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{{short description|Orbital gravitational observatory}}
{{Use dmy dates|date=December 2020}}
{{Infobox spaceflight
| name = Gravity Probe B
| image = Artist concept of Gravity Probe B.jpg
| image_caption =
| mission_type = Astrophysics
| operator = [[NASA]]/[[Stanford University]]
| website = {{url|https://einstein.stanford.edu}}
| COSPAR_ID = 2004-014A
| SATCAT = 28230
| mission_duration = {{time interval|20 April 2004 16:57:24|8 December 2010}} <ref name=GPB/>
| spacecraft_bus =
| manufacturer = [[Lockheed Martin]]
| dry_mass =
| launch_mass = {{convert|3100|kg|lb|abbr=on}}<ref name=GPB>{{cite web |url=http://einstein.stanford.edu/content/fact_sheet/GPB_FactSheet-0405.pdf |title=Gravity Probe B |work=NASA Facts |date=February 2005 |publisher=[[NASA]] and [[Stanford University]] |access-date=17 March 2011 }}</ref>
| dimensions = {{convert|6.4|×|2.6|m|ft|abbr=on}}<ref name=GPB/>
| power = 606 W<br/><small>Spacecraft: 293 W<br/>Payload: 313 W<ref name=GPB/></small>
| launch_date = {{start-date|20 April 2004, 16:57:24|timezone=yes}} UTC
| launch_rocket = [[Delta II|Delta II 7920-10C]]
| launch_site = [[Vandenberg Air Force Base|Vandenberg]] [[Vandenberg AFB Space Launch Complex 2|SLC-2W]]
| launch_contractor =
| disposal_type = Decommissioned
| deactivated = {{end-date|8 December 2010}}
| decay_date =
| orbit_epoch = <time>2004-04-20 15:57:00</time> UTC<ref name=NSSDC>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=2004-014A|title=Spacecraft orbit: Gravity Probe B|publisher=National Space Science Data Center|year=2004|access-date=18 January 2015}}</ref>
| orbit_reference = [[geocentric orbit|Geocentric]]
| orbit_regime = [[Low Earth orbit|Low Earth]]
| orbit_periapsis = {{convert|641|km|mi|abbr=on|sp=us}}<ref name=NSSDC/>
| orbit_apoapsis = {{convert|645|km|mi|abbr=on|sp=us}}<ref name=NSSDC/>
| orbit_inclination = 90.007º<ref name=GPB/>
| orbit_semimajor = {{convert|7027.4|km|mi|abbr=on|sp=us}}
| orbit_eccentricity = 0.0014<ref name=GPB/>
| orbit_period = 97.65 minutes<ref>{{cite web |author1=G. Hanuschak |author2=H. Small |author3=D. DeBra |author4=K. Galal |author5=A. Ndili |author6=P. Shestople |url=http://einstein.stanford.edu/content/sci_papers/papers/Hanuschak_ION_Paper.pdf |title=Gravity Probe B GPS Orbit Determination with Verification by Satellite Laser Ranging |access-date=17 March 2011 }}</ref>
| apsis = gee
| insignia = Gravity Probe B logo (KSC-03PD3038).jpg
}}
'''Gravity Probe B''' ('''GP-B''') was a [[satellite]]-based experiment whose objective was to test two previously-unverified predictions of general relativity: the [[geodetic effect]] and [[frame-dragging]]. This was to be accomplished by measuring, very precisely, tiny changes in the direction of spin of four gyroscopes contained in an Earth-orbiting satellite at {{cvt|650|km}} of altitude, crossing directly over the poles.
The satellite was launched on 20 April 2004 on a [[Delta II]] rocket.<ref>
{{cite web
|url=http://einstein.stanford.edu/content/faqs/faqs.html#launch
|title=Frequently asked Questions |website=Gravity Probe B |publisher=Stanford University
|at=Answers to Spacecraft and Mission Operations Questions: 1. When and where was GP-B launched, and where can I find photos, video or news clips of the launch?
|access-date=14 May 2009
}}</ref> The spaceflight phase lasted until <time>2005</time>;<ref>
{{cite web
|url=http://einstein.stanford.edu/content/faqs/faqs.html#operations
|title=Frequently asked Questions |website=Gravity Probe B |publisher=Stanford University
|at=Answers to Spacecraft and Mission Operations Questions: 4. Where is the GP-B Mission Operations Center (MOC) for controlling the spacecraft in orbit?
|access-date=14 May 2009
}}</ref> Its aim was to measure [[spacetime|spacetime curvature]] near Earth, and thereby the [[stress–energy tensor]] (which is related to the distribution and the motion of matter in space) in and near Earth. This provided a test of [[general relativity]], [[gravitomagnetism]] and related models. The principal investigator was [[Francis Everitt]].
