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This is the first of five connected papers detailing progress on the Gravity Probe B (GP-B) Relativity Mission. GP-B, launched 20 April 2004, is a landmark physics experiment in space to test two fundamental predictions of Einstein’s general relativity theory, the geodetic and frame-dragging effects, by means of cryogenic gyroscopes in Earth orbit. Data collection began 28 August 2004 and science operations were completed 29 September 2005. The data analysis has proven deeper than expected as a result of two mutually reinforcing complications in gyroscope performance: (1) a changing polhode path affecting the calibration of the gyroscope scale factor C g against the aberration of starlight and (2) two larger than expected manifestations of a Newtonian gyro torque due to patch potentials on the rotor and housing. In earlier papers, we reported two methods, ‘geometric’ and ‘algebraic’, for identifying and removing the first Newtonian effect (‘misalignment torque’), and also a preliminary method of treating the second (‘roll-polhode resonance torque’). Central to the progress in both torque modeling and C g determination has been an extended effort on “Trapped Flux Mapping” commenced in November 2006. A turning point came in August 2008 when it became possible to include a detailed history of the resonance torques into the computation. The East-West (frame-dragging) effect is now plainly visible in the processed data. The current statistical uncertainty from an analysis of 155 days of data is 5.4 marc-s/yr (∼14% of the predicted effect), though it must be emphasized that this is a preliminary result requiring rigorous investigation of systematics by methods discussed in the accompanying paper by Muhlfelder et al. A covariance analysis incorporating models of the patch effect torques indicates that a 3–5% determination of frame-dragging is possible with more complete, computationally intensive data analysis.

Summary Thermal demagnetization of the natural magnetization of Carboniferous basalts from Scotland has successfully revealed the original direction of magnetization, although the specimens had badly scattered directions at room temperature.

The Gravity Probe B experiment was developed to test two predictions of General Relativity; the Geodetic and the frame-dragging precessions of a mechanical gyroscope due to the gravitational field of the Earth. This space-based, cryogenic experiment was carried into orbit on 20 April 2004 atop a Boeing Delta II rocket. On-orbit operations consisted of 4.3 months of experiment setup, 11.6 months of science data collection, and 1.4 months of post-science calibrations. Analysis of the science data is now in progress, scheduled to complete in 2007.

STEP is one of a number of missions now being developed to take advantage of the quiet space environment to carry out very sensitive gravitational experiments. Using pairs of concentric free-falling proof-masses, STEP will be able to test the equivalence principle (EP) to a sensitivity at least five orders of magnitude better than currently achievable on ground. The EP is a founding principle of general relativity and STEP is the most sensitive experiment of this type planned so far, aiming at 1 part in 1018. 2007 Published by Elsevier Ltd on behalf of COSPAR.

We describe two space based gravitational physics experiments, the Gravity Probe B Relativity Mission (GPB) and the Satellite Test of the Equivalence Principle (STEP). GP-B will perform precision tests of two independent predictions of general relativity, the geodetic effect and frame dragging. STEP will provide a precision test of a foundation of general relativity, the Equivalence Principle.

The Equivalence Principle states that gravitational mass and inertial mass are identical quantities. However, modern theory cannot reconcile gravity with the other three fundamental forces found in nature. One possibility is that the present theory of gravity, rooted in general relativity, is incomplete or incorrect. As a consequence of this, the Equivalence Principle may be violated at some level beyond the one part in ∼1012 that has been experimentally verified. The objective of the Satellite Test of the Equivalence Principle (STEP) is to extend the range of experiment to one part in 1018. This is accomplished by allowing two concentric, cylindrical test masses to “fall” around the Earth in a drag-free satellite. The test masses are cooled to < 2K and are supported by frictionless, superconducting, linear bearings. Relative motion between the test masses is measured by ultra-sensitive SQUID position detectors. The consequences of a confirmed violation will be to provide key information toward the possible unification of the four fundamental forces. A null result may rule out several competing theories and set better limits for new theories. Of the several STEP missions studied, MiniSTEP is the least expensive mission to date that still achieves the basic scientific goals. It uses four pairs of test masses (four differential accelerometers) to compare at least four different materials. An existing superfluid helium dewar, containing the four differential accelerometers, will be used along with a small, semi-production spacecraft to form the drag free satellite. The satellite will be placed in a 400 km, sun-synchronous, Earth orbit by a small commercial launch vehicle early in 2002. The nominal mission lifetime is four months.

