Gravity Probe B

Last updated

Gravity Probe B
Artist concept of Gravity Probe B.jpg
Mission typeAstrophysics
Operator NASA/Stanford University
COSPAR ID 2004-014A OOjs UI icon edit-ltr-progressive.svg
SATCAT no. 28230
Website einstein.stanford.edu
Mission duration6 years, 7 months and 17 days [1]
Spacecraft properties
Manufacturer Lockheed Martin
Launch mass3,100 kg (6,800 lb) [1]
Dimensions6.4 m × 2.6 m (21.0 ft × 8.5 ft) [1]
Power606 W
Spacecraft: 293 W
Payload: 313 W [1]
Start of mission
Launch date20 April 2004, 16:57:24 (2004-04-20UTC16:57:24Z) UTC
Rocket Delta II 7920-10C
Launch site Vandenberg SLC-2W
End of mission
DisposalDecommissioned
Deactivated8 December 2010 (2010-12-09)
Orbital parameters
Reference system Geocentric
Regime Low Earth
Semi-major axis 7,027.4 km (4,366.6 mi)
Eccentricity 0.0014 [1]
Perigee altitude 641 km (398 mi) [2]
Apogee altitude 645 km (401 mi) [2]
Inclination 90.007º [1]
Period 97.65 minutes [3]
Epoch UTC [2]
Gravity Probe B logo (KSC-03PD3038).jpg

Gravity Probe B (GP-B) was a satellite-based experiment 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-orbiting satellite at 650 km (400 mi) of altitude, crossing directly over the poles.

Contents

The satellite was launched on 20 April 2004 on a Delta II rocket. [4] The spaceflight phase lasted until ; [5] Its aim was to measure 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 , the frame-dragging effect had been confirmed to within 15% of the expected result, [6] and the NASA report indicated that the geodetic effect was confirmed to be better than 0.5%. [7]

In an article published in the journal Physical Review Letters in , the authors reported analysis of the data from all four gyroscopes results in a geodetic drift rate of −6601.8±18.3  mas/yr and a frame-dragging drift rate of −37.2±7.2 mas/yr, in good agreement with the general relativity predictions of −6606.1±0.28% mas/yr and −39.2±0.19% mas/yr, respectively. [8]

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 .

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 650 km (400 mi) 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 transported 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 10000, 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 and , claimed to have found the frame-dragging effect with an accuracy of about 20% and 10% respectively, [9] [10] [11] whereas Gravity Probe B aimed to measure the frame dragging effect to a precision of 1%. [12] 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. [13]

The launch was planned for 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 PDT ( UTC) the spacecraft was launched successfully. The satellite was placed in orbit at AM ( 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 . NASA initially requested a proposal for extending the GP-B data analysis phase through . The data analysis phase was further extended to using funding from Richard Fairbank, Stanford and NASA, and beyond that point using non-NASA funding only. [6] Final science results were reported in .

Experimental setup

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. Einstein gyro gravity probe b.jpg
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.
Gravity Probe B Confirms the Existence of Gravitomagnetism-en.jpg

The Gravity Probe B experiment comprised four London moment gyroscopes and a reference telescope sighted on IM Pegasi, a binary star in the 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 were housed in a dewar of superfluid helium, maintaining a temperature of under 2 kelvins (−271  °C ; −456  °F ). Near-absolute zero temperatures were required to minimize molecular interference, and enable the lead and niobium components of the gyroscope mechanisms to become superconductive.

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 Avogadro project). Approximately the size of ping pong balls, they were perfectly round to within forty atoms (less than 10 nm). If 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 (8 ft) high. [15] 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 SQUIDs. (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 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

A representation of the geodetic effect. Gravity Probe turning axis.gif
A representation of the geodetic effect.

The conceptual design for this mission was first proposed by an MIT professor, George Pugh, who was working with the U.S. Department of Defense in and later discussed by Leonard Schiff (Stanford) in at Pugh's suggestion, based partly on a theoretical paper about detecting frame dragging that Schiff had written in . It was proposed to NASA in , and they supported the project with funds in . This grant ended in after a long phase of engineering research into the basic requirements and tools for the satellite.

