The far-infrared (FIR) is one of the few wavelength ranges where no astronomical data with sub-arcsec resolution exist yet. Neither of the medium-term satellite projects like SPICA, Millimetron or OST will resolve this malady. Information at high spatial and spectral resolution in the FIR, taken from atomic fine-structure lines, highly excited CO, and especially from water lines would, however, open the door for transformative science. This calls for interferometric concepts. We present first results of our feasibility study IRASSI (Infrared Astronomy Satellite Swarm Interferometry) for a FIR space interferometer. Extending on the principal concept of the ESPRIT study, it features heterodyne interferometry within a swarm of 5 satellite elements. The satellites can drift in and out within a range of several hundred meters, thereby achieving spatial resolutions of <0.1 over the whole wavelength range of 1-6 THz. Precise knowledge on the baselines will be ensured by metrology methods employing laser-based optical frequency combs, for which preliminary ground-based tests have been designed by us. We first show how the science requirements translate into operational and design parameters. We have put much emphasis on the navigational aspects of such a free-flying satellite swarm operating in relatively close vicinity. We hence present work on the formation geometry, the relative dynamics of the swarm, and aspects of our investigation towards attitude estimation. Furthermore, we discuss issues regarding the real-time capability of the autonomous relative positioning system, which is an important aspect for IRASSI where, due to the large raw data rates expected, the interferometric correlation has to be done onboard. We also address questions regarding the spacecraft architecture and how a thermomechanical model is used to study the effect of thermal perturbations on the spacecraft. (abridged)
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