Gravity Probe B

  • Thread starter Paulanddiw
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I was looking at some web sites about the Gravity Probe B. Some show the spin axes of the gyroscopes oriented perpendicular to the earth or else in the direction of movement of the satelite. Wouldn't these orientations cause lots of precession? I'd expect the gyroscopes' axes of rotation to be orientated paralled to the axis of the orbit.
 
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Paulanddiw said:
...... I'd expect the gyroscopes' axes of rotation to be orientated paralled to the axis of the orbit.
The satellites are in free fall; there is no classical precession expected.

The gyroscope spin vectors (axis) are pointed approximately in the plane of the satellite's polar orbit, in the direction of the guide star. Oriented as such the measurement of the two predicted relativistic precessions are at right angles to each other, one along the plane of the orbit and one orthogonal to it in the direction of earth's spin.
 

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As I recall, there is a geodetic precession in the plane of the orbit (which is a polar orbit) which is fairly large, and a smaller component of the precession, due to the Earth's rotation, which is smaller. Because the orbit is polar, the geodetic precession effect in the orbital plane is in a different direction than the frame-dragging effect due to the Earth's rotation.

Preventing precession due to classical tidal forces is the reason that the gravity probe B gyroscopes had to be such nearly perfect spheres. Tidal forces can cause torque on an oblate rotating body, but not a spherical one.
 

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