In NASA changed plans for the 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 tests planned of a prototype on a shuttle flight were cancelled as well.

Gravity 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.

The total cost of the project was about $750 million. [16]

Mission timeline

Gravity Probe B before launch Gravity Probe-B before launch.jpg
Gravity Probe B before launch

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

Launch of GP-B from Vandenberg AFB and successful insertion into polar orbit.
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.
The science phase of the mission ended and the spacecraft instruments transitioned to the final calibration mode.
The calibration phase ended with liquid helium still in the dewar. The spacecraft was returned to science mode pending the depletion of liquid helium.
Phase I of data analysis complete
Analysis team realised that more error analysis was necessary (particularly around the polhode motion of the gyros) than could be done in the time to and applied to NASA for an extension of funding to the end of .
Completion of Phase III of data analysis
Announcement of best results obtained to date. Francis Everitt gave a plenary talk at the meeting of the American Physical Society announcing initial results: [17] "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." [18]
GP-B spacecraft decommissioned, left in its 642 km (399 mi) polar orbit. [19]
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. [20]
Publication of GP-B Special Volume (Volume 32, Issue 22) in the peer-reviewed journal, Classical and Quantum Gravity . [21]

On , 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 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 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 , 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 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 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 , 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.[ needs update ]

NASA review

A review by a panel of 15 experts commissioned by NASA recommended against extending the data analysis phase beyond . 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". [22]

Data analysis after NASA

NASA funding and sponsorship of the program ended on , but GP-B secured alternative funding from King Abdulaziz City for Science and Technology in Saudi Arabia [6] that enabled the science team to continue working at least through . On , 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 that the data from GP-B indeed confirms the two predictions of Albert Einstein's general theory of relativity. [23] The findings were published in the journal Physical Review Letters . [8] The prospects for further experimental measurement of frame-dragging after GP-B were commented on in the journal Europhysics Letters . [24]

See also

Related Research Articles

<span class="mw-page-title-main">Mariner 1</span> Failed NASA mission to Venus (1962)

Mariner 1, built to conduct the first American planetary flyby of Venus, was the first spacecraft of NASA's interplanetary Mariner program. Developed by Jet Propulsion Laboratory, and originally planned to be a purpose-built probe launched summer 1962, Mariner 1's design was changed when the Centaur proved unavailable at that early date. Mariner 1, were then adapted from the lighter Ranger lunar spacecraft. Mariner 1 carried a suite of experiments to determine the temperature of Venus as well to measure magnetic fields and charged particles near the planet and in interplanetary space.

<i>Giotto</i> (spacecraft) European mission to comets Halley and Grigg–Skjellerup (1985–1992)

Giotto was a European robotic spacecraft mission from the European Space Agency. The spacecraft flew by and studied Halley's Comet and in doing so became the first spacecraft to make close up observations of a comet. On 13 March 1986, the spacecraft succeeded in approaching Halley's nucleus at a distance of 596 kilometers. It was named after the Early Italian Renaissance painter Giotto di Bondone. He had observed Halley's Comet in 1301 and was inspired to depict it as the star of Bethlehem in his painting Adoration of the Magi in the Scrovegni Chapel.

<span class="mw-page-title-main">Gravity assist</span> Space navigation technique

A gravity assist, gravity assist maneuver, swing-by, or generally a gravitational slingshot in orbital mechanics, is a type of spaceflight flyby which makes use of the relative movement and gravity of a planet or other astronomical object to alter the path and speed of a spacecraft, typically to save propellant and reduce expense.

<span class="mw-page-title-main">Solar and Heliospheric Observatory</span> European space observatory

The Solar and Heliospheric Observatory (SOHO) is a European Space Agency (ESA) spacecraft built by a European industrial consortium led by Matra Marconi Space that was launched on a Lockheed Martin Atlas IIAS launch vehicle on 2 December 1995, to study the Sun. It has also discovered over 5,000 comets. It began normal operations in May 1996. It is a joint project between the European Space Agency (ESA) and NASA. SOHO was part of the International Solar Terrestrial Physics Program (ISTP). Originally planned as a two-year mission, SOHO continues to operate after almost 29 years in space; the mission has been extended until the end of 2025, subject to review and confirmation by ESA's Science Programme Committee.

