OSIRIS-REx Spots Unexplained Particle Ejections from Asteroid Bennu

NASA’s OSIRIS-REx spacecraft is orbiting the asteroid Bennu in preparation for its sample collection run. According to NASA, Bennu is proving much different than the team expected. Not only is the surface remarkably rugged, but the asteroid is also regularly ejecting puffs of dust and rock into space. This is a phenomenon never before observed on an asteroid.

OSIRIS-REx (the Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer) arrived in orbit of Bennu on Dec. 31 of last year. Just a few days later on Jan. 6, NASA detected the first hints of particle expulsion from the surface of Bennu (see above). At that point, the team increased the frequency of observations, finding the dust ejection was not a one-time event — Bennu does this continuously.

NASA has concluded the particle clouds don’t pose a danger to the spacecraft, giving OSIRIS-REx the all-clear to move in and investigate. The team tracked the particles from these clouds, most of which eventually fell back to the surface of Bennu. Although, some remained in orbit of the asteroid.

We know startlingly little about these puffs of regolith. Comets are known to experience frequent outgassing as the sun heats the frozen parts. However, Bennu is a carbonaceous (or C-type) asteroid composed of various rocks and minerals with a smattering of carbon-containing compounds. About 75 percent of known asteroids are C-type. What if they’re all expelling particles like this due to some previously unknown mechanism? This could change the way we understand asteroids like Bennu.

NASA won’t get the smooth, open surface depicted in this picture. Bennu is covered in dangerous boulders.

Bennu has been an exciting learning experience for NASA that drives home how little we know about asteroids. Like the Japanese Hayabusa 2 mission, NASA has found that Bennu’s surface is covered in craggy boulders, making the Touch-and-Go (TAG) sample collection more challenging. Originally, NASA believed it would be able to find an open area at least 25 meters across for the touchdown. The team will have to make do with a smaller, more hazardous landing zone.

In the coming months, NASA will conduct rehearsals of the sample collection operation. When the probe makes contact, it will blow a stream of nitrogen at the surface to launch small particles into the collection pod. The probe has enough nitrogen for three attempts. If all goes as planned, OSIRIS-REx will return its sample to Earth in 2023.

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