The first operation rehearsal for the first touchdown of Hayabusa2 was performed from September 10 - 12. The spacecraft reached an altitude of about 600 m from the surface of Ryugu and then began to ascend once again. During the operation, the images taken for navigation purposes with the Optical Navigation Camera Wide angle (ONC-W1) were released in real time. The last image captured is shown in Figure 1. This photograph was taken on September 12 at around 12:40 JST at an altitude of 635m from the surface of Ryugu.
In this image, the left-hand side (within the red circle in Figure 1b) appears brightened due to a phenomenon known as the "opposition effect". It is a phenomenon that occurs when the angle formed between the Sun - the celestial body surface - observation point (known as the "phase angle") is close to zero, resulting in the celestial body being illuminated from directly behind the observation point and creating a bright region. In the center of the bright part is a black dot but this is the shadow of Hayabusa2. The sunlight is behind the spacecraft, casting the shadow on the asteroid.
It is easier to see that this dot is the shadow of the probe in an animation. Figure 2 shows the image sequence of Hayabusa2's descent.
The animation shows Ryugu increase in size during the approach and the shadow of Hayabusa2 can be seen near the end. Notice that the shadow of Hayabusa2 does not move even as Ryugu rotates and as the distance decreases, the shadow expands. It also appears to be fixed on the screen. To understand this, please check out the "A little more detail" section below.
Hayabusa2 is about 6m in size and is casting a shadow on a roughly 900m object, 300 million km from the Earth. That this could be captured in a photograph is quite breathtaking, don't you think? Hayabusa2 may only be small, but the spacecraft is very brave!
■A little more detail
Here we explain a little more about the motion of the shadow of Hayabusa2 and opposition effect. The relative position of the celestial bodies when Figure 1 was captured is shown in Figure 3. Since Hayabusa2 is in the immediate vicinity of Ryugu, it is in the same location as the marker for Ryugu but approaches the asteroid along the line connecting Ryugu to the Earth.
If we enlarge the diagram near Ryugu, the system looks like that shown in Figure 4.
As shown in Figure 4, Hayabusa2 descends along the line connecting the Earth with the center of Ryugu. The Sun is shining diagonally from the right of the figure. As Hayabusa2 descends, the shadow of the spacecraft appears on the left side of Ryugu and moves to the right.
Since the sunlight can be thought of as parallel rays, the angle θ in Figure 4 hardly changes during the short time the spacecraft descends. This means that the direction in which the shadow can be seen remains almost the same during the descent so that it appears as if the position of the shadow is fixed in the animation shown in Figure 2.
We know that the Sun is in the opposite direction to where shadows fall and that the surroundings directly opposite the Sun appear brighter than their surroundings. This is the opposition effect. Figure 5 shows a simplified schematic of this situation. When the angle formed by the direction of the Sun's rays and the line of sight (α: phase angle) is close to zero, the light reflected by the celestial body will be at its strongest.
The opposition effect was also seen when Hayabusa explored Itokawa (Figure 6).
If we approach Ryugu still closer, we should see the shadow of Hayabusa2 clearly as with the shadow of Hayabusa in Figure 6. This may be what we see on our next approach.