From a CBC article:
"Imagine the Earth as if it were immersed in honey. As the planet rotates, the honey around it would swirl, and it's the same with space and time," Francis Everitt, the Stanford University physicist who led the mission, said in a statement.
Gravity Probe B was designed to test the predictions by taking measurements from four ultra-precise gyroscopes. Those instruments contain rotors that NASA bills as the "most perfect spheres ever made by humans," allowing them to spin indefinitely without drifting from the alignment of their rotation.
The gyroscopes were aboard a spacecraft circling the Earth in a polar orbit — that is, it travelled perpendicular to the Earth's rotation, passing over both poles during each orbit.
The experiment was set up so the gyroscopes' axes of rotation should always point in the same direction — toward the "guide" star IM Pegasi — provided the Earth's gravity did not affect space and time.
However, the researchers detected tiny, measurable changes in the direction of the gyroscopes' spins, confirming that the Earth's gravity does:
- Warp space and time around it, which is called the geodetic effect.
- Pull space and time with it as it rotates, an effect called frame dragging.
NASA began planning the mission in 1963, making it one of the longest-running projects in the U.S. space agency's history. The satellite was decommissioned in December 2010, after the final parcels of data were collected.
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