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The IRASSI mission, aimed at studying pre-biotic conditions in Earth-like planets, is composed of five free-flying telescopes operating around the Sun-Earth L2 point. To address the challenge of finding suitable relative positions in three-dimensional space which satisfy science goals over long periods of time, a reconfiguration procedure has been devised. The method assigns optimized post-maneuver positions of the individual telescopes for the next to-be-observed target. A mesh-adaptive direct search algorithm is selected for the optimization and two cost functions are analyzed: a fuel-based and a Δ V - based one, both pursuing a balanced use of these resources across the fleet. The effect of simulation variables such as initial wet mass and fuel mass balance, thruster settings and cost-function weighting parameters is evaluated with respect to overall used fuel and fuel balance, Δ V and Δ V balance and maneuver duration. Results show that for wet-mass-imbalanced cases, cost functions should be tuned differently for optimal fuel and Δ V management. This is not necessarily the case for homogeneous or fuel-mass imbalanced cases. The analysis also suggests that the Δ V -based function produces more consistent optimal Δ V -balancing potential across different sets of initial conditions and shows less variability in individual maneuvers than the fuel-based function.

IRASSI is an astronomy mission aimed at observing stellar disks and protoplanetary regions using a constellation of five free-flying telescopes operating around the Sun-Earth/Moon L2 point. In these regions of the cosmos, important chemical/physical phenomena can be observed in the far-infrared, which may advance our current understanding of how pre-biotic conditions in Earth-like planets are formed. The distinguishing aspect of the IRASSI interferometer, however, is its free-flying concept. Rather than relying on a fixed formation configuration and relative position control, IRASSI relies on continuously drifting baselines (e.g., inter-satellite distances) and the knowledge of these baselines during the science observations. However, not all IRASSI targets require the same formation strategy and therefore the planning of the sequence of observation tasks and of the corresponding reconfiguration and science maneuvers has been investigated. The manuscript provides thus an overview of the planning architecture devised for IRASSI, called iSCOUT, composed of different modules: 1) Target Planner Module (TPM), 2) Reconfiguration Module (ReM), 3) Collision- and invisibility Avoidance Module (CAM), 4) Baseline Pattern Module (BPM). The first module (TPM), generates an optimized sequence of targets within a specified window period, taking into account variables such as visibility of the targets at each time step, retargeting time based on the current state, target priority or number of previously observed targets of the same type. The second module, ReM, assigns positions to the telescopes based on the target sequence generated by the TPM. Each translation maneuver is optimized to achieve a specified initial formation configuration, which is both perpendicular to the target direction and three-dimensional. Parameters such as fuel use and fuel-balance as well as ΔV and ΔV-balance are used for optimizing the reconfiguration of the formation. The third module, CAM, ensures that the telescopes following the ReM trajectories do not violate relative proximity restrictions or intervisibility requirements during the reconfiguration. In such an event, the CAM computes a new path in order to avoid collisions and invisibility instances. The last module, BPM, calculates the drifting trajectories that achieve the required patterns in the so-called ‘(U, V)-plane’ during the science observations. (U, V)-plane projection metrics and several operational constraints are used for the optimization of the trajectories. Each module has been individually optimized and the paper describes the main outcomes thereof. Results concerning and end-to-end iSCOUT simulation for the IRASSI mission are presented and future work goals complete the paper.

With recent advances in robotics and space technology for the exploration of our Solar System, innovative mission concepts are being proposed to further extend our knowledge of our neighbouring planets and their satellites. VaMEx, abbreviation for Valles Marineris Explorer, is an example of such an initiative. VaMEx aims to survey the canyons of the Valles Marineris, for the duration of one year. A cluster of robotic elements is tasked with the planetary exploration activities. Due to the hazardous environment, it is required that these elements navigate autonomously, broadcast position information and exchange science data during the operational phase of the mission. To support these functions, a VaMEx space segment composed of one (or more) orbiter satellite(s) is proposed. Three possible swarm localization schemes were investigated: 1) radio positioning, 2) cartographic triangulation of ground features from optical images and 3) cartographic triangulation of radar persistent scatters. For each of the proposed navigation concepts, a suitable orbit constellation solution was assessed. Each solution was evaluated in terms of orbit stability, control effort and communications performance using metrics specific to each of the three concepts (length of pass, visibility gaps, geometry constraints, etc.). With the described navigation concept and mission analysis results, a preliminary operational architecture for the VaMEx space segment is conceived, taking into account existing technologies, communications performance, overall mission complexity and feasibility.

37 pags., 12 figs., 1 tab., The far-infrared (FIR) regime is one of the wavelength ranges where no astronomical data with sub-arcsecond spatial resolution exist. None of the medium-term satellite projects like SPICA, Millimetron, or the Origins Space Telescope will resolve this malady. For many research areas, however, information at high spatial and spectral resolution in the FIR, taken from atomic fine-structure lines, from highly excited carbon monoxide (CO), light hydrides, and especially from water lines would open the door for transformative science. A main theme will be to trace the role of water in proto-planetary discs, to observationally advance our understanding of the planet formation process and, intimately related to that, the pathways to habitable planets and the emergence of life. Furthermore, key observations will zoom into the physics and chemistry of the star-formation process in our own Galaxy, as well as in external galaxies. The FIR provides unique tools to investigate in particular the energetics of heating, cooling, and shocks. The velocity-resolved data in these tracers will reveal the detailed dynamics engrained in these processes in a spatially resolved fashion, and will deliver the perfect synergy with ground-based molecular line data for the colder dense gas., Open Access funding enabled and organized by Projekt DEAL. The investigations by H.L., D.B., L.B., R.F. and M.L. have been funded by the German Space Agency (DLR) under FKZ 50NA1816– 19. H.B. acknowledges support from the European Research Council under the Horizon 2020 Framework Program via the ERC Consolidator Grant CSF-648505. H.B. also acknowledges support from the Deutsche Forschungsgemeinschaft in the Collaborative Research Center (SFB 881) “The Milky Way System” (subproject B1). J.R.G. thanks the Spanish MICIU for funding support under grant AYA2017-85111-P. V.O.-O. and U.G. were supported by the Collaborative Research Centre 956, sub-projects C1 and D2, funded by the Deutsche Forschungsgemeinschaft (DFG), project ID 184018867