Few facts in science are more surprising and none has had a longer history than the apparent equivalence of the two kinds of mass in physics, gravitational and inertial. From Galileo and Newton to Eotvos and Einstein, it has been a compelling issue both theoretically and experimentally. Ground-based tests have now a precision of about 1 part in 10 12 . Even with this extraordinary agreement, there are profound theoretical reasons for carrying the measurements further. Our generation has the unique oppurtunity to make an advance of a factor of a million in testing the Equivalence Principle in space.

Most of the Flight Payload hardware for the Gravity Probe B Relativity Mission is currently being manufactured. The design, fabrication, and integration of this hardware has already been subjected to an extensive program of full scale prototyping and testing in order to provide maximum assurance that the payload will meet all requirements. Full scale prototyping is considered to be a crucial aspect of the payload development because of the complexity of the payload, the stringency of its requirements, and the necessity for integration of a warm cryostat probe into a dewar maintained at liquid helium temperature. This latter requirement is derived from the fact that the dewar contains a superconducting ultralow magnetic field shield which provides an ambient magnetic field environment for the probe of

The Equivalence Principle, which states that gravitational mass and inertial mass are different ways of measuring the same property, is the experimental foundation for modern gravitational theory. Modern theory cannot reconcile gravity with the other fundamental forces. One possibility is that the present theory of gravity, general relativity, is incomplete or incorrect. If this is the case, the Equivalence Principle may be violated at some level beyond the one part in 1012 that has been experimentally verified. The goal of the Satellite Test of the Equivalence Principle (STEP) is to improve this result to better than one part in 1018. In STEP two or more concentric, cylindrical test masses “fall” around the Earth in a drag-free satellite. The masses are cooled to < 2K and supported by frictionless, superconducting, linear bearings. Ultra-sensitive SQUID position detectors measure their relative motion. A confirmed violation could provide key information toward the possible unification of the four fundamental forces; conversely, a null result can rule out competing theories or constrain new theories. MiniSTEP is the least expensive mission proposed to date that still achieves STEP's basic scientific goal. It compares four different materials in four differential accelerometers. Cost saving is achieved by operating the accelerometers sequentially, thereby avoiding duplicated equipment. The drag-free satellite will be built around an existing superfluid helium dewar and a small, semi-production spacecraft. A small commercial launch vehicle will place the satellite in a 400 km, sun-synchronous, polar, Earth orbit. Nominal mission lifetime is four months - but a six to eight month life is expected.

Gravity Probe B is an experiment to measure the geodetic and frame-dragging precessions, relative to the “fixed” “stars”, of a gyroscope placed in a 650 km altitude polar orbit about the earth. For Einstein's general relativity, the precessions are calculated to be 6.6 arcsec/yr for the geodetic precession and 0.042 arcsec/yr for the frame-dragging precession. The goal of the experiment is to measure these precessions to better than 0.01% and 1%, respectively. This paper gives an overview of the experiment and a discussion of the flight hardware development and its status. This paper also includes an estimate of the geodetic and frame-dragging errors expected for the experiment.

The Satellite Test of the Equivalence Principle (STEP) will test the equality of fall of objects in Earth orbit to an accuracy approaching one part in 108 by measuring the difference in rate of fall of test cylinders in cryogenic differential accelerometers in a drag-free satellite. This paper describes the current baseline design and principles used in the design of the STEP mission.

