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A flawlessly executed mission on a mysterious asteroid — thanks to Hayabusa2’s sampler horn

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Position of the sampler horn aboard Hayabusa2 and closeup of the mechanism, which is used to collect samples
The sampler projector, for firing projectiles, is housed inside the horn

Have you noticed the black cylinder extending from the front end of the asteroid explorer Hayabusa2? That’s its “sample horn,” a mechanism for collecting rock and sand samples from the surface of the asteroid Ryugu. And it’s a very ingenious device indeed. It took it only seconds to secure a sample, no matter what surprises the asteroid held.
The sampler horn system was developed by NEC, while the actual mechanism was designed and manufactured by Sumitomo Heavy Industries, Ltd. It takes a pragmatic approach unaffected by the nature of the asteroid’s surface. When the explorer lands on an asteroid and the tip of the sampler horn touches the surface, a signal is sent to the spacecraft telling it that it has touched down. The projector inside then shoots a spherical projectile made of rare metal, which breaks the surface and sends fragments flying. The resulting cloud of debris ends up in the catcher.
Most people probably imagine that sand is collected from an asteroid by scooping it up with a shovel. To use a shovel, though, you need a surface covered in sand; and because asteroids have such a weak gravitational field, the spacecraft could end up rising off the ground, shovel and all. Plus using a shovel is a complicated operation. The spacecraft has to touch down on the surface with its “legs” and then scoop up sand with the equivalent of its “hands”. For those reasons NASA’s asteroid explorer OSIRIS-REx, for example, took a different tack. It collected asteroid samples by releasing a burst of nitrogen gas from the end of its sampling arm and capturing the sand stirred up as a result. But that approach, too, assumes that the asteroid is sandy. In short, a shovel is not the preferred option when you won’t know whether the asteroid’s surface is covered in fine sand or big boulders until the probe actually gets there.

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The rare-metal projectile fired by the sampler projector

Ryugu, as it turned out, wasn’t very sandy. It was covered in boulders, making it a difficult place to land. But it made no difference to the sampler horn whether it was sandy or rocky. It simply shot a projectile into the surface and instantly bagged the material stirred up by the impact. That was the brilliant solution that engineers came up with to the tricky problem of collecting samples from a celestial object whose nature was still a mystery.
Did you realize, by the way, that Hayabusa2’s “projector”— the mechanism that fires the projectiles — is one of a pair of twins? Its twin brother was stowed away on Earth for four years after Hayabusa2’s launch. Then, in December 2018 — two months before its counterpart in space was to go into action — it was tested in a vacuum chamber simulating conditions in space to determine whether it still worked properly.* It was thus key to making sure that the projector could be relied on to execute its mission, even after so long.
Hayabusa2 touched down on the asteroid Ryugu in February and July 2019, and it was confirmed that the projectile fired exactly as it was supposed to both times. And so the intrepid explorer, like the bird of prey after which it is named — hayabusa means peregrine falcon in Japanese — succeeded in swiftly capturing some precious fragments of an asteroid. Which it never could have done without the sampler horn.

*Details of the projectile firing test performed using the sampler projector stored on Earth were released by JAXA in a briefing to reporters about the asteroid explorer Hayabusa2 on February 6, 2019.

Written by Ayano Akiyama
Published: November 20, 2020

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