Calibration update of the NIRS3 spectrometer and global near-infrared photometric properties of asteroid (162173) Ryugu: application for the Hayabusa2# extended mission

Asteroid (162173) Ryugu, visited by the JAXA Hayabusa2 mission, was extensively observed during the proximity phase with the NIRS3 near-infrared spectrometer (1.8 – 3.2 mm) [1,2]. The radiometric calibration of NIRS3 was initially obtained from ground-based measurements of a commercial blackbody. After the two touchdown (TD) maneuvers of the Hayabusa2 spacecraft, the instrumental response of the NIRS3 instrument was altered likely due to contamination by dust particles. Thorough analysis of all the Ryugu data obtained during the proximity phase showed systematic residual variations between data obtained during mission phases, indicating the need for further refinement of the radiometric calibration. The data obtained after TD1 were found to be about 5% brighter than the data before TD1, and the data after TD2 about 10% brighter. After improving the relative calibration of the NIRS3 dataset at the different mission phases, we also check and improve the absolute calibration of the instrument using the Moon observations acquired in December 2015 during the Hayabusa2 Earth gravity assist. Calibration method is as follows: (i) we used the photometrically-corrected radiance (i₀=30 deg, e₀=0 deg, α₀=30 deg) lunar map derived from Kaguya/SP and Chandrayaan-1/M³ observations [4,5]; (ii) we applied the ROLO correction factor [3], which we updated to apply it to the SP data up to 2053 nm; (iii) from the NIRS3 footprint position on the Moon, we computed the average SP and M³ spectra and projected them to the NIRS3 observation geometries using their respective photometric model [4,5]; (iv) we finally derived a scale factor to match the two lunar models and the NIRS3 data in the wavelength range 1820 – 2053 nm. NIRS3 reflectance was underestimated by about 8% compared with the original calibration [1]. The updated calibration will support the future observations of the asteroids (98943) Torifune and 1998 KY26, the two targets of the Hayabusa2# extended mission.

These new radiometrically corrected data enabled us to derive updated global near-infrared photometric properties of Ryugu, using all data from the proximity phase acquired between June 2018 to November 2019. After removing observations acquired with extreme illumination/observation geometries (i, e > 70 deg) and performing a 3D averaging binning (i, e, α), we derived the global disk-resolved phase curves and fitted these phase curves with the Hapke IMSA model for all wavelength channels, hence obtaining the variations of the Hapke parameters with the wavelength. The obtained parameters with this full dataset are consistent with previous studies [6,7]. An east-west dichotomy of the phase ratio and phase reddening is observed, likely consistent with a higher surface roughness in Ryugu eastern hemisphere.

The photometric parameters are used to produce photometrically corrected (i₀=30 deg, e₀=0 deg, α₀=30 deg) data. These radiometric and photometrically corrected NIRS3 data will be released through NASA PDS and JAXA DARTS.

Acknowledgments: This work is supported by the JAXA Hayabusa2# International Visibility Enhancement Project.

References: [1] Kitazato et al. (2019), Science, 364, 6437 [2] Iwata et al. (2017), SSR, 208 [3] Kouyama et al. (2016), PSS, 124 [4] Yokota et al. (2011), Icarus, 215, 2 [5] Besse et al. (2013), Icarus, 222 [6] Domingue et al. (2021), PSJ, 2:178 [7] Pilorget et al. (2021), Icarus, 355