Peter Galison's Image and Logic is an admirable and question-raising book. The questions concern two distinct human activities: the enterprise we call physics and the enterprise we call history of science. Galison's theme is the evolution of particle detectors from C. T. R. Wilson's cloud chamber (1895, 1911) and Hans Geiger's discharge counter (1908, 1913) to their giant multimillion-dollar offspring of today. Image and Logic is Galison's demarcation of two competing traditions: a "mimetic" exactness imaging trajectories of individual particles, and a "logical" exactness analyzing large numbers of particle events. As time went on and financial stakes rose, the two merged to yield hybrid devices such as the Time Projection Chamber (TPC) developed for the Stanford Linear Accelerator Center. It was a transition from little to big physics inconceivable earlier. Midway came the amazingly productive "cottage industry" that Cecil Powell built on the nuclear emulsion technique of Marietta Blau. Galison's subtitle, "A Material Culture of Microphysics," points to the shifts, intellectual and social as well as material, affecting physicists immersed in this ever-changing world.

The most demanding goal of the Gravity Probe B Relativity Mission (GP-B) is the measurement of the parametrized post-Newtonian parameter γ to one part in 105. This goal requires a total experimental accuracy of ≤ 0.044 marcsec/yr. Analysis of and results from 100,000 hours of gyroscope operation on the ground show that the residual Newtonian drift will be < 0.17 marcsec/yr for a supported gyroscope in 10−9 m/s2, and < 0.020 marcsec/yr for an unsupported gyroscope in a fully inertial orbit. The expected error due to gyroscope drift is thus consistent with the measurement goal. The main gyroscope disturbance caused by cosmic radiation is charging of the rotor. A force modulation technique allows measurement of the charge of the gyroscope rotor to about 5 pC, while bipolar charge control to 10 pC is achieved using electrons generated by UV photoemission.

The foundation of modern gravitational theory is the Equivalence Principle. General Relativity is incompatible with theories of other fundamental forces such as QED, suggesting that it is incomplete. For example, there may be additional forces coupled to baryon number or spin. In this case the Equivalence Principle may be violated below the experimentally verified level of one part in 10 12 . A violation could provide crucial information for new theories. A team of US and European scientists has assembled to do the Satellite Test of the Equivalence Principle (STEP) with the goal of improving this measurement to 1 part in 10 18 . In STEP two or more test masses “fall” around the earth in a drag free satellite. A difference in the rate of fall appears as a periodic difference in their acceleration. The test masses are cooled to less than 2K and are supported by frictionless superconducting bearings. Ultra-sensitive SQUID position sensors measure their relative motion and their common motion is removed by adjustments during acceleration maneuvers. Any Equivalence Principle signal is separated from major disturbances by rotation of the spacecraft. STEP is planned to be launched by 2004, with nominal mission lifetime of 6 months.

The NASA/Stanford Relativity Mission Gravity Probe B (GP-B) experiment will provide two extremely precise tests of General Relativity based on observations of electrically suspended gyroscopes in a satellite in a 650 km circular polar orbit around the Earth. The project is now nearing completion. Final assembly of the instrument will take place later this year and launch is scheduled for October 2000. GP-B will provide a very accurate measurement of the frame-dragging effect, with its subtle connections to gravitomagnetism and Mach's principle. In addition to measuring frame dragging to 0.3%, it will measure the geodetic effect to approximately 1 part in 105. GP-B is a controlled physics experiment where error terms such as the Newtonian drifts of gyroscopes are reduced to negligible values, and where the apparatus is under the experimenters' control.

We unify the gravitational field with its source by considering a new type of 5D manifold in which space and time are augmented by an extra dimension which induces 4D matter. The classical tests of relativity are satisfied, and for solitons we obtain new effects which can be tested astrophysically. The canonical cosmological models are in agreement with observations, and we gain new insight into the nature of the big bang. Our inference is that the world may be pure geometry in 5D.

We treat in an alternate way a problem recently considered by Beesham [1]. We find that anisotropic Bianchi I inflationary cosmologies with variable gravitational and cosmological “constants” admit de Sitter expansion at least for late times.

