Surprise! NASA’s OSIRIS-REx probe catches asteroid spewing bits into space

This view of asteroid Bennu ejecting particles from its surface on Jan. 19 was created by combining two images taken by NASA’s OSIRIS-REx spacecraft. Other image processing techniques were applied, such as cropping and adjusting the brightness and contrast of each image. (NASA / Goddard / Univ. of Arizona / Lockheed Martin Photo)

NASA’s OSIRIS-REx spacecraft has spotted something that hasn’t been seen up close on an asteroid before: plumes of particles erupting into space.

The mission’s scientists shared pictures of the plumes as well as the unexpectedly rugged terrain on the asteroid, known as Bennu, today at the Lunar and Planetary Science Conference in The Woodlands, Texas. They also published a set of seven papers about their findings in the journal Nature.

“The discovery of plumes is one of the biggest surprises of my scientific career,” Dante Lauretta, OSIRIS-REx principal investigator at the University of Arizona, said in a news release. “And the rugged terrain went against all of our predictions. Bennu is already surprising us, and our exciting journey there is just getting started.”

Bits of material were first spotted floating up from Bennu on Jan. 6, shortly after OSIRIS-REx went into an orbit that brought about a mile away from the quarter-mile-wide asteroid. Lauretta and his colleagues determined that the particles didn’t pose a hazard to the spacecraft, and they’re continuing to analyze the plumes and their possible causes.

“We don’t know the mechanism that is causing this right now,” Lauretta said.

Many of the particles were ejected clear of the asteroid and sailed out into space, 70 million miles from Earth, but the team tracked some particles that orbited Bennu as tiny satellites before returning to the asteroid’s surface.

The plumes make Bennu one of about a dozen active asteroids that exhibit comet-like behavior. OSIRIS-REx’s scientists didn’t expect to see such activity when the probe was launched in 2016.

They also didn’t expect to see so many boulders strewn across Bennu’s surface. And that could be a problem.

The main purpose of the OSIRIS-REx mission is to perform a touch-and-go maneuver, collect samples from the surface and put them in a capsule that would be delivered to Earth during a flyby in 2023.

OSIRIS-REx’s mission planners expected to identify a clear area measuring 82 feet wide as the target for the touch-and-go. But because the surface is so rugged, the team hasn’t been able to find a hazard-free spot that wide. Now the team is looking for smaller areas to target, and adjusting its sampling plan to do a more precise touch-and-go maneuver.

“Throughout OSIRIS-REx’s operations near Bennu, our spacecraft and operations team have demonstrated that we can achieve system performance that beats design requirements,” said Rich Burns, the mission’s project manager at NASA’s Goddard Space Flight Center. “Bennu has issued us a challenge to deal with its rugged terrain, and we are confident that OSIRIS-REx is up to the task.”

In addition to the particle plumes and rocky terrain, OSIRIS-REx’s scientists are reporting a change in Bennu’s rotation rate, due to a cycle of heating and cooling known as the Yarkovsky-O’Keefe-Radzievskii-Paddack effect, or YORP effect. It turns out that Bennu’s spin is speeding up at a rate of roughly one second every 100 years. Scientists have also detected a spot of magnetite on Bennu’s surface, which serves as further evidence that liquid water once interacted with the rock that made up Bennu’s parent body.

OSIRIS-REx stands for “Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer.” The probe is due to spend another two years studying Bennu and securing its sample from the surface. If all goes according to plan, it will begin the trip back to Earth in March 2021 and drop off the sample capsule during an Earth flyby in September 2023.

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