M. A. Taber

The Gravity Probe B test of general relativity

The Gravity Probe B mission provided two new quantitative tests of Einsteins theory of gravity, general relativity (GR), by cryogenic gyroscopes in Earths 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.

Development of the Gravity Probe B flight mission

Abstract 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 Einsteins 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.

Ultralow magnetic fields and Gravity Probe B gyroscope readout

Abstract We describe the generation of an ultralow magnetic field of −11 Tesla in the flight dewar of the Gravity Probe B Relativity Mission. The field was achieved using expanded-superconducting-shield techniques and is maintained with the aid of a magnetic materials control program. A high performance magnetic shield system is required for the proper function of gyroscope readout. The readout system employs a dc SQUID to measure the London moment generated by the superconducting gyro rotor in order to resolve sub-milliarcsecond changes in the gyro spin direction. In addition to a low residual dc magnetic field, attenuation of external field variation is required to be 10 12 at the gyro positions. We discuss the measurement of the dc magnetic field and ac attenuation factor and the performance of the readout system.

Flight Performance of Gravity Probe B Cryogenic System

Gravity Probe B (GP‐B) is a cryogenic and space‐based test of Einstein’s General Theory of Relativity by means of precision gyroscopes, The GP‐B spacecraft was launched into a polar orbit from Vandenberg AFB on April 20, 2004. The launch and operation of GP‐B represented the culmination of forty years of planning, technology development, hardware fabrication, and testing. The superfluid liquid helium became depleted on September 29, 2005, giving a lifetime of 17.3 months compared to the requirement of 16.5 months and a thermal model prediction of 16.6 months. The flight dewar contained 2320 liters of ∼1.8 K superfluid helium at launch and housed the science instrument consisting of four precision gyroscopes and a telescope. A porous plug phase separator effected the venting of the helium boiloff gas. This venting helium was used to operate 16 thrusters, which are the actuators that effect precision pointing on a fixed star and adjust the orbit to be drag free or close to true zero‐g.

Gravity Probe B payload verification and test program

Abstract 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

Applications of superconductivity to space-based gravitational experiments

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.

Gravity Probe B cryogenic payload

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.

Experimental techniques for gyroscope performance enhancement for the Gravity Probe B relativity mission

The Gravity Probe B relativity mission experiment is designed to measure the frame dragging and geodetic relativistic precessions in a 650 km polar orbit. We describe some of the advanced experimental techniques used to achieve the required gyroscope accuracy of between 0.05 and . The subjects discussed are: (i) the development of high-precision gyroscopes with drift rates of less than , (ii) a low-temperature bake-out procedure resulting in a helium pressure of less than at 2.5 K, (iii) a read-out system using DC SQUID magnetometers with a noise figure of at 5 mHz and (iv) AC and DC magnetic shielding techniques which produce an AC attenuation factor in excess of and a residual DC field of less than .