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Session 71 - Space Astronomy in the Next Millennium.
Display session, Wednesday, January 17
North Banquet Hall, Convention Center
The Laser Interferometer Space Antenna (LISA) is a mission designed for detailed studies of gravitational waves with frequencies of roughly 0.0001 to 1 Hz. It is a Cornerstone mission in the proposed ESA Horizon 2000 Plus program. Investigations of possible US contributions to LISA currently are in progress under the NASA program for studies of new mission concepts in astrophysics.
LISA is a formation of 6 spacecraft in the shape of an equilateral triangle 5 million km on a side, with 2 spacecraft at each vertex. The formation is in orbit around the Sun, following about 20^\circ behind the Earth. Orbital parameters are chosen so that the distances between the spacecraft stay nearly constant for years. Each spacecraft is made drag-free by keeping it centered on a carefully shielded test mass at its center. Laser distance measurements are made between the test masses using roughly 0.5 W per beam and about 40 cm diameter telescopes. Laser phase noise can be determined by observing the apparent changes in the 5 million km arm lengths, since the real variations in the arm lengths at the frequencies of interest are very small. The desired gravitational wave signals are obtained from the corrected differences in the arm lengths.
For 1 yr of observations and a S/N of 5, the LISA sensitivity reaches its maximum of 10^-23 at frequencies of 0.001 to 0.03 Hz. This sensitivity is sufficient to detect sources such as a 5 to 10\,M_ødot black hole orbiting around and coalescing with a roughly 10^6\,M_ødot black hole at z=1. Scenarios in which such events would be observed appear to be quite plausible if 0.01% or more of the stars in the cusps around supermassive black holes in galactic nuclei are black holes. Coalescences of two supermassive black holes after galactic or pregalactic mergers also may be observed. For short period galactic binaries, hundreds to thousands of individual sources will be resolved and their distribution in the galaxy determined.
