Inertial Sensor Design for the Laser Interferometer Space Antenna (LISA)

Abstract

The Engineering Model of the Inertial Sensor for the Laser Interferometer Space Antenna (LISA) mission is under development. The LISA test masses must be kept free of stray acceleration noise to within 3x10 m/s/vHz in order to obtain the low frequency gravitational wave sensitivity goal. We present here the technological solutions needed to map the scientific requirements into engineering specifications. We also include the results of thermal and gravitational analyses to support the design choices. Finally we report on the on-ground verification approach: the torsion pendulum test bench will allow extensive testing of some critical features and the measurement of the forces acting on the test mass to within an order of magnitude of the goals of the first validation mission. Introduction The Laser Interferometer Space Antenna (LISA) mission is a joint ESA/NASA mission with launch in 2011. The mission is aimed at detecting and observing gravitational waves from massive black holes and galactic binaries with periods in the range of a few seconds to a few hours. Measurements are therefore performed in the frequency range of 10 Hz to 10 Hz. The detection of gravitational waves will demonstrate the consequences of the Einstein's theory of general relativity. The measurement will be carried out using three interferometric arms forming an equilateral triangle. The LISA orbit configuration consists in fact of a constellation of three satellites, also called “spacecrafts” (S/C) in heliocentric orbit, adjusted such that the three S/Cs keep an equilateral triangle formation with 5*10Km side. Each S/C contains a pair of inertial sensors (IS). In each IS, as shown in Figure 1, there is a test mass (TM) maintained in pure geodesic motion. The TM is the end mirror of a single-arm interferometer (the other endmirror is in one of the other two S/Cs). A gravitational wave will slightly modulate the distance between TMs, which is continuously monitored by means of a laser interferometry. Figure 1 LISA layout LISA Scientific Requirements LISA top science requirements are defined in order to have the TM that falls under the effect of the large scale gravitational field only within the minimum achievable acceleration noise, relative to a free falling frame in the measurement frequency range. The LISA 54th International Astronautical Congress of the International Astronautical Federation, the International Academy of Astronautics, and the International Institute of Space Law 29 September 3 October 2003, Bremen, Germany IAC-03-Q.1.07 Copyright © 2003 by the author(s). Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. Released to IAF/IAA/AIAA to publish in all forms. 2 sensitivity performance is limited at low frequency by stray forces perturbing the TM out of their geodesics. In the long wavelength limit for the gravitational signal, and for small signals, the separation ∆x between the two end-mirror masses, of mass m, of one interferometer arm obeys the equation of motion