GRAVITY PROBE - B

Mission Objectives:

Gravity Probe B (GP-B) mission was launched to experimentally test the

General Theory of Relativity. It consists of four spherical Gyroscopes

and a Telescope in a Satellite orbiting about 600 km in a polar orbit

above the Earth. Its primary purpose is to measure the geodetic effect

and the frame-dragging effect predicted by the General Theory of

Relativity.

The Geodetic effect:

The amount by which the Earth warps the local space-time around it

The frame-dragging effect:

The amount by which the rotating Earth drags its local space-time

around with it.

The GP-B experiment tests these two effects by precisely measuring the

displacement angles of the spin axes of the four gyros over the course

of a year and by comparing these experimental results with predictions

from Einstein's theory.

Experimental Design:

At the start of the experiment, both the telescope and the spin axis of

each gyroscope are aligned with a distant reference point—a guide

star. The telescope is then kept aligned with the guide star for a year, as

the spacecraft makes over 5,000 orbits around the Earth, and measures

the change in the spin-axis alignment of each gyro over this period in

both the plane of the orbit (the geodetic precession) and orthogonally

in the plane of the Earth's rotation (frame-dragging precession).

The predicted geodetic gyro-spin-axis precession is a tiny angle of 6,606

milliarcseconds (0.0018 degrees) in the orbital plane of the spacecraft.

The orthogonal frame-dragging precession is predicted to be 39

milliarcseconds (1.1x10-5 degrees).

Challenges:

The main constraints in doing such an experiment were:

1. Gyroscopes had to be perfectly Spherical and of uniform density.

The gyroscopes mad for the experiment set the record for the

most spherical man-made object till that time.

2. The coating of superconducting coating on sphere had to be

uniform so that the SQUID magnetometers could measure the

angle change accurately.

3. The temperature of the system had to be less than 1.95 K so as to

maintain superconductivity.

4. All non-gravitational residual accelerations such as solar

radiation, atmospheric friction, magnetic fields, electro-magnetic

radiation from earth had to be cancelled out which made the

satellite the first drift free satellite which was achieved using

thrusters and feedback from the gyros which gave 6 degrees of

freedom to the satellite.

The table shows the tolerance required versus tolerance achieved in

the mission:

https://einstein.stanford.edu/highlights/status1.html

Results:

Analysis of the data from all four gyroscopes resulted in a geodetic

drift rate of -6,601.8±18.3 mas/year and a frame-dragging drift rate

of -37:2±7.2 mas/year, to be compared with the GR predictions of -

6,606.1 mas/year and -39.2 mas/year, respectively ('mas' is

milliarcseconds; 1 mas= 4.848 X10-9 radians or 2.778 X10-7 degrees).