{"history":[],"article":{"id":"e6f1926f-b3cd-47a3-b65a-2ab853fe0c86","title":"Gravity Probe B","slug":"gravity-probe-b","content":":::summary\n![Gravity Probe B](https://upload.wikimedia.org/wikipedia/commons/b/bb/Artist///concept///of///Gravity///Probe///B.jpg)\n**Name:** Gravity Probe B \n**Mission Type:** Astrophysics \n**Operator:** [NASA](https://bliptext.com/articles/nasa)/[Stanford University](https://bliptext.com/articles/stanford-university) \n**Website:** https://einstein.stanford.edu \n**Cospar Id:** 2004-014A \n**Satcat:** 28230 \n**Manufacturer:** [Lockheed Martin](https://bliptext.com/articles/lockheed-martin) \n**Launch Mass:** 3100 kg \n**Dimensions:** 6.4 × \n**Power:** 606 W Spacecraft: 293 W Payload: 313 W \n**Launch Date:** 20 April 2004, 16:57:24 UTC \n**Launch Rocket:** [Delta II 7920-10C](https://bliptext.com/articles/delta-ii) \n**Launch Site:** [Vandenberg](https://bliptext.com/articles/vandenberg-air-force-base) [SLC-2W](https://bliptext.com/articles/vandenberg-afb-space-launch-complex-2) \n**Disposal Type:** Decommissioned \n**Deactivated:** 8 December 2010 \n**Orbit Epoch:** 2004-04-20 15:57:00 UTC \n**Orbit Reference:** [Geocentric](https://bliptext.com/articles/geocentric-orbit) \n**Orbit Regime:** [Low Earth](https://bliptext.com/articles/low-earth-orbit) \n**Orbit Periapsis:** 641 km \n**Orbit Apoapsis:** 645 km \n**Orbit Inclination:** 90.007º \n**Orbit Semimajor:** 7027.4 km \n**Orbit Eccentricity:** 0.0014 \n**Orbit Period:** 97.65 minutes \n**Apsis:** gee \n**Insignia:** Gravity Probe B logo (KSC-03PD3038).jpg \n:::\n\n# \n**Gravity Probe B** (**GP-B**) was a [satellite](https://bliptext.com/articles/satellite)-based experiment to test two unverified predictions of general relativity: the [geodetic effect](https://bliptext.com/articles/geodetic-effect) and [frame-dragging](https://bliptext.com/articles/frame-dragging).\nThis 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 650 km of altitude, crossing directly over the poles.\nThe satellite was launched on 20 April 2004 on a [Delta II](https://bliptext.com/articles/delta-ii) rocket.\nThe spaceflight phase lasted until 2005 ; Its aim was to measure [spacetime curvature](https://bliptext.com/articles/spacetime) near Earth, and thereby the [stress–energy tensor](https://bliptext.com/articles/stress-energy-tensor) (which is related to the distribution and the motion of matter in space) in and near Earth.\nThis provided a test of [general relativity](https://bliptext.com/articles/general-relativity), [gravitomagnetism](https://bliptext.com/articles/gravitomagnetism) and related models.\nThe principal investigator was [Francis Everitt](https://bliptext.com/articles/francis-everitt).\nInitial results confirmed the expected geodetic effect to an accuracy of about 1%.\nThe 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).\nWork continued to model and account for these sources of error, thus permitting extraction of the frame-dragging signal.\nBy August 2008, the frame-dragging effect had been confirmed to within 15% of the expected result, and the December 2008 [NASA](https://bliptext.com/articles/nasa) report indicated that the geodetic effect was confirmed to be better than 0.5%.\nIn an article published in the journal [Physical Review Letters](https://bliptext.com/articles/physical-review-letters) in 2011, the authors reported analysis of the data from all four gyroscopes results in a geodetic drift rate of −6601.8 mas and a frame-dragging drift rate of −37.2 mas, in good agreement with the general relativity predictions of −6606.1 mas/yr and −39.2 mas/yr, respectively.\n\n# Overview\nGravity Probe B was a relativity [gyroscope](https://bliptext.com/articles/gyroscope) experiment funded by NASA.\nEfforts were led by the [Stanford University](https://bliptext.com/articles/stanford-university) physics department with [Lockheed Martin](https://bliptext.com/articles/lockheed-martin) as the primary subcontractor.\nMission scientists viewed it as the second relativity experiment in space, following the successful launch of [Gravity Probe A](https://bliptext.com/articles/gravity-probe-a) (GP-A) in 1976.\nThe mission plans were to test two unverified predictions of general relativity: the geodetic effect and [frame-dragging](https://bliptext.com/articles/frame-dragging).