<span class="mw-page-title-main">Satellite laser ranging</span>

In satellite laser ranging (SLR) a global network of observation stations measures the round trip time of flight of ultrashort pulses of light to satellites equipped with retroreflectors. This provides instantaneous range measurements of millimeter level precision which can be accumulated to provide accurate measurement of orbits and a host of important scientific data. The laser pulse can also be reflected by the surface of a satellite without a retroreflector, which is used for tracking space debris.

<span class="mw-page-title-main">Curved spacetime</span> Mathematical theory of the geometry of space and time

In physics, curved spacetime is the mathematical model in which, with Einstein's theory of general relativity, gravity naturally arises, as opposed to being described as a fundamental force in Newton's static Euclidean reference frame. Objects move along geodesics—curved paths determined by the local geometry of spacetime—rather than being influenced directly by distant bodies. This framework led to two fundamental principles: coordinate independence, which asserts that the laws of physics are the same regardless of the coordinate system used, and the equivalence principle, which states that the effects of gravity are indistinguishable from those of acceleration in sufficiently small regions of space. These principles laid the groundwork for a deeper understanding of gravity through the geometry of spacetime, as formalized in Einstein's field equations.

<span class="mw-page-title-main">GRACE and GRACE-FO</span> Joint American-German space mission to map Earths gravitational field

The Gravity Recovery and Climate Experiment (GRACE) was a joint mission of NASA and the German Aerospace Center (DLR). Twin satellites took detailed measurements of Earth's gravity field anomalies from its launch in March 2002 to the end of its science mission in October 2017. The two satellites were sometimes called Tom and Jerry, a nod to the famous cartoon. The GRACE Follow-On (GRACE-FO) is a continuation of the mission on near-identical hardware, launched in May 2018. On March 19, 2024, NASA announced that the successor to GRACE-FO would be Gravity Recovery and Climate Experiment-Continuity (GRACE-C), to be launched in or after 2028.

<span class="mw-page-title-main">Artificial gravity</span> Use of circular rotational force to mimic gravity

Artificial gravity is the creation of an inertial force that mimics the effects of a gravitational force, usually by rotation. Artificial gravity, or rotational gravity, is thus the appearance of a centrifugal force in a rotating frame of reference, as opposed to the force experienced in linear acceleration, which by the equivalence principle is indistinguishable from gravity. In a more general sense, "artificial gravity" may also refer to the effect of linear acceleration, e.g. by means of a rocket engine.

<span class="mw-page-title-main">Explorer 32</span> NASA satellite of the Explorer program

Explorer 32, also known as Atmosphere Explorer-B (AE-B), was a NASA satellite launched by the United States to study the Earth's upper atmosphere. It was launched from Cape Canaveral on a Delta C1 launch vehicle, on 25 May 1966. It was the second of five "Atmosphere Explorer", the first being Explorer 17. Though it was placed in a higher-than-expected orbit by a malfunctioning second stage on its launch vehicle, Explorer 32 returned data for ten months before failing due to a sudden depressurization. The satellite reentered the Earth's atmosphere on 22 February 1985.

The Satellite Test of the Equivalence Principle (STEP) is a proposed space science experiment to test the equivalence principle of general relativity. The experiment is thought to be sensitive enough to test Einstein's theory of gravity and other theories.

<span class="mw-page-title-main">STEREO</span> Solar observation mission (2006–present)

STEREO is a solar observation mission. Two nearly identical spacecraft were launched in 2006 into orbits around the Sun that cause them to respectively pull farther ahead of and fall gradually behind the Earth. This enabled stereoscopic imaging of the Sun and solar phenomena, such as coronal mass ejections.

