Ryugu samples bound for Australia

The Japanese space agency’s Hayabusa 2 spacecraft has collected the second of two samples from near-Earth asteroid 162173 Ryugu and is preparing to return them to Earth in a capsule that will land in Australia late next year.

The sample return will be the culmination of an ambitious six-year mission that has already deployed more landing probes than any other space mission in history, and even fired a 2.5-kilogram projectile at the asteroid at a speed of 2 kilometers per second – fast enough to excavate a 10-meter crater on the asteroid’s surface.

From the moment Hayabusa 2 braked into its rendezvous with the asteroid in June 2018, the mission found much that was unexpected. To begin with, the 900-metre-wide asteroid was top-shaped, with a prominent equatorial ridge, similar to asteroid 101955 Bennu, subsequently visited by NASA’s OSIRIS-REx mission. 

Its surface was also extremely dark, reflecting only slightly more than 2% of incoming sunlight, Shogo Tachibana from the University of Tokyo, who is in charge of sample collection and analysis, told the 2019 Microscopy & Microanalysis conference in Portland, US, yesterday. 

That’s one of several lines of evidence, he says, that indicates Ryugu may have formed from rubble blasted out of a parent body whose rocks may have been heated to as high as 500°C.

But most importantly, its surface was studded with boulders – so many that the first sample-collection landing was delayed by four months while mission controllers searched for a safe site. “There are a lot of dangerous boulders,” Tachibana says. 

In the interim, the spacecraft deployed two rovers – or, more technically, hoppers – which landed on the asteroid’s cobbled surface, then bounced around in its low gravity in a series of gentle leaps, in an effort to survey the surface from multiple locations.

Another lander settled on a single site for 17 hours, until its batteries died – long enough to take measurements through two of the asteroid’s 7.6-hour days. In the process, it was able to study the “thermal inertia” of a nearby stone, watching how the rock’s temperature rose and fell as it cycled from day to night.  

From that, Tachibana says, the researchers concluded that the stone was far less massive than its size would suggest, indicating that it must be made of frothy material that might be about 50% empty bubbles. 

It’s a finding, he adds, that tracks with measurements made by Hayabusa 2 itself, which found that the entire asteroid has such a low density that it must have an overall porosity of more than 50%.

“These are the results we have so far found,” Tachibana says.

On 21 February, the spacecraft found a safe place to land and grabbed a sample in a touch-and-go mission that appears to have gone as planned. 

Then, on 11 July, it did it again. But this time, it aimed for a landing zone near the crater created by the impact projectile’s cannon-like blast. 

The goal, Tachibana says, was to collect material blown out of the crater by the impact, hoping that it would include material from the subsurface – a nice alternative to the technologically challenging task of trying to drill into the asteroid’s interior, something which, to date, no asteroid-rendezvous mission has even attempted. 

All told, the goal is to collect a total of 100 milligrams of pebbles, sand and dust. 

Once safely back on Earth, Tachibana says, these can be used to study everything from the formation of dust grains before the birth of the Solar System to the evolution of the protoplanet whose breakup pieces are now assembled in Ryugu. 

Such studies would also include the question of whether these materials were once heated enough to produce the mildly dehydrated surface we now see, or whether the protoplanet from which they came simply didn’t have a lot of water in the first place. “This should be one of the issues that sample analysis should solve,” Tachibana says.

When completed, he says, these analyses should help us understand not only the history of the asteroid, but the evolutionary history of the Solar System, including the role asteroids played in delivering water and organic chemicals – important precursors to life – to the proto-Earth.

The Ryugu asteroid, he says, is named for a castle under the sea in an old Japanese folktale. “And that is why we are returning samples,” he says, “in order to understand the formation of the ocean, and life.”

Related reading:

• Some clues to Ryugu’s odd colours