There are many advantages to space-based interferometry, but monolithic, single-spacecraft platforms set limits on the collecting area and baseline length. These constraints can be overcome by distributing the optical elements of the interferometer over a system of multiple spacecraft flying in precise formation, opening up new realms of angular resolution and sensitivity. While the principles of interferometry are the same as for structurally-connected systems, formation-flying interferometers must integrate a wide range of technologies to provide an optically stable platform capable of finding, tracking and measuring fringes. This paper discusses some of the key differences between formation-flying and structurally-connected interferometers, including formation configurations, controlling beam shear, station-keeping, and the importance of delay and delay rate estimation in determining the instrument sensitivity.
Proposed future formation-flying interferometer missions include the Terrestrial Planet Finder (TPF), Darwin, the Submillimeter Probe of the Evolution of Cosmic Structure (SPECS), the Stellar Imager, the Micro-Arcsecond Xray Imaging Mission (MAXIM), and its precursor, MAXIM Pathfinder. In addition, Life Finder and Planet Imager have been identified as two formation-flying missions capable of detailed characterization of habitable exo-planets. The parameters for these missions are compared and described briefly.