A satellite that will put Albert Einstein's Theory of Relativity to the test is ready to be launched.
Einstein's theories about space have not been proved
Nasa hopes Gravity Probe B will lift off from California on 17 April.
Since it was first proposed in 1959, the project has been aborted and delayed because of technical hiccups many times.
Now it is ready to test two of Einstein's theories about the nature of space and time, and how the Earth distorts them.
The unmanned satellite will orbit 640km (400 miles) above Earth, measuring any slight changes in gravity.
The satellite will carry four ping-pong-sized balls made from quartz and sealed in a vacuum.
The scientists behind the project say they are the most perfect spheres ever made.
Gravity Probe B has been planned for 45 years (picture Nasa)
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, said Anne Kinney, director Nasa's division of astronomy and physics.
Once in space the balls will be sent spinning. If Einstein is correct, there should be slight changes to the balls' orientation, or 'spin axis'.
Scientists will carefully measure the expected tiny changes in the balls' movements.
Einstein proposed in 1916 that space and time form a structure that can be curved by the presence of a body.
Gravity Probe B will test how space and time are warped by the presence of the Earth, and how the Earth's rotation twists and drags space-time around with it.
The warping effect has been measured before, but the twisting effect, called frame-dragging, has never been directly detected. The Nasa mission aims to examine both.
Francis Everitt, the principal investigator of the project, said: "Aren't Einstein's theories all established and confirmed? After all it was 50 years ago that Einstein himself died and it's 100 years next year when he developed his first theory of relativity. Don't we already know it all? The answer is no."
If there are no more delays, the probe's mission should be completed in 16 months' time.