Radar tomography of asteroid deep interior. JuRa / HERA to Didymos and the Radars to Apophis: status of the instruments

Our knowledge of the internal structure of asteroids relies entirely on inferences from remote sensing observations of the surface and theoretical modeling. Many fundamental questions are still unresolved. 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. The characterization of the asteroids’ internal structure is crucial for science, planetary defense and exploration.

In orbit sounding Radars are the most mature instruments capable of achieving the objective of characterizing the internal structure and heterogeneity, both for the benefit of science as well as for planetary defense or exploration. It is the goal of JuRa on cruise to probe Didymos binary system in 2027 and aim of two instruments under development for the ESA Ramses Mission and the Caltech Mission is to study the deep interior of Apophis in 2029.

 

JuRa

JuRa, the Juventas radar, onboard the ESA HERA mission, is a monostatic radar, BPSK coded and inherited from CONSERT/Rosetta. The center frequency of 60 MHz provides the capability to probe the internal structure to a depth of 100 meters or more, with a limited vertical resolution of about 20 m. Multipass processing makes it possible to create a 3D tomographic image of the interior. HERA was launched in October 2024 to deeply investigate the Didymos binary system and in particular its moonlet Dimorphos, five years after the DART/NASA impact. On the ESA mission HERA, the Juventas 6U CubeSat is carrying the Juventas Radar (JuRa).

JuRa maps the backscatter coefficient (sigma zero - σ0) of the surface and of the subsurface, which quantifies the returned power per unit surface or volume. It is related to the degree of heterogeneity on the wavelength scale and to the dielectric contrast of heterogeneities, giving access to both, the sub-meter texture of the constituent material and larger scale structures.

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 in terms of size distribution from sub-metric to global scale. The second objective is to estimate the average permittivity and to monitor its spatial variation in order to obtain information on its composition and porosity. The characterization of the constitutive material and its spatial organization both on the main and on the moon will help to better model the formation and stability conditions of the binary system and the response of Dimorphos to the DART impact.

 

Radar to Apophis

Knowledge of the internal structure of Apophis is crucial to better understand its accretion and dynamical evolution, to better study its stability conditions, and to model its response to the gravitational constraints imposed by its approach to induced by Earth close approach.

For the RAMSES mission, the radar is operating in monostatic mode on one of the CubeSats and deeply inheriting from JuRa on Juventas/Hera, with minor modification. The radar is currently under development and under responsibility of Tyvak International, prime contractor for the CubeSat development. For the electronics (1U, 1kg), minor optimizations are proposed based on the experience from JuRa (mainly thermal optimization, adjustment of the network position and software modifications). The interfaces are revised depending on the carrying platform inheriting from Hera/Milani. For the antenna, a crossed dipole is proposed, as for JuRa on Juventas, providing measurements in full polarization. The cross dipole is consisting of 4 booms of 65g each. An optimization of the boom length and matching network will be required depending on the platform, on the antenna accommodation and on the solar array geometry.

The Caltech Mission to Apophis is under study by Caltech and JPL. For his mission, the radar is a customized version of JuRa, operating in bistatic mode: the two satellites are maintained opposite from each other around Apophis, using semi-autonomous navigation based on optical cameras. Electronics on the two platforms measure the signal transmitted throughout Apophis, like done for CONSERT on Rosetta/ESA. Using the synchronization provided by the Inter Satellite Link (ISL), the radar provides an absolute measurement of the propagation delay between the platforms through the asteroid. This provides a direct measurement of the dielectric permittivity, which is related to composition and microporosity of the inner structure of Apophis. Partial coverage will provide slices of the body with an average characterization and its spatial variability to characterize large scale structures. Dense coverage will provide a larger diversity of observation angles, the bistatic mode will then allow a complete 3D tomography to recover the permittivity contrast throughout the volume.

In addition to modifications depending on the new platform and the antenna accommodation and the adjustments based on JuRa lesson learned, the implementation of the bistatic mode requires a deeper revisit of the software and firmware to preserve synchronization of the two electronics, while the ISL is ensuring the control of the timing and the frequency.

In this talk we will present the instruments, their status, performances and goals as well as the science objectives in the context of the different targets.