Bennu and Ryugu: a tale of two asteroids

Successful Japanese and US space missions have given space scientists a unique opportunity to study two near-Earth asteroids at the same time.

Earlier this month, NASA’s OSIRIS-REx mission rendezvoused with asteroid 101955 Bennu, around which it plans to brake into close orbit on New Year’s Eve. Meanwhile, on June 27, Japan’s Hayabusa2 mission reached asteroid 162173 Ryugu, above which it has since been hovering at distances as short as 60 metres.

Already, the two teams are beginning to compare and contrast their findings, and as the missions progress, each will have the unprecedented opportunity to adjust their operations to take advantage of the other’s discoveries.

“This was not planned,” says Humberto Campins, a planetary scientist from the University of Central Florida, Orlando, US, and co-investigator for OSIRIS-REx. 

Already, the comparisons are coming in. 

Both asteroids, for example, are flattened at the poles, looking in profile a bit like a cross between a diamond and a hacky sack. But oddly, Ryugu, the slower-rotating of the two, is also the most flattened, which is not something one would expect. 

“It’s an asteroid dynamicist’s dream to have a mystery like this to solve,” Dante Lauretta, OSIRIS-REx’s principal investigator, said last week at a meeting of the American Geophysical Society in Washington, D.C.

Both asteroids are also liberally strewn with boulders – a challenge to mission planners on each team, who hope to descend close enough to scoop up samples for return to Earth. 

“It’s certainly more rugged than we had expected,” Lauretta says. 

But while boulders on Ryugu look to be unpleasantly ubiquitous, Bennu’s terrain is more varied, with more-obvious places where a safe touchdown might be possible. And, Lauretta notes, the OSIRIS-REx spacecraft is only recently arrived. 

“We have a long time to explore and make the crucial decisions about where to go,” he says.

The biggest difference has to do with water. Even though scientists believe them to be portions of the same parent body, which broke apart between 800 million and one billion years ago, Bennu appears to be water rich, while Ryugu is less so. 

Within a week of arrival, infrared spectrometers on OSIRIS-REx were finding evidence of water-bearing minerals, and not just in localised patches. 

“We saw this in every single one of the spectra we have taken to date,” says Amy Simon, of NASA’s Goddard Space Flight Centre in Greenbelt, Maryland. 

One of the reasons Bennu was chosen as OSIRIS-REx’s target, Lauretta says, was the hope it would be rich in water, and possibly in organic compounds. {%recommended 7395%}

That’s because, while asteroids like Bennu currently pose hazards if they hit the Earth, it is possible that early in the history of our planet enough of them rained down to bring not only water, but the chemical building blocks for life.

The building blocks haven’t yet been detected on Bennu, Lauretta says, “but it definitely looks like we’ve gone to the right place”. 

Meanwhile, Bennu is also proving to be a geologically interesting worldlet. Of particular interest is a giant boulder, tens of metres in diameter, resting on its surface like a giant egg. 

“We don’t know how a large boulder would end up like that,” Campins says. “It could be it was on top of some high point and broke off and rolled down. Or it could have been a satellite of Bennu that somehow spiralled in. It’s producing all kinds of interesting conversations.”

Also fueling speculation is that however different the two asteroids are, both are thought to be fragments of the same asteroid family, called the Polana family – named for 142 Polana, which at 55 kilometres across is the largest surviving relic. 

If so, why are they so different? Perhaps, says Campins, they came from different layers in the interior of the family’s parent body.

Meanwhile, science isn’t OSIRIS-REx’s only goal. The “RI” in its name stands for resource identification. “We want to identify potential resources to go mine them in the future,” Campins explains. 

The most important of these is water, now known to be present in Bennu’s hydrated minerals. But it isn’t the only one. 

“The moment you can produce both water and building materials cheaper from asteroids than from launching them from Earth,” Campins says, “there are going to be a lot of clients that are going to want to buy it.

“I think that sooner than many people think, there will be a self-sustaining space economy, with asteroid mining selling building materials to Earth orbit and lunar operations.”

How soon? “I might as well stick my neck out,” Campins says. “I’d say within 10 to 15 years.” 

If he’s right, that’s only 10 years after OSIRIS-REx’s sample is scheduled to be returned to Earth, in 2023. 

Meanwhile NASA’s next immediate problem will be learning to fly the OSIRIS-REx spaceship around an asteroid so small its surface gravity is only 1/100,000 that of the Earth. 

That’s so small, Lauretta says, that in orbit the force of gravity isn’t much different from the radiation pressure from the sun or a number of other factors normally to small to worry about. 

“That’s why the navigation team gets two months to learn to fly,” Lauretta says. 

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