OSIRIS-REx Team Explores Activity of Asteroid Bennu

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NASA’s OSIRIS-REx spacecraft, launched in 2016, is currently orbiting the near-Earth asteroid (101955) Bennu with the aim of briefly touching on the surface and obtaining a sample from the asteroid in October 2020, and then returning to Earth. Shortly after entering orbit around Bennu, the spacecraft’s instruments detected asteroid activity in the form of particles emanating from the surface. The OSIRIS-REx science team has since documented multiple discrete particle ejection events, orbiting particles, and particle sub-orbital trajectories. In a series of eleven papers published in the Journal of Geophysical Research: Planets and the journal Earth and Space Science, the team reports detailed observations of this activity.

This mosaic image of asteroid Bennu is composed of 12 images collected on December 2, 2018 by OSIRIS-REx’s PolyCam instrument from a range of 15 miles (24 km). Image credit: NASA / NASA’s Goddard Space Flight Center / University of Arizona.

This mosaic image of asteroid Bennu is composed of 12 images collected on December 2, 2018 by OSIRIS-REx’s PolyCam instrument from a range of 15 miles (24 km). Image credit: NASA / NASA’s Goddard Space Flight Center / University of Arizona.

“We thought that Bennu’s boulder-covered surface was the wild card discovery at the asteroid, but these particle events definitely surprised us,” said OSIRIS-REx principal investigator Professor Dante Lauretta, a researcher in the Lunar and Planetary Laboratory at the University of Arizona.

“We’ve spent the last year investigating Bennu’s active surface, and it’s provided us with a remarkable opportunity to expand our knowledge of how active asteroids behave.”

“We’ve been studying asteroids for a long time, and no one had ever seen this phenomenon before — these little particles getting shot off of the surface,” said Professor Daniel Scheeres, a scientist in the Smead Department of Aerospace Engineering Sciences at the University of Colorado Boulder.

“We want to know what that means for the bigger picture of how asteroids live their lives,” said Dr. Jay McMahon, also from the Smead Department of Aerospace Engineering Sciences at the University of Colorado Boulder.

OSIRIS-REx observed and tracked more than 300 particle ejection events on Bennu. Some particles escape into space, others briefly orbit the asteroid, and most fall back onto its surface after being launched.

Ejections most often occur during Bennu’s local two-hour afternoon and evening timeframe.

During a number of observation campaigns between January and September 2019 dedicated to detecting and tracking mass ejected from the asteroid, a total of 668 particles were studied, with the vast majority measuring between 0.5 and 1 cm (0.2-0.4 inches), and moving at about 20 cm (7.9 inches) per second. In one instance, a speedy outlier was clocked at about 3 m (9.8 feet) per second.

On average, the team observed one to two particles kicked up per day, with much of the material falling back onto the asteroid.

“To give you an idea, all of those 200 particles we observed during the first event after arrival would fit on a 10 x 10 cm (4 x 4 inches) tile,” said Dr. Carl Hergenrother, a researcher in the Lunar and Planetary Laboratory at the University of Arizona.

“The fact that we can even see them is a testament to the capabilities of our cameras.”

The scientists investigated various mechanisms that could cause these phenomena, including released water vapor, impacts by small space rocks known as meteoroids and rocks cracking from thermal stress.

The two latter mechanisms were found to be the most likely driving forces, confirming predictions about Bennu’s environment based on ground observations preceding the space mission.

As the asteroid completes one rotation every 4.3 hours, boulders on its surface are exposed to a constant thermo-cycling as they heat during the day and cool during the night. Over time, the rocks crack and break down, and eventually particles may be thrown from the surface.

The fact that particle ejections were observed with greater frequency during late afternoon, when the rocks heat up, suggests thermal cracking is a major driver.

The timing of the events is also consistent with the timing of meteoroid impacts, indicating that these small impacts could be throwing material from the surface.

Either, or both, of these processes could be driving the particle ejections, and because of the asteroid’s microgravity environment, it doesn’t take much energy to launch an object from Bennu’s surface.

Of the particles the authors observed, some had suborbital trajectories, keeping them aloft for a few hours before they settled back down, while others fly off the asteroid to go into their own orbits around the sun.

“In one instance, we tracked one particle as it circled the asteroid for almost a week,” Dr. Hergenrother said.

“The spacecraft’s cameras even witnessed a ricochet.”

“One particle came down, hit a boulder and went back into orbit,” he said.

“If Bennu has this kind of activity, then there is a good chance all asteroids do, and that is really exciting.”

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