The Gravity Probe B (GP-B) satellite is a collaboration between NASA and Stanford University and was launched in 2004 with an aim to study two effects predicted by general relativity, a theory first put forth by Einstein in 1915. In addition to the geodetic effect, the theory also predicts that massive bodies will pull space and time along with them as they rotate -- an effect called frame dragging.

Now analysis of the data from GP-B has confirmed the geodetic effect with an accuracy of better than one percent. Although the same effect has already been measured by NASA's Cassini mission, the results indicate that the much subtler frame-dragging effect should be confirmed by further data analysis by the end of this year. Frame-dragging has also been measured before by NASA's LAGEOS satellites with an accuracy of ten percent, and it is currently unclear whether GP-B data will yield a more accurate result.

Gravity Probe B used superconducting quantum interference devices (SQUIDs) to measure tiny changes in the orientations of four perfectly-spherical, quartz gyroscopes as the experiment orbited the Earth for one year. The gyroscopes were housed inside a vacuum chamber and were maintained at 1.8 Kelvin during the measurements using liquid helium. The probe also includes a telescope that was trained on a distant "guide star" to provide a reference direction for measurements on the gyroscopes. General relativity predicts that the frame-dragging effect will cause the direction of the gyroscopes to change by a tiny 0.041 of an arc second.

Prior to launch, however, the satellite suffered numerous delays, and now there is the possibility that the accuracy of its data will not surpass that of other experiments performed before now. "On one level one can say that [Gravity Probe B] is a fantastic triumph of engineering -- nobody has ever done an experiment like this before," Clive Speake, a physicist from the University of Birmingham, told Physics Web. "On the other hand, one can’t do these experiments for fun. We have to wait until the frame-dragging result comes out."