<span class="mw-page-title-main">Geosat</span>

The GEOSAT was a U.S. Navy Earth observation satellite, launched on March 12, 1985 into an 800 km, 108° inclination orbit, with a nodal period of about 6040 seconds. The satellite carried a radar altimeter capable of measuring the distance from the satellite to sea surface with a relative precision of about 5 cm. The initial phase was an 18-month classified Geodetic Mission (GM) have a ground-track with a near-23-day repeat with closure to within 50 kilometers. The effect of atmospheric drag was such that by fall 1986 GEOSAT was in an almost exact 23-day repeat orbit.

The flyby anomaly is a discrepancy between current scientific models and the actual increase in speed observed during a planetary flyby by a spacecraft. In multiple cases, spacecraft have been observed to gain greater speed than scientists had predicted, but thus far no convincing explanation has been found. This anomaly has been observed as shifts in the S-band and X-band Doppler and ranging telemetry. The largest discrepancy noticed during a flyby is tiny, 13.46 mm/s.

<span class="mw-page-title-main">Geodetic effect</span> Precession of satellite orbits due to a celestial bodys presence affecting spacetime

The geodetic effect represents the effect of the curvature of spacetime, predicted by general relativity, on a vector carried along with an orbiting body. For example, the vector could be the angular momentum of a gyroscope orbiting the Earth, as carried out by the Gravity Probe B experiment. The geodetic effect was first predicted by Willem de Sitter in 1916, who provided relativistic corrections to the Earth–Moon system's motion. De Sitter's work was extended in 1918 by Jan Schouten and in 1920 by Adriaan Fokker. It can also be applied to a particular secular precession of astronomical orbits, equivalent to the rotation of the Laplace–Runge–Lenz vector.

<span class="mw-page-title-main">GRAIL</span> NASA orbiter mission to the Moon (2011–2012)

The Gravity Recovery and Interior Laboratory (GRAIL) was an American lunar science mission in NASA's Discovery Program which used high-quality gravitational field mapping of the Moon to determine its interior structure. The two small spacecraft GRAIL A (Ebb) and GRAIL B (Flow) were launched on 10 September 2011 aboard a single launch vehicle: the most-powerful configuration of a Delta II, the 7920H-10. GRAIL A separated from the rocket about nine minutes after launch, GRAIL B followed about eight minutes later. They arrived at their orbits around the Moon 25 hours apart. The first probe entered orbit on 31 December 2011 and the second followed on 1 January 2012. The two spacecraft impacted the Lunar surface on December 17, 2012.

The Gravity Probe B mission timeline describes the events during the flight of Gravity Probe B, the science phase of its experimental campaign, and the analysis of the recorded data.

<span class="mw-page-title-main">Francis Everitt</span> British physicist

Charles William Francis Everitt is a US-based English physicist working on experimental testing of general relativity.

<span class="mw-page-title-main">LARES (satellite)</span>

LARES is a passive satellite system of the Italian Space Agency.

Frame-dragging is an effect on spacetime, predicted by Albert Einstein's general theory of relativity, that is due to non-static stationary distributions of mass–energy. A stationary field is one that is in a steady state, but the masses causing that field may be non-static ⁠— rotating, for instance. More generally, the subject that deals with the effects caused by mass–energy currents is known as gravitoelectromagnetism, which is analogous to the magnetism of classical electromagnetism.

Spacecraft attitude control is the process of controlling the orientation of a spacecraft with respect to an inertial frame of reference or another entity such as the celestial sphere, certain fields, and nearby objects, etc.

