David Hipkins

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.

Measurement of the London moment

We report on the measurement of the London moment produced in a rotating spherical shell superconductor. In the process of qualifying gyroscopes for the Gravity Probe B Relativity Mission we investigated the question of how a magnetic moment develops in rotating superconductors. We have been able to perform experiments which show that the dipole moment appears independent of the initial rotational state of the superconductor.

The Gravity Probe B electrostatic gyroscope suspension system (GSS)

A spaceflight electrostatic suspension system was developed for the Gravity Probe B (GP-B) Relativity Missions 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 instruments 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 spacecrafts 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.

Patch effect in drag-free satelites

To compensate for the non–gravitational orbital disturbances drag free satellites monitor and control their position with respect to a reference body enclosed inside their structure. The body, shielded from the environment, follows a free fall trajectory when its motion can be ideally considered decoupled from that of the spacecraft. Lessons learned from Gravity Probe B and the design of the Satellite Test of the Equivalence Principle experiment strongly motivate the study of the force and torque between the reference body and the spacecraft due to uneven distributions of electrostatic potentials. Additional interest to that comes also from prospective space experiments as Microscope and the Laser Interferometer Space Antenna.

Flux flushing for the GP-B gyroscopes

We conduct several experiments aimed at reducing the amount of magnetic flux trapped in superconducting gyroscopes. Most of experiments are conducted in an ambient field of 0.2±0.1 μG. We notice that the amount of flux trapped in the superconducting gyroscopes are sensitive to the thermal cycling procedures. We report our observations and discuss its implication for the Gravity Probe B Experiment.