Hayabusa2 Eyes Difficult Landing on Asteroid Ryugu’s Rocky Surface

Covered in primordial rocks and boulders, the near-Earth asteroid’s material could provide important insights into how planetary systems form.

Hayabusa2, MASCOT, Ryugu, asteroid
An illustration of Hayabusa2 and the asteroid Ryugu. | DLR German Aerospace Center
An illustration of Hayabusa2 and the asteroid Ryugu. | DLR German Aerospace Center

As the Japanese spacecraft Hayabusa2 prepares to land on the surface of the asteroid Ryugu this coming January, results are starting to come in from data collected by one of its payloads.

The main goal of the mission is to return a sample of asteroid material back to Earth to help scientists better understand the history of the solar system. Ryugu is among 17,000 certified near-Earth asteroids, and its material could provide important insights into how planetary systems form.

Hayabusa2 is supplementing its mission with smaller spacecraft. A German-built shoebox-sized lander, dubbed the Mobile Asteroid Surface Scout (MASCOT), separated safely from Hayabusa2 on Oct. 2 for a 17-hour adventure on the surface — the lifespan of its lithium-ion battery.

“With MASCOT, we have the unique opportunity to study the solar system’s most primordial material directly on an asteroid,” said Ralf Jaumann, a planetary researcher with the German Aerospace Center (DLR), after the landing.

DLR later reported that MASCOT had realized all of its mission objectives. The instruments worked beautifully, MASCOT was able to hop to another spot when commanded, and all of the data is now back on Earth and ready for analysis.

It will take months to do preliminary studies of the data, but already there are some early indications.

Hayabusa2, Ryugu, MASCOT, asteroid
An image taken by MASCOT after the lander's first contact with asteroid Ryugu's surface. | DLR/JAXA

“Ryugu seems to be covered with big boulders,” said MASCOT project leader Tra-Mi Ho in a statement. “In comparison with meteorites such as the Murchison, which has been used as reference for the composition on Ryugu, it looks like the boulders on Ryugu are more porous.”

The asteroid was also found to be very dark in color. Images of the surface appear gray because they were enhanced to show as much detail as possible.

MASCOT’s data will be supplemented with other Hayabusa2 mini-landers, as well as the Hayabusa2 spacecraft itself.

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Hayabusa2’s planned touchdown this year was delayed due to Ryugu’s terrain. The asteroid is rockier and has fewer landing zones than was anticipated.

Before Hayabusa2 arrived at Ryugu, DLR and other agencies examined the record of asteroids already looked at by spacecraft, such as Itokawa, Steins, Lutetia, Eros. Admittedly, some of these past explorations were flybys and did not give a high-resolution view of the surface.

Yet scientists anticipated that the surface of Ryugu would be covered in pebble-sized grains, similar to what Hayabusa (Hayabusa2’s predecessor mission) observed on Itokawa after arriving in 2005. Ryugu proved to be something different.

“All observations [including the ones of MASCOT] indicate Ryugu to be rocky all over,” Ho said. “There seem to be no wide, smooth terrains. However, for the safe landing and sampling maneuver of Hayabusa2, a safe landing area of [only] 100 meters [330 feet] is required.”

The spacecraft needs to carefully choose a spot that’s relatively clear of boulders. The difficulty will be to find a 100-meter area with boulders that are smaller than 50 centimeters to aid with sample collection.

“Unlike other asteroids we have visited, Ryugu has no powder, no fine-grain regolith. That makes selecting a place to sample more challenging,” said Deborah Domingue, a member of the Hayabusa2 science team. “We are helping characterize the surface to optimize landing site selection.”

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While MASCOT travels on Ryugu are complete, its design could be adapted to visit one of Mars’s moons — Phobos or Deimos. These Martian moons have never had a surface mission, although images of them have been captured during spacecraft flybys. Germany, Japan, and France are developing a Phobos rover to fly on a future Japanese mission called Martian Moons eXploration, set to launch in the early 2020s.

MASCOT was a hopper, but the Phobos rover will be more of a driver.

“Based on the higher gravity on Phobos, the engineers decided for the wheeled locomotion system,” Ho said. But on worlds with lower gravity, MASCOT's design would be helpful — although Ho noted that a longer battery life would be would be critical to collecting more data.

Hayabusa2 has been doing some landing practice runs in the last while, but soon it will turn its attention to two months of automated science operations. This is because Ryugu has grown too close to the sun for Earth’s radio commands to safely transmit without interference. Detailed science operations will resume afterwards.