Gravity Probe B, launched 20 April 2004, is a space experiment testing two fundamental predictions of Einstein's theory of general relativity (GR), the geodetic and frame-dragging effects, by means of cryogenic gyroscopes in Earth orbit. Data collection started 28 August 2004 and ended 14 August 2005. Analysis of the data from all four gyroscopes results in a geodetic drift rate of $\ensuremath{-}6601.8\ifmmode\pm\else\textpm\fi{}18.3\text{ }\text{ }\mathrm{mas}/\mathrm{yr}$ and a frame-dragging drift rate of $\ensuremath{-}37.2\ifmmode\pm\else\textpm\fi{}7.2\text{ }\text{ }\mathrm{mas}/\mathrm{yr}$, to be compared with the GR predictions of $\ensuremath{-}6606.1\text{ }\text{ }\mathrm{mas}/\mathrm{yr}$ and $\ensuremath{-}39.2\text{ }\text{ }\mathrm{mas}/\mathrm{yr}$, respectively (``mas'' is milliarcsecond; $1\text{ }\text{ }\mathrm{mas}=4.848\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}\text{ }\text{ }\mathrm{rad}$).

We show that the Gravity Probe B (GP-B) and the Satellite Test of Equivalence Principle (STEP) missions, with their disturbance-free orbits and precise tracking, will facilitate determination of at least two effects of post-Newtonian gravitation. First, for Einstein's classical test of general relativity, the advance of periapse of an orbit, the GP-B determination will be 3 parts in 1000. Second, for the eccentricity of objects orbiting the Earth as it orbits the sun, the GP-B determination will be 200 times more sensitive than lunar laser ranging measurements and at least 5 times more sensitive than Lageos due to GP-B's lower orbit. Nordtvedt shows that the annual variation in the eccentricity of such an orbit is zero only if general relativity is correct in its choice of parameters in the Parameterized Post-Newtonian generalized formulation of metric theories of gravitation. This test will discriminate between relativity and other theories at a level of 6 parts in 104. The perigee advance test also provides the most sensitive available test of the exotic β parameter associated in competing theories of gravitation with the second moment of the Earth's gravitational self-energy.

We consider Einstein's equations with variable gravitational couplingG and cosmological term Λ. For a power-law time-dependence ofG, the cosmological term varies in proportion to the inverse square of the time, provided the equation of state is not that of vacuum. There is then no dimensional constant associated with Λ. For a vacuum equation of state the model is compatible with classical inflation for a wide class of functionsG(t) and Λ(t). For non-power-law behaviour ofG(t), it is possible to have a scale factor that increases exponentially without a vacuum equation of state. For this case the energy density associated with Λ decreases exponentially, while at time zero it is equal with opposite sign to the regular energy density, so there is zero total energy initially.

The Gravity Probe-B (GP-B) spacecraft, designed to test predictions of general relativity, will fly in the mid 1990s. It will carry four electrostatically suspended gyroscopes in a cryogenic environment and will have a drag-free control system to minimize disturbances on the gyroscopes. The Stanford Test of Equivalence Principle (STEP) spacecraft, to fly later, will carry a set of test masses under very similar conditions. The possibility of using differential measurements of the GP-B gyroscopes suspension forces and the STEP tests mass displacement readout to form single-axis gravity gradiometers is explored. It is shown that the noise in the suspension systems is sufficiently small in the relevant frequency range, and that enough information is collected to compensate for the spacecrafts' attitude motion. Finally, using Breakwell's flat-earth approximation, these experiments are compared to other geodesy experiments and predict the contribution they can make to the knowledge of the Earth's geopotential.

NASA’s Gravity Probe B Mission is a test of two predictions of Einstein’s General Theory of Relativity based on observations on very precise cryogenic gyroscopes in a satellite in a 650 km polar orbit about the Earth. Construction and the first round of testing of the flight payload was completed in December 1999. Of the 32 planned qualification tests 28 were passed with complete success, meeting or in several instances surpassing the program requirements. However, one test very unexpectedly revealed a problem in the thermal performance of the Dewar/Probe system which has required a significant redesigin and rework, now successfully completed. Gravity Probe B is scheduled for launch on April 1, 2002. This article reviews from the physicist's viewpoint the experience of living through a space flight program.

