LISA is a space-based gravitational wave observatory that is anticipated to launch in 2034. Its constellation of three spacecraft will be located at the vertices of an equilateral triangle that will follow the Earth about the Sun. The spacecraft will be deployed in three heliocentric elliptical orbits that, to first order in their eccentricity, maintain the observatory's size and configuration. How LISA accomplishes this is an ideal illustration of basic orbital dynamics. The physics underlying the choice of orbits is explained at a level suitable for introductory undergraduate physics students.
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According to Kepler's 3rd law, the orbital period of a binary black hole is given by , where is the separation of the bodies and is their total mass. The bodies merge when , the sum of their Schwarzschild radii. The maximum orbital frequency before coalescence is therefore . Since the GW frequency , the minimum radiated wavelength is directly proportional to . Just as for the detection of radio waves, the longer the wavelength, the larger the antenna must be.