  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)
