NASA’s Gravity Probe B spacecraft will test Einstein’s general theory of relativity.

Gravity Probe B will test two concepts of the theory: that Earth — and almost any body in space — creates a dimple in the universe’s space-time fabric; and that the rotation of the Earth twists that fabric.

It will attempt to measure those effects by aligning itself with a distant star and then measuring tiny changes in the direction of its four spinning gyroscopes with respect to the line of the star.