Notice: This is an English translation of the Japanese article on August 6, 2018..

After a 300 million km journey from the Earth, Hayabusa2 arrived safely at asteroid Ryugu; a celestial target only 900m in size. A 900m target at a distance of 300 million km is the equivalent to a 6cm target at a distance of 20 thousand km. This means that arriving at Ryugu was like aiming at a 6cm target in Brazil from Japan. Extremely accurate navigation is therefore necessary, and this task fell on the shoulders of the Hayabusa2 Project's OPNAV team.

OPNAV stands for "Optical Navigation" or "光学航法" (kougaku-kouhou) in Japanese. Actually, a more accurate term would be "Hybrid Navigation using Optical and Radiometric Observations", but usually this long phrase is simply referred to as "Optical Navigation". Optical Navigation is a technique to accurately estimate the trajectory of both the spacecraft and the target celestial body by imaging the celestial target with an onboard camera, and communicating with the spacecraft via radio waves. Back in May, we also used the star tracker onboard Hayabusa2 instead of the onboard camera. This was described here, in our article entitled "How the Star Tracker image of Ryugu was used for optical navigation"

After the ion engine operation for the forward cruise was completed on June 3, 2018, optical navigation using the Optical Navigation Camera - Telescopic (ONC-T) began for the approach to Ryugu. The routine for optical navigation operation is summarized in Figure 1.

• Figure 1: The work loop for optical navigation.
  Image credit: JAXA

In Figure 1, the first four teams listed on the far left accurately measure the position of Ryugu with respect to the background stars from the image captured by the ONC-T. Apart from JAXA, the ground observation team included members who were experts in asteroid observation. The researchers who were involved in this part of the optical navigation were Professor Masateru Ishiguro (Seoul National University), Drs Shin-ichiro Okumura and Seitaro Urakawa (Japan Spaceguard Association) and Dr Daisuke Kuroda (Kyoto University). These four researchers measured the position of Ryugu independently from the images and sent this data to the "navigation team".

The navigation team estimates the trajectory of the spacecraft and Ryugu based on the location data for Ryugu sent by the previous team, and the radio navigation data (range and range rate). The estimated trajectory is then passed to the "guidance team". The guidance team then designs the future trajectory of the spacecraft. The navigation team and guidance team are the "people behind the OPNAV" and we will introduce the members later.

The spacecraft trajectory created by the guidance team is sent to the "operation team" who generate the command to send to the spacecraft. The primary people in charge of the operation team at that time were Drs Go Ono, Tomohiro Yamaguchi and Naoko Ogawa from JAXA. The generated command is sent to the spacecraft and the trajectory is corrected. The onboard camera then takes a new image of Ryugu and the loop repeats.

On June 3, this work loop had been performed ten times. In other words, it took ten optical navigation cycles to guide Hayabusa2 safely to the arrival at Ryugu. Since ten loops were performed over about 25 days, this meant operations were conducted every 2 to 3 days. Figure 2 summarized the result of the optical navigation.

• Figure 2: Progression of the estimation accuracy for the orbits of the spacecraft and asteroid made using optical navigation. The accuracy in the position of Ryugu (①) and the relative position of Ryugu from Hayabusa2 (②) are indicated. The numerical values written under the spacecraft indicate the distance and relative speed of the spacecraft to the asteroid at that time. There is a 1σ error in the listed value.
  Image credit：JAXA

As you can see from Figure 2, the error in the position for Ryugu was initially 140km (that is, Ryugu's location could be 140km more or less than the measured value) and the error in the relative position of Ryugu from Hayabusa2 was 500km. However, as the optical navigation was repeatedly performed, the error gradually decreased. Ultimately, the relative position error of Ryugu from Hayabusa2 was reduced to just 0.1km.

Let's now meet the people behind the OPNAV (Figure 3).

• Figure 3: The people behind the OPNAV. This photograph was taken after the OPNAV operation TCM07 on June 23, 2018. From the left in the front row, we see Tsuda, Onishi, Oki and Kikuchi, starting from the left in the middle row is Kato, Taniguchi, Matsuoka, and from the left in the back two is Takeuchi, Miyahara, Oi, and Takao (Family name only is given with titles omitted).
  Image credit：JAXA

The OPNAV team members are Dr Hiroshi Takeuchi (JAXA), Mr Masatoshi Matsuoka, Mr Takaaki Kato, Mr Toshihiko Oi (NEC), Mr Tadashi Taniguchi, Mr Takafumi Ohnishi, Mr Masaya Nakano, Mr Nobuhiro Miyahara, Ms Chiaki Aoshima, Mr Nobuaki Fujii, Ms Tomoko Yagami (Fujitsu). In addition, the OPNAV guidance team members are Project Manager Yuichi Tsuda, Dr Shota Kikuchi, Mr Yusuke Oki and Mr Yuki Takao (JAXA).

So, what is next for the people behind the OPNAV...?

Hayabusa2 Project
2018.08.28