos

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