On the basis of the recent observations and operations in the vicinity of asteroid Ryugu by the Hayabusa2 spacecraft, the project team have decided to postpone the touchdown (TD) from the end of October this year (2018) to after January next year.
The upcoming main mission operations in 2018 will be:
・October, 14 - 15: TD1-R1-A（equivalent to the second TD rehearsal）
・October, 24 - 25: TD1-R3（equivalent to the third TD rehearsal）
・Late November to December: operation for solar conjunction
The exact timing of the first touchdown will be considered during the operation period of solar conjunction, based on the results found during the rehearsal operations up to TD1-R3.
This decision to postpone touchdown is based on two main reasons. The first involves the improved understanding of the surface conditions on Ryugu by the operations conducted so far, and the second is the increased knowledge of the precision of the navigation guidance of the spacecraft.
As we discovered upon arrival at the asteroid, the surface of Ryugu is covered with numerous boulders with no wide, flat areas. The landing candidate sites of L08, L07 and M04 were therefore selected as locations offering a relatively safe region for touchdown on this difficult surface. Further examination of these candidate sites revealed that based on the data so far, a section of L08 seemed to be the best spot. The project team named this area L08-B (Figure 1).
In the area where the spacecraft will touchdown, it is dangerous to have boulders with a height greater than about 50cm. Since the length of the sampler horn is about 1m and the spacecraft will be to be slightly inclined during the touchdown, there is a possibility that if a boulder with a height above about 50cm is present, it will strike the main body of the spacecraft or the solar panels. Viewed from the position in Figure 2, there is no boulder larger than 50cm in the area L08-B. L08-B is the widest part within all the candidate sites without a boulder larger than 50cm.
The difficulty is that area L08-B is only about 20m in diameter. Originally, it was assumed that a safe region for touchdown would be a flat area with a radius of about 50m (100m in diameter). This has now become a radius of just 10m; a fairly severe constraint. On the other hand, during the descent to an altitude of about 50m during the MINERVA-II1 and MASCOT separation operations, we were able to confirm that the spacecraft can be guided within a position accuracy of about 10m for a height 50m above the surface of Ryugu (Figure 3). This is a promising feature for touchdown.
Although the spacecraft can be controlled with a position error of 10m at an altitude down to 50m, there remains the question of whether this accuracy can be retained as the spacecraft descends to the surface. This must be confirmed before touchdown operations. Therefore, the touchdown itself will be postponed until next year, during which time we will have two touchdown rehearsals; TD1-R1-A and TD1-R3.
The rehearsals are designed to check the accuracy of navigation guidance at altitudes less than 50m. For this check, the characteristics of the LRF (Laser Range Finger: used to measure the altitude in near distance) need to be tested. Originally, this was planned for the first touchdown rehearsal (TD1-R1: September 10 – 12, 2018). However, during this operation the LIDAR (laser altimeter) had an issue that the mode did not changed from the far mode to the near mode and the operation of the LRF could not be confirmed. The LIDAR was confirmed to have no issue in subsequent operations, leaving only the LRF to be tested. This will be performed with TD1-R1-A.
In TD1-R3, the item check list will depend on the results of TD1-R1-A. If TD1-R1-A has successfully confirmed the characteristics of the LRF, then the measurement results from the LRF will be used during control of the TD1-R3 operation and if possible, a target marker (TM) will be separated and the spacecraft confirmed to be able to properly track the target marker.
The results of these operations will be investigated in detail to plan the first touchdown. Furthermore, data from the surface of Ryugu taken by the MINERVA-II1 rovers and MASCOT lander will also provide important information for the touchdown. Although the first touchdown schedule is changed, we have enough time margins in our schedule, and our aim is a more reliable and safe operation through a comprehensive study of all the sources of information obtained so far during the Hayabusa2 project.
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