Development of a multispectral stereo-camera system comparable to Hayabusa2 Optical Navigation Camera (ONC-T) for observing samples returned from asteroid (162173) Ryugu

TL;DR

This paper presents a multispectral stereo-camera system designed to analyze asteroid samples with spectral and 3D shape data, closely matching the capabilities of Hayabusa2's ONC-T camera, enabling detailed characterization of returned samples.

Contribution

The study introduces a novel multispectral microscopic imaging system with 3D shape reconstruction that accurately replicates the spectral bands of the Hayabusa2 ONC-T camera.

Findings

The system achieves a 3% spectral reflectance error.

The 3D shape model has a 5% error margin.

Sample measurements show spectra consistent with asteroid observations.

Abstract

Hayabusa2 collected 5.4 g of samples from the asteroid (162173) Ryugu and brought them back to Earth. Obtaining multiband images of these samples with the spectral bands comparable to those used for remote-sensing observations is important for characterizing the collected samples and examining how representative the sample is compared with spacecraft observations of Ryugu as a whole. In this study, we constructed a multiband microscopic camera system that enables both visual multispectral imaging at 390 (ul), 475 (b), 550 (v), 590 (Na), 700 (w), and 850 nm (x), and three-dimensional (3D) shape reconstruction of individual grain samples based on stereo imaging. The imaging system yields the images of 4096 x 2160 pixels with the pixel resolution of 1.93 {\mu}m/pix and field of view of 7.9 x 4.2 mm. Our validation measurements demonstrate that our multispectral imaging system, which observes the samples with the spectral bands comparable to those on the telescopic optical navigation camera (ONC-T) on Hayabusa2, yields reflectance spectra with a relative error of 3% and a 3D model with an error of 5%. These results indicate that the multiband imaging system with a 3D shape reconstruction capability yields accurate spectral and shape data on the returned samples. Using this instrument, we conducted multispectral measurements of two Ryugu samples (grains in the dishes A3 and C1) acquired from two locations on the asteroid. The average spectra of the measured Ryugu samples were flat and consistent with the global averaged spectrum of Ryugu. The 550-nm band (v-band) reflectance of the returned grains in the dishes was 2.4% on average, higher than that of the global averaged spectrum of Ryugu observed with ONC-T. This apparent difference could be because the returned grains have greater specular reflectance.