\nThis 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 650 km altitude, crossing directly over the poles.\nThe gyroscopes were intended to be so free from disturbance that they would provide a near-perfect [spacetime](https://bliptext.com/articles/spacetime) reference system.\nThis 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.\nThe geodetic effect is an effect caused by space-time being \"curved\" by the mass of the Earth.\nA gyroscope's axis when [parallel transported](https://bliptext.com/articles/parallel-transport) around the Earth in one complete revolution does not end up pointing in exactly the same direction as before.\nThe angle \"missing\" may be thought of as the amount the gyroscope \"leans over\" into the slope of the space-time curvature.\nA 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.\nSuch a cone is made by cutting out a thin \"pie-slice\" from a circle and gluing the cut edges together.\nThe spatial geodetic precession is a measure of the missing \"pie-slice\" angle.\nGravity Probe B was expected to measure this effect to an accuracy of one part in 10,000, the most stringent check on general relativistic predictions to date.\nThe much smaller frame-dragging effect is an example of [gravitomagnetism](https://bliptext.com/articles/gravitomagnetism).\nIt is an analog of [magnetism](https://bliptext.com/articles/magnetism) in [classical electrodynamics](https://bliptext.com/articles/classical-electrodynamics), but caused by rotating masses rather than rotating electric charges.\nPreviously, only two analyses of the [laser-ranging](https://bliptext.com/articles/laser-ranging) data obtained by the two [LAGEOS](https://bliptext.com/articles/lageos) satellites, published in 1997 and 2004, claimed to have found the frame-dragging effect with an accuracy of about 20% and 10% respectively, whereas Gravity Probe B aimed to measure the frame dragging effect to a precision of 1%.\nA recent analysis of [Mars Global Surveyor](https://bliptext.com/articles/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.\nThe launch was planned for 19 April 2004 at [Vandenberg Air Force Base](https://bliptext.com/articles/vandenberg-air-force-base) but was scrubbed within 5 minutes of the scheduled launch window due to changing winds in the upper atmosphere.\nAn unusual feature of the mission is that it only had a one-second launch window due to the precise orbit required by the experiment.\nOn 20 April, at 9:57:23 AM [PDT](https://bliptext.com/articles/time-zone) (16:57:23 [UTC](https://bliptext.com/articles/coordinated-universal-time)) the spacecraft was launched successfully.\nThe satellite was placed in orbit at 11:12:33 AM (18:12:33 UTC) after a cruise period over the south pole and a short second burn.\nThe mission lasted 16 months.\nSome preliminary results were presented at a special session during the [American Physical Society](https://bliptext.com/articles/american-physical-society) meeting in April 2007.\nNASA initially requested a proposal for extending the GP-B data analysis phase through December 2007.\nThe data analysis phase was further extended to September 2008 using funding from [Richard Fairbank](https://bliptext.com/articles/richard-fairbank), Stanford and NASA, and beyond that point using non-NASA funding only.\nFinal science results were reported in 2011.\n\n# Experimental setup\nThe Gravity Probe B experiment comprised four [London moment gyroscopes](https://bliptext.com/articles/gyroscope) and a reference [telescope](https://bliptext.com/articles/telescope) sighted on [IM Pegasi](https://bliptext.com/articles/im-pegasi), a [binary star](https://bliptext.com/articles/binary-star) in the constellation [Pegasus](https://bliptext.com/articles/pegasus-constellation).\nIn [polar orbit](https://bliptext.com/articles/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.\nThe gyroscopes were housed in a [dewar](https://bliptext.com/articles/dewar-flask) of [superfluid helium](https://bliptext.com/articles/superfluid-helium-4), maintaining a temperature of under 2 K. Near-[absolute zero](https://bliptext.com/articles/absolute-zero) temperatures were required to minimize molecular interference, and enable the [lead](https://bliptext.com/articles/lead) and [niobium](https://bliptext.com/articles/niobium) components of the gyroscope mechanisms to become [superconductive](https://bliptext.com/articles/superconductivity).