The Japan Aerospace Exploration Agency (JAXA) spacecraft Hayabusa2 has once again successfully collected samples of the asteroid Ryugu!
On July 10, the washing-machine-sized spacecraft began its agonizingly slow descent to the asteroid’s surface. At 40 centimeters per second, it could have been outrun by a mosquito (though, to be fair, a mosquito wouldn't last long in the depths of space). Then, an hour after midnight UTC on July 11, it made contact with the surface.
It then fired a 5-gram bullet made of the element tantalum into the asteroid at over a thousand kilometers per hour. The impact blasted material off the surface, where it was collected into a horn on the spacecraft. This debris was then packed away, stored for a later return to Earth planned in late 2020.
Everything about this is amazing, but first let me show you the video of the event! It’s a time-lapse, starting when Hayabusa2 was about 8.5 meters above Ryugu's surface and ending some eight minutes later when it was 150 meters away. The moment it shoots the bullet is pretty obvious, Watch:
That. Is. So. Cool.
The first touchdown was done in late February 2019, and the procedure here was very similar (you can read about it in the article I wrote then, which explains how this was done, why the bullet was made of tantalum, and more). But there's a big difference between the two, and it has to do with what happened in between the events.
There are lots of mission goals for Hayabusa2, including getting samples of rocks from the surface. But an important one is to get samples from below the surface, away from radiation from the Sun, cosmic rays, and micrometeorite impacts. To get this subsurface sample, Hayabusa2 switched weapons from a gun to a cannon: In April it shot a 2-kilogram copper projectile into the surface at a speed of over 7,000 kph into Ryugu! This blasted a crater over 10 meters across, digging up material from below the surface to be collected.
In May it approached the surface again to drop a target marker, a shiny object filled with small plastic grains to absorb the impact and prevent repeated bouncing on the surface. Hayabusa2 uses LIDAR to measure its distance from the surface, but sideways motion is more difficult to measure. The target marker acts as a benchmark so that it can more easily see its motion as it approaches. The marker was dropped 20 or so meters away from the impact crater, in a location that had been observed carefully after the impact to make sure it was safe to touch down there. Jagged rocks and an uneven surface are significant dangers to the spacecraft, and the mission controllers wanted to make absolutely sure they could make the second touchdown with minimal risk.
And that's why the second touchdown was done. The first was successful, but the second was important for two reasons. One is that it could collect samples from a different spot than the first touchdown; you want to make sure your samples are representative of the surface. If you collect only one it's possible that just by chance you picked a weird spot, and the material, while interesting, may not be good to use to extrapolate to the asteroid as a whole.
The second reason is that they had confirmed subsurface debris from the impact had settled in the candidate landing spot, so collecting from that site would mean getting that important material.
Since the touchdown was successful, we can hope that some of that precious cargo is now on board the spacecraft. Later this year, Hayabusa2 will complete its mission at Ryugu. It'll fire up its low-impulse ion engines and set course for Earth. A year later it will fly past our fair planet and release the sample return container, which will then ram through Earth's atmosphere, slowing enough that parachutes can be deployed to slow it further. It will land in the Australian desert to be picked up and returned to Japan where the samples can then be examined in detail.
This is all pretty amazing. Ryugu is a rubble pile asteroid a mere 900 meters across, and was roughly 200 million kilometers from Earth during the second touchdown. This is not like dropping by the local supermarket; this was a highly planned and detailed mission that has been executed incredibly well.
In fact, if you want more technical details on the second touchdown I recommend reading JAXA's Hayabusa2 blog, as well as a great article over at The Planetary Society website. That will help you appreciate just how great an achievement this mission is.
And there's still a lot more to do! Stay tuned for more information about Hayabusa2, Ryugu, and humanity's attempts — successful attempts, I'll add — to better understand asteroids, the building blocks of the solar system.