A spaceflight electrostatic suspension system was developed for the Gravity Probe B (GP-B) Relativity Mission's cryogenic electrostatic vacuum gyroscopes which serve as an indicator of the local inertial frame about Earth. The Gyroscope Suspension System (GSS) regulates the translational position of the gyroscope rotors within their housings, while (1) minimizing classical electrostatic torques on the gyroscope to preserve the instrument's sensitivity to effects of General Relativity, (2) handling the effects of external forces on the space vehicle, (3) providing a means of precisely aligning the spin axis of the gyroscopes after spin-up, and (4) acting as an accelerometer as part of the spacecraft's drag-free control system. The flight design was tested using an innovative, precision gyroscope simulator Testbed that could faithfully mimic the behavior of a physical gyroscope under all operational conditions, from ground test to science data collection. Four GSS systems were built, tested, and operated successfully aboard the GP-B spacecraft from launch in 2004 to the end of the mission in 2008.

The Gravity Probe B mission provided two new quantitative tests of Einstein's theory of gravity, general relativity (GR), by cryogenic gyroscopes in Earth's orbit. Data from four gyroscopes gave a geodetic drift-rate of −6601.8 ± 18.3 marc-s yr−1 and a frame-dragging of −37.2 ± 7.2 marc-s yr−1, to be compared with GR predictions of −6606.1 and −39.2 marc-s yr−1 (1 marc-s = 4.848 × 10−9 radians). The present paper introduces the science, engineering, data analysis, and heritage of Gravity Probe B, detailed in the accompanying 20 CQG papers.

The Gravity Probe B (GP-B) satellite used electrostatically suspended gyroscopes to test two predictions of general relativity. Here, we describe the satellite's proportional thrusters, which utilized boil-off helium gas for attitude and translation Control (ATC). The evolution of the design and its successful effect on orbit performance is reported.

Gravity Probe B (GP-B) was a cryogenic, space-based experiment testing the geodetic and frame-dragging predictions of Einstein's theory of general relativity (GR) by means of gyroscopes in Earth orbit. This first of three data analysis papers reviews the GR predictions and details the models that provide the framework for the relativity analysis. In the second paper we describe the flight data and their preprocessing. The third paper covers the algorithms and software tools that fit the preprocessed flight data to the models to give the experimental results published in Everitt et al (2011 Phys. Rev. Lett. 106 221101–4).

The results of the Gravity Probe B relativity science mission published in Everitt et al (2011 Phys. Rev. Lett. 106 221101) required a rather sophisticated analysis of experimental data due to several unexpected complications discovered on-orbit. We give a detailed description of the Gravity Probe B data reduction. In the first paper (Silbergleit et al Class. Quantum Grav. 22 224018) we derived the measurement models, i.e., mathematical expressions for all the signals to analyze. In the third paper (Conklin et al Class. Quantum Grav. 22 224020) we explain the estimation algorithms and their program implementation, and discuss the experiment results obtained through data reduction. This paper deals with the science data preparation for the main analysis yielding the relativistic drift estimates.

This paper gives a detailed account of the Gravity Probe B cryogenic payload comprised of a unique Dewar and Probe. The design, fabrication, assembly, and ground and on-orbit performance will be discussed, culminating in a 17 month 9 day on-orbit liquid helium lifetime.

In 1918, J. Lense and H. Thirring calculated that a moon in orbit around a massive rotating planet would experience a nodal dragging effect due to general relativity. We describe an experiment to measure this effect by means of two counter-orbiting drag-free satellites in polar orbit about the earth. For a 2 1/2 year experiment, the measurement should approach an accuracy of 1%. An independent measurement of the geodetic precession of the orbit plane due to the motion about the sun may also be possible to about 10% accuracy. In addition to precision tracking data from existing ground stations, satellite-to-satellite Doppler data are taken at points of passing near the poles to yield an accurate measurement of the separation distance between the two satellites. New geophysical information on both earth harmonics and tidal effects is inherent in this polar ranging data.

