The rocket launched successfully on its second attempt

The US space agency's $700m (�387m) probe launched on a Boeing Delta 2 rocket from Vandenberg Air Base in California at 1657 GMT.

The probe will orbit around the Earth for more than a year on its mission.

Gravity Probe B will test Einstein's ideas about space and time and how the Earth distorts them.

Mission controllers grounded the probe for 24 hours due to uncertainty about flight software during the countdown on Monday.

It will carry four near-perfect spheres in gyroscopes to help verify two key elements of Einstein's theory.

The probe will align itself with its "guide star" IM Pegasi, so that the spin axes of the spheres point to this star.

Over the course of the year their spin axes will be monitored for tiny changes that could be caused by the effects Einstein described.

Spinning around

The first element of relativity to be tested by the probe is the geodetic effect. This proposes that any body in space warps local space and time.

The second of these is that massive objects spinning in space drag local space and time around with them as they rotate, an effect known as "frame dragging".

The probe will test whether space-time can be distorted by Earth's rotation

"We've seen two of the three aspects of warped space-time. We've seen the warping of space and the warping of time. We have never seen, in any clean way, the dragging of space into motion," said Kip Thorne of the California Institute of Technology.

These effects have important implications for the nature of matter and the structure of the Universe.

Once in space, the unmanned satellite will enter a polar orbit 640km (400 miles) above Earth.

To ensure accuracy, the balls must be kept chilled to near absolute zero, inside the largest vacuum flask ever flown in space and isolated from any disturbances in the quietest environment ever produced.

The idea of the mission was first proposed in 1959, but the project has been aborted and delayed many times due to budget reviews.