http://www.cnn.com/2004/TECH/space/0....ap/index.html
At the spacecraft's heart are four pingpong-sized balls of quartz, the most perfect spheres ever made. To ensure accuracy, the balls must be kept chilled to near absolute zero, in the vacuum of the largest thermos ever flown in space, and isolated from any disturbances in the quietest environment ever produced, said Anne Kinney, director of the National Aeronautics and Space Administration's division of astronomy and physics.
He proposed in 1916 that space and time form a structure that can be curved by the presence of a body, like the Earth, warping it like the dimple created by the heft of bowling ball resting on a soft mattress. That distortion accounts for gravity.
If theory holds, the mass and rotation of the Earth, 397 miles below the probe, should throw the alignment of the spinning balls off kilter in subtle but measurable ways.
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