We are now finally approaching the time for touchdown, but just before this, we would like to show everyone the images from the BOX-B operation that was conducted in January.
The BOX-B operation is where the altitude (distance from the surface) is kept at about 20 km while the spacecraft moves in the north-south and east-west direction to observe Ryugu from different angles (for more details.).
Previous BOX-B operations were carried out from August to September last year (see http://www.hayabusa2.jaxa.jp/en/topics/20180927e_BoxB). At that time, we observed the south pole side of Ryugu (first BOX-B observation) and the evening side of Ryugu (second BOX-B observation). This time, we performed two further BOX-B observations to view the asteroid when the Sun is in opposition (third BOX-B observation) and the north pole side of Ryugu (fourth in total BOX-B observation).
Images from opposition
The third in total BOX-B operation was conducted from January 5 ~ 12. Here, we performed something known as an opposition observation. In this observation, Hayabusa2 is in the same direction as the Sun when seen from the asteroid. In other words, if you were standing on the asteroid, you would see Hayabusa2 overlapping with the Sun. If you replace Ryugu with the Earth and Hayabusa2 with the Moon, this would be a total solar eclipse. However, at the distance of the BOX-B observations, Hayabusa2 cannot hide the Sun and would be seen as a small point projected on the Sun from Ryugu. More exactly, opposition is when the angleθ(the phase angle) between the spacecraft and Sun in Figure 1 becomes zero.
Figure 2 shows the image captured during these observations. The photograph was taken at about 19:12 JST on January 8, 2019, using the Optical Navigation Camera – Telescopic (ONC-T). The atmosphere in this shot is quite different from the photos we have published so far. Overall, the contrast is low and the image appears flat and featureless, making it difficult to see the surface conditions. This is known as the “opposition effect”; a phenomenon in which a celestial body with an uneven surface or one covered with regolith (gravel) appears brighter when the phase angle is close to zero. This occurs because as the phase angle shrinks, the many fine shadows caused by the rough surface disappear.
As you can see in Figure 2, the brightness in the vicinity of the center of Ryugu is the same as that in the peripheral regions. It is difficult to gain any kind of stereoscopic feeling of Ryugu’s surface from this image. In Japanese, we have the expression “bon no yoo na tsuki” or “the moon is like a round tray”. This is because when the Moon is full, the brightness is also equal at the edges and the center which gives it the appearance of a flat disc rather than a sphere. More professionally, this means that “there is no peripheral dimming” which is due to the opposition effect.
There is one point that shines white in this image. This is the target marker (TM-B) that was dropped to the surface of Ryugu in October last year, shining as it reflects the Sun’s light. We were able to confirm the target marker reflects the Sun’s light from a distance of 20 km away. That is exactly the site where Hayabusa2 will try to touchdown. For comparison, Figure 3 shows an image not in the direction of opposition.
As we have taken many pictures from the direction of the Sun previously, you may wonder why it is only this time that the photographs are like that shown in Figure 2. For example, Figure 4 shows an image taken with the Optical Navigation Camera – Wide angle (ONC-W1) during our touchdown rehearsal, TD1-R3, when the target marker was dropped.
You can see a black dot near the center of Figure 4, which is the shadow of the spacecraft. Where this shadow is present, the phase angle (θ in Figure 1) is zero. The region around the shadow has brightened as a result of the opposition effect. However, as you move outwards from the shadow position, you can see surface features on the asteroid as normal. This is because the phase angle has become larger further from the shadow position, so no opposition effect appears.
The photograph in Figure 4 was taken when Hayabusa2 was close to the asteroid. But during this BOX-B observation, the phase angle is close to zero over the entire asteroid as we are imaging from a distance of about 20 km (the maximum phase angle reaches about 1.5 degrees). This is the reason why the entire asteroid appears featureless in this image due to the opposition effect.
Images near the north pole
The fourth BOX-B operation took place between January 18 ~ 31 where we observed the north pole of Ryugu. In the BOX-B operation performed last year from August to September, we observed Ryugu’s south pole region, so this time we moved to observe around the north pole. The captured image is shown in Figure 5.
Figure 5 shows a diagonally imaged photograph of Ryugu, captured by moving the spacecraft towards the direction of the north pole. The upper side of the image shows the north pole and reveals a landscape dominated with many large boulders. The white band extending to the left and right slightly below the center of the image is the equatorial ridge (Ryujin Ridge). The arrow tip marks the planned touchdown site and you can see this site is on the main ridge.
This is the first time we have images the northern hemisphere of Ryugu. In this observation, we acquired data on the equatorial region of Ryugu, the southern and northern hemisphere. Imaging the entire area is very important for creating accurate global shape models for Ryugu.
*：Please use the displayed credit when reproducing these images. In the case where an abbreviated form is necessary, please write "JAXA, University of Tokyo & collaborators".