Gravity Probe B
Gravity Probe hot three space missions of NASA to test the general theory of relativity by Albert Einstein.
Gravity Probe A ( GP -A) flew on 16 June 1976, an extremely accurate atomic clock in a steep ballistic trajectory with 10,000 km peak altitude. During the nearly two-hour flight, the transition of the clock by means of microwave connection with two identical clocks were compared on the ground. For the occasion, the signal probe was impressed on a received signal from the ground by a transponder and sent back. This method avoided the disturbing effect of the Doppler effect, allowing the measurement of the gravitational redshift based on the equivalence principle with an accuracy of 0.02%. In 1965, the accuracy was still at 1%, as measured by Mössbauer effect over a total drop of only 15 meters. A little later the GPS satellite system allowed much more precise measurements.
In the satellite Gravity Probe B ( GP -B) there were free-floating four rapidly rotating quartz spheres. The extremely accurate observation of their rotational axes gave information on torques caused by two relativistic effects. The idea comes from the theorist Leonard Schiff, who discussed it in the early 1960s with William Fairbank Sr.. The scientific management of the NASA mission was a former employee of Fairbank at Stanford University, Francis Everitt. The evaluation of the 2004/2005 recorded, severely disturbed data continued until 2011. The results were as expected in accordance with the theory, but not further than previous measurements by means of the geodetic satellites LAGEOS and GRACE.
Gravity Probe C is a possible future mission, which is still in the early planning phase and will consist of two satellites orbiting in the equatorial orbits in the opposite direction of the Earth. According to general relativity theory are by gravitomagnetic effects (caused by the Earth's rotation ) the orbital periods differ by about 100 ns. To be able to count out other effects, the gravitational field of the earth needs to be investigated in more detail before the mission.
Details for Gravity Probe B
The satellite mission Gravity Probe B ( GP -B) should - at the time of their planning by Fairbank first time - allow experimental verification of two statements of general relativity:
According to the predictions of the physicist, the rotational axes of the four gyroscopes should because of the space-time curvature per year tend to 6606.1 milliarcseconds addition, the Lense- Thirring through effect to 39.2 milliarcseconds in a different direction. The measurement of such small changes of the rotation axis is an extreme challenge to the experimental technique. The specially developed for this mission consisted of quartz gyroscopes balls the size of a ping-pong ball (3.8 cm ), which rotated in a vacuum at 10,000 revolutions per minute. They were cooled to 1.8 K in order to make its surface coated with niobium superconducting and to be generated by the London moment magnetic field in the direction of the axis of rotation. Changes the axis of rotation were determined by highly sensitive superconducting quantum interference detectors, so-called SQUIDs recognized. External magnetic fields were reduced by a double shield of superconducting material to 240 dB. In this way, changes were of a milliarcsecond measurable ( at 10 hours integration time). At this angle head of a pin appears at a distance of 1,000 km.
The four balls were in the axis of rotation of the satellite and rotated equal in pairs or in opposite directions. The axes of rotation were compared with the measured satellite and covered with a small telescope in its axis of rotation on the spectroscopic binary IM Pegasi. For the attitude control of the satellite about its axis and during the occultations of IM Pegasi through the earth several star tracker and gyroscopes were used. IM Pegasi was chosen because it is close to the plane of the Earth's equator, is bright enough for accurate bearings and its strong radio emissions detectable by VLBI, so that its movement to the reference system further quasars could be obtained.
The satellite was successfully launched on 20 April 2004 by the U.S. Air Force Base Vandenberg aboard a Delta II 7920 rocket. The orbit of the satellite resulted in a height of about 740 km above the poles.
On 28 August 2004, the preparations were completed for the actual measurements. The previously performed calibration measurements, however, had already shown that unforeseen effects (later described as misalignment torque and roll- resonance torque polhode ) affected the direction of rotation of the gyroscopes. After the cause was understood, the effects could be modeled and initially calculated out roughly. It was an unwanted coupling between the ball and the wall surface by inhomogeneous electric fields due to local variations in the work function. In addition, an interaction of this field with the activity of the resulted electrostatic centering of the ball in a damping of its precessional motion, which makes the analysis of the measured values and delayed difficult.
The from the actual measurements are not precise enough detectable coupling constants with the wall, so with the rotating satellites were measured separately in a provided at the end of the mission calibration by the axis of rotation of the satellite was roughly adjusted wrong, what the influences multiplied. It was followed by a further measuring phase until the helium for cooling was used in late 2005.
The public acknowledgment of the problems took Everitt itself
Shortly before the NASA mission ended in 2007, he applied for an extension of one year, as a further evaluation promised more accurate results. This has been criticized by other researchers, who feared for the financing of their missions with the argument that LAGEOS had already allows similar precise tests of the theory of relativity. A 15 - member panel of NASA approved only meager funds which have been supplemented by a private donation from Fairbanks son and agent of the University. The final report of the mission appeared in December 2008, but the evaluation was continued, including with funding from Saudi Arabia. The final report of the University appeared with renewed public attention in May 2011.
The effect of the space-time curvature (theoretical: 6606.1, all in milliarcseconds / year) had to be determined with a precision of 0.01 %, the Lense- Thirring weaker effect ( theory: 39.2 ) to 1%. The result was 6601.8 ± 18.3 (0.28 %), and 37.2 ± 7.2 (19 %), σ error information in each one. Already in 2004, had the prediction of the Lense- Thirring effect at 1 % hit with an uncertainty of 5% evaluation gathered by over eleven years LAGEOS - path data.