The Japanese spacecraft Hayabusa 2 has touched down on Ryugu, the asteroid it's been orbiting since last summer, and collected a sample from its surface.
Practice makes perfect.
Last week, Hayabusa 2 dove to grab a sample from the surface of the asteroid 162173 Ryugu, swooping down like its namesake peregrine falcon. It all went perfectly according to plan: the moment the sample horn made contact a tantalum bullet fired at the surface, knocking away asteroid material that flew up into the horn. Then the spacecraft flew up and away. Hayabusa 2 has now sealed the sample collection compartment, saving the sample for return to Earth in December 2020.
It all seems so easy, but that ease didn’t happen without a lot of work and worry. Thirteen years ago, Hayabusa 2’s predecessor, Hayabusa, suffered some kind of damage during its two sampling attempts, and the bullet-firing mechanism didn’t work. It took extraordinary creativity and persistence for Hayabusa’s engineers to bring the spacecraft back to Earth, and the sample capsule it returned contained only a few micrograms of dust. The memory of those struggles weighed heavily on those who worked on Hayabusa 2, one of whom remarked that the successful sample grab was “revenge” for Hayabusa’s travails.
Finding the right spot to sample Ryugu was more challenging than anticipated. Itokawa had possessed large “ponds,” relatively smooth deposits of dust and small gravel. Hayabusa sampled one of these. But when Hayabusa 2 arrived at Ryugu on June 27, 2018, no such ponds were visible — the asteroid was uniformly rocky.
The team had to develop new sampling site selection criteria, because no site on Ryugu matched the criteria they’d developed before launch. The scientists defined two extremely narrow landing sites where they’d have to navigate the spacecraft between potentially damaging rocks. The team tried out descending toward those sites twice. Back on Earth, they tested whether the bullet-firing mechanism would liberate enough material from a rocky surface; it appeared to, so they proceeded, and it all worked perfectly — probably.
We won’t know for absolute sure just how successful the sampling operation was until Hayabusa 2 returns to Earth. The spacecraft has no way of measuring just how much material made it into the chamber. However, all the spacecraft telemetry, from the velocity profile of the descent and ascent to the temperature changes inside the sample horn, are consistent with successful sampling.
Hayabusa 2 could actually collect two more samples, placing them into two more collection compartments in the sample return capsule. Now, the mission has to decide whether obtaining the additional sample would be worth the additional risk.
Hayabusa 2 also has a (literally) flashy experiment yet to perform. It has a deployable Small Carry-on Impactor (SCI), a mostly copper spacecraft containing explosives that can create an artificial crater on Ryugu’s surface. Hayabusa 2 will shelter from the explosion behind the asteroid’s bulk, but it would be a shame to set off a firework and not get to see it explode. So Hayabusa 2 also has a deployable camera, DCAM3, which it will release shortly after SCI deploys to (hopefully) obtain views of the crater’s formation. If the experiment is successful, the team would follow by sampling material from the crater. Being able to directly compare material from the asteroid’s weathered surface to its relatively pristine interior would be a boon to scientists, who must often determine bulk composition from remote observations of asteroids’ surfaces.
Hayabusa 2 must wait for planetary alignment to start its journey back to Earth, so it will remain in proximity to Ryugu until November or December. Its return will take about a year. After dropping the sample in the Australian desert on December 7, 2020, the spacecraft could potentially go on to an extended mission, a flyby past another asteroid. Hopefully, the sample capsule will contain lots of asteroid material, which Japan will share with researchers and labs around the world.