NASA's OSIRIS-REx spacecraft is not due to bring home samples from the asteroid Bennu until some time in 2023, but already it is turning up some fascinating discoveries about this distant world. The latest data streamed back to Earth paints a picture of an unexpectedly rocky place with an affinity for blowing off some steam, forcing scientists back to the drawing board ahead of a sampling attempt due to take place next year.
The OSIRIS-REx probe slipped into orbit around Bennu on the final day of 2018 and immediately began scanning the surface with its suite of instruments from relatively close range. Observations made from Earth based on radar measurements and Bennu's ability to conduct and store heat had suggested a largely smooth surface, which would offer scientists plenty of options to choose from for their upcoming sampling maneuver, known as Touch-and-Go (TAG).
But the latest imagery relayed back to Earth shows it to be anything but. Rather, the team has found the asteroid's exterior to be far more rugged than they'd anticipated, a feature that doesn't lend itself particularly well to trying to land on its surface. New up-close imagery shows in detail just how rough and boulder-strewn Bennu's surface is, forcing a rethink on how the spacecraft can safely snaffle some of its material.
"The first three months of OSIRIS-REx's up-close investigation of Bennu have reminded us what discovery is all about — surprises, quick thinking, and flexibility," said Lori Glaze, acting director of the Planetary Science Division at NASA Headquarters in Washington.
The team had originally targeted a clearing on the surface with an 82-ft (25 m) radius for the sampling maneuver, but that is now out of the question given the high density of boulders. It has aptly renamed the sampling maneuver to Bullseye TAG, and will now have to target a much more confined space, though it remains optimistic of pulling it off.
"Bennu has issued us a challenge to deal with its rugged terrain, and we are confident that OSIRIS-REx is up to the task," said Rich Burns, the project manager of OSIRIS-REx at NASA's Goddard Space Flight Center in Greenbelt, Maryland.
Another revelation from the new images that has got scientists thinking is the appearance of plumes erupting from the asteroid's surface. These were first spotted in images from January 6 and confirmed in follow up images over the two months since.
These constitute the first ever close-up images of particles being ejected from an asteroid's surface, and join a number of other interesting discoveries made about Bennu in recent months. A team from the Southwest Research Institute published a paper this week describing new water-bearing minerals on the asteroid, building on a similar discovery from December last year, while research published last week described how the Bennu's orbit appears to be speeding up.
"The discovery of plumes is one of the biggest surprises of my scientific career," said Dante Lauretta, OSIRIS-REx principal investigator at the University of Arizona, Tucson. "And the rugged terrain went against all of our predictions. Bennu is already surprising us, and our exciting journey there is just getting started."
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