Sasha Buchman

The gravity-probe-b relativity gyroscope experiment: Development of the prototype flight instrument

The Gravity-Probe-B Relativity Gyroscope Experiment (GP-B) will measure the geodetic and frame-dragging precession rates of gyroscopes in a 650 km high polar orbit about the earth. The goal is to measure these two effects, which are predicted by Einsteins General Theory of Relativity, to 0.01% (geodetic) and 1% (frame-dragging). This paper presents the development progress for full-size prototype flight hardware including the gyroscopes, gyro readout and magnetic shielding system, and an integrated ground test instrument. Results presented include gyro rotor mass-unbalance values (15–86 nm) due the thickness variations of the thin niobium coating on the rotor, interior sphericities (163–275 nm peak-to-valley) of fused-quartz gyro housings produced by tumble lapping, gyro precession rates (gyroscopes at 5 K) which imply low mass-unbalance components parallel to the gyro axis (23–62 nm), and demonstration of a magnetic shielding factor of 2×1010 for the gyro readout system with one shielding component missing (the gyro rotor). All of these results are at or near flight requirements for the GP-B Science Mission, which is expected to be launched in 1995.

Advanced gravitational reference sensor for high precision space interferometers

LISA and the next generation of space-based laser interferometers require gravitational reference sensors (GRS) to provide distance measurements to picometre precision for LISA, and femtometre precision for the proposed Big Bang Observatory (BBO). We describe a stand-alone GRS structure that has the benefits of higher sensitivity and ease of fabrication. The proposed GRS structure enables high precision interferometric links in three-dimensional directions. The GRS housing provides the optical reference surface onto which the transmitted laser beam, and the independent received laser beam are referenced. The stand-alone GRS allows balanced optical probing of the distance of the proof mass relative to the housing at a power and wavelength that differ from the transmitted and received wavelengths and with picometre sensitivity without radiation pressure imbalance. The single parameter that reduces proof mass disturbance forces is the gap spacing. Optical readout allows the use of a large gap between the GRS housing and proof mass. We propose using rf-modulated optical interferometry to measure both relative displacement and absolute distance. Further we propose to use a reflective grating beamsplitter within the GRS and on the external optical bench. The reflective grating design eliminates the in-path transmissive optical components and the dn/dT related optical path effects, and simplifies the optical bench structure. Inside the GRS, a near-Littrow mounted grating enables picometre precision measurement at microwatts of optical power. Preliminary experimental results using a grating beamsplitter interferometer are presented, which demonstrate an optical sensing sensitivity of 30 pm Hz−1/2.

Kelvin probe measurements : investigations of the patch effect with applications to ST-7 and LISA

One of the possible noise sources for the space-based gravitational wave detector LISA (the Laser Interferometer Space Antenna), associated with its test masses, is that due to spatial variations in surface potential (or patch effect) across the surfaces of the test mass and its housing. Such variations will lead to force gradients which may result in a significant acceleration noise term. Another noise source is that due to temporal variations in the surface potential, which in conjunction with any ambient dc voltage or net free charge on the test mass may also produce a significant acceleration noise term. The ST-7 demonstrator mission is designed to test technologies for LISA, including the gravitational reference sensor, which contains a gold-coated gold/platinum (Au/Pt) alloy test mass, surrounded by a housing that carries the electrodes for sensing and control. We have used a Kelvin probe at the Goddard Space Flight Center to make spatial and temporal measurements of contact potential differences for a selection of materials (Au/Pt, beryllia, alumina, titanium) and coatings (gold, diamond-like carbon, indium tin oxide, titanium carbide). Our investigations indicate that subject to certain assumptions all of these coatings appear to satisfy the ST-7 requirement that patch effect spatial variations should be less than 100 mV. The data also revealed evidence of behavioural trends with pressure and possible contamination effects. Regarding temporal variations, the current accuracy of the instrument is limiting the measurements at a level above the likely LISA requirements. We discuss our results and draw some conclusions of relevance to LISA.

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.

