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More than 4 years after launch and a half year surveying asteroid Ryugu in space, Japan's Hayabusa2 spacecraft is ready for its biggest moment yet: sample collection. The spacecraft is scheduled to touch down on Ryugu at 08:15 Japan time on 22 February (21 February 23:15 UTC, 18:15 EST). If all goes well, Hayabusa2 will gently touch Ryugu with its meter-long sample horn, fire a bullet made of tantalum into the surface, and capture the resulting cloud of dust and debris.
Hayabusa2's touchdown site is a 6-meter-wide circle named L08-E1. It's near Ryugu's equator, roughly between Kintaro crater to the west and Brabo crater to the east. Here's a JAXA diagram with the landing site as a red box:
Here's a wide shot I made using a NavCam image, with the landing site circled. Brabo crater is visible as the big indentation on the right:
And finally, here's an animation I made using prior touchdown rehearsal images that pauses to circle the landing site:
Even though it's the smaller of thetwo final regions the team was considering for sample collection, L08-E1 won out because the rocks there are smaller than 60 centimeters, and it's closer to thetarget marker Hayabusa2 dropped, meaning the spacecraft can keep it in view for longer as it descends.
The entire landing procedure lasts roughly 2 days, from when Hayabusa leaves and eventually returns to its 20-kilometer home position. Here's a timetable of events in Earth-received time; just subtract 19 minutes from any cell to find out when something actually happened on the spacecraft. JAXA notes that these times are not fixed and may change.
Update 21 Feb: JAXA updated some of their times in thelatest press briefing. Those updated times are now shown in italics.
(Note: JAXA has not listed an estimated time for Hayabusa2's return to home position.)
The moment that Hayabusa2's meter-long sample horn touches the surface, it fires a bullet made of tantalum from within the horn. The 5-gram bullet hits Ryugu at about 300 meters per second, stirring dust and small particles up the horn, through a 90-degree turn, and into a sample catcher. The bullet is made from tantalum so scientists can distinguish pieces of it from the sample material.
Originally, the Hayabusa2 team expected Ryugu to be covered in regolith, meaning the bullet shouldn't have had any problem spreading a sample up into the spacecraft. But theMINERVA-II1 andMASCOT rovers showed that the surface was mostly covered with gravel. Would the sample collection procedure still work?
To find out, the teamfired a flight spare bullet into a container of Ryugu-like gravel, and filmed the results. Not only did they collect material, "the resulting sample amount exceeded the initial assumption that would be released from the surface." The pre-mission plan was for Hayabusa2 to collectat least 100 milligrams of fine-grained material, and several millimeter-sized chunks. I’m not clear on whether the team is saying the new test exceeded those original assumptions, or just their pre-test predictions. Either way, the team is happy, and they're confident they'll get even more material during the actual sample collection, because Ryugu’s gravity is so much weaker than Earth’s.
The sample collection process literally takes just a second; Hayabusa2 fires its thrusters just 1 second after touchdown to avoid tipping over. The sample horn has a lip around the bottom opening to catch a few small pieces of gravel as they tumble back out. When the spacecraft decelerates later in its ascent, those pieces will have a second chance to rise up and into the sample catcher.
After touchdown, Hayabusa2 makes its way back to the home position. There's no way to know for sure how big a sample the spacecraft captures until the return capsule is opened back on Earth. If the tantalum bullet fires as planned, they are almost certain to have collected a good sample. In the case of the first Hayabusa mission, the bullet didn’t fire, but a few particles of asteroid Itokawastill made their way into the sample container.
Amazingly, there’s more mission excitement yet to come. The spacecraft can collect three samples in total, but the team is waiting to see how this first attempt goes before deciding whether to make a second attempt.
Beyond that, there’s another experiment to look forward to. The Small Carry-on Impactor (SCI) contains a box of explosives and a copper bullet (this mission has lots of bullets!) to create an artificial crater on the surface. The team could choose to collect a sample from that crater.
When the spacecraft finishes collecting its sample(s), a spring pushes the sample catcher into the return capsule, which sits inside the reentry capsule. Hayabusa2 will spend most of 2019 at Ryugu, waiting for the planets to align, before returning to Earth in 2020.