This paper describes a method of spinning an electrostatically supported cryogenic gyroscope to speeds in excess of 200 cps. The gyroscope has been designed as part of the preliminary work on a satellite experiment to test general relativity originally suggested by Schiff [1]. The gyro, shown schematically in Fig. 1, consists of a 1.5-in.-diameter quartz ball with a homogeneity of 1 part in 105, which is coated with a thin film of superconducting niobium and supported in a vacuum by three orthogonal pairs of electrodes.† The spacing between the electrodes and the ball is approximately 0.0038 cm, and the support voltage in the earth’s gravitational field is approximately 500 V rms. The use of a superconducting ball is dictated by the readout system of the gyroscope, which locates the axis by measuring the direction of the magnetic field associated with the “London moment” of a spinning superconductor [2]. An operating speed approaching 200 cps is required to give adequate drift performance and readout sensitivity. The operating speed is made somewhat lower than that in other types of precision gyroscopes in order to limit the centrifugal distortion of the ball.

The role of gravitational energy in the evolution of the universe is examined. In co-moving coordinates, calculation of the Landau-Lifshitz pseudotensor for FRW models reveals that: (i) the total energy of a spatially closed universe irrespective of the equation of state of the cosmic fluid is zero at all times, (ii) the total energy enclosed within any finite volume of the spatially flat universe is zero at all times, (iii) during inflation the vacuum energy driving the accelerated expansion and ultimately responsible for the creation of matter (radiation) in the universe, is drawn from the energy of the gravitational field. In a similar fashion, certain cosmological models which abandon adiabaticity by allowing for particle creation, use the gravitational energy directly as an energy source. ? 1995 Plenum Publishing Corporation.

According to widespread expectations, we will shortly witness a new era in experimental gravitation. More accurate measurements of classical, weak field effects will soon be performed. Examples of these classical effects are the bending of light rays by the solar mass (or its modern counterparts, based on radio propagation effects), and the relativistic perturbations of the orbit of Mercury. In addition, we will shortly probe a new domain, where predicted but not yet measured effects (e.g. the Lense-Thirring effect, gravitational waves) will provide significant new tests of general relativity and its foundations. Particularly promising are the relativistic tests in space. Some of these experiments require the use of dedicated missions (e.g. GPB, STEP, LISA), while others are part of complex missions dedicated to astronomy or space exploration in general (e.g. Cassini, BepiColombo, GAIA). The aim of this book is to provide a detailed review of the subject of experimental gravitation in space. These are the proceedings of an international advanced school which took place in 1999 in Bad Honnef. A positive aspect of this book is that both experimental techniques and theoretical background are presented side by side. Particular emphasis is given to tests of the Lense-Thirring effect and the equivalence principle, and to applications of spaceborne atomic clocks. In such a rapidly developing field, it is almost inevitable that some experimental techniques are left out, and the editors quite rightly decided to focus on only some of the topics in the field of experimental gravitation. The book starts with an excellent review by Nordtvedt on solar system tests of general relativity. The second section is dedicated to the Lense-Thirring effect, offering a very good introduction for readers unfamiliar with gravitomagnetism. On the experimental side, this part is dominated by the review by Everitt et al on the status of GPB. Also well covered is the equivalence principle (EP). We mention Haugan and Lammerzahl's review of the role of the EP in gravitational field theories. From the experimental side, Nordtvedt gives an update on the status of the LLR experiment, now able to test the validity of the EP to an accuracy of the order of 10-13, along with a confirmation of the de Sitter precession and an upper limit on the time variation of Newton's constant. In the future, experiments like STEP will be able to test the EP to an accuracy several orders of magnitude better than that currently available from LLR. Lockerbie et al give an updated review of the status of STEP, which gives a clear idea of the very challenging technical progress required in order to meet these expectations. I found the section on gravitational waves somewhat disappointing. It includes theoretical papers by Blanchet et al on the generation of gravitational radiation, and by Grishchuk on the cosmological background. However, the experimental part is very poorly covered, with a single presentation by Rudiger et al on the GEO600 ground-based laser interferometer (thus, not even a space experiment). Completely missing are contributions on space experiments based on laser interferometry (e.g. LISA) or on the Doppler technique (e.g. Cassini). The book would have certainly gained from the inclusion of a detailed discussion on the status and plans of such detectors. Finally, like other titles from Springer-Verlag's series on Lecture Notes in Physics, this book is very well produced, although it would certainly have benefited from accurate editing before going to press. Several typos were found (the most evident case being the misspelling of Thirring in the title of Part II). Cited references are extensive and very useful, although, again, they could have been produced with a single and more accurate format throughout the book. In summary, I strongly recommend this book, in particular to professionals and graduate students interested in learning some theoretical aspects of modern experimental gravitation. Giacomo Giampieri

