John Hanson

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.

ST-7 gravitational reference sensor: analysis of magnetic noise sources

A next generation gravitational reference sensor is being developed by Stanford University for the disturbance reduction system (DRS). The DRS will demonstrate the technology required for future gravity missions, including the planned LISA gravitational-wave observatory. The GRS consists of a freely floating test mass, a housing, sensing electrodes and associated electronics. Position measurements from the GRS are used to fly the spacecraft in a drag-free trajectory, where spacecraft position will be continuously adjusted to stay centred about the test mass, essentially flying in formation with it. 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 GRS will have an inherent acceleration noise level more than four orders of magnitude lower than previously demonstrated in space. To achieve such a high level of performance, the interaction of the magnetized test mass with the magnetic fields produced by the spacecraft must be considered carefully. It is shown that a new noise source due to the interaction of the time-varying magnetic field gradient and the permanent dipole of the test mass must be added to the noise analysis. A simple current loop model shows that the design of the spacecraft and instrument electronics must be done with attention to the magnetic noise produced.

Ground testing and flight demonstration of charge management of insulated test masses using UV-LED electron photoemission

The UV LED mission demonstrates the precise control of the potential of electrically isolated test masses that is essential for the operation of space accelerometers and drag free sensors. Accelerometers and drag free sensors were and remain at the core of geodesy, aeronomy, and precision navigation missions as well as gravitational science experiments and gravitational wave observatories. Charge management using photoelectrons generated by the 254 nm UV line of Hg was first demonstrated on Gravity Probe B and is presently part of the LISA Pathfinder technology demonstration. The UV LED mission and prior ground testing demonstrates that AlGaN UV LEDs operating at 255 nm are superior to Mercury vapor lamps because of their smaller size, lower draw, higher dynamic range, and higher control authority. We show flight data from a small satellite mission on a Saudi Satellite that demonstrates AC charge control (UV LEDs and bias are AC modulated with adjustable relative phase) between a spherical test mass and its housing. The result of the mission is to bring the UV LED device Technology Readiness Level (TRL) to TRL 9 and the charge management system to TRL 7. We demonstrate the ability to control the test mass potential on an 89 mm diameter spherical test mass over a 20 mm gap in a drag free system configuration. The test mass potential was measured with an ultra high impedance contact probe. Finally, the key electrical and optical characteristics of the UV LEDs showed less than 7.5 percent change in performance after 12 months in orbit.

UV LED charge control of an electrically isolated proof mass in a Gravitational Reference Sensor configuration at 255 nm

Data from a satellite mission will show that compact, low-power AlGaN Ultraviolet LEDs operating at 255 nm are effective for precise control of the potential of an electrically isolated proof mass with applications in gravitational reference sensors.

mSTAR: Testing special relativity in space using high performance optical frequency references

The proposed space mission mini Space-Time Asymmetry Research (mSTAR) aims at a test of special relativity by performing a clock-clock comparison experiment in a low-Earth orbit. Using clocks with instabilies at or below the 1·10-15 level at orbit time, the Kennedy-Thorndike coefficient will be measured with an up to two orders of magnitude higher accuracy than the current limit set by ground-based experiments. In the current baseline design, mSTAR utilizes an optical absolute frequency reference based on molecular iodine and a length-reference based on a high-finesse optical cavity. Current efforts aim at a space compatible design of the two clocks and improving the long-term stability of the cavity reference. In an ongoing Phase A study, the feasibility of accommodating the experiment on a SaudiSat 4 bus is investigated.

mSTAR: Testing Lorentz invariance in a low Earth orbit with high performance optical frequency standards

fundamental physics test, Kennedy-Thorndike, clocks, ultra-stable cavities, iodine spectroscopy, space instrumentation