With four near-perfect spheres -- the roundest objects ever made, according to NASA -- the probe will try to show whether the Earth, which is known to warp both time and space with its mass, also twists them like tornado winds as it rotates.
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"This test of relativity is very simple in concept, but when you get down to the technology of how to do it, it's a testimony to perseverance to say the least," said Stanford professor Brad Parkinson, who heads the engineering team.

As it turns out, the effect is dramatic around something as massive as a black hole, but rather minimal when measuring the space around something Earth-sized. So the measuring instruments had to be extremely precise, said Parkinson.

To an astronaut orbiting the Earth, the effect is undetectable, but Einstein's theory says that a small bit of space is actually lost as space is spun around on itself.

"You would find if you could measure the radius and the circumference (of that orbit) there would be a small defect which I like to call the missing inch," said Francis Everitt, a theoretical physicist from Stanford, the principal investigator on the NASA project.