Initial results confirmed the expected geodetic effect to an accuracy of about 1%. The expected frame-dragging effect was similar in magnitude to the current noise level (the noise being dominated by initially unmodeled effects due to nonuniform coatings on the gyroscopes). Work continued to model and account for these sources of error, thus permitting extraction of the frame-dragging signal. By <time datetime="2008-08">August 2008</time>, the frame-dragging effect had been confirmed to within 15% of the expected result,<ref name = "Gugliotta2009">
{{cite news
| last = Gugliotta | first = G.
| title = Perseverance Is Paying Off for a Test of Relativity in Space
| work = [[The New York Times]]
| date = 16 February 2009
| url = https://www.nytimes.com/2009/02/17/science/17gravity.html?_r=1
| access-date =18 February 2009
}}</ref> and the <time datetime="2008-12">December 2008</time> [[NASA]] report indicated that the geodetic effect was confirmed to be better than 0.5%.<ref>
{{cite web
|author1=Everitt, C.W.F. |author2=Parkinson, B.W. |url=http://einstein.stanford.edu/content/final_report/GPB_Final_NASA_Report-020509-web.pdf
|title=Gravity Probe B Science Results—NASA Final Report
|date=2009
|access-date=2 May 2009
}}</ref>
In an article published in the journal ''[[Physical Review Letters]]'' in <time>2011</time>, the authors reported analysis of the data from all four gyroscopes results in a geodetic drift rate of {{val|−6601.8|18.3|ul=mas|upl=yr}} and a frame-dragging drift rate of {{val|−37.2|7.2|u=mas|up=yr}}, in good agreement with the general relativity predictions of {{val|−6606.1|0.28|errend=%|u=mas/yr}} and {{val|−39.2|0.19|errend=%|u=mas/yr}}, respectively.<ref name=PRL/>
==Overview==
Gravity Probe B was a relativity [[gyroscope]] experiment funded by NASA. Efforts were led by the [[Stanford University]] physics department with [[Lockheed Martin]] as the primary subcontractor. Mission scientists viewed it as the second relativity experiment in space, following the successful launch of [[Gravity Probe A]] (GP-A) in <time>1976</time>.
The mission plans were to test two unverified predictions of general relativity: the geodetic effect and [[frame-dragging]]. This was to be accomplished by measuring, very precisely, tiny changes in the direction of spin of four gyroscopes contained in an Earth satellite orbiting at {{cvt|650|km}} altitude, crossing directly over the poles. The gyroscopes were intended to be so free from disturbance that they would provide a near-perfect [[spacetime]] reference system. This would allow them to reveal how space and time are "warped" by the presence of the Earth, and by how much the Earth's rotation "drags" space-time around with it.
The geodetic effect is an effect caused by space-time being "curved" by the mass of the Earth. A gyroscope's axis when [[parallel transport]]ed 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. A more precise explanation for the space curvature part of the geodetic precession is obtained by using a nearly flat cone to model the space curvature of the Earth's gravitational field. Such a cone is made by cutting out a thin "pie-slice" from a circle and gluing the cut edges together. The spatial geodetic precession is a measure of the missing "pie-slice" angle. Gravity Probe B was expected to measure this effect to an accuracy of one part in {{val|10,000}}, the most stringent check on general relativistic predictions to date.
The much smaller frame-dragging effect is an example of [[gravitomagnetism]]. It is an analog of [[magnetism]] in [[classical electrodynamics]], but caused by rotating masses rather than rotating electric charges. Previously, only two analyses of the [[laser-ranging]] data obtained by the two [[LAGEOS]] satellites, published in <time>1997</time> and <time>2004</time>, claimed to have found the frame-dragging effect with an accuracy of about 20% and 10% respectively,<ref>
{{cite arXiv
|last1=Ciufolini | first1=I.
|last2=Lucchesi | first2=D.
|last3=Vespe | first3=F.
|last4=Chieppa | first4=F.
