Hayabusa2 is continuing to approach asteroid Ryugu using optical navigation. Figure 1 shows Ryugu photographed by the ONC-T (Optical Navigation Camera - Telescopic) on June 10, 2018, from a distance of about 1500km. The celestial body shining brightly in the center of the image is Ryugu. Compared with images taken on June 6, the position in the sky (relative to background stars) where Ryugu is visible has changed. The astronomical magnitude of Ryugu is now -5.7 mag.
Figure 2 shows a photograph taken at approximately the same time but using an exposure of about 0.09 seconds. Here, only Ryugu is imaged as a point and the background stars are now too faint to be seen. In expanding the section of image that contains Ryugu, the asteroid can be seen to have a diameter of about 5 or 6 pixels but this is still not sufficient to see the shape. However, from this image is does seem that Ryugu is not elongated like Itokawa.
Hayabusa2 will continue to draw closer to Ryugu. Please keep following us for future images!
Does it seem strange that in the caption of the ONC images we refer to the "ground observation team"? Primarily before the launch of the spacecraft, the ground observation team observed Ryugu (which was then referred to as 1999 JU3) and other potential celestial bodies using ground-based telescopes. The team were responsible for providing important data for selecting prospective candidates for the Hayabusa2 mission. After Ryugu was selected as the destination for Hayabusa2, the team observed Ryugu (to get light curve and spectrum observation data) and used this to investigate the physical properties of the asteroid.
The images from ONC are now being used by the ground observation team to accurately measure the position of Ryugu based on the location of the background stars in the captured image. In addition, they also create images for the public that we can all see. Members of the ground observation team are led by Principal Investigator, Professor Masateru Ishiguro at Seoul National University and consist of about 30 researchers from many domestic and international organizations. In addition to Professor Ishiguro, Dr. Daisuke Kuroda from Kyoto University and Drs Shin-ichiro Okumura and Seitaro Urakawa from the Japan Spaceguard Association work on the ONC image analysis.