On June 3, 2018, ion engine operation was completed and the final approach to the asteroid begun. By photographing the asteroid with the Optical Navigation Camera, optical navigation (precisely “hybrid navigation using optical and radiometric observations”) can be used to approach Ryugu while accurately estimating the trajectory of the spacecraft and asteroid.
Figure 1 below shows an image taken with the ONC-T (Optical Navigation Camera - Telescopic). This photo was taken on June 6 at approximately 04:15 JST from the spacecraft in the direction of Ryugu. Ryugu appears extremely bright due to a long exposure time of 178 seconds, which was used in order to image background stars. Because of this, Ryugu has become blurred and smears are generated. The brightness of Ryugu here is about -5 magnitude (a brightness scale used for stars).
In order to image Ryugu properly, a shorter exposure time is needed. Figure 2 shows a photograph taken at approximately the same time with an exposure time of about 0.09 seconds. On Ryugu is seen as a point, without any stars in the background. An enlargement of that part of the image is seen on the left side of Figure 2, with the image of Ryugu being about 3 pixels in diameter.
The distance from the spacecraft to Ryugu at the time the images were taken is about 2600km. In the ONC-T image, one pixel is about 22 arcseconds. Therefore, in Figure 2, the length of one pixel would be approximately 0.3km. This means we still cannot yet know the shape of the asteroid.