 Our last test is the most recent. It was designed to measure the twisting of space around a rotating mass. This twisting is called frame dragging, where space is literally dragged along with the rotating mass. The effect was derived in 1918 by physicist Joseph Lenz and Hans Thering. It is known as the Lenz-Thering effect. They predicted that the rotation of a massive object would distort the spacetime metric, making the orbit of a nearby test particle precess like a gyroscope. This does not happen with Newtonian gravity, where the gravitational field of a body depends only on its mass, not on its rotation. Up until now we have been using the Schwarzschild metric, which does not show this effect. It wasn't until 1963 that a mathematician named Roy Kerr discovered the significantly more complicated metric for rotating bodies that made it possible to calculate the precession one can expect from a given mass and rotation of an object, like the Earth. To test this effect, NASA developed a satellite called Gravity Probe B and put it into orbit 264 kilometers above the Earth in 2004 where it operated for a year. It used a set of super sensitive gyroscopes to measure precession due to frame dragging. It also included a non-gravitational drag identification gyro and compensation micro thrusters to maintain a non-gravitational drag free environment. It compensated for solar radiation drag and atmospheric disturbances drag. By 2011, data analysis had confirmed that frame dragging did occur and measured it to within 15% of the amount predicted by the Kerr metric for Einstein's field equations.