IRASSI: InfraRed astronomy satellite swarm interferometry — Mission concept and description

A current focus of modern astronomy is the characterization of the physical properties and of the chemical processes which can lead to prebiotic conditions in Earth-like planets. In order to identify such conditions, the first step is to observe regions in space which could originate Earth-like planets, such as stellar disks. The involved chemical processes are visible in the far-infrared radiation spectrum — more specifically in the spectral range of 1 to 6 THz. In order to perform observations in the far-infrared frequencies with high resolution, sophisticated instrumentation needs to be used. This spectrum is attenuated by the atmosphere and therefore can only be observed directly from space. Due to the high requirements placed on the spatial resolution, interferometry has gained popularity in recent years. Interferometric systems employ arrays of telescopes to extract information about a source with high resolution, by super-imposing electromagnetic wavefronts which are phase-shifted and measuring their interference. Such a system relies on the determination of the baseline of the telescopes with an accuracy proportional to the observed wavelength. In the far-infrared, this corresponds to accuracies in the micrometer level. This paper presents the IRASSI mission, whose aim is the observation of stellar disks and protoplanetary regions so as to understand the genesis of planets, star formation and evolution processes. IRASSI is a multidisciplinary interferometric telescope mission to the second Lagrange point, L2, of the Sun-Earth/Moon system. The constellation is composed of 5 spacecraft. The operating principle of IRASSI is that by dynamically changing the baseline distances between the spacecraft during scientific observations, one can measure the interference of the wavefronts at different locations. This technique allows the observation of the far-infrared phenomena at better resolution than that obtained with a single spacecraft. The outline of the IRASSI mission was built on precursor mission studies and concepts, such as ESPRIT and DARWIN. Unlike DARWIN, for instance, IRASSI does not require active control of the formation because it uses heterodyne detection in combination with a ranging system, which can provide inter-satellite distances with a very high accuracy. The present paper introduces therefore the mission concept of IRASSI, followed by a detailed description of the in-orbit operational concept at L2, while addressing how such mission fills in the gap of information regarding the observations in the far-infrared. The main mission analysis results obtained thus far are subsequently presented, and a hypothesized mechanical configuration is described. The key technical challenges posed by such endeavor are identified, complemented by an overview of the future work. The concluding remarks of the IRASSI study are then provided.