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, 43 pages (preprint format), 10 figures, 6 tables; in press, to appear in Advances in Space Research, Special Issue on Space-VLBI (eds. Leonid Gurvits et al.)

The far-infrared (FIR) regime is one of the few wavelength ranges where no astronomical data with sub-arcsecond spatial resolution exist yet. Also medium-term satellite projects like SPICA, Millimetron or OST will not resolve this malady. For many research areas, however, information at high spatial and spectral resolution in the FIR, taken from atomic fine-structure lines, from highly excited CO and especially from water lines would open the door for transformative science. These demands call for interferometric concepts. We present here first results of our feasibility study IRASSI (Infrared Astronomy Satellite Swarm Interferometry) for an FIR space interferometer. Extending on the principal concept of the previous study ESPRIT, 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

IRASSI is an interferometry-based mission concept, composed of five free-flying telescopes orbiting the Sun-Earth/Moon second Lagrange point, L2. IRASSI aims to observe particular regions of the sky, where Earth-like planets are likely to form, such as stellar disks and cold dust clouds. In these regions, important chemical and physical phenomena observable in the far-infrared frequencies (1 to 6 THz) can be recorded, enabling scientists to study star formation and evolution processes, as well as early planetary origins. Such phenomena must be resolved finely by the telescopes, with the angular resolution goal set to 0.1 arcsec or lower. To achieve such granular resolution magnitudes, the interferometer relies on dynamically changing baselines (i.e., physical separation) during the scientific observations. This is coupled with heterodyne detection, which collects and correlates detected signals of a celestial object from all telescopes simultaneously. With such a setup, the telescopes can combine their detected signals at different locations, improving the obtained angular resolution of the observations. In order to precisely correlate the detected far-infrared signals, the baseline vectors between the telescope reference points must be determined with an accuracy of 5 μm. This requirement poses a challenge to existing navigation technology. To overcome this challenge, a ranging system is employed to provide the distance measurements at micrometer-level accuracy between ranging sensors located at each satellite. A further important task is the determination of the lever arms (between the ranging sensor and the telescope reference point) and its orientation in space. Since the position estimation at L2 cannot be provided in absolute coordinates with such high accuracy, the navigation concept consists of two interacting components: (I) the absolute position estimation, with respect to Earth; and (II) the relative position estimation, determining satellite's positions, with respect to each other. For relative positioning, a geometric approach is currently under development, because it is capable of real-time processing. In future work, a semi-kinematic approach is aimed for, i.e., process dynamics is included, in order to increase robustness in case of measurement outliers or missing measurements. For absolute positioning, the Earth-based navigation method has been selected due to its potential to satisfy high accuracy expectations for the initialization of the relative position algorithms. The paper begins with a brief description of the IRASSI mission, where the issue of baseline accuracy determination is introduced. The Nominal Operations phase of the mission is presented in detail, with a focus on formation dynamics. A description of the navigation concepts, including the absolute and relative position estimation, is thereafter provided. The absolute and relative position estimation is analyzed in terms of their dynamic models, observation models and estimation methods. Furthermore, their performance relative to the requirements and overcoming challenges are discussed and compared in terms of their response time and accuracy. The key challenges in the frame of baseline measurement accuracy is then highlighted. Finally, the content of the paper is summarized and the scope of future work concludes the manuscript.

Space-based interferometry has gained prominence in recent years, largely because higher spatial resolutions of celestial observations can be achieved with multi-telescope formations compared to those achieved with a fixed-aperture, single telescope. IRASSI is a space interferometer composed of five spacecraft, whose aim is to observe particular chemical and physical processes in cold regions of space, such as dust clouds and stellar disks, in the far-infrared frequencies. Ultimately, the goal is to study the genesis of planets, star formation and evolution processes in these cold regions and to understand how prebiotic conditions in Earth-like planets are created. IRASSI will orbit the second Lagrange point, L2, of Sun-Earth/Moon system. The operating principle of IRASSI is based on free-drifting baselines, which dynamically change during the observations and measure therefore the incoming wavefront of a celestial target at different locations in space. This process relies on very accurate measurements of the baselines - at micrometer level - rather than on precise control of the formation. Naturally, a free-flying formation comes with a set of challenges, namely identifying a nominal formation geometry, that is, a suitable dispersion of the telescopes in three-dimensional space. In addition, understanding how this free-drifting geometry is expected to change is crucial, particularly if this may affect the operation of the telescope instruments and thus the quality of the final synthesized images. The present paper describes therefore the major requirements for establishing a desired formation geometry and proposes a preliminary nominal formation for the observations. The relative dynamics of the free-drifting spacecraft are modeled and evaluated. Final considerations regarding formation control are presented and the paper concludes with a summary of the work and outlook for the future.

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.