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Raumfahrt - JAXA´s Asteroid Explorer Hayabusa-2 auf Asteroid 1999 JU3 gelandet-Update-17

12.07.2019

Approach to the 2nd touchdown
–Part 3: To go or not to go–

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.

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  • Figure 1: Change in the surface reflectivity due to the artificial crater formed with the Small Carry-on Impactor (SCI). The black regions indicate areas that have darkened after the collision. The planned touchdown point is in the vicinity of C01-C in the figure; a region that has darkened after the generation of the artificial crater. That is, it is thought that subsurface material from the artificial crater has been deposited in this region. (※ Image credit: JAXA, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, University of Aizu, AIST and Kobe University.)

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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The 2nd touchdown

The 2nd touchdown operation will take place from July 9 – 11. Here, we describe the operation events and schedule (the operation is referred to as the “2nd touchdown” but denoted PPTD).

For discussion about the 2nd touchdown operation, please take a look at these recent articles on our website.  (1) Approach to the 2nd touchdown –Part 1: observations near the touchdown point−
 (2) Approach to the 2nd touchdown –Part 2: details of the touchdown point−
 (3) Approach to the 2nd touchdown –Part 3: to go, or not to go−

The first shift for the 2nd touchdown starts on July 9, where the set-up for the descent will be completed. The actual descent will begin on July 10 at 10:46 JST (on-board time). Initially, the descent will begin at a velocity of 40cm/s, matching that in previous operations. At 21:06 JST, the altitude will reach about 5km, whereupon the descent will slow to about 10 cm/s. On July 11 at 09:40 JST, the altitude will have reduced to 30m and the spacecraft will start to hover. Touchdown will be at about 10:05 JST at the earliest, and about 10:45 JST at the latest. Immediately after touchdown, the spacecraft will rise at a speed of about 65 cm/s and return to the home position on July 12. The entire touchdown operation is shown in Figure 1

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  • Figure 1: Outline of the 2nd touchdown operation. The switch to using the Laser Range Finder (LRF) will take place during the descent from 30m. The time of touchdown indicated with the * is estimated to be around 10:05 JST (onboard time) at the earliest and about 10:45 JST at the latest (image credit: JAXA).

Figure 2 shows the operation at low altitude in detail. When the altitude reaches 30m on July 11 at 09:40 JST, the spacecraft will hover and capture the position of the target marker. This is done autonomously, so we do not know in advance when the target marker will be captured but if this is about 09:51, then the spacecraft will start to descend immediately. At this point, the altitude is being measured by the laser altimeter (LIDAR) but during this next descent, the task will be taken over by the Laser Range Finder (LRF). The spacecraft will then descend while keeping the target marker in the center of the field of view. At 09:57 JST, the spacecraft will reach an altitude of 8.5m and begin to hover again.

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  • Figure 2: Operation sequence at low altitude.
    From an altitude of 8.5m to touchdown, the spacecraft will follow a parabolic trajectory as it descends due to the pull from Ryugu, but will appear to descend almost directly downwards as viewed from the asteroid surface. The times indicated by the asterisk * are an example for the case where the operation proceeds at the fastest time and the actual time could be delayed by up to about 40 minutes. (Image credit: JAXA)

After hovering at an altitude of 8.5, the first step is to change the attitude of the spacecraft to the landing attitude. The landing attitude is basically an attitude parallel to the surface of the asteroid but –as with the first touchdown—the attitude is slightly raised on the ion engine side (opposite the sample capsule) of the spacecraft. In the project team, we refer to this as a ‘tail-up’ posture. Changing the attitude would cause the target marker to move towards the edge of the field of view, but the spacecraft moves horizontally while changing attitude to keep the target marker centralized. After the attitude change, the spacecraft moves further horizontally to directly above the center of the touchdown point. And, when all conditions are satisfied, the spacecraft starts to descend at 10:03 JST, reaching the surface at 10:05 JST, completes the touchdown and immediately begins to rise.

However, please remember that the times for altitudes of 30m or less described in Figure 2 and the above description are for an example case where the spacecraft operation progresses most swiftly. When the 30m altitude is reaches, the spacecraft will operate autonomously. As the spacecraft will check each operation before proceeding, the time needed may be longer. This is why the touchdown time is about 10:05 JST at the earliest but could be as late as about 10:45 JST. The same is true for the other times listed, which may be up to 40 minutes behind the times given here.

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Navigation Images from the 2nd touchdown
operation (PPTD) (Real time delivery)

hayabusa2-hb-1

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Ryugu stereoscopic image by
Dr Brian May

Dr Brian May —astrophysicist and the lead guitarist of the British rock band, Queen— has previously created stereoscopic images (article1, article2) of asteroid Ryugu that can be viewed in 3D. He has now created new images showing the whole of Ryugu that allow a clear view of the large Otohime Saxum rock formation.

Dr Brian May said in a message that:
“Claudia Manzoni and I are proud to be part of the ground-breaking HAYABUSA 2 team. These Stereoscopic images of Ryugu are the closest to actually ‘being there’ that humanity will experience in our lifetimes.”

Claudia Manzoni is a colleague of Brain May in stereoscopic image processing.

Below are the images of Ryugu in 3D stereoscopic vision that were created by Brian May and Claudia Manzoni. The two sets of images show the Otohime Saxum seen from two different directions. Each image set shows the image pairs in parallel stereoscopy and cross-eyed stereoscopy where the left and right eye images are switched, so they are correctly viewed when cross-eyed (typically easier when viewing for the first time). Can you see our asteroid in 3D?

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JAXA/Hayabusa2/Claudia Manzoni/Brian May

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2nd touchdown image bulletin

oday (July 11), the Hayabusa2 spacecraft performed a 2nd touchdown on the surface of asteroid Ryugu. The touchdown occurred at 10:06 JST at the onboard time and was successful. Below we show images taken before and after the touchdown. As this is a quick bulletin, more detailed information will be given in the future.

 

■ Images taken with the Optical Navigation Camera – Wide angle (ONC-W1)

Immediately after touchdown, we captured images with the ONC-W1. Here are two bulletin images from this camera.

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mage take on July 11 2019 at 10:06:32 JST (onboard time) with the ONC-W1.
(Image credit ※: JAXA, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, University of Aizu and AIST.)

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This image was taken on July 11 2019 at 10:08:53 JST (onboard time) with the ONC-W1.
(Image credit ※: JAXA, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, University of Aizu and AIST.)

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CAM-H operated before and after touchdown, capturing images 4 seconds before touchdown, the moment of touchdown and 4 seconds after touchdown. (CAM-H is the camera that was developed and installed on Hayabusa2 through public donations. The field of view is downwards beside the sampler horn.)

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  • Image taken 4 seconds before touchdown with CAM-H (image credit: JAXA).

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The moment of touchdown captured with CAM-H(image credit:JAXA).

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  • Image taken 4 seconds after touchdown with CAM-H (image credit: JAXA).

Cooperation: Kimura lab., Tokyo University of Science
(The technology for CAM-H is the result of previous collaborative research between JAXA and the Tokyo University of Science.)

"JAXA, University of Tokyo & collaborators".

Quelle: JAXA

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