\nAt 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 [Avogadro project](https://bliptext.com/articles/alternative-approaches-to-redefining-the-kilogram)).\nApproximately the size of [ping pong](https://bliptext.com/articles/ping-pong) balls, they were perfectly round to within forty atoms (less than 10 nm).\nIf one of these spheres were scaled to the size of the Earth, the tallest mountains and deepest ocean trench would measure only 2.4 m high.\nThe spheres were made of [fused quartz](https://bliptext.com/articles/fused-quartz) and coated with an extremely thin layer of [niobium](https://bliptext.com/articles/niobium).\nA primary concern was minimizing any influence on their spin, so the gyroscopes could never touch their containing compartment.\nThey 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 [SQUIDs](https://bliptext.com/articles/squid).\n(A spinning superconductor generates a magnetic field precisely aligned with the rotation axis; see [London moment](https://bliptext.com/articles/london-moment).)\nIM Pegasi was chosen as the guide star for multiple reasons.\nFirst, it needed to be bright enough to be usable for sightings.\nThen it was close to the ideal positions near the [celestial equator](https://bliptext.com/articles/celestial-equator).\nAlso important was its well-understood motion in the sky, which was helped by the fact that this star emits relatively strong [radio signals](https://bliptext.com/articles/radio-astronomy).\nIn 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.\n# [At the time, the fused quartz gyroscopes created for Gravity Probe B were the most nearly perfect spheres ever created by humans. The gyroscopes differ from a perfect sphere by no more than 40 atoms of thickness. One is pictured here refracting the image of Albert Einstein in background. | upload.wikimedia.org/wikipedia/commons/9/9b/Einstein///gyro///gravity///probe///b.jpg]\n# [Gravity Probe B | upload.wikimedia.org/wikipedia/commons/7/7a/Gravity///Probe///B///Confirms///the///Existence///of///Gravitomagnetism-en.jpg]\n\n# History\nThe conceptual design for this mission was first proposed by an [MIT](https://bliptext.com/articles/mit) professor, George Pugh, who was working with the [U.S. Department of Defense](https://bliptext.com/articles/u-s-department-of-defense) in 1959 and later discussed by [Leonard Schiff](https://bliptext.com/articles/leonard-schiff) ([Stanford](https://bliptext.com/articles/stanford-university)) in 1960 at Pugh's suggestion, based partly on a theoretical paper about detecting frame dragging that Schiff had written in 1957.\nIt was proposed to NASA in 1961, and they supported the project with funds in 1964.\nThis grant ended in 1977 after a long phase of [engineering](https://bliptext.com/articles/engineering) research into the basic requirements and tools for the satellite.\nIn 1986 NASA changed plans for the [Space Shuttle](https://bliptext.com/articles/space-shuttle), which forced the mission team to switch from a shuttle-based launch design to one that was based on the Delta 2, and in 1995 tests planned of a prototype on a shuttle flight were cancelled as well.\nGravity Probe B marks the first time that Stanford University has been in control of the development and operations of a space satellite funded by NASA.\nThe total cost of the project was about $750 million.\n\n# Mission timeline\nThis is a list of major events for the GP-B experiment.\nOn 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.\nIn 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.\nConsequently, the date for the final release of data was pushed back several times.\nIn 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 as to whether any useful data could be extracted in the future to test this effect.\nIn June 2007, a detailed update was released explaining the cause of the problem, and the solution that was being worked on.\nAlthough 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 [contact potential](https://bliptext.com/articles/volta-potential), which gave the sphere an electrostatic axis.