Last week the Japan Aerospace Exploration Agency’s (JAXA) spacecraft Hayabusa2 fired a bullet into the asteroid Ryugu, kicking up debris to catch and eventually bring back to Earth. Faced with surprising terrain, mission members back home had spent months testing the maneuver by shooting at a collection of faux meteorites.
When Hayabusa2 was launched in 2014, researchers expected to find sand-like regolith covering Ryugu. But when the spacecraft arrived at its target in June 2018, observations revealed a much rougher surface, with centimeter-sized gravel and larger boulders. The team delayed the first collection attempt, originally scheduled for late October, to allow the mission scientists time to find a safe area for the spacecraft to touch down on the asteroid. The delay also gave them time to investigate how the collection equipment would function given the heavier pieces it might have to catch.
A Stand-in for Ryugu
Originally, the spacecraft planned to dart down to the surface, fire a bullet at the asteroid from close range, and scoop up ejected debris in its sampling horn. But the newfound rocks raised questions about how effective the attempt might be. Before launch, tests revealed that the 5-gram metal bullet, made from tantalum, was capable of crushing a single large rock similar to the meteorites, with the sampler capable of gathering up small pieces. But researchers weren’t sure how the closely packed gravel around the impact site would respond.
Back on Earth, the team had a projector identical to the one on board Hyabasu2. They intended to fire it simply to ensure it still worked properly after 4 years in storage, like the one aboard the spacecraft. Faced with a strange new surface, the team found a new use for the projector: testing whether the spacecraft would be able to collect a sample under existing plans or whether they might need to change things up. The JAXA team recently revealed details about the experiment in English on 14 February.
Asteroids being in short supply on Earth, the first step was preparing large chunks of artificial asteroidal material. Carbonaceous chondrite meteorites, strewn across Earth, are thought to be small pieces broken off of larger C-type asteroids like Ryugu. Researchers created a simulated material that mimicked the strength, density, and composition of these meteorites, all stacked on top of each other like the rocks on Ryugu. In December, they placed the projector and target gravel inside of a vacuum chamber, which was depressurized to mimic space. And then they fired it.
A bullet from a projector, identical to the one aboard Hayabusa2, strikes a pile of rocks simulating the surface of Ryugu. Credit: JAXA
The results were impressive. Traveling at 300 meters per second, the bullet shattered the large gravel pieces. Fragments flew into nearby gravel, shattering them as well. In the experiment, the entire process generated more collectable material than originally estimated.
A Shot and a Catch
The test results seemed to prove out last week when Hayabusa2 collected its first sample. On 22 February, the spacecraft swooped down to the asteroid and, according to JAXA officials, emerged with fragments in its grasp.
Hayabasu2 isn’t the only mission that plans to bring asteroid samples back to Earth. NASA’s Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer (OSIRIS-REx) mission entered orbit around Bennu at the start of 2019, and team members have been watching the JAXA team’s progress.
When the observations of Ryugu came back last summer, “our first thought was, wow, we hope Bennu isn’t quite that rugged,” Melissa Morris, OSIRIS-Rex’s deputy program scientist at NASA headquarters, told Eos. Their luck held—Bennu has a few large rocks but nothing like Ryugu. OSIRIS-REx is slated to collect its sample in late 2020 or early 2021 before it returns to Earth. In the meantime, the NASA team is celebrating the success of their JAXA colleagues.
“We’re all just really excited and happy for the Hayabusa2 team and JAXA,” Morris said. “Kudos and congratulations to them for dealing with this type of challenge.”