|title=Detection of Lense–Thirring Effect Due to Earth's Spin
|eprint=gr-qc/9704065
|date=1997
}}</ref><ref>
{{cite news
|title=Einstein's warp effect measured
|url=http://news.bbc.co.uk/2/hi/science/nature/3762852.stm
|work=BBC News
|date=21 October 2004
|access-date=14 May 2009
}}</ref><ref>
{{cite journal
|last=Peplow |first=M.
|title=Spinning Earth twists space
|journal=[[Nature (journal)|Nature News]]
|date=2004
|doi=10.1038/news041018-11
}}</ref> whereas Gravity Probe B aimed to measure the frame dragging effect to a precision of 1%.<ref>{{cite web |title=Overview of the GP-B Mission |url=http://einstein.stanford.edu/MISSION/mission1.html |website=Gravity Probe B |publisher=Stanford University |year=2011 |access-date=18 January 2015}}</ref> A recent analysis of [[Mars Global Surveyor]] data has claimed to have confirmed the frame dragging effect to a precision of 0.5%, although the accuracy of this claim is disputed.<ref>
{{cite journal
|last=Krogh |first=K.
|title=Comment on 'Evidence of the gravitomagnetic field of Mars'
|journal=[[Classical and Quantum Gravity]]
|volume=24
|issue = 22
|pages=5709–5715
|date=November 2007
|doi=10.1088/0264-9381/24/22/N01
|bibcode = 2007CQGra..24.5709K |arxiv=astro-ph/0701653
|s2cid=12238950
}}</ref>
The launch was planned for <time datetime="2004-04-19">19 April 2004</time> 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 <time datetime="2004-04-20 09:57:23-07:00">20 April, at 9:57:23 AM</time> [[Time zone|PDT]] (<time datetime="2004-04-20 16:57:23Z">16:57:23</time> [[Coordinated Universal Time|UTC]]) the spacecraft was launched successfully. The satellite was placed in orbit at <time datetime="2004-04-20 11:12:33-07:00">11:12:33</time> AM (<time datetime="2004-04-20 18:12:33Z">18:12:33</time> UTC) after a cruise period over the south pole and a short second burn. The mission lasted 16 months.
Some preliminary results were presented at a special session during the [[American Physical Society]] meeting in <time datetime="2007-04">April 2007</time>. NASA initially requested a proposal for extending the GP-B data analysis phase through <time datetime="2007-12">December 2007</time>. The data analysis phase was further extended to <time datetime="2008-09">September 2008</time> using funding from [[Richard Fairbank]], Stanford and NASA, and beyond that point using non-NASA funding only.<ref name = "Gugliotta2009"/> Final science results were reported in <time>2011</time>.
== Experimental setup ==
[[Image:Einstein gyro gravity probe b.jpg|thumb|At the time, the fused quartz [[gyroscopes]] created for Gravity Probe B were the most nearly perfect [[sphere]]s ever created by humans.<ref>
{{cite web
|last = Barry
|first = P.L.
|title = A Pocket of Near-Perfection
|url = https://science.nasa.gov/headlines/y2004/26apr_gpbtech.htm
|publisher = [[NASA|Science@NASA]]
|date = 26 April 2004
|access-date = 20 May 2009
|url-status = dead
|archive-url = https://web.archive.org/web/20090512163541/http://science.nasa.gov/headlines/y2004/26apr_gpbtech.htm
|archive-date = 12 May 2009
}}</ref> The gyroscopes differ from a perfect sphere by no more than 40 [[atom]]s of thickness. One is pictured here [[refraction|refracting]] the image of [[Albert Einstein]] in background.]]
[[Image:Gravity Probe B Confirms the Existence of Gravitomagnetism-en.jpg|thumb]]
The ''Gravity Probe B'' experiment comprised four [[Gyroscope#London moment|London moment gyroscopes]] and a reference [[telescope]] sighted on [[IM Pegasi]], a [[binary star]] in the constellation [[Pegasus (constellation)|Pegasus]]. In [[polar orbit]], with the gyro spin directions also pointing toward IM Pegasi, the frame-dragging and geodetic effects came out at right angles, each gyroscope measuring both.
