By Jonathan Amos
BBC News Online science staff
The contract has been signed that will lead to the building of one of the most ambitious space missions ever flown.
Lisa Pathfinder should launch in 2008
The Lisa Pathfinder will demonstrate technologies that will be necessary to detect gravitational waves in space.
Being able to see these "ripples in the fabric of space-time" should allow scientists to probe the Universe to within one second of the Big Bang.
The European Space Agency has awarded the contract to build Lisa Pathfinder to EADS-Astrium in Stevenage, UK.
It is the first Esa science mission built in Britain since the remarkable Giotto probe, which obtained the first close-up images of the heart of a comet - Comet Halley - in 1986.
"Lisa could open a new window on the Universe, giving us completely different kinds of information to that obtained through all of the satellites and all of the Earth-based observatories we've ever had," Dr Harry Ward, from the Institute of Gravitational Physics at Glasgow University, told BBC News Online.
Gravitational waves were predicted by Einstein's general theory of relativity, published in 1916. However, although astronomers have indirect evidence of their existence, none has yet been detected directly.
The movement of truly massive bodies, such as black holes and colliding galaxies, are expected to disturb the space-time around them, sending gravitational waves radiating outwards.
But unlike electromagnetic waves - the light seen by traditional telescopes - gravitational waves are extremely weak. If one were to pass through your body it should simultaneously stretch your space in one direction whilst squashing it in a direction that is at right angles.
Professor Southwood has given the UK the prime role; the first since Giotto
Current Earth-based observatories hunting for this disturbance bounce lasers down L-shaped tunnels that are hundreds or thousands of metres long. And their instrumentation is fantastically sensitive, aiming to find deviations that can be equivalent to one one-thousandth of the width of a proton, one of the particles that make up all atoms.
The Lisa (Laser Interferometer Space Antenna) mission will do something very similar in space - but with three spacecraft flying 5 million km apart in an equilateral triangle formation.
The laser beams travelling between the spacecraft will measure distances between free-floating gold blocks held in place by carefully controlled electrostatic fields.
Again, to detect the disturbance in space caused by a passing wave, the size of the deviation that must be detected is astonishingly small - about a billionth of a millimetre over the distance of 5 million km.
Back in time
The spacecraft must be insulated from all extraneous "noise" that might swamp the measurements, from the gravity from their own components to the pressure of sunlight which will try to push the satellites off course.
The fixes scientists and engineers have come up with for all these problems will have to be tested first - and that is where Lisa Pathfinder comes in.
It will be the demonstrator. For example, Pathfinder will assess the performance of three models of thrusters which must maintain the satellites' precise positions as their lasers watch over the gold blocks.
These propulsion systems either "leak" a cold gas or "spit" atoms to generate from 0.1 to 50 microNewtons of thrust; you would need 10,000 of them to lift a piece of paper on Earth.
The thrusters produce tiny forces
"Lisa Pathfinder is an entirely new type of spacecraft," said Dr Mike Healy, EADS-Astrium's director of Earth observation, navigation and science. "It challenges Europe in precision technologies; Lisa can be viewed as the 'Swiss watch' of spacecraft."
If Pathfinder proves those technologies and the main mission does indeed fly and detects gravitational waves, the prize would be remarkable.
"Our standard model of the Universe predicts the production of a noise of background gravitational waves just after the Big Bang - just like the microwave background emitted 300,000 years after the Big Bang," explained Professor Stefano Vitale, from Trento University, Italy, and the lead scientist on Pathfinder.
"The difference with the gravitational background is that it would have been emitted [0.000,000,000,000,000,000,000,000,000,000,01] seconds after the Big Bang. So the moment we are able to detect this, we will have a very early picture of the Universe."
A successful Lisa would give astronomers access to information they can only dream about now - allowing them, for example, to map the "surfaces" of black holes.
Electomagnetic (light) waves from just a few hundred thousand years after the Big Bang; Lisa will see a much earlier time
Colin Paynter, managing director of EADS-Astrium, signed the 80 million euros (£50m) contract to build Lisa Pathfinder with Professor David Southwood, Esa's director of science.
"To find answers you have to search the unknown," Professor Southwood said.
"Lisa takes us into totally new activities; a totally new kind of astronomy. Pathfinder is the necessary technological step that kicks us off into an unknown Universe."
Pathfinder should launch in 2008. It will draw on the expertise of institutions from across Europe and has substantial input from the US space agency (Nasa).