\nThis created a classical dipole torque on each rotor, of a magnitude similar to the expected frame dragging effect.\nIn addition, it dissipated energy from the [polhode motion](https://bliptext.com/articles/polhode) by inducing currents in the housing electrodes, causing the motion to change with time.\nThis meant that a simple time-average polhode model was insufficient, and a detailed orbit by orbit model was needed to remove the effect.\nAs 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 24 hours, to exacerbate any potential problems.\nThis data proved invaluable for identifying the effects.\nWith 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.\nStanford agreed to release the raw data to the public at an unspecified date in the future.\nIt 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.\nBecause 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.\n# [Gravity Probe B before launch | upload.wikimedia.org/wikipedia/commons/c/c0/Gravity///Probe-B///before///launch.jpg]\n\n## NASA review\nA review by a panel of 15 experts commissioned by NASA recommended against extending the data analysis phase beyond 2008.\nThey 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\".\n\n## Data analysis after NASA\nNASA funding and sponsorship of the program ended on 30 September 2008, but GP-B secured alternative funding from [King Abdulaziz City for Science and Technology](https://bliptext.com/articles/king-abdulaziz-city-for-science-and-technology) in Saudi Arabia that enabled the science team to continue working at least through December 2009.\nOn 29 August 2008, the 18th meeting of the external GP-B Science Advisory Committee was held at Stanford to report progress.\nThe Stanford-based analysis group and NASA announced on 4 May 2011 that the data from GP-B indeed confirms the two predictions of Albert Einstein's general theory of relativity.\nThe findings were published in the journal [Physical Review Letters](https://bliptext.com/articles/physical-review-letters).\nThe prospects for further experimental measurement of frame-dragging after GP-B were commented on in the journal [Europhysics Letters](https://bliptext.com/articles/europhysics-letters).","current_revision":null,"created_by":"kACnuQmH5k9Nr5VNVEpVeg7XpvUmMctD","created_at":"2025-01-25T19:57:59.638Z","updated_at":"2025-01-25T19:57:59.638Z","search_vector":"'b':3 'graviti':1 'probe':2","search_content":" \n \n**Name:** Gravity Probe B \n**Mission Type:** Astrophysics \n**Operator:** [NASA](https://bliptext.com/articles/nasa)/[Stanford University](https://bliptext.com/articles/stanford-university) \n**Website:** https://einstein.stanford.edu \n**Cospar Id:** 2004-014A \n**Satcat:** 28230 \n**Manufacturer:** [Lockheed Martin](https://bliptext.com/articles/lockheed-martin) \n**Launch Mass:** 3100 kg \n**Dimensions:** 6.4 × \n**Power:** 606 W Spacecraft: 293 W Payload: 313 W \n**Launch Date:** 20 April 2004, 16:57:24 UTC \n**Launch Rocket:** [Delta II 7920-10C](https://bliptext.com/articles/delta-ii) \n**Launch Site:** [Vandenberg](https://bliptext.com/articles/vandenberg-air-force-base) [SLC-2W](https://bliptext.com/articles/vandenberg-afb-space-launch-complex-2) \n**Disposal Type:** Decommissioned \n**Deactivated:** 8 December 2010 \n**Orbit Epoch:** 2004-04-20 15:57:00 UTC \n**Orbit Reference:** [Geocentric](https://bliptext.com/articles/geocentric-orbit) \n**Orbit Regime:** [Low Earth](https://bliptext.com/articles/low-earth-orbit) \n**Orbit Periapsis:** 641 km \n**Orbit Apoapsis:** 645 km \n**Orbit Inclination:** 90.007º \n**Orbit Semimajor:** 7027.4 km \n**Orbit Eccentricity:** 0.0014 \n**Orbit Period:** 97.65 minutes \n**Apsis:** gee \n**Insignia:** Gravity Probe B logo (KSC-03PD3038).jpg \n \n\n# \n**Gravity Probe B** (**GP-B**) was a [satellite](https://bliptext.com/articles/satellite)-based experiment to test two unverified predictions of general relativity: the [geodetic effect](https://bliptext.com/articles/geodetic-effect) and [frame-dragging](https://bliptext.com/articles/frame-dragging).\nThis 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 650 km of altitude, crossing directly over the poles.\nThe satellite was launched on 20 April 2004 on a [Delta II](https://bliptext.com/articles/delta-ii) rocket.