LISA - an ESA cornerstone mission for a gravitational wave observatory

The European Space Agency has selected LISA, a gravitational wave observatory, as a cornerstone mission in its future science program Horizons 2000. This observatory will complement the development of ground-based gravitational wave detectors currently under construction. A spaceborne detector will enable the observation of low-frequency gravitational waves in a frequency range from to which is totally inaccessible to ground-based experiments. This frequency range is unique in that it is expected to contain signals from massive black holes, galactive binary stars, as well as the most violent events in the Universe. LISA will attain this low-frequency sensitivity by employing laser interferometric distance measurements over a very long baseline of . Three of these baselines form an equilateral triangle with spacecraft at each vertex. The cluster of spacecraft is in an Earth-like orbit around the Sun trailing the Earth by . The spacecraft contain infrared light-emitting Nd:YAG lasers and freely floating test masses made from a special platinum - gold alloy with vanishing magnetic susceptibility. The spacecraft are being kept centred on their test masses by using drag-free technology and field-emission electric propulsion, thus letting the test masses follow purely inertial orbits