The relativity mission, also known as gravity probe B (GP-B), uses high-precision electrostatically suspended cryogenic gyroscopes for measuring the relativistic precessions of the frame of reference in a 650 km polar orbit. A 2 K environment is used to ensure the thermal stability and to implement the readout technique based on the magnetic dipole moment generated by a rotating superconductor. Analysis and results from more than 100 000 h of gyroscope operation show that the residual Newtonian drift is less than 0.17 marcsec/yr for a supported gyroscope in 10 −9 m/s 2 , and less than 0.020 marcsec/yr for a gyroscope in a fully inertial orbit.

Gravity Probe B (GP-B) is a landmark physics experiment in space designed to yield precise tests of two fundamental predictions of Einstein's theory of general relativity, the geodetic and frame-dragging effects, by means of cryogenic gyroscopes in Earth orbit. Launched on 20 April 2004, data collection began on 28 August 2004 and science operations were completed on 29 September 2005 upon liquid helium depletion. During the course of the experiment, two unexpected and mutually-reinforcing complications were discovered: (1) larger than expected 'misalignment' torques on the gyroscopes producing classical drifts larger than the relativity effects under study and (2) a damped polhode oscillation that complicated the calibration of the instrument's scale factor against the aberration of starlight. Steady progress through 2006 and 2007 established the methods for treating both problems; in particular, an extended effort from January 2007 on 'trapped flux mapping' led in August 2007 to a dramatic breakthrough, resulting in a factor of ~20 reduction in data scatter. This paper reports results up to November 2007. Detailed investigation of a central 85-day segment of the data has yielded robust measurements of both relativity effects. Expansion to the complete science data set, along with anticipated improvements in modeling and in the treatment of systematic errors may be expected to yield a 3–6% determination of the frame-dragging effect.

The Gravity Probe B (GP-B) experiment, under development at Stanford University for over twenty years, is designed to measure with high precision two extremely important effects of Einstein’s general theory of relativity as observed with gyroscopes in Earth orbit. These are respectively the frame-dragging effect exerted by a spinning body (the Earth) on an angular momentum vector (the set of gyroscopes), and the relativistic “geodetic” effect resulting from the motion of the gyroscope in its orbit through the curvature in space-time produced by the Earth’s mass. Because the frame-dragging effect is very small — in this case, only 0.042 arcseconds per year as defined with respect to the line of sight to a distant guide star — we must meet several stringent conditions in order to measure it to the 1% precision or better that is the goal of the experiment. In particular we need make the Gravity Probe B spacecraft “drag free” and need carry on it a GPS (Global Positioning System) receiver for precise orbit tracking. In doing this in a satellite of relatively low altitude (650 km) in polar orbit around the Earth, we create a system that will also, as an unexpected “coexperiment”, improve knowledge of the Earth’s gravity field by a factor of 100 to 1000.

Techniques based on superconductivity are crucial in providing the means of achieving the high accuracy and low noise required by experimental tests of gravitational theories. We discuss applications of superconductivity to two space-based experiments: the Gravity Probe B Relativity Mission (GP-B), and the Satellite Test of the Equivalence Principle (STEP). Superconducting shields attenuate the dc magnetic field to less than 10−11 T and provide an ac shielding factor in excess of 1012. The readout of the GP-B gyroscopes is based on the London magnetic dipole generated by a rotating superconductor and detected with state-of-the-art dc SQUIDs, which are also used in STEP.