The gyroscopes
At the time of their manufacture, the gyroscopes were the most nearly spherical objects ever made (two gyroscopes still hold that record, but third place has been taken by the silicon spheres made by the [[Alternative approaches to redefining the kilogram#Avogadro project|Avogadro project]]). Approximately the size of [[ping pong]] balls, they were perfectly round to within forty atoms (less than {{val|10|u=nm}}). If one of these spheres were scaled to the size of the Earth, the tallest mountains and deepest ocean trench would measure only {{convert|2.4|m|ft|sigfig=1|abbr=on}} high.<ref>
{{cite web
|last=Hardwood |first=W.
|title=Spacecraft launched to test Albert Einstein's theories
|url=http://www.spaceflightnow.com/delta/d304/
|work=[[Spaceflight Now]]
|date=20 April 2004
|access-date=14 May 2009
}}</ref> The spheres were made of [[fused quartz]] and coated with an extremely thin layer of [[niobium]]. A primary concern was minimizing any influence on their spin, so the gyroscopes could never touch their containing compartment. They were held suspended with electric fields, spun up using a flow of helium gas, and their spin axes were sensed by monitoring the magnetic field of the superconductive niobium layer with [[SQUID]]s. (A spinning superconductor generates a magnetic field precisely aligned with the rotation axis; see [[London moment]].)
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 near the [[celestial equator]]. Also important was its well-understood motion in the sky, which was helped by the fact that this star emits relatively strong [[Radio astronomy|radio signals]]. In preparation for the setup of this mission, astronomers analyzed the radio-based position measurements with respect to far distant quasars taken over several years to understand its motion as precisely as needed.
== History ==
[[Image:Gravity Probe turning axis.gif|thumb|A representation of the geodetic effect.]]
The conceptual design for this mission was first proposed by
In
Gravity Probe B marks the first time
The total cost of the project was about $750 million.<ref>{{cite web |url=http://www.sciencenews.org/view/generic/id/73870/title/Gravity_Probe_B_finally_pays_off_ |title=Gravity Probe B finally pays off |author=Devin Powell |date=4 May 2011 |work=Science News |access-date=7 May 2011 |archive-date=30 September 2012 |archive-url=https://web.archive.org/web/20120930181448/http://www.sciencenews.org/view/generic/id/73870/title/Gravity_probe_B_finally_pays_off_ |url-status=dead }}</ref>
== 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>
== See also ==
{{Portal|Spaceflight}}
* [[Frame-dragging]]
* [[Gravity Probe A]]
* [[Gravitomagnetism]]
* [[Modified Newtonian dynamics]]
* [[Tests of general relativity]]
* [[Timeline of gravitational physics and relativity]]
== References ==
{{reflist}}
== External links ==
{{commons category|Gravity Probe B}}
* [http://www.nasa.gov/mission_pages/gpb/index.html Gravity Probe B web site at NASA] {{Webarchive|url=https://web.archive.org/web/20220122125058/https://www.nasa.gov/mission_pages/gpb/index.html |date=22 January 2022 }}
* [https://web.archive.org/web/20090922110819/http://www.rdrop.com/users/green/school/framdrag.htm Gravity Probe B Web site at Stanford]
* [http://news.bbc.co.uk/2/hi/science/nature/3639193.stm#graphic Graphic explanation of how Gravity Probe B works]
* [https://web.archive.org/web/20110901094350/http://www.ksc.nasa.gov/elvnew/gpb/ NASA GP-B launch site]
* [https://web.archive.org/web/20090512163541/http://science.nasa.gov/headlines/y2004/26apr_gpbtech.htm NASA article on the technologies used in Gravity Probe B]
* {{webarchive |url=https://web.archive.org/web/20090922110819/http://www.rdrop.com/users/green/school/framdrag.htm |date=22 September 2009 |title=Frame Dragging }}
* [http://www.phy.duke.edu/~kolena/framedrag.html General Relativistic Frame Dragging]
* [https://www.washingtonpost.com/wp-dyn/articles/A52713-2004Oct21.html Layman's article on the project progress]
* [http://iopscience.iop.org/0264-9381/32/22 IOP Classical and Quantum Gravity, Volume 32, Issue 22, Special Focus Issue on Gravity Probe B]
* [http://libarchstor.uah.edu:8081/repositories/2/resources/108 Gravity Probe B Collection, The University of Alabama in Huntsville Archives and Special Collections]
{{Orbital launches in 2004}}
[[Category:Tests of general relativity]]
[[Category:Physics experiments]]
[[Category:Satellites orbiting Earth]]
[[Category:Spacecraft launched in 2004]]
[[Category:Spacecraft launched by Delta II rockets]]
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