Observation of spurious forces
Current gravitational space projects such as GRACE and GOCE and future missions designed for measurements of gravitational fields all require, at their core, test masses that are shielded by the surrounding spacecraft from all non-gravitational disturbances. There are two fundamental modes of operation:
- Accelerometer mode, where the test mass itself is electro-statically actuated and held centered by an active control. The necessary force of actuation on the test mass is measured and corresponds to the non-gravitational perturbations.
- Drag-free mode, on the other hand, operates on the basis of a truly free-flying test mass where the spacecraft is commanded to follow the test mass, maintaining its position centered and shielded of non-gravitational noise. An array of micro-newton thrusters is used to control the spacecraft around the free-flying test mass. Drag-free conditions are more demanding but also scientifically more valuable.
In both cases the test masses must be carefully protected from all spurious, non-gravitational forces such as electrostatic surface effects, magnetic fields, gas damping force noise, etc., which must be measured in order to develop sensor systems which are free from their disturbances. The main challenge in developing and improving these systems on ground is the presence of much larger forces, in particular the Earth’s gravitational pull, in contrast to the conditions in orbit where disturbances are about 10 orders of magnitude smaller. Given the dominant effect of gravity along the vertical axis, the best approach to measure such tiny forces on ground is to choose orthogonal observation frames along one or two horizontal axes. To this end, a torsion balance is the preferred experimental framework. It is an indispensable tool for measurements at the level of femto-Newtons as required for the development and testing of novel optical gravitational sensor concepts and components for future drag-free space missions and as such it constitutes an important building block for geo-Q. We plan to construct and operate in the new HITec building a torsion balance with state-of-the-art sensitivity. It will consist of a symmetric four-arm payload suspended in vacuum from a torsion fiber made from tungsten or fused silica. Adequate choice of materials and pendulum geometry allows for the suspended test mass to be nearly free along the torsional degree of freedom such that tiny rotations will translate into quasi-free linear motions at the end of each arm. Multiple active stabilizations of the rotational operating point, the pendulum swing motion and the platform tilt are necessary and foreseen to reach sensitivities at the oder of 10−13. . . 10−14 ms−2/√Hz at mHz frequencies. The aim is to achieve an ultra-low-force environment in one and later two horizontal, translational degrees of freedom such that small spurious forces acting on test masses can be investigated, and low-noise optical sensors (A07) can be tested. In later development stages, it is foreseen that also other components, such as μN thrusters or mechanical actuators, designed for spacecraft with stringent vibration requirements can be tested. The unique feature of our torsion balance will be the use of laser interferometric readout techniques. Adopting laser interferometry as the sensing technique for the entire facility – test mass and pendulum structure – will enable an angular readout of the test mass with less disturbing interactions and lower noise than capacitive readout which is typically used. Additionally, it will allow control of the main pendulum mode and platform tilt with higher precision and high bandwidth. Highly sensitive, miniaturized optomechanical accelerometers with fiber-optical sensing, a technology to be transferred to this project and geo-Q by NIST through the Mercator fellow, will complement and further improve the sensing and control capabilities. This combined implementation of dedicated laser interferometry and highly sensitive optomechanical accelerometers is a novel approach to stabilize and record the pendulum motion and will potentially enable to overcome the limits of presently used readout techniques.
Scientists working on this project
phone: +49 511 762-17033
Presentations, Talks and Posters
Wang Q. (2016): Proceeding of the Torsion Pendulum, 2nd geo-Q General Assembly, 22 February, 2016