Gravity Probe B

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) altitude, crossing directly over the poles.

Gravity Probe B
Mission typeAstrophysics
OperatorNASA/Stanford University
COSPAR ID2004-014A
SATCAT no.28230
Websiteeinstein.stanford.edu
Mission duration17.5 months[1]
Spacecraft properties
ManufacturerLockheed 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
RocketDelta II 7920-10C
Launch siteVandenberg SLC-2W
End of mission
DisposalDecommissioned
Deactivated8 December 2010 (2010-12-09)
Orbital parameters
Reference systemGeocentric
RegimeLow Earth
Semi-major axis7,027.4 km (4,366.6 mi)
Eccentricity0.0014[1]
Perigee altitude641 km (398 mi)[2]
Apogee altitude645 km (401 mi)[2]
Inclination90.007º[1]
Period97.65 minutes[3]
Epoch UTC[2]
 

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 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 with solar panels folded.

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 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 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] However, Lorenzo Iorio claimed that the level of total uncertainty of the tests conducted with the two LAGEOS satellites has likely been greatly underestimated.[13][14][15][16][17][18] A recent analysis of Mars Global Surveyor data has claimed to have confirmed the frame dragging effect to a precision of 0.5%,[19] although the accuracy of this claim is disputed.[20][21] Also the Lense–Thirring effect of the Sun has been recently investigated in view of a possible detection with the inner planets in the near future.[22][23]

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.[24] 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.

The Gravity Probe B experiment comprised four London moment gyroscopes and a reference telescope sighted on HR8703 (also known as IM Pegasi), a binary star in the constellation Pegasus. In polar orbit, with the gyro spin directions also pointing toward HR8703, 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.[25] 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.

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 in history that a university has been in control of the development and operations of a space satellite funded by NASA.

Total cost of this project was about $750 million.[26]

Mission timeline
Main article: Gravity Probe B mission timeline

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:[27] "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."[28]
GP-B spacecraft decommissioned, left in its 642 km (399 mi) polar orbit.[29]
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.[30]
Publication of GP-B Special Volume (Volume #32, Issue #22) in the peer-reviewed journal, Classical and Quantum Gravity.[31]

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.

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".[32]

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 ensuing SAC report to NASA states:

The progress reported at SAC-18 was truly extraordinary and we commend the GPB team for this achievement. This has been a heroic effort, and has brought the experiment from what seemed like a state of potential failure, to a position where the SAC now believes that they will obtain a credible test of relativity, even if the accuracy does not meet the original goal. In the opinion of the SAC Chair, this rescue warrants comparison with the mission to correct the flawed optics of the Hubble Space Telescope, only here at a minuscule fraction of the cost.

— SAC #18 Report to NASA

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.[33] 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.[34]

See also

References
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