Disturbance reduction system: testing technology for drag-free operation

The Disturbance Reduction System (DRS) is designed to demonstrate technology required for future gravity missions, including the planned LISA gravitational-wave observatory, and for precision formation-flying missions. The DRS is based on a freely floating test mass contained within a spacecraft that shields the test mass from external forces. The spacecraft position will be continuously adjusted to stay centered about the test mass, essentially flying in formation with the test mass. Any departure of the test mass from a gravitational trajectory is characterized as acceleration noise, resulting from unwanted forces acting on the test mass. The DRS goal is to demonstrate a level of acceleration noise more than four orders of magnitude lower than previously demonstrated in space. The DRS will consist of an instrument package and a set of microthrusters, which will be attached to a suitable spacecraft. The instrument package will include two Gravitational Reference Sensors comprised of a test mass within a reference housing. The spacecraft position will be adjusted using colloidal microthrusters, which are miniature ion engines that provide continuous thrust with a range of 1-20 mN with resolution of 0.1 mN. The DRS will be launched in 2007 as part of the ESA SMART-2 spacecraft. The DRS is a project within NASAs New Millennium Program.

Electrostatic charging of space-borne test bodies used in precision experiments

Space-borne physics experiments involving the measurement of small motions of test bodies are likely to be limited by disturbance forces. Of particular concern are forces arising from electrostatic charging of the test body due to interactions with particle radiation. Estimates of charging rates have been computed using Monte Carlo particle-transport codes in combination with semi-empirical particle flux models. Results are presented for the STEP and LISA geometries, and are extrapolated for GP-B. The consequences of the charging are assessed for each experiment, and a method for alleviating the problem is discussed which uses the photoemission technique already in the hardware development phase for GP-B.

UV LED operation lifetime and radiation hardness qualification for space flights

We report measurements of ultraviolet light emitting diode (UV LED) performance under conditions simulating operation in an orbiting satellite. UV LED light output maintained within less than 3% observational uncertainty, over more than 19,000 hours of operation in a nitrogen atmosphere, and over 8,000 hours operation at a pressure of less than 10-7 torr vacuum. In addition, irradiation with 63 MeV protons to a total fluence of 2×1012 protons/cm2 does not degrade the UV light output. Spectrally, the emissive center-wavelength and spectral shape are unchanged after proton irradiation within the precision of our measurement. These results qualify the UV LED operation lifetime and radiation hardness for space flights

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.

A superconducting microwave oscillator clock for use on the Space Station

The International Space Station provides an ideal platform for high precision frequency measurements, both as applied to scientific experiments and as regards basic time metrology. Consequently, a variety of clocks are being developed for flight on the Space Station, a majority of which are atomic clocks with very good stability for time periods in excess of 1,000 seconds. We plan to significantly augment the scope and capability of the ensemble of space clocks by adding to it SUperconducting Microwave Oscillator, SUMO. SUMO is a space version of the superconducting cavity stabilized oscillator, SCSO; the most precise clock developed to date for the interval range of around 100seconds. Short descriptions are given of the SUMO project and the SCSO technology and the principal disturbances causing frequency fluctuations are outlined. The planned improvements to the SCSO and preliminary results of the development work for SUMO are also highlighted.

The effects of patch-potentials on the gravity probe B gyroscopes

Gravity probe B (GP-B) was designed to measure the geodetic and frame dragging precessions of gyroscopes in the near field of the Earth using a drag-free satellite in a 642 km polar orbit. Four electrostatically suspended cryogenic gyroscopes were designed to measure the precession of the local inertial frame of reference with a disturbance drift of about 0.1 marc sec/yr-0.2 marc sec/yr. A number of unexpected gyro disturbance effects were observed during the mission: spin-speed and polhode damping, misalignment and roll-polhode resonance torques, forces acting on the gyroscopes, and anomalies in the measurement of the gyro potentials. We show that all these effects except possibly polhode damping can be accounted for by electrostatic patch potentials on both the gyro rotors and the gyro housing suspension and ground-plane electrodes. We express the rotor and housing patch potentials as expansions in spherical harmonics Y(l,m)(θ,φ). Our analysis demonstrates that these disturbance effects are approximated by a power spectrum for the coefficients of the spherical harmonics of the form V(0)(2)/l(r) with V(0) ≈ 100 mV and r ≈ 1.7.