\nThe spaceflight phase lasted until 2005 ; Its aim was to measure [spacetime curvature](https://bliptext.com/articles/spacetime) near Earth, and thereby the [stress–energy tensor](https://bliptext.com/articles/stress-energy-tensor) (which is related to the distribution and the motion of matter in space) in and near Earth.\nThis provided a test of [general relativity](https://bliptext.com/articles/general-relativity), [gravitomagnetism](https://bliptext.com/articles/gravitomagnetism) and related models.\nThe principal investigator was [Francis Everitt](https://bliptext.com/articles/francis-everitt).\nInitial results confirmed the expected geodetic effect to an accuracy of about 1%.\nThe 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).\nWork continued to model and account for these sources of error, thus permitting extraction of the frame-dragging signal.\nBy August 2008, the frame-dragging effect had been confirmed to within 15% of the expected result, and the December 2008 [NASA](https://bliptext.com/articles/nasa) report indicated that the geodetic effect was confirmed to be better than 0.5%.\nIn an article published in the journal [Physical Review Letters](https://bliptext.com/articles/physical-review-letters) in 2011, the authors reported analysis of the data from all four gyroscopes results in a geodetic drift rate of −6601.8 mas and a frame-dragging drift rate of −37.2 mas, in good agreement with the general relativity predictions of −6606.1 mas/yr and −39.2 mas/yr, respectively.\n\n# Overview\nGravity Probe B was a relativity [gyroscope](https://bliptext.com/articles/gyroscope) experiment funded by NASA.\nEfforts were led by the [Stanford University](https://bliptext.com/articles/stanford-university) physics department with [Lockheed Martin](https://bliptext.com/articles/lockheed-martin) as the primary subcontractor.\nMission scientists viewed it as the second relativity experiment in space, following the successful launch of [Gravity Probe A](https://bliptext.com/articles/gravity-probe-a) (GP-A) in 1976.\nThe mission plans were to test two unverified predictions of general relativity: the geodetic effect and [frame-dragging](https://bliptext.com/articles/frame-dragging).\nThis 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 650 km altitude, crossing directly over the poles.\nThe gyroscopes were intended to be so free from disturbance that they would provide a near-perfect [spacetime](https://bliptext.com/articles/spacetime) reference system.\nThis 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.\nThe geodetic effect is an effect caused by space-time being \"curved\" by the mass of the Earth.\nA gyroscope's axis when [parallel transported](https://bliptext.com/articles/parallel-transport) around the Earth in one complete revolution does not end up pointing in exactly the same direction as before.\nThe angle \"missing\" may be thought of as the amount the gyroscope \"leans over\" into the slope of the space-time curvature.\nA 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.\nSuch a cone is made by cutting out a thin \"pie-slice\" from a circle and gluing the cut edges together.\nThe spatial geodetic precession is a measure of the missing \"pie-slice\" angle.\nGravity Probe B was expected to measure this effect to an accuracy of one part in 10,000, the most stringent check on general relativistic predictions to date.\nThe much smaller frame-dragging effect is an example of [gravitomagnetism](https://bliptext.com/articles/gravitomagnetism).\nIt is an analog of [magnetism](https://bliptext.com/articles/magnetism) in [classical electrodynamics](https://bliptext.com/articles/classical-electrodynamics), but caused by rotating masses rather than rotating electric charges.\nPreviously, only two analyses of the [laser-ranging](https://bliptext.com/articles/laser-ranging) data obtained by the two [LAGEOS](https://bliptext.com/articles/lageos) satellites, published in 1997 and 2004, claimed to have found the frame-dragging effect with an accuracy of about 20% and 10% respectively, whereas Gravity Probe B aimed to measure the frame dragging effect to a precision of 1%.\nA recent analysis of [Mars Global Surveyor](https://bliptext.com/articles/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.