The expected collisional history for NEOs meant that we anticipated that Ryugu would have a “rubble-pile” structure: rather than a coherent body, it’s made of rocks held loosely together by the asteroid’s gravity. We had expected to see a surface pitted with craters and boulders, interspersed with flatter regions and covered with a layer of finer-grained regolith. However, we were shocked to find that Ryugu appears to have almost no fine-grained regolith. A global look at Ryugu shows a homogeneous spread of boulders larger than 8 meters, while closer inspection reveals a high density of boulders about 1 meter in size. The largest boulder, 130 meters across and named Otohime Saxum, sits near the south pole.

Itokawa (the destination of Hayabusa) also had a surface covered in boulders. However, this asteroid had a flat plain with a width of about 50 meters covered with approximately centimeter- sized grains of regolith. We had expected a similar open space on Ryugu, and its absence made landing a serious challenge.

Roving an asteroid

Hayabusa2 has 4 different types of surface operations. The spacecraft carried 3 rovers, a European- built lander, and a Small Carry-on Impactor (SCI) for generating an artificial crater to expose subsurface material. The spacecraft itself also needed to touch the surface to gather material.

After almost 2 months of remote observations, the Japanese and international team members of the Hayabusa2 Joint Science Team gathered at ISAS to choose landing sites. Data from optical navigation cameras and a laser altimeter had been used to create a 3-dimensional model of the asteroid to guide the selection. Considerations such as surface temperature, local slope, and boulder density were major factors for the landing site selections, which also had to be safely separated in distance so as not to interfere with one another.

The day of intense discussion resulted in choosing sites on the asteroid’s northern side for the rovers, the southern side for the lander, and close to the equator for touchdown. The sites were also separated in longitude around the asteroid.

About a month later on 21 September, Hayabusa2 descended to a low altitude of about 55 meters over Ryugu’s surface and separated 2 of the rovers, MINERVA-II-1A and -1B, later named HIBOU and OWL. They were designed to test motion in a low-gravity environment. On Ryugu, there is not enough friction for wheels to roll.

The rovers therefore have an internal weight that rotates and rebounds to create a force that hops the rover across the surface. The rovers are solar powered and autonomous, hopping when their batteries are sufficiently charged. Cameras and thermal sensors send data back to Hayabusa2, providing a close-up look at the rugged landscape.

HIBOU and OWL transmitted data for 113 and 10 Ryugu days respectively and then went quiet. They may have moved into a shadowed region on the asteroid surface and may have been unable to recharge their batteries. It may be possible to regain contact with one or both as the Sun shifts on Ryugu’s surface later in the year.

On 3 October, Hayabusa2 descended once again to deploy the Mobile Asteroid Surface Scout (MASCOT) lander. The German (DLR) and French (CNES) space agencies provided this powerful, shoebox-sized laboratory. The lander was equipped with 4 instruments to image the surface, measure surface temperature and any magnetic field, and analyze the surface composition. Powered by a lithium battery, MASCOT was designed to last about 16 hours, or 2 asteroid days. It exceeded this expectation by operating for 17 hours, and the results are currently being analyzed by the European team.

The original plan was for Hayabusa2 to collect a sample at the end of October 2018. However, with the surface being more treacherous than expected, we decided to postpone that in order to further analyze a safe way to operate the touchdown. At the end of 2018, everyone on the team celebrated with a limited-edition “touchdown beer,” created by the Yatsugatake Brewery in Kiyosato. As Japan welcomed in the new year, we began preparations for descending.

Sample Grab

To confirm the technique for touchdown and to investigate the landscape in more detail, we performed 3 rehearsal descents in September and October 2018. During the third rehearsal, Hayabusa2 descended to an altitude of 12 meters and dropped a target marker, a highly reflective, baseball-sized object. A flashlight on the spacecraft can illuminate the marker while optical navigation cameras take photos. The reflective surface makes the marker easy to spot. The spacecraft can track the marker’s position and use that to navigate precisely during touchdown.