Pioneering UK-led gravity probe is designed to open a unique window on the universe but the technology can only be tested in space. Fingers crossed for LISA-Pathfinder

“This is the first of a new breed of spacecraft,” says Cesar Garcia, Esa Project manager for the LISA-Pathfinder mission, “It is exquisite.”

He is speaking to me in the giant cleanroom at Airbus Defence and Space in Stevenage. On Friday 27 February, LISA-Pathfinder was painstakingly lifted onto its propulsion module, and secured in place. Over the weekend, the whole assembly was packaged in a high-tech shipping crate and is now on its way to IABG (Industrieanlagen Betriebsgesellschaft), near Munich, Germany, for final work.

“We have tested everything that it is realistic to test on Earth,” says Garcia. For the rest, the engineers and scientists have relied on calculations and computer models. In September, it will be time to swap the virtual world of simulations for the reality of space.

LISA-Pathfinder marks a resurgence of the UK’s space science industry. It is the first UK-led Esa science mission since Giotto, which visited Halley’s comet in 1986. It will be launched from Europe’s spaceport at Kourou, French Guiana, and is designed to trace the gravitational field of a small region of the solar system.

Einstein envisioned a gravitational field as a landscape of invisible contours, called the spacetime continuum. LISA-Pathfinder will reveal these contours.

No one has built a spacecraft or an instrument like this before. Therefore its primary mission is simply to see if it works at all.

Facebook Twitter Pinterest The LISA-Pathfinder spacecraft is lowered onto its propulsion unit in the cleanroom at Airbus Defence and Space, Stevenage. Photograph: Stuart Clark

“If everything checks out as we hope, this will open the way for a wave of gravitational missions,” says David Southwood, who was the director of science at Esa between 2001-2011, when a large part of the mission’s development took place, “Everyone will start building them but, of course, it’s having the nerve to do it first.”

On that score, LISA-Pathfinder certainly has balls. Two of them. They are the test masses that will respond to the Solar System’s gravitational field. Each weighs 2kg and has been hewn from a €200,000 ingot of gold-platinum alloy. They will essentially float freely inside the spacecraft, moving only as gravity dictates.

“It’s rather similar to Galileo dropping objects from the leaning tower of Pisa,” says Southwood.

When LISA-Pathfinder was approved in 2000, launch was envisaged in 2006. The technology to reach the necessary accuracy, however, has proved so challenging that delays were inevitable.

To watch how the test masses are moving, the spacecraft will bounce a laser beam off them, detecting their motions down to 100,000th of the width of a human hair. It will then shift its own position to keep the test masses centred within it, using thrusters so fine that they could not even blow a piece of paper off a desk on Earth. Yet, in the frictionless environment of space, they can move the whole spacecraft, which will weigh 1900kg at launch.

As well as developing the technology, the engineers faced another unique challenge. “We have had to weigh every single nut and every single bolt to calculate the gravitational field of the spacecraft itself,” says Ian Honstvet, LISA-Pathfinder project manager for Airbus.

That’s because even the minuscule gravity created by a single unaccounted-for cable-tie would be enough to devastate the results. So, the engineers have built a precise computer model of the spacecraft’s components to allow their influence to be subtracted from the results.

Scientists and engineers expect that the primary mission will last between six and twelve months. After that, if there is fuel left, the spacecraft could be sent on a looping, year-long journey around the Earth to make one of the most exciting measurements in modern physics – and provide a much needed reality check on whether most of the matter is in a mysterious unknown form called dark matter.

Our whole view of the universe is built on the assumption that gravity behaves the same in the remotest regions of space as it does on Earth but this has never been tested. The trouble is that the galaxies in the universe all rotate faster than the visible matter contained within them allows.

Most astronomers think that this means swaths of dark matter, hypothetical subatomic particles, must be there to provide the extra gravity to spin the galaxies. But despite decades of efforts and billions spent on the search, no one has managed to convincingly detect a single piece of the stuff. As a result, a small but growing number of scientists are wondering whether the problem lies in our understanding of gravity.

LISA-Pathfinder could test these opposing views by passing through the point between the Earth and our sun where gravity cancels out. If it sees an unexpected gravitational signal here, it would be a landmark in physics; a Nobel prize-winning scientific revolution every bit as huge as those of Newton and Einstein. Our understanding of the whole universe would change direction overnight, and probably we could discount dark matter.

Conversely, if it sees nothing unusual, it will be a tremendous confidence boost that despite all the detection failures, we do need dark matter to explain the motion of galaxies. Either result is profoundly important. So is the technology that the mission will test first.

LISA-Pathfinder is our bridge to the future. In parallel with the gravitational wave observatories being built on Earth, it will open up a whole new way of studying the universe. Be excited.

Stuart Clark is author of The Unknown Universe (Head of Zeus). Find him on Twitter @DrStuClark.

