Direct observations of Didymos' regolith and internal structure with LFR RADAR on JUVENTAS Cubesat for ESA Hera mission

Our knowledge of the internal structure of asteroids entirely relies on inferences from remote sensing observations of the surface and theoretical modeling while this question is crucial for planetary defense. Is the body a monolithic piece of rock or a rubble-pile? How high is the porosity? What is the typical size of the constituent blocks? Are these blocks homogeneous or heterogeneous? How is constituted the regolith covering its surface? After several asteroid orbiting missions, these basic questions remain open while they are crucial to better understand asteroid accretion and dynamical evolution, to better model mechanics of granular materials in low gravity and asteroids' stability conditions, and more generally speaking to plan any interaction of a spacecraft which such bodies for planetary defense, exploration or science. Probing of the interior is also crucial for mineralogy. Therefore, surface properties as observed by optical remote sensing may not be representative of the interior mineralogy and chemical composition due to both Space weathering and thermal cycling

The Low Frequency Radar (LFR) on the JUVENTAS CubeSat for HERA / ESA mission to Didymos Binary Asteroid is a unique opportunity to access direct measurement of its internal structure. LFR has been developed and optimized to fathom asteroid from a small platform. This instrument is inherited from CONSERT/Rosetta and has been redesigned in the frame of the AIDA and HERA ESA mission. Onboard JUVENTAS, LFR is operating in monostatic mode to probe down to the first hundreds of meters of the subsurface and would achieve the full tomography of the Didymos' moonlet. - The first objective of the LFR is characterizing the moonlet interior, to identify internal structure and to characterize size distribution and heterogeneity of constitutive blocks from sub metric to global - The second objective is estimating average permittivity and monitoring its spatial variation especially in the crater area. - The same characterization applied to the main of the binary system is among secondary objectives. - Supporting shape modeling and determination of the dynamical state by radar ranging is another secondary objectives.

This paper will present the instrument concept and measurement strategy, its performances and the expected science return.