\nThe launch was planned for 19 April 2004 at [Vandenberg Air Force Base](https://bliptext.com/articles/vandenberg-air-force-base) but was scrubbed within 5 minutes of the scheduled launch window due to changing winds in the upper atmosphere.\nAn unusual feature of the mission is that it only had a one-second launch window due to the precise orbit required by the experiment.\nOn 20 April, at 9:57:23 AM [PDT](https://bliptext.com/articles/time-zone) ( 16:57:23 [UTC](https://bliptext.com/articles/coordinated-universal-time)) the spacecraft was launched successfully.\nThe satellite was placed in orbit at 11:12:33 AM ( 18:12:33 UTC) after a cruise period over the south pole and a short second burn.\nThe mission lasted 16 months.\nSome preliminary results were presented at a special session during the [American Physical Society](https://bliptext.com/articles/american-physical-society) meeting in April 2007.\nNASA initially requested a proposal for extending the GP-B data analysis phase through December 2007.\nThe data analysis phase was further extended to September 2008 using funding from [Richard Fairbank](https://bliptext.com/articles/richard-fairbank), Stanford and NASA, and beyond that point using non-NASA funding only.\nFinal science results were reported in 2011.\n\n# Experimental setup\nThe Gravity Probe B experiment comprised four [London moment gyroscopes](https://bliptext.com/articles/gyroscope) and a reference [telescope](https://bliptext.com/articles/telescope) sighted on [IM Pegasi](https://bliptext.com/articles/im-pegasi), a [binary star](https://bliptext.com/articles/binary-star) in the constellation [Pegasus](https://bliptext.com/articles/pegasus-constellation).\nIn [polar orbit](https://bliptext.com/articles/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.\nThe gyroscopes were housed in a [dewar](https://bliptext.com/articles/dewar-flask) of [superfluid helium](https://bliptext.com/articles/superfluid-helium-4), maintaining a temperature of under 2 K. Near-[absolute zero](https://bliptext.com/articles/absolute-zero) temperatures were required to minimize molecular interference, and enable the [lead](https://bliptext.com/articles/lead) and [niobium](https://bliptext.com/articles/niobium) components of the gyroscope mechanisms to become [superconductive](https://bliptext.com/articles/superconductivity).\nAt 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 [Avogadro project](https://bliptext.com/articles/alternative-approaches-to-redefining-the-kilogram)).\nApproximately the size of [ping pong](https://bliptext.com/articles/ping-pong) balls, they were perfectly round to within forty atoms (less than 10 nm).\nIf one of these spheres were scaled to the size of the Earth, the tallest mountains and deepest ocean trench would measure only 2.4 m high.\nThe spheres were made of [fused quartz](https://bliptext.com/articles/fused-quartz) and coated with an extremely thin layer of [niobium](https://bliptext.com/articles/niobium).\nA primary concern was minimizing any influence on their spin, so the gyroscopes could never touch their containing compartment.\nThey 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 [SQUIDs](https://bliptext.com/articles/squid).\n(A spinning superconductor generates a magnetic field precisely aligned with the rotation axis; see [London moment](https://bliptext.com/articles/london-moment).)\nIM Pegasi was chosen as the guide star for multiple reasons.\nFirst, it needed to be bright enough to be usable for sightings.\nThen it was close to the ideal positions near the [celestial equator](https://bliptext.com/articles/celestial-equator).\nAlso important was its well-understood motion in the sky, which was helped by the fact that this star emits relatively strong [radio signals](https://bliptext.com/articles/radio-astronomy).\nIn 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.