(ed.). Gravitational waves (IOP, 2001)(422s).

120 LISA: A proposed joint ESA–NASA gravitational-wave missionare the changes with time in the distances between test masses in the differentspacecraft. Capacitive sensors on the inside of a housing surrounding each testmass sense the position of the spacecraft with respect to the test mass. Thisposition information is fed via a proportional control servosystem to micronewtonthrusters that keep the spacecraft essentially fixed with respect to the test mass.Each test mass plus the housing around it and the associated electronics willbe referred to as a ‘free mass sensor’. The names ‘inertial sensor’, ‘drag-freesensor’, and ‘disturbance reduction sensor’ also have been used frequently, buteach choice has some drawbacks. ‘Inertial sensor’ causes confusion with thegyroscopic sensors used in inertial navigation systems. ‘Drag-free sensor’ or‘disturbance reduction sensor’ emphasizes the role of the sensor in permittingexternal forces on the spacecraft to be accurately compensated for, but do notindicate the importance of keeping the spurious accelerations of the test mass farbelow the residual accelerations of the spacecraft.At frequencies below roughly 3 mHz, the threshold sensitivity of the LISAantenna will be determined mainly by the spurious accelerations of the testmasses, despite the care that is taken to minimize such disturbances. However,at higher frequencies the main limitation will come from how well the relativechanges in the 5000 000 km distances between the test masses can be measured.The optical interferometry system for measuring these distances will be describedin the next section. The free mass sensors and the way they are used will bediscussed in detail in a later section.In describing the operation of the gravitational-wave antenna, it is useful todistinguish between the ‘main spacecraft’ and the ‘instrument package’. Here theinstrument package will mean the Y-shaped thermal shield and everything insideit. In addition, it will include the lasers and a thermal radiation plate they arelocated on that is attached to the bottom of the Y. With this arrangement, theheat generated by the lasers can be radiated by the radiation plate out through alarge hole in the bottom of the spacecraft to space. Everything else, including thespacecraft structure and all the other equipment mounted on it, will be referred toas the main spacecraft.Probably the most important factor in the design of the LISA mission, afterminimizing spurious forces on the test masses, is the need to keep the temperaturedistribution throughout each main spacecraft and instrument package as constantas possible. This is essential for two main reasons. One is that mass displacementsdue to local temperature changes within the desired measurement band, roughly10 −6 Hz to 1 Hz, will change the gravitational force on the test mass and look likea real signal [10, 11]. The other is that a variation in the temperature differencebetween the two sides of the housing in a free mass sensor will cause a differencein the thermal radiation pressure force on the two sides of the test mass, and thusgive a spurious acceleration.The thermal design of the LISA spacecraft can be understood on the basisof the following rough model. The main spacecraft will have some thermal timeconstant for responding to changes in the incident solar flux or in the electrical