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The Laser Interferometer Space Antenna (LISA) is a next-generation project proposed by NASA to detect some of the weakest (and perhaps most interesting) gravitational waves. Gravitational waves are "ripples in spacetime" caused by large-scale events like galaxy coalescences, black hole collisions, and neutron star binary systems. The project uses the scientific technique of laser interferometry. Three separate spacecraft form three corners of a large triangle in space. Each of the three spacecraft establishes an independent orbit around the Sun, trailing the Earth's orbit by about 20 degrees. When a low-frequency gravitational wave passes, LISA will be able to detect slight changes in the distance between the corners of the triangle. The mission is expected to launch in 2011, with a mission life of about 5 years.
LISA uses a scientific technique called laser interferometry. Interferometry uses two or more laser 'arms' to detect tiny fluctuations in distance. Here's how it works: A laser beam from one of the spacecraft is pointed toward a detector on another spacecraft, 5 million kilometers away. The laser beam precisely measures a distance between the two spacecraft. When the distance changes, an interference pattern is formed, and LISA has detected a gravitational wave. The three arms of the spacecraft work together to confirm each other's observations, as well as to get more detailed information about the passing waveform.
Why is LISA so big? The answer is that it has to be big because gravitational waves are so weak. Gravititational waves are essentially propagating strain waves. A strain is a small change in a distance divided by that distance. LISA can measure a change in length approximately 1000 times smaller than the diameter of an atom (or about 30 times the size of the nucleus of an atom!). To measure the very small strains expected for gravitational waves, the distance between spacecraft must be very large - about 5 million km. This is approximately 13 times the distance between the earth and the moon. This observatory will be very sensitive!