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Gravity Probe B measures the theory of relativity.

Gravity Probe B is a satellite-based mission which launched in 2004. Gravity Probe B spaceflight phase lasted until 2005, and data analysis is currently underway. Gravity Probe B's aim is to measure spacetime curvature near Earth, and thereby the stress-energy tensor (the distribution, and especially the motion, of matter) in and near Earth, and thus to test related models in application of Einstein's general theory of relativity.

Gravity Probe B.
Gravity Probe B with solar panels folded.

Initial results confirm the expected geodetic effect to an accuracy of about 1%. The expected frame-dragging signal is similar in magnitude to the current noise level (the noise being dominated by currently unmodeled effects). Work is continuing throughout 2007 to model and account for these sources of unintended signal, thus permitting extraction of the frame-dragging signal if it exists at the expected level.

Overview of Gravity Probe B.

Gravity Probe B was a relativity gyroscope experiment funded by NASA. Efforts were headed up by the physics department at Stanford University with Lockheed Martin as the primary subcontractor.

Mission scientists view it as the second gravity experiment in space, following the successful launch of Gravity Probe A (GP-A) in 1976.

Some preliminary results were presented at a special session during the American Physical Society (APS) meeting, 14 to 17 April 2007. NASA has requested a proposal for extending the GP-B data analysis phase through December 2007.

The mission plans were to test two unverified predictions of general relativity:

The experiment planned to check, very precisely, tiny changes in the direction of spin of four gyroscopes contained in an Earth satellite orbiting at 650 km (400 statute miles) altitude and crossing directly over the poles. So free are the gyroscopes from disturbance that they provided a near-perfect space-time reference system. They were intended to measure how space and time are "warped" by the presence of the Earth, and, more profoundly, if and how much the Earth's rotation "drags" space-time around with it; the so-called frame-dragging effect or gravitomagnetism, a field generated by the rotation of Earth and similar to magnetism in electrodynamics.

Previously, only two analyses of the laser-ranging data obtained by the two LAGEOS satellites, published in 1997 and 2004, claimed to have found the frame-dragging effect with an accuracy of about 20 percent and 10 percent respectively, whereas Gravity Probe B aims to measure the effect to a precision of 1 percent. A recent analysis of Mars Global Surveyor data has claimed to have confirmed the effect to a precision of 0.5%,although the accuracy of this claim is disputed.

The probe should also detect the so-called geodetic effect, a much larger effect caused by space-time being 'curved' by the mass of the Earth. A gyroscope's axis when parallel transported around the Earth in one complete revolution does not end up pointing in exactly the same direction as before. The angle 'missing' may be thought of as the amount the gyroscope 'leans over' into the slope of the space-time curvature. Gravity Probe B should measure this effect to an accuracy of one part in 10,000, the most stringent check on general relativistic predictions to date.

The launch was planned for April 19, 2004 at Vandenberg Air Force Base but was scrubbed within 5 minutes of the scheduled launch window due to changing winds in the upper atmosphere. An unusual feature of the mission is that it only had a one-second launch window due to the precise orbit required by the experiment. On April 20 at 9:57:23 a.m. PDT (16:57:23 UTC) the spacecraft was launched successfully. The satellite was placed in orbit at 11:12:33 a.m. (18:12:33 UTC) after a cruise period over the south pole and a short second burn. The mission lasted 16 months.

Experimental setup of Gravity Probe B.

Gravity Probe B experiment.
One of the most perfect spheres ever created by humans. A fused quartz gyroscope for the Gravity Probe B experiment which differs from a perfect sphere by no more than 40 atoms of thickness, refracting the image of Einstein in background.

The Gravity Probe B experiment comprises four gyroscopes and a reference telescope sighted on HR8703 (also known as IM Pegasi), a Binary star in the constellation Pegasus. In polar orbit, with the gyro spin directions also pointing toward HR8703, the frame-dragging and geodetic effects came out at right angles, each gyroscope measuring both.

The gyroscopes are housed in a dewar of superfluid helium, maintaining a temperature of under 2 kelvins (-271 degrees Celsius, -456 degrees Fahrenheit). Near-absolute zero temperatures are required in order to minimize molecular interference, and enable the Lead and niobium components of the gyroscope mechanisms to become superconductive.

The gyroscopes are the most spherical objects ever made. Approximately the size of ping pong balls, they are perfectly round to within forty atoms. They are composed of fused quartz and coated with an extremely thin layer of niobium. The gyros' spin axes are sensed by monitoring the magnetic field of the superconductive niobium layer with SQUIDs.

IM Pegasi was chosen as the guide star for multiple reasons. First, it needed to be bright enough to be usable for sightings. Then it was close to the ideal positions at the Equator of the sky coordinates. Also important was its well understood motion in the sky, which was helped by the fact that this star emits relatively strong radio signals. As a preparation for the setup of this mission, astronomers analyzed the radio-based position measurements taken over the last few years to understand its motion as precisely as needed.

History of Gravity Probe B.

The conceptual design for this mission was first proposed by an MIT professor, George Pugh who was working with the U.S. Department of Defense in 1959 and later discussed by Leonard Schiff (Stanford) in 1960 at Pugh's suggestion. It was proposed to NASA in 1961, and it supported the project with funds in 1964. This grant ended in 1977 after a long phase of engineering research into the basic requirements and tools for the satellite.

In 1986 NASA changed plans for the shuttle, which forced the mission team to switch from a shuttle-based launch design to one that is based on the Delta 2, and in 1995 tests planned of a prototype on a shuttle flight were cancelled as well.

Gravity Probe B marks the first time in history that a university has been in control of the development and operations of a space satellite funded by NASA.

Gravity Probe B mission timeline.

This is a list of major events for the GP-B experiment.

Future of Gravity Probe B.

On Febuary 9th 2007 it was announced that a number of unexpected signals had been received and that these would need to be separated out before final results could be released. Consequently, the date for the final release of data has been pushed back from April of 2007 to December of 2007.

Speculation on some internet sites, such as PhysicsForums.org, has centered around the source and nature of these anomalous signals. Several posters and alternative theorists (some skeptical of Gravity Probe B and its methodology) have indicated that understanding these signals may be more interesting than the original goal of testing GR.

Stanford has agreed to release the raw data to the public at an unspecified date in the future. It is likely that this data will be examined by independent scientists and independently reported to the public well after the December 2007 release. Because future interpretations of the data by scientists outside of Gravity Probe B may differ from the official results, it may take several more years for of all the data received by Gravity Probe B to be completely understood.

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