Astronomy Picture of the Day

Explanation: Does gravity have a magnetic counterpart? Spin any electric charge and you get a magnetic field. Spin any mass and, according to Einstein, you should get a very slight effect that acts something like magnetism. This effect is expected to be so small that it is beyond practical experience and ground laboratory measurement. In a bold attempt to directly measure gravitomagnetism, NASA launched in 2004 the smoothest spheres ever manufactured into space to see how they spin. These four spheres, each roughly the size of a ping-pong ball, are the key to the ultra-precise gyroscopes at the core of Gravity Probe B. Last week, after accounting for persistent background signals, the results were announced -- the gyroscopes precessed at a rate consistent with the gravitational predictions of Einstein's General Theory of Relativity. The results, which bolster existing findings, may have untold long term benefits as well as shorter term benefits such as better clocks and global positioning trackers.