FIGURE 3.1 Artist’s conception of the Laser Interferometer Space Antenna, consisting of three spacecraft in a triangular configuration separated by 5 million km. LISA is aimed primarily at studying strong-field gravity, the coalescence of massive black holes in galactic nuclei, and compact binary star systems in the Milky Way. Courtesy of W.Folkner, Jet Propulsion Laboratory, California Institute of Technology.

mass, the precision thrusters, and the high-precision interferometry. The most critical area is the development of the sensor and control system to maintain the LISA spacecraft in pure inertial (drag-free) orbits. The technical challenge is to reduce the nongravitational disturbing forces (such as fluctuating electric fields and fluctuating radiation pressure from thermal gradients) on a reference mass used to guide the motion of the spacecraft containing the optical components. The concept of surrounding a reference mass by a spacecraft shell that follows its motion and shields it from nongravitational forces was tested in the TRIAD program almost two decades ago at acceleration levels approximately 10⁷ times greater than needed for LISA. The Gravity Probe B cryogenic gyro