1302 days after the launch from Tanegashima Space Center on December 3, 2014, Hayabusa2 has finally arrived at the target asteroid Ryugu. The arrival time was 9:35 am JST on June 27, 2018. From here, we can begin to fully explore Ryugu.
After the end of the ion engine operations on June 3, 2018, Hayabusa2 began the final asteroid approach phase. Optical navigation was used to precisely aim for the asteroid's location. During the approach, the chemical propulsion thrusters were used to perform nine Trajectory Correction Maneuvers (TCM) to control the velocity of the spacecraft, with a tenth TCM made at the above time for arrival. After the final TCM10, the relative speed between Hayabusa2 and Ryugu was 1 cm/s or less and arrival at the asteroid was declared.
Figure 1 shows Doppler data at the time of arrival. The Doppler data can estimate the speed of the spacecraft along the line-of-sight from the Earth (i.e. the speed along a straight line connecting our planet to Hayabusa2) using the frequency of radio waves received from the spacecraft. In Figure 1, the difference between the measured value and the planned value for the orbital speed at arrival is shown before and after the final TCM. (The value plotted on the vertical axis is actually double that difference.) After the TCM, the speed difference has become nearly zero, indicating that the relative speed with respect to Ryugu is almost zero as planned. The time of the TCM in Figure 1 is not 9:35 am JST since the spacecraft is at a distance of about 1.9 astronomical units (about 280 million km), and information takes about 16 minutes to transmit.
After viewing the Doppler data, Project Manager Yuichi Tsuda checked the information (telemetry data) from the spacecraft and confirmed arrival at asteroid Ryugu at 09:54 JST. We then took a group photo to commemorate the moment (Figure 2).
Now at a distance of about 20 km, the environment of Ryugu is clearly visible. Figure 3 is the image of Ryugu captured by the ONC-T (Optical Navigation Camera - Telescopic) on June 26, the day before arrival. You can easily see the shape is like that of an abacus bead, with large and small craters and many boulders.
The images of Ryugu shared so far have shown a surface with a whitish color, as in Figure 3. But what is the actual color? The ONC-T can take images with light within a specific wavelength band using seven different filters. By carefully combining this data, a natural color image can be created. Figure 4 shows color photographs of both the Earth and Ryugu created in this same way.
As you can see from Figure 4, the actual Ryugu is a very dark object. In ground observations of the asteroid, the albedo (reflectivity of light) of Ryugu was estimated to be just 0.05, suggesting this was a dark object which the images from the spacecraft now support.
From now on, Hayabusa2 will stay near Ryugu, observe the asteroid, release the lander and rovers, touch down and perform the impact experiment. Since these mission operations differ considerably from the activities prior to arrival, we created a new mission logo for Hayabusa2 (Figure 5). The color has changed from a system of blues to pink and purple. The outermost vermilion shades represent the palace of Ryugu; the location in the Japanese folk tale of Urashima Taro that asteroid Ryugu takes its name. The inner purple is for the nobles of the palace and Princess Otohime, while the central light blue is for the princess's feathered robe. These changing colors show the enthusiasm of the Project members to explore the whole of Ryugu. The logo also shows an illustration of the asteroid, with the large craters and boulders that have now been seen on the surface.
The time for exploration has finally come. There are a variety of challenging and long operations ahead, but the plan is to thoroughly investigate asteroid Ryugu.
Our last image to show is a movie where we can watch Ryugu become steady bigger as we approach (Figure 6).
We hope you are looking forward to future exploration results!
※ If you wish to use the images here, please include the displayed credit. In the case where an abbreviated form is necessary, please use "JAXA, University of Tokyo & collaborators".