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

Description of the LISA mission 121power dissipated in its equipment. The Y-shaped thermal shield is mountedfrom the main spacecraft by low-thermal-conductivity flexural strips or stressedfibreglass bands, and forms a second passive thermal isolation stage. The thirdstage is the support cylinder around each optical assembly and the fourth stageis the optical bench that contains the free mass sensor. Since low-thermalconductivitysupports are used between each successive stage, and with lowinfrared emissivity coatings on as much of the surfaces as possible, quite longthermal time constants can be achieved for each stage.With the above type of multistage passive thermal isolation approach, quitelow levels of temperature fluctuations can be achieved at frequencies of 0.1 mHzand higher. But there are practical limits to how long the various thermal timeconstants can be made. To achieve high thermal stability at lower frequencies,some active thermal control will be needed. Even a fairly crude servosystem forthe temperature of the Y-shaped thermal shield would help considerably, and someactive control of the temperature of the main spacecraft probably will be desirablealso. Such active temperature control systems may be added to the baselinemission design later to meet a goal of improved antenna sensitivity at frequenciesbelow 0.1 mHz, but are not included in the present mission requirements.A fundamental feature of the mission design is the high stability of the solarenergy dissipation in the spacecraft. In an ecliptic-based reference frame thatrotates once per year about the pole of the ecliptic, the three spacecraft forma nearly equilateral triangle in a plane that is tipped at 60 ◦ to the ecliptic, asdiscussed earlier. Thus, the direction to the Sun makes a nearly constant 30 ◦angle with the normal to the sunward side of each spacecraft, where the solarcells providing power for the spacecraft are located. Because of the stable solarillumination geometry, only fluctuations in the intrinsic solar intensity and inthe electrical power dissipation in the various parts of the spacecraft producesignificant temperature variations. Measures such as keeping the microwavetransmitters operating at the same power level whether data is being transmittedor not are taken to reduce thermal changes. Also, the power needed in the opticalassemblies and particularly on the optical benches is kept as low as possible, andis highly regulated.10.1.3 Optics and interferometry systemThe present parameters assumed for the LISA optical system are the use of 1 Wof output power from cw NdYAG lasers operating at 1.064 µm wavelength, andof 30 cm diameter telescopes to transmit and receive the laser beams betweendifferent spacecraft. Perhaps the most impressive thing about the mission is thatthese fairly modest parameter values lead to roughly 2 × 10 8 photoelectrons s −1being detected at the far end of a 5000 000 km arm, which would correspondto 2 × 10 −21 precision in measuring changes in the arm length in one second ifthere were no other sources of error. Since almost all expected types of LISAgravitational-wave sources can be observed for a year or more, the potential