Abstract
LISA (laser interferometer space antenna) is designed to observe gravitational waves from violent events in the Universe in a frequency range from 
to 
which is totally inaccessible to ground-based experiments. It uses highly stabilized laser light (Nd:YAG, 
) in a Michelson-type interferometer arrangement.
A cluster of six spacecraft with two at each vertex of an equilateral triangle is placed in an Earth-like orbit at a distance of 1 AU from the Sun, and 
behind the Earth. Three subsets of four adjacent spacecraft each form an interferometer comprising a central station, consisting of two relatively adjacent spacecraft (200 km apart), and two spacecraft placed at a distance of 
from the centre to form arms which make an angle of 
with each other. Each spacecraft is equipped with a laser.
A descoped LISA with only four spacecraft has undergone an ESA assessment study in the M3 cycle and the full six-spacecraft LISA mission has now been selected as a cornerstone mission in the ESA Horizon 2000-plus programme.