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

A detector in space 39LISA has been named a cornerstone mission of the European Space Agency(ESA), and NASA has recently formed its own team to study the same mission,with a view toward a collaboration with ESA. LISA will be sensitive in a rangefrom 0.3 mHz to about 0.1 Hz, and it will be able to detect known binarystar systems in the Galaxy and binary coalescences of supermassive black holesanywhere they occur in the universe. A joint ESA–NASA project looks verylikely, aiming at a launch around 2010. A technology demonstration missionmight be launched in 2005 or 2006.LISA’s technology is fascinating. We can only allude to the most interestingparts of the mission here. A full description can be found in the pre-Phase A studydocument [13]. The most innovative aspect of the mission is drag-free control. Inorder to guarantee that the interferometry is not disturbed by external forces, suchas fluctuations in solar radiation pressure, the mirror that is the reference pointfor the interferometry is on a free mass inside the spacecraft. The spacecraft actsas an active shield, sensing the position of the free mass, firing jets to counteractexternal forces on itself and ensure that it does not disturb the free mass. Thejets themselves are remarkable, in that they must be very weak compared to mostspacecraft’s control jets, and they must be capable of very precise control. Theywill work by expelling streams of ions, accelerated and controlled by a highvoltageelectric field. Fuel for these jets is not a problem: 1 g will be enough fora mission lifetime of ten years!LISA interferometry is not done with reflection from mirrors. When a laserbeam reaches one spacecraft from the other, it is too weak to reflect: the sendingspacecraft would only get the occasional photon! Instead, the incoming lightis sensed, and an on-board laser is slaved to it, returning an amplified beamwith the same phase and frequency as the incoming one. No space mission hasyet implemented this kind of laser-transponding. The LISA team had to ensurethat there was enough information in all the signals to compensate for inevitablefrequency fluctuations among all six on-board lasers.A further serious problem that the LISA team had to solve was howto compensate for the relative motions of the spacecraft. The laser signalsconverging on a single spacecraft from the other two corners will be Dopplershifted so that their fringes change at MHz frequencies. This has to be sensed onboard and removed from the signal that is sent back to Earth, which can only besampled a few tens of times per second.When LISA flies it will, on a technical as well as a scientific level, be aworthy counterpart to its Earth-based interferometer cousins!3.4.1 LISA’s capabilitiesIn the low-frequency LISA window, most sources will be relatively long lived,at least a few months. During an observation, LISA will rotate and change itsvelocity by a significant amount. This will induce Doppler shifts into the signals,and modulate their amplitudes, so that LISA should be able to infer the position,