After the generation of the artificial crater on the surface of asteroid Ryugu using the Small Carry-on Impactor (SCI) on April 5, four operations were conducted to observe near the new crater. As was mentioned in Part 2, the third descent operation also successfully dropped a target marker at the touchdown candidate site. Finally, here we present “To go, or not to go, that is the question”.
Although the first touchdown was successful, going for a second touchdown is “the question” because touchdown is a high-risk operation. This is especially true in the case of Ryugu, which has no large, flat areas. The spacecraft therefore needs precise control to avoid a collision in rocky locations. In short, just because we have succeeded in the past does not mean we can easily do so again.
The spacecraft is operating far into space, in a harsh environment and with a communication time too long for us to correct problems if they occur. We always operate alongside the risk of failure or breakdown. Therefore, our project members will always feel uneasy about the prospect of performing a touchdown. But being vaguely anxious does not make any progress. The situation needs to be considered from a scientific and technical standpoint.
Two major issues need to be considered. The first is whether the second touchdown has significant scientific and engineering merit. If there is little extra to be gained, and as the first touchdown was already successful, there is no point in performing this twice. A second issue is the risk of the touchdown operations. If the risk is high, then the descent would be reckless.
First, let’s consider the scientific and engineering value of the second touchdown. From the observations around the site of the artificial crater, it was clear that there is ejecta from the crater in the region where the second touchdown is planned (Figure 1). In other words, if we go ahead with the touchdown, we will reliably be able to collect subsurface material from Ryugu. This is high scientific value. In addition, this would also result in samples being collected from multiple locations on the asteroid. This also adds to the scientific value as it gives more universal information about Ryugu, rather than the possibility you may have collected material from an unusual spot. From an engineering perspective, this will be the world’s first collection of samples from multiple locations and also the first sample from below the surface. This naturally means the value is high. Combined, this confirmed that the science and engineering value of a second touchdown is significant.
Next to consider is the risk of the operation. If this is too high, there is an argument that this is not a chance that should be taken. We therefore first selected places that touchdown could be performed near the artificial crater, and proceeded to collect detailed information on the topography of these touchdown candidate points during the low altitude descent operations. We also were able to drop a target marker at one of these locations. This eventually became the planned touchdown site.
The planned touchdown site is about 20m away towards the north from the artificial crater generated with the SCI. This is an area with a radius of about 3.5m, which was named C01-Cb by the project. There are dangerous boulders around the area and also substantial rock piles in C01-Cb. After estimating the height of these rocks, creating a three-dimensional map and confirming the danger during a touchdown operation, it was judged that there would not be a problem if the spacecraft were to touchdown in this region.
A further technical issue was that the amount of light received by the optical systems on Hayabusa2 (the Optical Navigation Camera – Wide angle, ONC-W1, and laser range finder, LRF) decreased during the first touchdown. This is thought to be due to dust that soared upwards at the time of touchdown and adhered to the instruments. To cope with this problem, we decided to compensate for the decrease in the amount of received light by lowering the altitude at which to switch to the affected optical system. We confirmed that this approach works well during the low altitude operations.
As a result of the above examination, it was confirmed that the risk during the second touchdown is equal or less than the risk of the first touchdown. Since the second touchdown is of high scientific and engineering value, we decided the project should perform a second touchdown to collect a sample from asteroid Ryugu. This was approved by ISAS on June 21 and by JAXA as a whole on June 25, whereupon is was decided to do a second touchdown.
The second touchdown will be attempted on July 11. We will proceed with our mission with care, but boldly go.
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