Instruments

LISA Technology Package

The LTP represents one arm of the LISA interferometer, in which the distance between the two proof masses is reduced from 5 million kilometres to 35 centimetres. As in LISA, the proof masses fulfil a double role: they serve as mirrors for the interferometer and as inertial references for the drag-free control system.

Illustration of the LTP

The LTP drag-free control system consists of an inertial sensor, a proportional micro-propulsion system and a control loop. The two identical proof masses, each one a 46 mm cube, are housed in individual vacuum cans. The displacement of the cubes with respect to their housing is measured by capacitive sensing in three dimensions. These position signals are used in a feedback loop to command proportional micro-propulsion thrusters to enable the spacecraft to remain centred on the proof mass. Field Emission Electric Propulsion (FEEP) thrusters will be used as actuators.

Although the proof masses are shielded from non-gravitational forces by the spacecraft, cosmic rays and solar flare particles can significantly charge them, leading to electrostatic forces that can corrupt the measurements by the inertial sensor. A system of fibre-coupled UV lamps will discharge the proof masses at regular intervals. As surface effects on the proof masses can cause electrostatic forces, the proof masses have to be coated very carefully to avoid contamination. In order not to damage the coating during launch, a caging mechanism is used to maintain the proof masses in a safe position during launch.

This caging mechanism is particularly challenging: it is required to hold the mass with a very large force during the launch (3000 N), without damaging its surfaces, and needs to be able to release the test mass at the right position, with high accuracy (60 µm) and very steadily (release speed less than 5 µm s^-1, or 18 mm per hour!) in order to allow the electrostatic suspension system to take over the mass control once on orbit.

The proof mass can also be disturbed by the presence of a very small amount of gas inside the enclosure. If the gas pressure is higher than 10^-5 Pa, equivalent to the pressure of the near Earth outer space, a combined effect of this residual pressure with a small temperature unevenness would produce the so called radiometer effect which would disturb the experiment. The vacuum enclosure therefore needs to be designed and built with special getter pumps capable of maintaining the required vacuum level for the entire mission.

Using two proof masses, the reference point for the drag-free system can be chosen to be on each of the two masses or at any point between them. Having two proof masses also allows verification of the performance of the drag-free control loop by sensing the movement of the second proof mass relative to the spacecraft while the spacecraft follows the first proof mass.

The position of the proof masses, with respect to the spacecraft or each other, is measured by an interferometric system that is capable of picometre (10^-12 m) precision in the frequency band 10^-3 - 10^-1 Hz. The interferometric measurement system utilises a heterodyne Mach-Zehnder architecture, with all components rigidly bonded to an ultra-low expansion base-plate, manufactured from Zerodur. 

Disturbance Reduction System

The Disturbance Reduction System (DRS) is a NASA-supplied system, which contributes to the LISA Pathfinder mission goals and uses the European LTP. The DRS consists of two clusters of colloidal thrusters that use ionised droplets of a colloidal solution accelerated in an electric field to provide micro-propulsion, and drag-free control software residing on a dedicated computer. The DRS will use the sensor information of the LTP (test masses position and attitude) to control the spacecraft attitude with independent drag-free software and will use the colloidal thrusters as actuators.