The LISA mission comprises three identical spacecraft positioned 5 million kilometres apart in an equilateral triangle. LISA is a giant Michelson interferometer in space with a third arm to give independent information on the two polarisations of gravitational waves and for redundancy. LISA will observe gravitational radiation in the range 10-4 to 10-1 Hz, which covers most of the interesting sources of gravitational radiation, namely massive black holes and galactic binaries.
Each spacecraft contains two optical assemblies oriented at an angle of 60 degrees. The two assemblies on one spacecraft point towards an identical assembly on the other two. In this way, the three spacecraft form two independent Michelson interferometers, providing redundancy. A 1 Watt laser beam (1064 nm wavelength) is transmitted to the corresponding remote spacecraft via a 40 cm-aperture f/1 telescope. The same telescope is used to focus the very weak beam (approximately 10-12 W) coming from the distant spacecraft and to direct the light to a sensitive photodetector (a quadrant photodiode), where it is superimposed with a fraction of the original local light.
At the heart of each assembly is a vacuum enclosure containing a free-flying polished platinum-gold 40 mm cube - the proof mass - that serves as the optical reference (mirror) for the light beams. A passing gravitational wave will change the length of the optical path between the proof masses of one arm of the interferometer relative to the other arm. The distance fluctuations are measured to a precision of 4x10-11 m (averaged over 1 second) which, when combined with the large separation between the spacecraft, allows LISA to detect relative displacements caused by gravitational waves down to a level in the order of Δλ/λ =10-23 in 1 year of observation with a signal-to-noise ratio of 5.
The spacecraft serves mainly to shield the proof masses from the adverse effects of solar radiation pressure so that they follow a purely gravitational orbit. Although the position of the spacecraft does not enter directly into the measurement, it is nevertheless necessary to keep all spacecraft moderately accurately centred on their respective proof masses to reduce spurious local noise forces. This is achieved by a drag-free control system consisting of an accelerometer (or gravitational sensor) and a system of µN thrusters.
Capacitive sensing in three dimensions measures the displacements of the proof masses relative to the spacecraft. These position signals are used in a feedback loop to command Field Emission Electric Propulsion (FEEP) thrusters to follow the proof masses precisely. As a reference point for the drag-free system, one or the other mass (or any point between) can be chosen. The FEEP thrusters are also used to control the attitude of the spacecraft relative to the incoming optical wave fronts using signals derived from the quadrant photodiodes.
Although the spacecraft shields its proof masses from non-gravitational forces, cosmic rays and solar flare particles can cause a significant charging of the proof masses. A discharging system, consisting of a fibre-coupled UV light source, will therefore be operated at regular intervals.