JuRa Radar on HERA: tomography processing development and validation using scaled measurements on asteroid analog models.

Our knowledge of the internal structure of asteroids relies entirely on inferences from remote sensing observations of the surface and theoretical modeling. Is the body a monolithic piece of rock or a rubble-pile, and how high is the porosity? What is the typical size distribution of the constituent blocks? Are these blocks homogeneous or heterogeneous? Direct measurements of an asteroid’s deep interior structure are needed to better understand asteroid accretion and their dynamic evolution for science, planetary defense and exploration. In orbit Radars sounding is the most mature instruments capable of achieving the objective of characterizing the internal structure and heterogeneity [1].

This is the goal of JuRa, the Juventas radar, onboard the ESA HERA mission. JuRa is a monostatic radar, BPSK coded at 60MHz carrier frequency and 20MHz bandwidth [2], [3], [4]. The main objective of JuRa is to characterize the asteroid interior, to identify internal geological structure such as layers, voids and sub-aggregates, to bring out the aggregate structure and to characterize its constituent blocks size distribution from sub-metric to global scale. The second objective is to estimate the average permittivity and its spatial variation to retrieve information on its composition and porosity.

In less than a year, the JuRa radar will begin probing the internal structures of Dimorphos and Didymos. The instrument is in good shape, and observation strategies are currently under development. In parallel, data retrieval and tomographic processing methods are being developed within the JuRa instrument team.

 

In addition to radar signal compression and radiometric calibration, a crucial aspect of preprocessing is signal co-registration. The final processing is performed in the body-fixed frame, and the goal is to perform this geometric transformation precisely in order to ensure the phase coherence of the signal required for any radar processing. In other words, this involves determining not only the trajectory but also the system’s dynamic state (position of the spin axis, precession, etc.) with a precision better than 1 meter (lambda = 5m). Different radar processing techniques allow for refinement of the geometric information, such as autofocus, which tests phase coherence within a single acquisition sequence, or interferometric methods, which quantify the phase difference between sequences [5]. These techniques for classical Earth observation need to be adapted to the specific observational geometry of a binary asteroid.

The main processing step is SAR synthesis, which corresponds to signal backpropagation. The backpropagation in a vacuum, which does not account for differences in permittivity within the asteroid, is fast and useful for co-registration and geometric correction, but its performance remains remain limited for imaging the interior [6]. Full tomographic processing is needed to inverse the interior and consist in an iterative minimization under the constraints of a direct problem [7], [8].

Geometry correction and tomography are currently undergoing development and validation. To this end, we have developed a scaled experiment using analog models. Different models on the scale of an asteroid were produced using 3D printing with materials of controlled permittivity. These ~15 cm models were then characterized in an anechoic chamber over the 3–18 GHz frequency range. This chamber covers virtually the entire range of observation geometries at a fixed distance ~1.5m. The measured signals can then be used to validate the JuRa processing chain [9], [10]. 

In this presentation, we will first review the current status of the instrument, followed by a focus on the development and validation of the processing.  

[1]         A. Herique et al., « Direct observations of asteroid interior and regolith structure: Science measurement requirements », Advances in Space Research, vol. 62, n^o 8, p. 2141‑2162, oct. 2018, doi: 10.1016/j.asr.2017.10.020.

[2]         P. Michel et al., « The Hera Space Mission in the Context of Small Near-Earth Asteroid Missions in the Past, Present and Future », Space Sci Rev, vol. 221, n^o 5, p. 70, juill. 2025, doi: 10.1007/s11214-025-01195-1.

[3]   H. Goldberg et al., « The Juventas CubeSat in Support of ESA’s Hera Mission to the Asteroid Didymos », 2019, 33rd Annual AIAA/USU  Conference on Small Satellites, SSC19-WKIV-05

[4]         A. Herique, « JuRa: The Juventas Radar on Hera mission to probe internal structure of Didymos and Dimorphos asteroids. », SSR submitted. .

[5]         W. Carrara, Spotlight synthetic aperture radar. in Signal Processing Algorithms. Artech House, 1995.

[6]         O. Gassot, A. Herique, Y. Rogez, W. Kofman, S. Zine, et P.-P. Ludimbulu, « SPRATS: a versatile Simulation and Processing RAdar ToolS for planetary missions », in 2020 IEEE Radar Conference (RadarConf20), Florence, Italy: IEEE, sept. 2020, p. 1‑5. doi: 10.1109/RadarConf2043947.2020.9266488.

[7]         A. Dufaure, C. Eyraud, L.-I. Sorsa, Y. O. Yusuf, S. Pursiainen, et J.-M. Geffrin, « Imaging of the internal structure of an asteroid analogue from quasi-monostatic microwave measurement data - I. The frequency domain approach », A&A, vol. 674, p. A72, juin 2023, doi: 10.1051/0004-6361/202244777.

[8]         Y. Berquin, A. Hérique, Y. Rogez, W. Kofman, et S. Zine, « Internal structure imaging of Didymos with JuRa using homogeneous asteroid models », Copernicus Meetings, EPSC-DPS2025-788, juill. 2025. doi: 10.5194/epsc-dps2025-788.

[9]         L.-I. Sorsa, C. Eyraud, A. Hérique, M. Takala, S. Pursiainen, et J.-M. Geffrin, « Complex-structured 3D-printed wireframes as asteroid analogues for tomographic microwave radar measurements », Materials & Design, p. 109364, déc. 2020, doi: 10.1016/j.matdes.2020.109364.

[10]       T. Pajala, C. Eyraud, A. Hérique, J.-M. Geffrin, et S. Pursiainen, « Rubble pile asteroid radar analogue model for Dimorphos — The asteroid moon of 65803 Didymos », Acta Astronautica, vol. 246, p. 462‑476, sept. 2026, doi: 10.1016/j.actaastro.2026.04.027.