July 7 is the Japanese star festival of Tanabata. However, the original Tanabata event was held on July 7 of the lunisolar calendar, which translates to August 25 of this year. When Tanabata is celebrated on the date corresponding to the lunisolar calendar day, the festival is called “traditional tanabata”. Here, we would like to introduce images of the Milky Way captured by Hayabusa2 on Tanabata celebrated on July 7 of the current calendar. (These images were previously shared on twitter.)

On April 6 of this year, we took a picture of the starry sky to check the Optical Navigation Camera – Telescopic (ONC-T). This is a check that is performed when possible from time-to-time, but on April 6, we were able to capture a beautiful image of the Milky Way (figure 1).

Figure 1: The Milky Way captured by the ONC-T. The image was taken on April 6, 2020 at around 09:10 JST.
(Image credit: JAXA/AIST/University of Tokyo/Kochi University/Rikkyo University/Nagoya University/Chiba Institute of Technology/Meiji University/University of Aizu).

Since this image shows the Milky Way, the frame is packed with stars. The ONC-T has a maximum exposure time of 178 seconds and the range shown in Figure 1 is approximately 6.3 sq. degrees. Although we have not checked in detail how many stars are in the photo frame, ONC-T can image stars down to a brightness of about 12 magnitudes. In this image, we used a wide band filter that captures light from the widest possible wavelength range for the camera and set the longest possible exposure time for observation. This setting is the same as that used for the first light captured from Ryugu at a distance of 1.3 million km in February 2018 (http://www.hayabusa2.jaxa.jp/topics/20180301_e/). It has been a tumultuous two years since then!

We added blue to Figure 1 to create a more artistic image (Figure 2). This is the same photograph as in Figure 1, but the atmosphere feels a little different.

Figure 2: Image of the Milky Way captured by the ONC-T (Figure 1) colored blue.
(Image credit: JAXA/AIST/University of Tokyo/Kochi University/Rikkyo University/Nagoya University/Chiba Institute of Technology/Meiji University/University of Aizu).

Do you know where in the Milky Way is shown in this picture? We posed this as a challenge to people on twitter, and an impressive number of people guessed correctly! The answer is shown in Figure3: the image shows a location near the toes of Ophiuchus.

Figure 3: Location of the region imaged by the ONC-T. The area within the □ is the range captured in Figures 1 and 2. (Created with Stella Navigator.)

A bright star can be seen in the center of the upper half of Figures 1 and 2. This is the star system, θ-Ophiuchi (へびつかい). On the left side of the observed region is Sagittarius (いて in Figure 3) and to the right is Scorpio (さそりin Figure 3). This location is close to the center of the Milky Way Galaxy and is the region in which the most stars can be seen in the Milky Way.

From the perspective of Hayabusa2, stars can be seen in all directions. It seems Hayabusa2 is wrapped in stars as it travels towards the Earth. It is almost time to arrive back home!

•This article was supported by the ONC Team’s Toru Kouyama (Principal Researcher, AIST) and Seiji Sugita (Professor, University of Tokyo, ONC Team PI).