In Search of Gravitomagnetism: "NASA's Gravity Probe B spacecraft left Earth today in search of a force of nature, long suspected but never proven: gravitomagnetism. Gravitomagnetism is produced by stars and planets when they spin. 'It's similar in form to the magnetic field produced by a spinning ball of charge,' explains physicist Clifford Will of Washington University. Replace charge with mass, and magnetism becomes gravitomagnetism."

"What does gravitomagnetism do? 'It can make the orbits of satellites precess,' says Will, 'and it would cause a gyroscope placed in Earth orbit to wobble.' Both effects are small and difficult to measure."

"Gravity Probe B, developed by scientists at Stanford University, NASA and Lockheed Martin, … circles Earth in a polar orbit 400 miles high. Onboard are four gyroscopes, each one a sphere, 1.5 inches in diameter, suspended in vacuum and spinning ten thousand times per minute.

"If Einstein's equations are correct and gravitomagnetism is real, the spinning gyroscopes should wobble as they orbit the earth. Their spin axes will shift, little by little, until a year from now they point 42 milli-arcseconds away from where they started. Gravity Probe B can measure this angle with a precision of 0.5 milli-arcseconds, or about 1%.

"Any angle measured in milli-arcseconds is tiny. Consider this: One arcsecond equals 1/3600th of a degree. One milli-arcsecond is 1000 times smaller than that. The half milli-arcsecond precision expected for Gravity Probe B corresponds to the thickness of a sheet of paper held edge-on 100 miles away."