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P. L. Bender (JILA, Univ. of Colorado)
The LISA antenna will operate by measuring changes in the distances between freely moving test masses inside 3 different spacecraft forming a triangle 5 million km on each side that is located 50 million km behind the Earth in orbit around the Sun. At frequencies below about 3 millihertz, the instrumental sensitivity is limited mainly by spurious accelerations of the test masses due to fluctuating forces: electrical, magnetic, gravitational, radiation pressure, molecular impact, and cosmic ray impact forces. About half a dozen of the error sources expected to give the largest effects will be discussed briefly. Redundancy and reliability are major considerations in the design of the mission, and a ten year extended mission lifetime is being designed for.
Each test mass will be a cube about 4.4 cm on a side. A housing provides a cubical cavity around the test mass with roughly 5 cm interior dimensions. Six pairs of capacitor plates on the inside of the housing and on opposite sides of the test mass permit accurate measurement of changes in the relative translation and rotation of the housing with respect to the test mass. Continuously variable micronewton thrusters buck out the solar radiation pressure force on the spacecraft and keep the housing from moving with respect to the test mass with a precision of about 3 nanometers per root Hz, or roughly 30 picometers rms over 10,000 s periods.This precise station keeping, plus the extremely stable temperature inside the spacecraft because it always keeps the same orientation with respect to the Sun, greatly reduce the fluctuating forces on the test mass.
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Bulletin of the American Astronomical Society, 35 #3
© 2003. The American Astronomical Soceity.