The Laser Interferometer Space Antenna consists of three spacecraft orbiting the sun in a triangular configuration, separated from each other by five million kilometers. Each spacecraft will contain freely falling "proof masses" protected from all other forces except for gravity. The relative motion of the masses can be measured by combining laser beams shining between the spacecraft. Passing gravitational waves will ripple space and time, revealing their presence by altering the motion of the proof masses. Click on image to view large resolution. Credit: NASA