The asteroid explorer Hayabusa2 encountered the Cb-type asteroid 162173 Ryugu between June 2018 and November 2019. The Telescopic Optical Navigation Camera (ONC-T) [1, 2] onboard Hayabusa2 observed Ryugu with 7 bandpass filters ranging in wavelength of 0.40-0.95 mm. On 8 January 2019, ONC-T observed the asteroid in the opposition geometry from ~20 km distance, through one rotation period (7.6 hr) in 7-band. The local solar phase angle of each pixel ranges from 0.0° to ~1.7°. Using this dataset, we present the map of normal albedo , defined as the radiance factor (I/F) at phase angle 0°.
The data number of the raw image pixels are converted  to I/F. The observation geometry of each pixel is calculated using the Ryugu shape model . We fit a linear function to the phase function plot within the narrow phase angle range (0.2-1.7°). Using this function, each pixel’s I/F was extrapolated to normal albedo. Finally, we create a mosaic map of normal albedo for each band.
From this map, a v-band (0.55 μm) average normal albedo is derived as 4.06 ± 0.10%. This value is consistent with a geometric albedo (albedo in disk-integrated definition) of 4.0 ± 0.5%, derived from recent photometry study . The recent color studies [e.g. 1, 4, 5, 6] of Ryugu reported that the spectral slope from b-band (0.48 μm) to x-band (0.86μm) exhibits the greatest regional variation on Ryugu. Our normal albedo map also confirms this spectral character of Ryugu. Additionally, we found that the v-band normal albedo is well correlated with the spectral slope.
Since the observed brightness at the opposition condition is less affected by shadows or topographic undulation than other geometries, the derived map successfully shows the albedo distribution under minimal noise conditions. This approach may be useful for a future mission of a bumpy surface asteroids.
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