Imaging of the internal structure of an asteroid analogue from quasi-monostatic microwave measurement data

Abstract

Context.The internal structure of small Solar System bodies (SSSBs) is still poorly understood, although it can provide important information about the formation process of asteroids and comets. Space radars can provide direct observations of this structure.Aims.In this study, we investigate the possibility to infer the internal structure with a simple and fast inversion procedure applied to radar measurements. We consider a quasi-monostatic configuration with multiple measurements over a wide frequency band, which is the most common configuration for space radars. This is the first part (Paper I) of a joint study considering methods to analyse and invert quasi-monostatic microwave measurements of an asteroid analogue. This paper focuses on the frequency domain, while a separate paper focuses on time-domain methods.Methods.We carried out an experiment in the laboratory equivalent to the probing of an asteroid using the microwave analogy (multiplying the wavelength and the target dimension by the same factor). Two analogues based on the shape of the asteroid 25143 Itokawa were constructed with different interiors. The electromagnetic interaction with these analogues was measured in an anechoic chamber using a multi-frequency radar and a quasi-monostatic configuration. The electric field was measured on 2372 angular positions (corresponding to a sampling offering complete information). We then inverted these data with two classical imaging procedures, allowing us to reach the structural information of the analogues interior. We also investigated reducing the number of radar measurements used in the imaging procedures, that is both the number of transmitter-receiver pairs and the number of frequencies.Results.The results show that the 3D map of the analogues can be reconstructed without the need for a reference target. Internal structural differences are distinguishable between the analogues. This imaging can be achieved even with a reduced number of measurements. With only 35 well-selected frequencies over 321 and 1257 transmitter-receiver pairs, the reconstructions are similar to those obtained with the entire frequency band.