Gravity Probe B, a satellite launched by NASA in 2004 tested two of Einstein's General Relativity Theory predictions regarding gravity and space-time. "One is the geodetic effect-the amount by which the mass of the Earth warps the local space-time in which it resides. The other effect, called frame-dragging, is the amount by which the rotating Earth drags local space-time around with it. According to Einstein's theory, over the course of a year, the geodetic warping of Earth's local space-time causes the spin axes of each gyroscope to shift from its initial alignment by a minuscule angle of 6.606 arc-seconds (0.0018 degrees) in the plane of the spacecraft's orbit. Likewise, the twisting of Earth's local space-time causes the spin axis to shift by an even smaller angle of 0.039 arc-seconds (0.000011 degrees)-about the width of a human hair viewed from a quarter mile away-in the plane of the Earth's equator." (Spaceflight Now)
The experiment was conceived in the late '50s. It took more than 40 years to develop the technology that would run the otherwise simple experiment. After one year of collecting data, it took 18 months for extracting the necessary information for computing the geodetic effect. This proved to be in the range of 1% of Einstein's predictions. For the frame-dragging effect another 6 months are required for data processing. Models of electrostatic generated torques and sensor effects have to be carefully developed for reaching the required instruments' accuracy (4 inertial gyroscopes, a million times more precise than any other gyro ever built).
image credit: NASA