Collecting a sample from asteroid Ryugu is going to be dicey

The scientists and engineers behind Japan's Hayabusa2 mission are giving themselves more time to prepare for a hair-raising sample collection from asteroid Ryugu's surface.

Hayabusa2 arrived at Ryugu in June, deployed three hopping rovers in September, and dropped a toaster-sized lander earlier this month. The spacecraft was scheduled to touch down on the asteroid and collect a sample later this month, but that has been delayed to early 2019 as Ryugu and Hayabusa2 prepare for solar conjunction, a roughly month-long blackout period where they are on the opposite side of the sun from Earth. 

"Once the angle between the spacecraft, Earth and Sun is less than about 6 degrees, the radio noise from the Sun interferes with communication too much to send a signal to Hayabusa2," said Elizabeth Tasker, an associate professor at the Japanese Aerospace Exploration Agency, JAXA. "As that angle shrinks even more, there is also a point where the Sun is physically in the way."

JAXA officials say the delay will give them more time to study Ryugu's surface in preparation for touchdown, while learning more about the performance of their spacecraft.

Tip-to-tip on its solar panels, Hayabusa2 is 6 meters wide, with a central core the size of a refrigerator. To collect a sample, the spacecraft's meter-long sampler horn, which is lined with folds to capture asteroid dust, must physically touch the surface. Upon touchdown, the horn fires small projectiles into Ryugu's surface to kick up and capture material.

Finding an area flat enough to do this has proved difficult. Ryugu is just a kilometer wide, and littered with boulders. The team needs a spot with nothing taller than 50 centimeters. Anything higher and they risk banging the spacecraft off of a rock.

Hayabusa2 navigates using a combination of optical cameras and laser pulses. The cameras help the spacecraft aim in the right direction, while the laser pulses — LIDAR higher than 30 meters, and a laser range finder closer than that — tell the spacecraft how close it's getting to the surface.

Earlier in the mission, JAXA officials assumed the touchdown site needed to be 50 meters wide. The navigation system is only accurate to a certain degree; landing Hayabusa2 is like throwing a dart at a bulls-eye when you're only guaranteed to hit the dartboard. 

After zeroing in on three candidate landing sites, mission managers found the widest possible landing site was just 20 meters across — much smaller than their original 50-meter-wide criteria. Fortunately, Hayabusa2's LIDAR is proving itself to be more accurate than hoped. Following a touchdown rehearsal Oct. 15, the team believes they may only need a 10-meter-wide landing spot. Another trial run is scheduled for Oct. 24.

Photos from small probes dropped on the surface show there may not be much fine-grained material for Hayabusa2 to collect. But mission manager Makoto Yoshikawa says that won't be a problem.

"The sampling method of Hayabusa2 (which is the same as Hayabusa) can be used for any kind of surface, from sands and pebbles to rocks," said Tasker, relaying an answer from Yoshikawa.

A final sample collection date has not been set. Hayabusa2 will remain at Ryugu for most of 2019 before returning to Earth in 2020.