References

  1. 1 2 3 4 5 6 "Gravity Probe B" (PDF). NASA Facts. NASA and Stanford University. February 2005. Retrieved 17 March 2011.
  2. 1 2 3 "Spacecraft orbit: Gravity Probe B". National Space Science Data Center. 2004. Retrieved 18 January 2015.
  3. ↑ G. Hanuschak; H. Small; D. DeBra; K. Galal; A. Ndili; P. Shestople. "Gravity Probe B GPS Orbit Determination with Verification by Satellite Laser Ranging" (PDF). Retrieved 17 March 2011.
  4. ↑ "Frequently asked Questions". Gravity Probe B. Stanford University. 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?. Retrieved 14 May 2009.
  5. ↑ "Frequently asked Questions". Gravity Probe B. Stanford University. Answers to Spacecraft and Mission Operations Questions: 4. Where is the GP-B Mission Operations Center (MOC) for controlling the spacecraft in orbit?. Retrieved 14 May 2009.
  6. 1 2 3 Gugliotta, G. (16 February 2009). "Perseverance Is Paying Off for a Test of Relativity in Space". The New York Times . Retrieved 18 February 2009.
  7. ↑ Everitt, C.W.F.; Parkinson, B.W. (2009). "Gravity Probe B Science Results—NASA Final Report" (PDF). Retrieved 2 May 2009.
  8. 1 2 Everitt; et al. (2011). "Gravity Probe B: Final Results of a Space Experiment to Test General Relativity". Physical Review Letters. 106 (22): 221101. arXiv: 1105.3456 . Bibcode:2011PhRvL.106v1101E. doi:10.1103/PhysRevLett.106.221101. PMID   21702590. S2CID   11878715.
  9. ↑ Ciufolini, I.; Lucchesi, D.; Vespe, F.; Chieppa, F. (1997). "Detection of Lense–Thirring Effect Due to Earth's Spin". arXiv: gr-qc/9704065 .
  10. ↑ "Einstein's warp effect measured". BBC News. 21 October 2004. Retrieved 14 May 2009.
  11. ↑ Peplow, M. (2004). "Spinning Earth twists space". Nature News . doi:10.1038/news041018-11.
  12. ↑ "Overview of the GP-B Mission". Gravity Probe B. Stanford University. 2011. Retrieved 18 January 2015.
  13. ↑ Krogh, K. (November 2007). "Comment on 'Evidence of the gravitomagnetic field of Mars'". Classical and Quantum Gravity . 24 (22): 5709–5715. arXiv: astro-ph/0701653 . Bibcode:2007CQGra..24.5709K. doi:10.1088/0264-9381/24/22/N01. S2CID   12238950.
  14. ↑ Barry, P.L. (26 April 2004). "A Pocket of Near-Perfection". Science@NASA. Archived from the original on 12 May 2009. Retrieved 20 May 2009.
  15. ↑ Hardwood, W. (20 April 2004). "Spacecraft launched to test Albert Einstein's theories". Spaceflight Now . Retrieved 14 May 2009.
  16. ↑ Devin Powell (4 May 2011). "Gravity Probe B finally pays off". Science News. Archived from the original on 30 September 2012. Retrieved 7 May 2011.
  17. ↑ "Exciting April Plenary Talks – Saturday, 14 April". Archived from the original on 20 February 2007. Retrieved 16 November 2006.
  18. ↑ Khan, B. (14 April 2007). "Was Einstein Right" (PDF). Stanford News. Stanford University . Retrieved 14 May 2009.
  19. ↑ "Gravity Probe-B Latest News". NASA. Archived from the original on 22 January 2022. Retrieved 20 February 2011.
  20. ↑ "GP-B STATUS UPDATE — May 4, 2011". Gravity Probe B. NASA and Stanford University. NASA Headquarters Science Update/Press Conference. Retrieved 6 May 2011.
  21. ↑ Clifford M Will (17 November 2015). "Focus issue: Gravity Probe B". Classical and Quantum Gravity. 32 (22). IOP: 220301. Bibcode:2015CQGra..32v0301W. doi: 10.1088/0264-9381/32/22/220301 .
  22. ↑ Hecht, J. (20 May 2008). "Gravity Probe B scores 'F' in NASA review". New Scientist . Retrieved 20 May 2008.
  23. ↑ "Stanford's Gravity Probe B confirms two Einstein theories". Stanford News. Stanford University. 4 May 2011.
  24. ↑ L. Iorio (November 2011). "Some considerations on the present-day results for the detection of frame-dragging after the final outcome of GP-B". Europhysics Letters. 96 (3): 30001. arXiv: 1105.4145 . Bibcode:2011EL.....9630001I. doi:10.1209/0295-5075/96/30001. S2CID   118532421.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.