On December 28 —the last day of work in 2018— the sampler team conducted an important experiment. As a final test before touchdown (TD), the team fired an identical bullet to that onboard Hayabusa2 into a simulated soil of the surface of Ryugu to test how much sample would be ejected.
Hayabusa2 uses a projector to inject metal bullets into the asteroid surface and release material, before passively collecting these samples through the sampler horn. This projector, including the pyrotechnic products, were manufactured with multiple flight spares (equivalent products manufactured at the same time as the flight model) (Figure 1).
The original purpose of this experiment was to confirm that one month before the TD operation, the flight spare projector was operating normally after it had been stored for a long period of four years.
As we now know, the expected topography of a powdery fine regolith was not found on the surface of Ryugu. But cm-sized or larger gravel was observed by the MASCOT and MINERVA-II1 rovers that landed on the asteroid surface. This is quite different from the prediction before launch, so it took time to investigate the safety of the spacecraft during TD. Additionally, it was necessary to review whether sample material would still be released from the asteroid surface as originally assumed.
Therefore, we decided to use the flight spare projector to perform an operation confirmation test, whereby we examined what happens if a bullet identical to that onboard Hayabusa2 is fired into a target that simulates the observed surface of Ryugu.
First, we prepared an artificial gravel in collaboration with Professor Hideaki Miyamoto at the University of Tokyo, Graduate School of Engineering. By simulating properties such as strength, density and composition, we replicated a carbonaceous chondrite meteorite, which is regarded as fragments of C-type asteroids similar to Ryugu. The target was formed by stacking up the artificial gravel with a similar size distribution as that observed on the surface of Ryugu based on images from the landers (Figure 2).
For this test, the Ryugu simulated target and the projector were installed in a vacuum chamber with an inner diameter of about 60cm. The bullet was then injected after depressurizing to 1/1000 atm or less (Figure 3).
In the ground test performed during the initial development, even large rocks with similar strengths to carbonaceous chondrite meteorites were crushed when a projectile made of metal (tantalum) with a mass of 5g was injected at about 300 m/s. It was confirmed that material formed from the resulting small pieces could be gathered by the sampler. So in this test, it was predictable that the bullet would crush material that it struck, but what would be the behavior of the gravel surrounding the focus of the shot?
From the results of the experiment, the fragments of gravel that were crushed were released into the surrounding gravel where they collided like billiards to break up the material. The resulting sample amount exceeded the initial assumption that would be released from the surface (Figure 4).
While the diameter of the collision site (crater) made by the impact of the projectile is smaller than when compared to that in a fine regolith layer, it was a sufficient size in comparison with the inner diameter of the open tip of the sampler horn. Although the experiment was carried out in the Earth’s gravity, the images from the high-speed camera (Figure 5) revealed that samples of various sizes from fragments of crushed powdery gravel to samples that can pass through the horn were released. Under microgravity, even more samples are expected to be introduced into the sampler horn, meaning that if we land on a terrain similar to the simulated target, we can sample the surface of Ryugu.
With test results obtained that exceeded expectation, the sampler team celebrated for a good new year.
※This experiment was supported by Professor Hideaki Miyamoto from the University of Tokyo and Dr. Sunao Hasegawa from ISAS.
Hayabusa2 Project Sampler Team
2019.02.14 (Translated 2019.02.18)