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Putting Einstein to the test

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3:11pm, April 24, 2007

From Jacksonville, Fla., at a meeting of the American Physical Society

NASA's longest-running mission is accomplished—almost.

Gravity Probe B went into orbit in 2004 after 4 decades of development and seven NASA reviews that often threatened to cancel it. It was meant to test general relativity, Albert Einstein's theory of gravity (SN: 11/5/05, p. 302: Available to subscribers at A matter of gravity; 11/1/03, p. 280: A Spin through Space-Time).

The $700 million probe incorporated numerous technologies to keep four gyroscopes—near-perfect spinning spheres the size of ping-pong balls—virtually free of external disturbances. Magnetic readouts tracked the gyros' rotation axes, which Newtonian physics predict would be perfectly stable. However, two distinct relativity effects would make the axes drift.

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