This study aims at advancing mathematical and computational techniques for reconstructing the interior structure of a small Solar System body via Computed Radar Tomography (CRT). We introduce a far-field model for full-wave CRT and validate it numerically for an orbiting distance of 5 km using a synthetic 3D target asteroid and sparse limited-angle data. As a potential future application of the proposed method, we consider the Deep Interior Scanning CUbeSat (DISCUS) concept in which the goal is to localize macroporosities inside a rubble pile near-Earth asteroid with two small spacecraft carrying a bistatic radar.

In this paper, we investigate full-wave computed radar tomography (CRT) using a rubble-pile asteroid model in which a realistic shape (Itokawa) is coupled with a synthetic material composition and structure model. The aim is to show that sparse bistatic radar measurements can distinguish details inside a complex-structured rubble-pile asteroid. The results obtained suggest that distinct local permittivity distribution changes such as surface layers, voids, low-permittivity anomalies, highpermittivity boulders, and cracks can be detected with bistatic CRT, when the total noise level in the data is around -10 dB with respect to the signal amplitude. Moreover, the bistatic measurement set-up improves the robustness of the inversion compared to the monostatic case. Reconstructing the smooth Gaussian background distribution was found to be difficult with the present approach, suggesting that complementary techniques, such as gravimetry, might be needed to improve the reliability of the inference in practice.

We have performed an initial stage conceptual design study for the Deep Interior Scanning CubeSat (DIS-CUS), a tandem 6U CubeSat carrying a bistatic radar as the main payload. DISCUS will be operated either as an independent mission or accompanying a larger one. It is designed to determine the internal macroporosity of a 260-600 m diameter Near Earth Asteroid (NEA) from a few kilometers distance. The main goal will be to achieve a global penetration with a low-frequency signal as well as to analyze the scattering strength for various different penetration depths and measurement positions. Moreover, the measurements will be inverted through a computed radar tomography (CRT) approach. The scientific data provided by DISCUS would bring more knowledge of the internal configuration of rubble pile asteroids and their collisional evolution in the Solar System. It would also advance the design of future asteroid deflection concepts. We aim at a single-unit (1U) radar design equipped with a half-wavelength dipole antenna. The radar will utilize a stepped-frequency modulation technique the baseline of which was developed for ESA's technology projects GINGER and PIRA. The radar measurements will be used for CRT and shape reconstruction. The CubeSat will also be equipped with an optical camera system and laser altimeter to support navigation and shape reconstruction. We provide the details of the measurement methods to be applied along with the requirements derived of the known characteristics of rubble pile asteroids. Additionally, an initial design study of the platform and targets accessible within 20 lunar distances is presented.

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The applied-field magnetoplasmadynamic thruster AF-MPD ZT1 was successfully put in operation at IRS. The thruster operated in steady-state mode with Argon as propellant at low electric arc power of 6 kW. Discharge voltage and power increased as expected almost linear with applied magnetic flux density. The variation of mass flow rate ratio between anode and cathode gas towards higher cathode gas fraction showed increasing thrust and thrust efficiency at applied magnetic flux density of 0.1 T and are comparable with DLR's X13 thruster. Additionally a steady-state 100 kW AF-MPD thruster SX3 was developed, set in operation and preliminary characterized at IRS. The SX3 thruster was operated at relatively low arc powers up to 30 kW and applied magnetic flux density of 0.1 T generated by the modified ZT coil. Due to low arc current and magnetic flux level, the AF-MPD ZT1 thruster achieved thrust of 70 mN and exhaust velocity up to 10 km/s at 6 kW arc power and up to 6 % thrust efficiency. The SX3 thruster reached thrust of 362 mN and thrust efficiency more than 12 % in steady state-operation at 25 kW arc power and at applied magnetic flux density of 0.1 T. However both thrusters have been operated at limited magnetic fluxes only. For SX3 a total operation time of more than 3600 s together with 30 ignitions could be accumulated. The electrodes, however, do not show significant erosion nor a respective degradation. The performances of thrusters provide an outlook for future investigations on AF-MPD thrusters at IRS and give a hint to improvement in thrust efficiency of presented devices with the new applied-field coil, which will be manufactured in the future to produce magnetic flux densities up to 0.6 T allowing further increase in thrust efficiency up to 30 %.