Gravity Probe B
A satellite developed by NASA and Stanford University to test two extraordinary,
unverified predictions of Einstein's general theory
of relativity. As the probe orbits at an altitude of about 640 km directly
over Earth's poles, tiny changes in the direction of spin of four onboard
gyroscopes are measured very precisely.
So free are the gyroscopes from disturbance that they provide an almost
perfect reference system. They measure how space and time are warped by
the presence of Earth (the so-called geodetic effect), and, more
profoundly, how Earth's rotation drags spacetime
around with it (known as frame-dragging). These effects, though
small for Earth, have far-reaching implications for the nature of matter
and the structure of the Universe.
The idea of the mission was first proposed in 1959, but the project was
delayed many times due to budget reviews before finally being launched in
2004. Preliminary results from the probe announced in April 2007 confirm
Einstein's geodetic effect to a precision of better than 1 percent. At that
time, scientists from Stanford were still trying to extract the signature
of frame-dragging from the data.
||Apr. 20, 2004
||641 km × 645 km × 90°
See also Gravity Probe A.
AND SPACE PROBES