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Gravity Probe B

Gravity Probe B with solar panels folded

Gravity Probe B (GP-B) is a satellite-based mission which launched in 2004. The spaceflight phase lasted until 2005, and data analysis is currently under way (as of May 2008) and may continue to into 2010[1]. Its aim is to measure spacetime curvature near Earth, and thereby the stress-energy tensor (the distribution, and especially the motion, of matter) in and near Earth, and thus to test related models in application of Einstein's general theory of relativity.

Initial results confirm the expected geodetic effect to an accuracy of about 1%. The expected frame-dragging signal is similar in magnitude to the current noise level (the noise being dominated by currently unmodeled effects). Work is continuing through September 2008, and potentially to as late as March 2010, to model and account for these sources of unintended signal, thus permitting extraction of the frame-dragging signal if it exists at the expected level.

OverviewEdit

Gravity Probe B was a relativity gyroscope experiment funded by NASA. Efforts were headed up by the Physics department at Stanford University with Lockheed Martin as the primary subcontractor.

Mission scientists view it as the second gravity experiment in space, following the successful launch of Gravity Probe A (GP-A) in 1976.

Some preliminary results were presented at a special session during the American Physical Society (APS) meeting, 14-17 April 2007. NASA initially requested a proposal for extending the GP-B data analysis phase through December 2007. The data analysis phase was further extended to September 2008, and possibly March 2010, when definitive science results on the frame dragging effect are expected.

The mission plans were to test two unverified predictions of general relativity:

The experiment planned to check, very precisely, tiny changes in the direction of spin of four gyroscopes contained in an Earth satellite orbiting at 650 km (400 statute miles) altitude and crossing directly over the poles. So free are the gyroscopes from disturbance that they provided a near-perfect space-time reference system. They were intended to measure how space and time are "warped" by the presence of the Earth, and, more profoundly, if and how much the Earth's rotation "drags" space-time around with it; the so-called frame-dragging effect or gravitomagnetism, a field generated by the rotation of Earth and similar to magnetism in electrodynamics.

Previously, only two analyses of the laser-ranging data obtained by the two LAGEOS satellites, published in 1997 and 2004, claimed to have found the frame-dragging effect with an accuracy of about 20 percent and 10 percent respectively,[2][3] whereas Gravity Probe B aims to measure the effect to a precision of 1 percent. A recent analysis of Mars Global Surveyor data has claimed to have confirmed the effect to a precision of 0.5%[4],although the accuracy of this claim is disputed[5].

The probe has also detected the so-called geodetic effect, a much larger 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 should measure this effect to an accuracy of one part in 10,000, the most stringent check on general relativistic predictions to date.

The launch was planned for April 19, 2004 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 April 20 at 9:57:23 a.m. PDT (16:57:23 UTC) the spacecraft was launched successfully. The satellite was placed in orbit at 11:12:33 a.m. (18:12:33 UTC) after a cruise period over the south pole and a short second burn. The mission lasted 16 months.

Experimental setupEdit

Einstein gyro gravity probe b

At the time, the most nearly perfect spheres ever created by humans.[6] A fused quartz gyroscope for the Gravity Probe B experiment which differs from a perfect sphere by no more than 40 atoms of thickness, refracting the image of Einstein in background.

The Gravity Probe B experiment comprises four gyroscopes and a reference telescope sighted on HR8703 (also known as IM Pegasi[7]), 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 are housed in a dewar of superfluid helium, maintaining a temperature of under 2 kelvins (−271 degrees Celsius, −456 degrees Fahrenheit). Near-absolute zero temperatures are required in order to minimize molecular interference, and enable the lead and niobium components of the gyroscope mechanisms to become superconductive.

At the time, the gyroscopes were the most nearly spherical objects ever made. Approximately the size of ping pong balls, they are perfectly round to within forty atoms (less than 10 nanometers). If scaled to the size of the earth, the tallest mountains would be 2.4 meters (eight feet) high.[8] They are composed of fused quartz and coated with an extremely thin layer of niobium. A primary concern is minimizing any influence on their spin, so the gyroscopes must never touch their containing compartment. They are held suspended with electric fields, spun up using a flow of helium gas, and their spin axes are 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 at the equator of the sky coordinates. Also important was its well understood motion in the sky, which was helped by the fact that this star emits relatively strong radio signals. As a preparation for the setup of this mission, astronomers analyzed the radio-based position measurements with respect to far distant quasars taken over the last few years to understand its motion as precisely as needed.

HistoryEdit

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 1959 and later discussed by Leonard Schiff (Stanford) in 1960 at Pugh's suggestion. It was proposed to NASA in 1961, and they supported the project with funds in 1964. This grant ended in 1977 after a long phase of engineering research into the basic requirements and tools for the satellite.

In 1986 NASA changed plans for the shuttle, which forced the mission team to switch from a shuttle-based launch design to one that is based on the Delta 2, and in 1995 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.

Mission timelineEdit

Main article: Gravity Probe B mission timeline

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

  • April 20, 2004
    • Launch of GP-B from Vandenberg AFB and successful insertion into polar orbit.
  • August 27, 2004
    • 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.
  • August 15, 2005
    • The science phase of the mission ended and the spacecraft instruments transitioned to the final calibration mode.
  • September 26, 2005
    • The calibration phase ended with liquid helium still in the dewar. The spacecraft was returned to science mode pending the depletion of the last of the liquid helium.
  • February 2006
    • Phase I of data analysis complete
  • September 2006
    • Analysis team realised that more error analysis, particularly around the Polhode motion of the gyros, was necessary than could be done in the time to April 2007, and applied to NASA for an extension of funding to the end of 2007.
  • December 2006
    • Completion of Phase III of data analysis
  • April 14, 2007
    • Announcement of best results obtained to date. Francis Everitt gave a plenary talk at the meeting of the American Physical Society announcing initial results:[9] "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." (Source: Gravity Probe B web site [10])

FutureEdit

On February 9, 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. In 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. Consequently, the date for the final release of data has been pushed back from April 2007 to December 2007, and subsequently to September 2008, and possibly March 2010. In 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 on whether any useful data could be extracted in the future to test this effect.

In June 2007, 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 was anticipated, and was thought to have been controlled for before the experiment, it is now known that the final layer of the coating on the spheres defined two halves of slightly different 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 q1qqqqqq, where amongst other activities, data was gathered with the spacecraft axis deliberately mis-aligned for 24 hours, to exacerbate any potential problems. This data proved invaluable for identifying the effects. With the electrostatic torque modelled as a function of axis misalignment, and the Polhode motion modelled at a sufficiently fine level, it is hoped to isolate the relativity torques to the originally expected resolution.

Stanford has 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 September 2008 release. Because future interpretations of the data by scientists outside of GPB may differ from the official results, it may take several more years for of all the data received by GPB to be completely understood.

NASA reviewEdit

A review by a panel of 15 experts commissioned by NASA has recommended against extending the data analysis phase beyond 2008. They warn 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"[1].

Data Analysis after NASAEdit

NASA funding and sponsorship of the program ends on September 30, 2008, but GP-B has secured alternative funding that will enable the science team to continue working at least through December 2009.

On Friday, August 29, 2008, 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

See alsoEdit

ReferencesEdit

External linksEdit

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