A satellite designed to test two fundamental predictions made by Albert Einstein about the universe is ready for launch, 45 years after it was first proposed, NASA and Stanford University officials said.

Since 1959, Gravity Probe B has overcome a half-dozen attempts at cancellation, countless technical hurdles and several delayed launches. The NASA-funded, university-developed spacecraft is now scheduled to begin its mission following an April 17 liftoff from Vandenberg Air Force Base, Calif.

The unmanned, Earth-orbiting satellite is designed to test two of Einstein's predictions about the nature of space and time, and how the Earth and other bodies warp and twist the fabric that combines the two.

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, Anne Kinney, director of the National Aeronautics and Space Administration's division of astronomy and physics, said Friday.

Once in space and set spinning, the orientation of the balls should change -- unless Einstein was wrong.

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.

Two years later, others suggested that the rotation of such a mass should drag space-time with it, twisting the structure of the fabric.

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.

The warping effect has been measured before. The twisting effect, called frame-dragging, has never been directly detected. Gravity Probe B aims to detect both.

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