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ore than three decades after it was conceived, one of the strangest satellites ever designed is slowly taking shape in a laboratory outside San Francisco. Its mission is to carry four spheres the size of ping pong balls into orbit, where it will set them spinning at a rate of 9000 revolutions per minute. The spinning spheres will act as tiny gyroscopes, and their axes will point towards IM Pegasus, a star in the constellation of Pegasus, a few degrees above the celestial equator. But during the course of the two-year mission, scientists expect the gyroscopes to be deflected by a force that has never been observed before.The mission is called Gravity Probe B and it is based on the strange ability of a gyroscope to point always in the same direction (which is why they are used as navigation aids for aircraft, submarines and even spacecraft). By this principle, the axes of the spinning spheres should remain locked on IM Pegasus as the satellite orbits the Earth. But physicists have other expectations. They say that a force predicted by Einstein's theory of relativity, will push the axes away from the star by an amount so small that it is difficult to imagine, let alone measure. The force is gravitomagnetism and, if it exists at all, is as different from ordinary gravity as electricity is from magnetism. Failure to find the force would be a devastating blow and could mean that scientists will have to rewrite Einstein's theory which has become one of the cornerstones of modern physics.When Isaac Newton first toyed with the notion of gravity he wasn't bothered by considerations such as the speed of light or the idea of space-time. As far as he was concerned, gravity acted instantaneously between the Earth and an apple and he had no reason to think otherwise. All this changed in 1905 when Albert Einstein published his theory of special relativity, which proclaimed that nothing can travel faster than the speed of light. Newton's theory of gravity troubled Einstein because it allowed gravitational forces to act instantaneously, even across the vast distances between galaxies that light would take hundreds of millions of years to cross.To resolve this conflict, in 1916 Einstein published his

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