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Link here. As much as they are alike, which is not surprising as they both come from the same asteroid family, the article notes the surprising differences.
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.
The similarities and differences in the samples both spacecraft will bring back will be most revealing.