Editing Gravity Probe B (section)

== Mission timeline ==
{{Main|Gravity Probe B mission timeline}}

[[File:Gravity_Probe-B_before_launch.jpg|thumb|Gravity Probe B before launch]]

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

; <time datetime="2004-04-20">20 April 2004</time> : Launch of GP-B from Vandenberg AFB and successful insertion into polar orbit.

; <time datetime="2004-08-27">27 August 2004</time> :  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.

; <time datetime="2005-08-15">15 August 2005</time> : The science phase of the mission ended and the spacecraft instruments transitioned to the final calibration mode.

; <time datetime="2005-09-26">26 September 2005</time> : The calibration phase ended with liquid helium still in the dewar. The spacecraft was returned to science mode pending the depletion of liquid helium.

; <time datetime="2006-02">February 2006</time> : Phase I of data analysis complete

; <time datetime="2006-09">September 2006</time> : Analysis team realised that more error analysis was necessary (particularly around the [[polhode|polhode motion]] of the gyros) than could be done in the time to <time datetime="2007-04">April 2007</time> and applied to NASA for an extension of funding to the end of <time>2007</time>.

; <time datetime="2006-12">December 2006</time> : Completion of Phase III of data analysis

; <time datetime="2007-04-14">14 April 2007</time> : Announcement of best results obtained to date. Francis Everitt gave a plenary talk at the meeting of the [[American Physical Society]] announcing initial results:<ref>{{cite web|url=http://www.aps.org/meetings/april/plenary.cfm |title=Exciting April Plenary Talks – Saturday, 14 April |access-date=16 November 2006 |url-status=dead |archive-url=https://web.archive.org/web/20070220162557/http://www.aps.org/meetings/april/plenary.cfm |archive-date=20 February 2007 }}</ref> "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."<ref>
{{cite web
 |last=Khan |first=B.
 |title=Was Einstein Right
 |url=http://einstein.stanford.edu/content/press_releases/SU/pr-aps-041807.pdf
 |publisher=[[Stanford University]] |work=Stanford News
 |date=14 April 2007
 |access-date=14 May 2009
}}</ref> 

; <time datetime="2010-12-08">8 December 2010</time> : GP-B spacecraft decommissioned, left in its {{cvt|642|km}} polar orbit.<ref>
{{cite web
 |title=Gravity Probe-B Latest News
 |url=http://www.nasa.gov/mission_pages/gpb/index.html
 |publisher=[[NASA]]
 |access-date=20 February 2011
 |archive-date=22 January 2022
 |archive-url=https://web.archive.org/web/20220122125058/https://www.nasa.gov/mission_pages/gpb/index.html
 |url-status=dead
}}</ref> 

; <time datetime="2011-05-04">4 May 2011</time> : GP-B Final experimental results were announced. In a public press and media event at NASA Headquarters, GP-B Principal Investigator, Francis Everitt presented the final results of Gravity Probe B.<ref>{{cite web|title=GP-B STATUS UPDATE — May 4, 2011 |at=NASA Headquarters Science Update/Press Conference |url=http://einstein.stanford.edu/highlights/status1.html |website=Gravity Probe B |publisher=[[NASA]] and [[Stanford University]] |access-date=6 May 2011}}</ref> 

; <time datetime="2015-11-14">19 November 2015</time> : Publication of GP-B Special Volume (Volume 32, Issue 22) in the peer-reviewed journal, ''[[Classical and Quantum Gravity]]''.<ref>{{cite journal |title=Focus issue: Gravity Probe B |journal=Classical and Quantum Gravity|author=Clifford M Will |date=17 November 2015 |volume=32 |issue=22 |pages=220301 |publisher=IOP |doi=10.1088/0264-9381/32/22/220301 |bibcode=2015CQGra..32v0301W|doi-access=free }}</ref> 

On <time datetime="2007-02-09">9 February 2007</time>, 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 <time datetime="2007-04">April</time> 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 was pushed back several times. In the data for the frame-dragging results presented at the <time datetime="2007-04">April 2007</time> 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 as to whether any useful data could be extracted in the future to test this effect.

In <time datetime="2007-06">June 2007</time>, 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 were anticipated, and were thought to have been controlled for before the experiment, it was subsequently found that the final layer of the coating on the spheres defined two-halves of slightly different [[Volta potential|contact 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|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 misaligned for <time datetime="PT24h">24 hours</time>, to exacerbate any potential problems. This data proved invaluable for identifying the effects. With the electrostatic torque modeled as a function of axis misalignment, and the polhode motion modeled at a sufficiently fine level, it was hoped to isolate the relativity torques to the originally expected resolution.

Stanford 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 final release by the project scientists. Because future interpretations of the data by scientists outside 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.{{Update inline|date=May 2020}}

=== NASA review ===
A review by a panel of 15 experts commissioned by NASA recommended against extending the data analysis phase beyond <time>2008</time>. They warned that the required reduction in noise level (due to classical torques and breaks in data collection due to solar flares) "is so large that any effect ultimately detected by this experiment will have to overcome considerable (and in our opinion, well justified) skepticism in the scientific community".<ref>
{{cite web
 |last=Hecht |first=J.
 |url=https://www.newscientist.com/article/dn13938-gravity-probe-b-scores-f-in-nasa-review.html
 |title=Gravity Probe B scores 'F' in NASA review
 |work=[[New Scientist]]
 |date=20 May 2008
 |access-date=20 May 2008
}}</ref>

=== Data analysis after NASA ===
NASA funding and sponsorship of the program ended on <time datetime="2008-09-30">30 September 2008</time>, but GP-B secured alternative funding from [[King Abdulaziz City for Science and Technology]] in Saudi Arabia<ref name = "Gugliotta2009"/> that enabled the science team to continue working at least through <time datetime="2009-12">December 2009</time>. On <time datetime="2008-08-29">29 August 2008</time>, the 18th meeting of the external GP-B Science Advisory Committee was held at Stanford to report progress. 

The Stanford-based analysis group and NASA announced on <time datetime="2011-05-04">4 May 2011</time> that the data from GP-B indeed confirms the two predictions of Albert Einstein's general theory of relativity.<ref>{{cite web |url=https://news.stanford.edu/2011/05/04/stanfords-gravity-probe-b-confirms-two-einstein-theories/ |title=Stanford's Gravity Probe B confirms two Einstein theories |website=Stanford News |publisher=Stanford University |date=4 May 2011}}</ref> The findings were published in the journal ''[[Physical Review Letters]]''.<ref name=PRL>{{cite journal | author=Everitt | display-authors=etal | date=2011| title=Gravity Probe B: Final Results of a Space Experiment to Test General Relativity| journal=Physical Review Letters| volume = 106 | issue = 22 | page=221101 | doi = 10.1103/PhysRevLett.106.221101 |arxiv =1105.3456 | bibcode=2011PhRvL.106v1101E | pmid=21702590| s2cid=11878715 }}
</ref> The prospects for further experimental measurement of frame-dragging after GP-B were commented on in the journal ''[[Europhysics Letters]]''.<ref>{{cite journal| author = L. Iorio|date=November 2011| title=Some considerations on the present-day results for the detection of frame-dragging after the final outcome of GP-B| journal=Europhysics Letters| volume=96| issue=3| page=30001| doi = 10.1209/0295-5075/96/30001|bibcode = 2011EL.....9630001I |arxiv = 1105.4145 |s2cid=118532421}}</ref>