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Nasa scientists are preparing the coolest, strangest and most sensitive experiment ever launched, to answer a question that has teased physicists for 88 years - does the Earth really twist space into a kink as it spins?

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Gravity Probe B is a spacecraft of superlatives. It has been the longest in the making, the first funding began more than 40 years ago. It has the coolest instrument ever, chilled by 613 gallons of liquid helium to a bleak -271C, just 2 degrees above absolute zero.

It will make the most sensitive observations so far attempted in space, the equivalent to measuring the thickness of a human hair from 440 yards. It has the most precise launch window of any spacecraft: just one second at 6.01pm BST on April 19.

And it has the most historic challenge: it will try to test some predictions made in 1916 by Albert Einstein's general theory of relativity. This is that planets and stars not only curve the structure of spacetime, they must also twist it as they spin on their axes. A colossal black hole would massively distort spacetime around it, and even a small planet like Earth should "drag" the structure of spacetime very slightly as it rotates.

This proposition has stood for almost nine decades. But Gravity Probe B, fitted with an exquisitely accurate telescope and a set of ultrasensitive gyroscopes in a huge vacuum flask 10 times emptier than the empty space around it, is heading into orbit 400 miles above Earth to put it to the test.

"If the predictions of general relativity are confirmed, I would feel a sense of satisfaction that not only Einstein's historic theory of gravity, but also the many years of work on the GP-B project, had all succeeded spectacularly," said Michael Ratner of the Harvard-Smithsonian Centre for Astrophysics.

"Conversely, if the Gravity Probe B results are inconsistent with Einstein's theory, I would be excited about the prospect of what might come next. It would motivate a fresh look at the foundations of physics."

At the heart of the experiment are four gyroscopes, each holding a spinning mass of quartz the size of a pingpong ball, and a telescope which will be fixed on a star called IM Pegasi.

The spacecraft will orbit the Earth every 97 minutes or so. The telescope is fixed on the guide star, while the gyroscopes will blindly measure the spacecraft's orientation through space over 13 months.

If Einstein's predictions are right, at the end of that period the telescope will still be fixed on the star's light, but the gyroscope axis will have shifted by a tiny fraction of a degree: 40.9 milliarc seconds.

But to devise an apparatus this sensitive has taken 40 years of effort. To detect such tiny shifts in spacetime, scientists have had to eliminate all other influences. The gyroscopes must be cold, free of gas, free of gravity, undisturbed by electrical and magnetic fields.

So the apparatus must be colder than the space around it, and emptier. The magnetic field inside it must be less than one millionth of the Earth's magnetic field, which means the apparatus must be held inside a lead balloon.

The gyro rotors have to be centred to a few millionths of an inch while spinning at 10,000 revs per minute. The 9ft vacuum chamber must operate at pressures 120 times less than near-Earth space, and the gyroscope spheres must be machined precisely.

"Gravity Probe B has the potential to uncover fundamental properties of the invisible universe, a universe which seems bizarre and alien to our everyday perceptions, yet one that Einstein tried to show us almost a century ago," said Anne Kinney, of Nasa's office of space science.

And Francis Everitt of Stanford University, principal investigator, said "Developing GP-B has been a supreme challenge requiring the skillful integration of an extraordinary range of new technologies."


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