\n# [At the time, the fused quartz gyroscopes created for Gravity Probe B were the most nearly perfect spheres ever created by humans. The gyroscopes differ from a perfect sphere by no more than 40 atoms of thickness. One is pictured here refracting the image of Albert Einstein in background. | upload.wikimedia.org/wikipedia/commons/9/9b/Einstein///gyro///gravity///probe///b.jpg]\n# [Gravity Probe B | upload.wikimedia.org/wikipedia/commons/7/7a/Gravity///Probe///B///Confirms///the///Existence///of///Gravitomagnetism-en.jpg]\n\n# History\nThe conceptual design for this mission was first proposed by an [MIT](https://bliptext.com/articles/mit) professor, George Pugh, who was working with the [U.S. Department of Defense](https://bliptext.com/articles/u-s-department-of-defense) in 1959 and later discussed by [Leonard Schiff](https://bliptext.com/articles/leonard-schiff) ([Stanford](https://bliptext.com/articles/stanford-university)) in 1960 at Pugh's suggestion, based partly on a theoretical paper about detecting frame dragging that Schiff had written in 1957.\nIt was proposed to NASA in 1961, and they supported the project with funds in 1964.\nThis grant ended in 1977 after a long phase of [engineering](https://bliptext.com/articles/engineering) research into the basic requirements and tools for the satellite.\nIn 1986 NASA changed plans for the [Space Shuttle](https://bliptext.com/articles/space-shuttle), which forced the mission team to switch from a shuttle-based launch design to one that was based on the Delta 2, and in 1995 tests planned of a prototype on a shuttle flight were cancelled as well.\nGravity Probe B marks the first time that Stanford University has been in control of the development and operations of a space satellite funded by NASA.\nThe total cost of the project was about $750 million.\n\n# Mission timeline\nThis is a list of major events for the GP-B experiment.\nOn 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.\nIn 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.\nConsequently, the date for the final release of data was pushed back several times.\nIn 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 as to whether any useful data could be extracted in the future to test this effect.\nIn June 2007, a detailed update was released explaining the cause of the problem, and the solution that was being worked on.\nAlthough 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 [contact potential](https://bliptext.com/articles/volta-potential), which gave the sphere an electrostatic axis.\nThis created a classical dipole torque on each rotor, of a magnitude similar to the expected frame dragging effect.\nIn addition, it dissipated energy from the [polhode motion](https://bliptext.com/articles/polhode) by inducing currents in the housing electrodes, causing the motion to change with time.\nThis meant that a simple time-average polhode model was insufficient, and a detailed orbit by orbit model was needed to remove the effect.\nAs 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 24 hours, to exacerbate any potential problems.\nThis data proved invaluable for identifying the effects.\nWith 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.\nStanford agreed to release the raw data to the public at an unspecified date in the future.\nIt 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.\nBecause 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.\n# [Gravity Probe B before launch | upload.wikimedia.org/wikipedia/commons/c/c0/Gravity///Probe-B///before///launch.jpg]\n\n## NASA review\nA review by a panel of 15 experts commissioned by NASA recommended against extending the data analysis phase beyond 2008.\nThey 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\".\n\n## Data analysis after NASA\nNASA funding and sponsorship of the program ended on 30 September 2008, but GP-B secured alternative funding from [King Abdulaziz City for Science and Technology](https://bliptext.com/articles/king-abdulaziz-city-for-science-and-technology) in Saudi Arabia that enabled the science team to continue working at least through December 2009.\nOn 29 August 2008, the 18th meeting of the external GP-B Science Advisory Committee was held at Stanford to report progress.\nThe Stanford-based analysis group and NASA announced on 4 May 2011 that the data from GP-B indeed confirms the two predictions of Albert Einstein's general theory of relativity.\nThe findings were published in the journal [Physical Review Letters](https://bliptext.com/articles/physical-review-letters).\nThe prospects for further experimental measurement of frame-dragging after GP-B were commented on in the journal [Europhysics Letters](https://bliptext.com/articles/europhysics-letters).","revisionCount":0},"hasMore":false,"nextCursor":null}