Gravity Probe B - Mission Timeline

Mission Timeline

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

• 20 April 2004: Launch of GP-B from Vandenberg AFB and successful insertion into polar orbit.
• 27 August 2004: GP-B entered its science phase. On mission day 129 all systems were configured to be ready for data collection, with the only exception being gyro 4, which needed further spin axis alignment.
• 15 August 2005: The science phase of the mission ended and the spacecraft instruments transitioned to the final calibration mode.
• 26 September 2005: The calibration phase ended with liquid helium still in the dewar. The spacecraft was returned to science mode pending the depletion of the last of the liquid helium.
• February 2006: Phase I of data analysis complete
• September 2006: Analysis team realised that more error analysis, particularly around the polhode motion of the gyros, was necessary than could be done in the time to April 2007, and applied to NASA for an extension of funding to the end of 2007.
• December 2006: Completion of Phase III of data analysis
• 14 April 2007: Announcement of best results obtained to date. Francis Everitt gave a plenary talk at the meeting of the American Physical Society announcing initial results: "The data from the GP-B gyroscopes clearly confirm Einstein's predicted geodetic effect to a precision of better than 1 percent. However, the frame-dragging effect is 170 times smaller than the geodetic effect, and Stanford scientists are still extracting its signature from the spacecraft data." — Gravity Probe B website
• 8 December 2010: GP-B spacecraft decommissioned, left in its 642 km (400 mi) polar orbit.
• 4 May 2011: GP-B Final experimental results were announced. In a public press and media event at NASA Headquarters, GP-B Principal Investigator, Francis Everitt announced the final results of Gravity Probe B.

On 9 February 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. In April it was announced that the spin axes of the gyroscopes were affected by torque, in a manner that varied over time, requiring further analysis to allow the results to be corrected for this source of error. Consequently, the date for the final release of data has been pushed back several times. In the data for the frame-dragging results presented at the April 2007 meeting of the American Physical Society, the random errors were much larger than the theoretical expected value and scattered on both the positive and negative sides of a null result, therefore causing skepticism on whether any useful data could be extracted in the future to test this effect.

In June 2007, a detailed update was released explaining the cause of the problem, and the solution that was being worked on. Although electrostatic patches caused by non-uniform coating of the spheres was anticipated, and was thought to have been controlled for before the experiment, it is now known that the final layer of the coating on the spheres defined two halves of slightly different potential, which gave the sphere an electrostatic axis. This created a classical dipole torque on each rotor, of a magnitude similar to the expected frame dragging effect. In addition, it dissipated energy from the polhode motion by inducing currents in the housing electrodes, causing the motion to change with time. This meant that a simple time-average polhode model was insufficient, and a detailed orbit by orbit model was needed to remove the effect. As it was anticipated that "anything could go wrong", the final part of the flight mission was calibration, where amongst other activities, data was gathered with the spacecraft axis deliberately mis-aligned for 24 hours, to exacerbate any potential problems. This data proved invaluable for identifying the effects. With the electrostatic torque modelled as a function of axis misalignment, and the polhode motion modelled at a sufficiently fine level, it is hoped to isolate the relativity torques to the originally expected resolution.

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 September 2008 release. Because future interpretations of the data by scientists outside of GP-B may differ from the official results, it may take several more years for all of the data received by GP-B to be completely understood.