Pre-Phase A Report - Lisa - Nasa

44 Chapter 2 Different Ways of Detecting Gravitational Waves
Figure 2.3 Conceptual design of the Gravitational Wave Interferometer
(GWI). Left: GWI structure after deployment in low-Earth orbit. Right:
Schematic of the interferometer system.
shielded proof masses in three widely separated drag-free spacecraft, using laser transponders
rather than mirrors at the ends of the interferometer arms to greatly reduce the shot
noise. Both, the separated spacecraft and monolithic approaches were mentioned in a 1976
committee report by Weiss et al. [73] and described by Weiss in a 1979 paper [74]. A more
complete discussion of a possible separated spacecraft interferometer with 1×106 km arm
lengths was given by Decher et al. [75] in 1980, but it required frequent major adjustments
to the orbit of one spacecraft and had other disadvantages.
The first suggestion of a mission using spacecraft orbits similar to those planned for LISA
was made in 1981 by Faller and Bender [76, 77]. It included the appraoch suggested by
Faller of using the apparent changes in length of one arm to determine the laser phase
noise, and then correcting the arm length difference based on the measured laser phase
noise. A full description of this concept, then tentatively named the Laser Antenna for
Gravitational-radiation Observation in Space (LAGOS), was given by Faller [78].
LAGOS had already many elements of the present-day LISA mission. It consisted of three
drag-free satellites, one master spacecraft in the center and two auxilliary spacecraft at
adistanceof106km from the central spacecraft, forming a triangle with an angle of
120◦ at the central spacecraft (see Figure 2.4). This configuration would be placed in
a circular heliocentric orbit at 1 AU from the Sun, about 4×107 km (15◦ ) ahead of the
Earth. With 100 mW laser power and 50 cm diameter telescopes for transmitting and
receiving the laser beams a strain sensitivity of δl/l =10−19 over the frequency range
from 10−4 to 10−1 Hz appeared feasible. The proof masses in the spacecraft were thought
to be cylinders of about 15 cm in length and diameter, freely floating inside a housing
of 25 cm. Displacements of the housing by 10 µm with respect to the proof masses would
3-3-1999 9:33 Corrected version 2.08